Issuu on Google+

seaweeds and other algae are common, large stands of seaweeds do not become established. Thus, sea urchins appear to limit the local distribution of seaweeds. This kind of interaction can be tested by "removal and addition~ experiments. In studies near Sydney, Australia, WI. J. Fletcher tested the hypothesis that sea urchins are a biotic factor limiting seaweed distribution. Because there are often other herbivores in the habitats where seaweeds may grow, Fletcher performed a series of manipulative field experiments to isolate the influence of sea urchins on seaweeds in his study area (see Figure 52.8). By removing sea urchins from certain plots and observing the dramatic increase in seaweed cover, he showed that urchins limited the distribution of seaweeds. In addition to predation and herbivory, the presence or absence of food resources, parasites, pathogens, and competing organisms can act as biotic limitations on species distribution. Some of the most striking cases of limitation occur when humans accidentally or intentionally introduce exotic predators or pathogens into new areas and wipe out native species. You will encounter examples ofthese impacts in Chapter 56, which discusses conservation ecology.

Water The dramatic variation in water availability among habitats is another important factor in species distribution. Species living at the seashore or in tidal wetlands Can desiccate (dry out) as the tide recedes. Terrestrial organisms face a nearly constant threat of desiccation, and the distribution of terrestrial species reflects their ability to obtain and conserve water. Desert organisms, for example, exhibit a variety of adaptations for acquiring and conserving water in dry environments, as described in Chapter 44.

Salinity As you learned in Chapter 7, the salt concentration ofwater in the environment affects the water balance of organisms through osmosis. Most aquatic organisms are restricted to either freshwater or saltwater habitats by their limited ability to osmoregulate (see Chapter 44). Although many terrestrial organisms can excrete excess salts from specialized glands or in feces, salt flats and other high-salinity habitats typically have few species of plants or animals.

Abiotic Factors

Sunlight

The last question in the flowchart in Figure 52.6 considers whether abiotic factors, such as temperature, water, salinity, sunlight, or soil, might be limiting a species' distribution. Ifthe physical conditions at a site do not allow a species to survive and reproduce, then the species wm not be found there. Throughout this discussion, keep in mind that the environment is characterized by both spatial heterogeneity and temporal heterogeneity; that is, most abiotic factors vary in space and time. Although two regions of Earth may experience different conditions at any given time, daily and annual fluctuations of abiotic factors may either blur or accentuate regional distinctions. Furthermore, organisms can avoid some stressful conditions temporarily through behaviors such as dormancy or hibernation.

Sunlight absorbed by photosynthetic organisms provides the energy that drives most ecosystems, and too little sunlight can limit the distribution of photosynthetic species. In forests, shading by leaves in the treetops makes competition for light especially intense, particularly for seedlings growing on the forest floor. In aquatic environments, every meter of water depth selectively absorbs about 45% of the red light and about 2% of the blue light passing through it. As a result, most photosynthesis in aquatic environments occurs relatively near the surface. Too much light can also limit the survival oforganisms. The atmosphere is thinner at higher elevations, absorbing less ultraviolet radiation, so the sun's rays are more likely to damage DNA and proteins in alpine environments (Figure 52,9). In other ecosystems, such as deserts, high light levels can increase temperature stress if animals are unable to avoid the light or to cool themselves through evaporation (see Chapter 40).

Temperature Environmental temperature is an important factor in the distribution of organisms be<ause of its effect on biological processes. Cells may rupture ifthe water they contain freezes (at temperatures below (fC), and the proteins of most organisms denature at temperatures above 45'C. In addition, feworganisms can maintain an active metabolism at very low orvery high temperatures, though extraordinary adaptations enable some organisms, such as thermophilic prokaryotes (see Chapter 27), to live outside the temperature range habitable by other life. Most organisms function best within a specific range of environmental temperature. Temperatures outside that range may force some animals to expend energy regulating their internal temperature, as mammals and birds do (see Chapter 40).

1154

UNIT EIGHT

Ecology

Rocks and Soil The pH, mineral composition, and physical structure of rocks and soil limit the distribution of plants and thus of the animals that feed on them, contributing to the patchiness of terrestrial ecosystems. The pH of soil and water can limit the distribution of organisms directly, through extreme acidic or basic conditions, or indirectly, through the solubility of nutrients and toxins. In streams and rivers, the composition of the substrate (bottom surface) can affect water chemistry, which in turn influences the resident organisms. In freshwater and marine environments, the


The sun's warming effect on the atmosphere, land, and water establishes the temperature variations, cycles of air movement, and evaporation of water that are responsible for dramatic latitudinaJ variations in climate. Figure 52.10, on the next WiO pages, summarizes Earth's climate patterns and how they are formed.

Regional, Local, and Seasonal Effects on Climate Proximity to bodies of water and topographic features such as mountain ranges create regional climatic variations, and smaller features of the landscape contribute to local climatic variation. Seasonal variation is another influence on climate.

.. Figure 52.9 Alpine tree. Organisms living at high elevations are exposed to high levels of ultraviolet radiation They face other challenges as well. including freezing temperatures and strong winds, which increase water loss and inhibit the growth of limbs on the windward side of trees.

structure of the substrate determines the organisms that can attach to it or burrow into it. Now that we have surveyed some ofthe abiotic factors that affect the distribution of organisms, let's focus on how those factors vary with climate, as we consider the major role that climate plays in determining species distribution.

Climate Four abiotic factors-temperature, precipitation, sunlight, and wind-are the major components of climate, the long-term, prevailing weather conditions in a particular area. Climatic factors, particularly temperature and water availability, have a major influence on the distribution ofterrestrial organisms. We Labrador can describe climate patterns on current two scaJes: macroclimate, patGulf terns on the global, regional, and stream local level; and microclimate, very fine patterns, such as those encountered by the community of organisms that live beneath a fallen log. First let's consider Earth's macroclimate.

Global Climate Patterns Earth's global climate patterns are determined largely by the input of solar energy and the planet's movement in space.

Bodies of Water Ocean currents influence climate along the coasts of continents by heating or cooling overlying air masses, which may then pass across the land. Coastal regions are also generally moister than inland areas at the same latitude. The cool, misty climate produced by the cold CaJifornia current that flows southward along the western United States supports a coniferous rain forest ecosystem in the Pacific Northwest and large redwood groves farther south. Similarly, the west coast of northern Europe has a mild climate because the Gulf Stream carries warm water from the equator to the North Atlantic, driven in part by the "great ocean conveyor belt" (Figure 52.11). As a result, northwest Europe is warmer during winter than New England, which is farther south but is cooled by the Labrador Current flowing south from the coast of Greenland. Because of the high specific heat ofwater (see Chapter 3), oceans and large lakes tend to moderate the climate of nearby land. Duringa hot day, when the land is warmer than the nearby body of water, air over the land heats up and

o

â&#x20AC;˘

water

)' ..

.. Figure 52.11 The great ocean conveyor belt. Water is warmed at the equator and flows along the ocean surface to the North Atlantic, where it cools, becomes denser, and sinks thousands of meters. The deep, cold water may not return to the ocean surface for as long as 1,000 years.

(HfJ,PTER fifTY¡ TWO

An lntrodu(tion to Ecology and the Biosphere

1155


â&#x20AC;˘ FIguro 52.10

Exploring Global Climate Patterns Latitudinal Variation in Sunlight Intensity Earth's curved shape causes latitudinal variation in the intensity of sunlight. Because sunlight strikes the tropics (those regions that lie between 23.5" north latitude and 23.5" south latitude) most directly, more heat and light per unit of surface area are delivered there. At higher latitudes, sunlight strikes Earth at an oblique angle, and thus the light energy is more diffuse on Earth's

surface. low angle of lncomll19 S1Jnlight

Seasonal Variation in Sunlight Intensity March equinox: Equator faces sun dIrectly: neither pole tilts toward sun; all regions on Earth e~penence

June solstice: Northern

Hemisphere lilts toward sun and has longest day and shortest night; Southern Hemisphere tilts away from sun and has shortest day and longest night.

0° (equator)

'!II.lb.~1

12 hours of daylight and 12 hours of

darkness.

~o.s

December solstice: Northern Hemisphere tilts iIWay from sun and has shortest day and longest night; Southern HemISPhere lilts toward sun and has longest day and shortest night. September equinox: Equator faces sun directly; neither pole tilts toward sun; all re(jlons on Earth eXpener1ce 12 hours of daylight and 12 hours of darkness.

Earth's tilt causes seasonal variation in the intensity of solar radiation. Because the planet is tilted on its axis by 23S relative to its plane oforbit around the sun, the tropics experience the greatest annual input of solar rndiation and the least seasonal variation. The seasonal variations of light and temperature increase toward the poles. 1156

UNIT UGHT

Ecology


Global Air Circulation and Precipitation Patterns

30 0 N

Descending dry air

Descending dry air

absorbs

0° (equator)

Ascending moist air

moisture

absorbs moisture

releases moisture

30°5

30" 23Y Arid

0' Tropics

zone

23.5° 300 Arid

zone

Intense solar radiation near the equator initiates a global pattern of air circulation and precipitation. High temperatures in the tropics evaporate water from Earth's surface and cause warm, wet air masses to rise (blue arrows) and flow toward the poles. The rising air masses release much of their water content, creating abundant precipitation in tropical regions. The high-altitude air masses, now dry, descend (brown arrows) toward Earth, absorbing moisture from the land and creating an arid climate conducive to the development of the deserts that are common at latitudes around 30' north and south. Some of the descending air then flows toward the poles. At latitudes around 60' north and south, the air masses again rise and release abundant precipitation (though less than in the tropics). Some of the cold, dry rising air then flows to the poles, where it descends and flows back toward the equator, absorbing moisture and creating the comparatively rainless and bitterly cold climates of the polar regions.

Global Wind Patterns

-"",,,,,'-

Westerlies.

..//

Air flowing close to Earth's surface creates predictable global wind patterns. As Earth rotates on its axis, land near the equator moves faster than that at the poles, deflecting the winds from the vertical paths shown above and creating more easterly and westerly flows. Cooling trade winds blow from east to west in the tropics; prevailing westerlies blow from west to east in the temperate zones, defined as the regions between the Tropic of Cancer and the Arctic Circle and between the Tropic of Capricorn and the Antarctic Circle.

~

C~"'PH~ fIHY·TWO

An Introduction to Ecology and the Biosphere

1157


e

Air cools at high elevation.

o Cooler air sinks over water.

OWarmair

over land rises.

o

Cool air over water moves inland, replacing rising warm air over land.

(--------=;.;?--J .4 Figure 52.12 Moderating effects of a large body of water on climate. This figure illustrates what happens on a hot summer day_

rises, drawing a cool breeze from the water across the land (Figure 52,12), At night, air over the now warmer water rises, drawing cooler air from the land back out over the water, replacing it with warmer air from offshore. The moderation of climate may be limited to the coast itself, however. In certain regions, such as southern California, cool, dry ocean breezes in summer are warmed when they contact the land, absorbing moisture and creating a hot, rainless climate just a few miles inland (see Figure 3.5). This climate pattern also occurs around the Mediterranean Sea, which gives it the name

Mediterranean climate. Mountains Mountains affect the amount of sunlight reaching an area and consequently the local temperature and rainfall. South-facing slopes in the Northern Hemisphere receive more sunlight than nearby north-facing slopes and are therefore warmer and drier. These abiotic differences influence species distribution; for example, in many mountains of western North America, spruce and other conifers occupy the cooler north-facing slopes, whereas shrubby, drought-resistant plants inhabit the south-facing slopes. In addition, every l,{X)}.m increase in elevation produces a temperature drop of approximately 6路C, equivalent to that produced by an 88O-km increase in latitude. This is one reason the biological communities of mountains are similar to those at lower elevations but farther from the equator. 1158

U"IT EIG~T

Ecology

When warm, moist air approaches a mountain, the air rises and cools, releasing moisture on the windward side ofthe peak (Figure 52.13). On the leeward side, cooler, dry air descends, absorbing moisture and prodUcing a "rain shadow," Deserts commonly occur on the leeward side of mountain ranges, a phenomenon evident in the Great Basin and the Mojave Desert of western North America, the Gobi Desert of Asia, and the small deserts found in the southwest corners of some Caribbean islands. Seasonality As described earlier, Earth's tilted axis of rotation and its annual passage around the sun cause strong seasonal cycles in middle to high latitudes (see Figure 52.10). In addition to these global changes in day length, solar radiation, and temperature, the changing angle ofthe sun over the course of the year affects local environments. For example, the belts ofwet and dry air on either side ofthe equator move slightly northward and southward with the changing angle of the sun, producing marked wet and dry seasons around 20' north and 20路 south latitude, where many tropical deciduous forests grow. In addition, seasonal changes in wind patterns produce variations in ocean currents, sometimes causing the upwelling of cold water from deep ocean layers. This nutrient-rich water stimulates the growth of surface-dwelling phytoplankton and the organisms that feed on them.

Microclimate Many features in the environment influence microclimates by casting shade, affecting evaporation from soil, or changing wind patterns. For example, forest trees frequently moderate the microclimate below them. Consequently, cleared areas generally experience greater temperature extremes than the

othe Asocean moist air moves in off and encounters mountains, it flows upward, cools at higher altitudes, and drops a large amount of water as precipitation.

e

On the leeward side of the mountains, there is little precipitation. As a result of this rain shadow, a desert is often present.

1-

Leeward side of mountain

Mountain range Ocean

.4

Figure 52.13 How mountains affect rainfall.


forest interior because ofgreater solar radiation and wind currents that are established by the rapid heating and cooling of open land. Within a forest, low·lying ground is usually wetter than high ground and tends to be occupied by different species of trees. A log or large stone can shelter organisms such as salamanders, worms, and insects, buffering them from the extremes of temperature and moisture. Everyenvironment on Earth is similarly characterized by a mosaic of small-scale differences in the abiotic factors that influence the local distributions of organisms.

D

o

Current range Predicted range Overlap

(a)4,SOC warming over next century

Long-Term Climate Change If temperature and moisture are the most important factors limiting the geographic ranges of plants and animals, then the global climate change currently under way will profoundly affect the biosphere (see Chapter 55). One way to predict the possible effects of climate change is to look back at the changes that have occurred in temperate regions since the last ice age ended. Until about 16,000 years ago, continental glaciers covered much of North America and Eurasia. As the climate warmed and the glaciers retreated, tree distributions expanded northward. A detailed record of these migrations is captured in fossil pollen deposited in lakes and ponds. (It may seem odd to think of trees "migrating;' but recall from Chapter 38 that wind and animals can disperse seeds, sometimes over great distances.) 1£ researchers can determine the climatic limits of current geographic distributions for organisms, they can make predictions about how distributions will change with climatic warming. A major question when applying this approach to plants is whether seed dispersal is rapid enough to sustain the migration of each species as climate changes. For example, fossils suggest that the eastern hemlock was delayed nearly 2,500 years in its movement north at the end of the last ice age. This delay in seed dispersal was partly attributable to the lack of"wings n on the seeds, causing the seeds to fall close to their parent tree. Let's look at a specific case of how the fossil record of past tree migrations can inform predictions about the biological impact of the current global warming trend. Figure 52.14 shows the current and predicted geographic ranges of the American beech (Fagus grandifolia) under two different climate-change models. These models predict that the northern limit of the beech's range will move 700-900 km northward in the next century, and its southern range limit will move northward an even greater distance. If these predictions are even approximately correct, the beech must move 7-9 km per year northward to keep pace with the warming climate. However, since the end of the last ice age, the beech has migrated into its present range at a rate of only 0.2 km per year. Without human as-

(b)6'soC warming over next century

... Figure 52.14 Current range and predicted range for the American beech (Fagus grandifolia) under two scenarios of climate change. The predicted range in each scenario is based on climate factors alone. Whal other factors might alter the distribution of this species?

D

sistance in moving into new ranges where they can survive as the climate warms, species such as the American beech may have much smaller ranges and may even become extinct. CONCEPT

CHECK

52.2

I, Give examples of human actions that could expand a species' distribution by changing its (a) dispersal or (b) biotic interactions. 2. Explain how the sun's unequal heating of Earth's surface influences global climate patterns. 3. You suspect that deer are restricting the distribution of a tree species by preferentially eating the seedlings of the tree. How might you test that hypothesis?

_','!ifni.

For suggested answers, see AppendiK A.

rz~~:~r;b~:~~s are diverse and dynamic systems that cover most of Earth

We have seen how both biotic and abiotic factors influence the distribution of organisms on Earth, Combinations ofthese factors determine the nature of Earth's many biomes, major terrestrial or aquatic life zones, characterized by vegetation type in terrestrial biomes or the physical environment in aquatic biomes. Well begin by examining Earth's aquatic biomes. Aquatic biomes account for the largest part of the biosphere in terms of area, and all types are found around the

CHAPTER fifTY· TWO

An Introduction to Ecology and the Biosphere

1159


globe (Figure 52.15), Ecologists distinguish between fresh· water biomes and marine biomes on the basis of physical and chemical differences. For example, marine biomes generally have salt concentrations that average 3%, whereas freshwater biomes are usually characterized by a salt concentration ofless than 0.1%.

The oceans make up the largest marine biome, covering about 75% of Earth's surface. Because of their vast size, they have an enormous impact on the biosphere, The evaporation of water from the oceans provides most of the planet's rainfall, and ocean temperatures have a major effect on world climate and wind patterns. In addition, marine algae and

lakes

Coral reefs

/"' Rivers

k--30'N

Oceanic pelagic and benthic zones Estuaries Intertidal zones

Tropic of Cancer

f--7,-Equator - - - { < ;,%,-r.,,,-----i! .. , ____~

Tropic 01 S.a[lric.?~n

_

\---30'5-------1

... Figure 52.15 The distribution of major aquatic biomes. Intertidal zone

• ••

Oceanic zone

Photic zone 200 m ----cl-'\"~-'<~""::::::.=cn Continental Pelagi shelf zone

Aph,'"

Photic zone

zone

j

Pelagic

zone Aphotic

zone

(a) Zonation in a lake. The lake environment IS generally classified on the basis of three physical CrIteria: light penetration (photic and aphotic zones), distance from shore and water depth (littoral and limnetic zones), and whether it is open water (pelagic zone) or bottom (benthic zone),

... Figure 52.16 Zonation in aquatic environments. 1160

U"IT EIG~T

Ecology

2,000-6,000 m------;~:::;~:J.:::b Abyssal zone (deepest regions of ocean floor)

(b) Marine zonation. like lakes. the marine environment is generally claSSIfied on the basis of light penetration (photic and aphotic zones), distance Irom shore and water depth (intertidal, neritic, and oceanic zones), and whether it is open water (pelagic zone) or bottom (benthic and abyssal zones).


photosynthetic bacteria supply a substantial portion of the world's oxygen and consume large amounts of atmospheric carbon dioxide. Freshwater biomes are closely linked to the soils and biotic components of the terrestrial biomes through which they pass or in which they are situated. The particular characteristics of a freshwater biome are also influenced by the patterns and speed of water flow and the climate to which the biome is exposed.

Winter

Stratification of Aquatic Biomes

Spring

Many aquatic biomes are physically and chemically stratified (layered), as illustrated for both a lake and a marine environment in Figure 52.16, on the facing page. Light is absorbed by both the water itself and the photosynthetic organisms in it, so its intensity decreases rapidly with depth, as mentioned earlier. Ecologists distinguish between the upper photic zone, where there is sufficient light for photosynthesis, and the lower aphotic zone, where little light penetrates. At the bottom of all aquatic biomes, the substrate is called the benthic zone. Made up of sand and organic and inorganic sediments, the benthic zone is occupied by communities of organisms collectively called the benthos. A major source of food for many benthic species is dead organic matter called detritus, which "rains" down from the productive surface waters of the photic zone. In the ocean, the part of the benthic zone that lies between 2,000 and 6,000 m below the surface is known as the abyssal zone. Thermal energy from sunlight warms surface waters to whatever depth the sunlight penetrates, but the deeper waters remain quite cold. In the ocean and in most lakes, a narrow layer ofabrupt temperature change called a thermocline separates the more uniformly warm upper layer from more uniformly cold deeper waters. Lakes tend to be particularly layered with respect to temperature, especially during summer and winter, but many temperate lakes undergo a semiannual mixing of their waters as a result of changing temperature profiles (Figure 52.17). This turnover, as it is called, brings oxygenated water from a lake's surface to the bottom and nutrienHich water from the bottom to the surface in both spring and autumn. These cyclic changes in the abiotic properties of lakes are essential for the survival and growth of organisms at all levels within this ecosystem. In both freshwater and marine environments, communities are distributed according to water depth, degree of light penetration, distance from shore, and whether they are found in open water or near the bottom. Marine communities, in particular, illustrate the limitations on species distribution that result from these abiotic factors. Plankton and many fish species occur in the relatively shallow photic zone (see Figure 52.16b). Because water absorbs light so well and the

o water In winter. the coldest the lake (O"() In

lies Just below the surface ice; water is progressively warmer at deeper levels of the lake, typically 4"( at the bottom.

,. ,.,.

f) In spring. as the sun melts the ice. the surface water warms to 4"( and sinks below the cooler layers immediately below, eliminating the thermal stratification. Spring winds mix the water to great depth, bringing oxygen to the bottom waters and nutrients to the surface.

() ,f'

f) In summer, the lake

Summer

regains a distindive thermal profile, with warm surface water separated from cold bottom water by a narrow vertical zone of abrupt temperature change, called a thermocline. Thermocline

o water In autumn, as surface cools rapidly, it

Autumn

,

() ,./'

Sinks below the underlying layers. remixing the water until the surface begins to freeze and the winter temperature profile is reestablished.

... Figure S2.17 Seasonal turnover in lakes with winter ice cover. Because of the seasonal turnover shown here, lake waters are well oxygenated at all depths in spring and autumn; in winter and summer, when the lake IS stratified by temperature. oxygen concentrations are lower in deeper waters and higher near the surface of the lake.

ocean is so deep, most of the ocean volume is virtually devoid of light (the aphotic zone) and harbors relatively little life, except for microorganisms and relatively sparse populations of fishes and invertebrates. Similar factors limit species distribution in deep lakes as well. Figure 52.18, on the next four pages, surveys the major aquatic biomes.

CHAPTER fifTY路 TWO

An Introduction to Ecology and the Biosphere

1161


• Figure 52.18

••

• Aquatic Biomes Lakes

Physical Environment Standing bodies of water range from ponds a few square meters in area to lakes covering thousands of square kilometers. Light decreases with depth, creating stratification (see Figure 52.16<1). Temperate lakes may have a seasonal thermocline (see Figure 52.17); tropical lowland lakes have a themlocline year-round.

Chemical Environment The salinity, oxygen concentration, and nutrient content differ greatly among lakes and can vary with season. Oligotrophic lakes are nutrient-poor and generally oxygen-rich: eutrophi<: lakes are nutrient-rich and often depleted of oxygen in the deepest zone in summer and if ice covered in winter. The amount of decomposable organic matter in bottom sediments is low in oligotrophic lakes and high in eutrophic lakes; high rates of decomposition in deeper layers of eutrophic lakes cause periodic oxygen depletion. Geologic Features Oligotrophic lakes may become more eutrophic over time as runoff adds sediments and nutrients. They tend to have less surface area relative to their depth than eutrophic lakes have. Photosynthetic Organisms Rooted and floating aquatic plants live in the littoral zone, the shallow, welllighted waters close to shore. Farther from shore, where water is too deep to support rooted aquatic plants, the limnetic zone is inhabited by a variety of phytoplankton and cyanobacteria. Heterotrophs In the limnetic zone, small drifting heterotrophs, or zooplankton, graze on the phytoplankton. The benthic zone is inhabited by assorted invertebrates whose species composition depends partly on oxygen levels. Fishes live in all zones with sufficient oxygen.

An oligotrophic lake in Grand Teton National Park, Wyoming

A eutrophIC lake Delta, Botswana

In

the Okavango

Human Impact Runofffrom fertilized land and dumping of wastes leads to nutrient enrichment, which can produce algal blooms, oxygen depletion, and fish kills.

Ph)'5ical Environment A wetland is a habitat that is inundated by water at least some of the time and that supports plants adapted to water-saturated soil. Some wetlands are inundated at all times, whereas others flood infrequently. cnemical Environment Becauseofhigh organic production by plants and decomposition by microbes and other organisms, both the water and the soils are periodically low in dissolved oxygen. Wetlands have a high capacity to filter dissolved nutrients and chemical pollutants. Geologic Features Basin wetlands develop in shallow basins, ranging from upland depressions to filled-in lakes and ponds. Riverine wetlands develop along shallow and periodically flooded banks of rivers and streams. Fringe wetlands occur along the coasts of large lakes and seas, where water flows back and forth because of rising lake levels or tidal action. Thus, fringe wetlands include both freshwater and marine biomes. Photosynthetic Organisms Wetlands are among the most productive biomes on Earth. Their water-saturated soils favor the growth of plants such as floating pond lilies and emergent cattails, many sedges, tamarack, and black spruce, which have adaptations enabling them to grow in water or in soil that is periodically anaerobic owing to the presence of unaerated water. Woody plants dominate the vegetation of swamps, while bogs are dominated by sphagnum mosses.

from crustaceans and aquatic insect larvae to muskrats, consume algae, detritus, and plants. Carnivores are also varied and may include dragonflies, otters, alligators, and owls.

Heterotrophs Wetlands are home to a diverse community of invertebrates, which in tum support a wide variety of birds. Herbivores,

Human Impact Draining and filling have destroyed up to 90% of wetlands, which help purify water and reduce peak flooding.

1162

U"IT EIGHT

Ecology

Okefenokee National Wetland Reserve in Georgia


Streams and Rivers Municipal, agricultural, and industrial pollution degrade water quality and kill aquatic organisms. Damming and flood control impair the natural functioning of stream and river e<:osystems and threaten migratory species such as salmon.

Human Impact

The most prominent physical characteristic of streams and rivers is their current. Headwater streams are generally cold, clear, turbulent, and swift. Farther downstream, where numerous tributaries may have joined, forming a river, the water is generally warmer and more turbid because of suspended sediment. Streams and rivers are stratified into vertical zones.

Physical Environment

Chemical Environment The salt and nutrient content of streams and rivers increases from the headwaters to the mouth. Headwaters are generally rich in oxygen. Downstream water may also contain substantial oxygen, except where there has been organic enrichment. A large fraction of the organic matter in rivers consists of dissolved or highly fragmented material that is carried by the current from forested streams.

Headwater stream channels are often narrow, have a rocky bottom, and alternate between shallow sections and deeper pools. The downstream stretches of rivers are generally wide and meandering. River bottoms are often silty from sediments deposited over long periods of time. Geologic Features

Photosynthetic Organisms Headwater streams that flow through grasslands or deserts may be rich in phytoplankton or rooted aquatic plants.

A great diversity of fishes and invertebrates inhabit unpolluted rivers and streams, distributed according to, and throughout, the vertical wnes. In streams flowing through temperate or tropical forests, organic matter from terrestrial vegetation is the primary source of food for aquatic consumers. Heterotrophs

...

--

A headwater stream in the Great Smoky Mountains

The Mississippi River far from its headwaters

An estuary is a transition area between river and sea. Seawater flows up the estuary channel during a rising tide and flows back down during the falling tide. Often, higher-density seawater occupies the bottom of the channel and mixes little with the lower-density river water at the surface.

Physical Environment

Salinity varies spatially within estuaries, from nearly that of fresh water to that of seawater. Salinity also varies with the rise and fall of the tides. Nutrients from the river make estuaries, like wetlands, among the most productive biomes.

Chemical Environment

Geologic Features Estuarine flow patterns combined with the sediments carried by river and tidal waters create a complex network of tidal channels, islands, natural levees, and mudflats.

Saltmarsh grasses and algae, including phytoplankton, are the major producers in estuaries.

Photosynthetic Organisms

Estuaries support an abundance of worms, oysters, crabs, and many fish spe<:ies that humans consume, Many marine invertebrates and fishes use estuaries as a breeding ground or migrate through them to freshwater habitats upstream, Estuaries are also crucial feeding areas for waterfowl and some marine mammals.

Heterotrophs

Pollution from upstream, and also filling and dredging, have disrupted estuaries worldwide.

Human Impact

Continued on next page

An estuary in a low coastal plain 01 Georgia C~"'PH~

flfTY路rwo

An Introduction to Ecology and the Biosphere

1163


• Figure 52.18 (continued)

••

• Aquatic Biomes

Chemical Environment Oxygen and nutrient levels are generally high and are renewed with each turn of the tides. Geologic Features The substrates of intertidal zones, which are generally either rocky or sandy, select for particular behavior and anatomy among intertidal organisms. The configuration of bays or coastlines influences the magnitude of tides and the relative exposure of intertidal organisms to wave action. Pnotosyntnetic Organisms A high diversity and biomass of attached marine algae inhabit rocky intertidal zones, especially in the lower zone. Sandy intertidal zones exposed to vigorous wave action generally lack attached plants or algae, while sandy intertidal zones in protected bays or lagoons often support rich beds of sea grass and algae.

Rocky intertidal zone on the Oregon coast Ph)'5ical Environment An intertidal zone is periodically submerged and exposed by the tides, twice daily on most marine shores. Upper zones experience longer exposures to air and greater variations in temperature and salinity. Changes in physical conditions from the upper to the lower intertidal zones limit the distributions of many organisms to particular strata, as shown in the photograph.

Heterotropns Many of the animals in rocky intertidal environments have structural adaptations that enable them to attach to the hard substrate. The composition, density, and diversity of animals change markedly from the upper to the lower intertidal wnes. Many of the animals in sandy or muddy intertidal zones, such as worms, clams, and predatory crustaceans, bury themselves and feed as the tides bring sources of food. Other common animals are sponges, sea anemones, echinoderms, and small fishes. Human Impact Oil pollution has disrupted many intertidal areas.

Oceanic Pelagic Zone Physical Environment The oceanic pelagic zone is a vast realm of open blue water, constantly mixed by wind-driven oceanic currents. Because of higher water clarity, the photic zone extends to greater depths than in coastal marine waters. Chemical Environment Oxygen levels are generally high. Nutrient concentrations are generally lower than in coastal waters. Because they are thermally stratified year-round, some tropical areas of the oceanic pelagic lOne have lower nutrient concentrations than temperate oceans. Turnover between fall and spring renews nutrients in the photic zones of temperate and high-latitude ocean areas.

Heterotrophs The most abundant heterotrophs in this biome are zooplankton. These protists, worms, copepods, shrimp-like krill, jellies, and the small larvae of invertebrates and fishes graze on photosynthetic plankton. The oceanic pelagic zone also includes free-swimming animals, such as large squids, fishes, sea turtles, and marine mammals. Human Impact Overfishing has depleted fish stocks in all Earth's oceans, which have also been polluted by waste dumping.

Geologic Features This biome covers approximately 70% of Earth's surface and has an average depth of nearly 4,000 m. The deepest point in the ocean is more than 10,000 m beneath the surface. Photosynthetic Organisms The dominant photosynthetic organisms are phytoplankton, including photosynthetic bacteria, that drift with the oceanic currents. Spring turnover and renewal of nutrients in temperate oceans produces a surge of phytoplankton growth. Because of the large extent of this biome, photosynthetic plankton account for about half of the photosynthetic activity on Earth.

1164

U"IT EIGHT

Ecology

Open ocean off the island of Hawaii


Physical Environment Coral reefs are formed largely from the calcium carbonate skeletons of corals. Shallow reef-building corals live in the photic zone of relatively stable tropical marine environments with high water clarity, primarily on islands and along the edge of some continents. They are sensitive to temperatures below about 18-20'C and above 30路C. Deep-sea coral reefs, found between 200 and 1,500 m deep, are less known than their shallow counterparts but harbor as much diversity as many shallow reefs do. Chemical Environment Corals require high oxygen levels and are excluded by high inputs of fresh water and nutrients. Geologic Filatures Corals require a solid substrate for attachment. A typiCll coral reef begins as a fringing reef on a young, high island, forming an offshore harrier reef later in the history of the island and becoming a coral atoll as the older island submerges. Photosynthetic Organisms Unicellular algae live within the tissues of the corals, forming a mutualistic relationship that provides the corals with organic molecules. Diverse multicellular red and green algae growing on the reef also contribute substantial amounts of photosynthesis.

A coral reef in the Red Sea

Heterotrophs Corals, a diverse group of cnidarians (see Chapter 33), are themselves the predominant animals on coral reefs. However, fish and invertebrate diversity is exceptionally high. Overall animal diversity on coral reefs rivals that of tropical forests.

Human Impact Collecting of coral skeletons and overfishing have reduced populations of corals and reef fishes. Global warming and pollution may be contributing to large-scale coral death. Development of coastal mangroves for aquaculture has also reduced spawning grounds for many species of reef fishes.

Marine Benthic Zone Physical Environment The marine benUtic lOne consists ofthe seafloor below the surface waters ofthe coastal, or neritic, zone and the offshore, pelagic zone (see Figure 52.16b). Except for shallow, near-coastal areas, the marine benthic zone receives no sunlight. Water temperature declines with depth, while pressure increases. As a result, organisms in the verydecp benthic, or abyssal, zone are adapted to continuous cold {about 3'C) and very high water pressure. Chemical Environment Except in some areas of organic enrichment, oxygen is present at sufficient concentrations to support a diversity of animals.

Geologic Features Soft sediments cover most of the benthic lone. However, there are areas of rocky substrate on reefs, submarine mountains, and new oceanic crust. Autotrophs Photosynthetic organisms, mainly seaweeds and filamentous algae, are limited to shallow benthic areas with sufficient light to support them. Unique assemblages oforganisms, such as those shown in the photo, are found near deep-sea hydrothennal vents on mid-ocean ridges. In these dark, hot environments, the food producers are chemoautotrophic prokaryotes {see Chapter 27) that obtain energy by oxidizing H~ formed by a reaction ofthe hot water with dissolved sulfate (50/-). Heterotrophs Neritic benthic communities include numerous invertebrates and fishes. Beyond the photic zone, most consumers depend entirely on organic matter raining down from above. Among the animals of the deep-sea hydrothermal vent communities are giant tube worms (pictured at left), some more than I m long. They are nourished by chemoautotrophic prokaryotes that live as symbionts within their bodies. Many other invertebrates, including arthropods and echinoderms, are also abundant around the hydrothermal vents. Human Impact Overfishing has decimated important benthic fish populations, such as the cod of the Grand Banks off Newfoundland. Dumping of organic wastes has created oxygen-deprived benthic areas.

A deep-sea hydrothermal vent community

C~"'PH~ fIHY路TWO

An Introduction to Ecology and the Biosphere

1165


CONCEPT

CHECI(

52.3

ity) that changes a community, removing organisms from it and altering resource availability. Frequent fires, for instance, can kill woody plants and keep a savanna from becoming the woodland that climate alone would otherwise support.

The first two questions refer to Figure 52.18. 1. Many organisms living in estuaries experience freshand saltwater conditions each day with the rising and falling of tides. What challenge does this pose for the physiology of the organisms? 2. Why are phytoplankton, and not benthic algae or rooted aquatic plants, the dominant photosynthetic organisms of the oceanic pelagic zone? 3. -'*,.)114 Water leaving a reservoir behind a dam is often taken from deep layers of the reservoir. Would you expect fish found in a river below a dam in summer to be species that prefer colder or warmer water than fish found in an undammed river? Explain.

Climate and Terrestrial Biomes We can see the great impact of climate on the distribution of organisms by constructing a climograph, a plot of the temperature and precipitation in a particular region. For example, Figure 52.20 is a dimograph of annual mean temperature and precipitation for some of the biomes found in North America. Notice that the range of precipitation in northern coniferous forests is similar to that in temperate forests, but the temperature ranges are different. Grasslands are generally drier than either kind of forest, and deserts are drier still. Factors other than mean temperature and precipitation also playa role in determining where biomes exist. For example, certain areas in North America with a particular combination of temperature and precipitation support a temperate broadleaf forest, but other areas with similar values for these variables support a coniferous forest. How do we explain this variation? First, remember that the climograph is based on annual averages. Often, however, the pattern of climatic variation is as important as the average climate. For example, some areas may receive regular precipitation throughout the year, whereas other areas with the same annual precipitation have distinct wet and dry seasons. A similar phenomenon may occur with respect to temperature. Other environmental characteristics, such as the type of bedrock in an area, may greatly affect mineral nutrient availability and soil structure, which in turn affect the kind of vegetation that can grow.

For suggested answers. see Appendix A

r;~::~;:c~~~~nd distribution of terrestrial biomes are controlled by climate and disturbance

All the abiotic factors discussed in this chapter, but especially climate, are important in determining why a particular terrestrial biome is found in a certain area. Because there are latitudinal patterns ofclimate over Earth's surface (see Figure 52.10), there are also latitudinal patterns of biome distribution (Figure 52.19). These biome patterns in turn are modified by disturbance, an event (such as a storm, fire, or human activ-

.

"

.... --- .. r-----.--â&#x20AC;˘

Tropic of Cancer

!----Equator----{f Tropic of

_______ ~~e~c9~

_

\--30'5-----iI\/ ;--------\:';t?---=-----i:7'\)~'l-_:i

.. Figure 52.19 The distribution of major terrestrial biomes. Although biomes are mapped here with >harp boundaries. biomes actually grade into one another, sometimes over large areas

1166

U"IT EIG~T

Ecology


Desert

2

,•

IS

Temperate broadleaf forest

0

Northern coniferous forest

E

• c ~

E ~c ~

Tropical forest

30

,

•~

Temperate grassland

Arctic and alpine tundra 100 200 300 Annual mean precipitation (em)

R II ~ 400

... Figure 52.20 A c1imograph for some major types of biomes in North America. The areas plotted here encompass the range of annual mean temperature and precipitation in the biomes.

General Features ofTerrestrial Biomes and the Role of Disturbance Most terrestrial biomes are named for major physical or climatic features and for their predominant vegetation. Temperate grasslands, for instance, are generally found in middle latitudes, where the climate is more moderate than in the tropics or polar regions, and are dominated by various grass species (see Figure 52.19). Each biome is also characterized by microorganisms, fungi, and animals adapted to that particular environment. For example, temperate grasslands are more likely than forests to be populated by large grazing mammals. Although Figure 52.19 shows distinct boundaries between the biomes, in actuality, terrestrial biomes usually grade into each other without sharp boundaries. The area of intergradation, called an ecotone, may be wide or narrow. Vertical layering isan important feature ofterrestrial biomes, and the shapes and sizes ofplants largely define that layering. In many forests, for example, the layers from top to bottom consist of the upper canopy, the low-tree layer, the shrub understory, the ground layer of herbaceous plants, the forest floor (litter layer), and the root layer. Nonforest biomes have similar, though usually less pronounced, layers. Grasslands have an herbaceous layer of grasses and forbs (small broadleaf plants), a litter layer, and a root layer. Layering of vegetation provides many different habitats for animals, which often occupy well-defined feeding groups, from the insectivorous birds and bats that feed above canopies to the small mammals, numerous worms, and arthropods that search for food in the litter and root layers. The species composition ofeach kind ofbiome varies from one location to another. For instance, in the northern conifer-

ous forest (taiga) of North America, red spruce is common in the east but does not occur in most other areas, where black spruce and white spruce are abundant. In an example of con· vergent evolution (see Figure 26.7), cacti living in North American deserts appear very similar to plants called euphorbs found in African deserts, although cacti and euphorbs belong to different evolutionary lineages. Biomes are dynamic, and disturbance rather than stability tends to be the rule. For example, hurricanes create openings for new species in tropical and temperate forests. In northern coniferous forests, gaps are produced when old trees die and fall over or when snowfall breaks branches. These gaps allow deciduous species, such as aspen and birch, to grow. As a result, biomes usually exhibit extensive patchiness, with several different communities represented in any particular area. In many biomes, the dominant plants depend on periodic disturbance. For example, natural wildfires are an integral component of grasslands, savannas, chaparral, and many coniferous forests. However, fires are no longer common across much of the Great Plains because tallgrass prairie ecosystems have been converted to agricultural fields that rarely burn. Before agricultural and urban development, much of the southeastern United States was dominated by a single conifer species, the longleaf pine. Without periodic burning, broadleaf trees tended to replace the pines. Forest managers now use fire as a tool to help maintain many coniferous forests. Figure 52.21, on the next four pages, summarizes the mao jor features of terrestrial biomes. As you read about the characteristics ofeach biome, remember that humans have altered much of Earth's surface, replacing original biomes with urban and agricultural ones. Most of the eastern United States, for example, is classified as temperate broadleafforest, but little of that original forest remains. Throughout this chapter, you have seen how the distributions of organisms and biomes depend on both abiotic and biotic factors. In the next chapter, we will begin to work our way down the hierarchy outlined in Figure 52.2, focusing on how abiotic and biotic factors influence the ecology of populations. CONCEPT

CHECK

52.4

I. Based on the climograph in Figure 52.20, what mainly differentiates dry tundra and deserts? 2. Identify the natural biome in which you Jive and summarize its abiotic and biotic characteristics. Do these reflect your actual surroundings? Explain. UI • If global warming increases average 3. temperatures on Earth by 4'C in this century, predict which biome is most likely to replace tundra in some locations as a result. Explain your answer.

-','Ilf

For suggested answers, see Appendix A.

CHAPTER fifTY· TWO

An Introduction to Ecology and the Biosphere

1167


• Figure 52.21

••

• Terrestrial Biomes Tropical Forest Distribution Equatorial and subequatorial regions. Precipitation In tropical rain forests, minfJll is relatively constant, about 200-4OCl cm annually. In tropical dry forests, precipitation is highly seasonal, about 150-200 cm annually, with a six- to seven-month dry season. Temperature Air temperatures are high year-round, averaging 25-29·C with little seasonal variation. Plants Tropical forests are vertically layered, and competition for light is intense. Layers in rain forests include emergent trees that grow above a closed canopy, the canopy trees, one or m'o layers of subcanopy trees, and shrub and herb layers. There are generally fewer layers in tropical dry forests. Broadleaf evergreen trees are dominant in tropical min forests, whereas tropical dry forest trees drop their leaves during the dry season. Epiphytes

such as bromeliads and orchids generally cover tropical forest trees but are less abundant in dry forests. Thorny shrubs and succulent plants are common in some tropical dry forests. Animals Earth's tropical forests are home to millions of species, including an estimated 5-30 million still undescribed species of insects, spiders, and other arthropods. In fJct, animal diversity is higher in tropical forests than in any other terrestrial biome. The animals, including amphibians, birds and other reptiles, mammals, and arthropods, are adapted to the vertically layered environment and are often inconspicuous. Human Impact Humans long ago established thriving communities in tropical forests. Rapid population growth leading to agriculture and development is now destroying some tropical forests.

Desert Distribution Deserts occur in bands near 30· north and south latitude or at other latitudes in the interior of continents (for instance, the Gobi Desert of north central Asia). Precipitation Precipitation is low and highly variable, generally less than 30 cm per year. Temperature Temperature is variable seasonally and daily. Maximum air temperature in hot deserts may exceed 50"C; in cold deserts air temperature may fall below -30T. Plants Desert landscapes are dominated by low, widel}'scattered vegetation: the proportion ofbare ground is high compared with other terrestrial biomes. The plants include succulents such as cacti, deeply rooted shrubs, and herbs that grow during the infrequent moist periods. Desert plant adaptations include heat and desiccation 1168

U"IT EIGHT

Ecology

tolerance, water storage, and reduced leafsurface area. Physical defenses, such as spines, and chemical defenses, such as toxins in the leaves ofshrubs, are common. Many ofthe plants exhibit C4 or CAM photo-synthesis (see Chapter 10). Animals Common desert animals include many kinds of snakes and lizards, scorpions, ants, beetles, migratory and resident birds, and seed-eating rodents. Many species are nocturnal. Water conservation is a common adaptation, with some species surviving on water from metabolic breakdown of carbohydrates in seeds. Human Impact Long-distance transport ofwater and deep groundwater wells have allowed humans to maintain substantial populations in deserts. Conversion to irrigated agriculture and urbanization have reduced the natural biodiversity ofsome deserts.


Savanna Distribution Equatorial and subequatorial regions, Precipitation

Rainfall, which

is seasonal, averages 30-50 cm

per )'ear. The dl'}' season can last up to eight or nine months. Temperature The U\"anna is warm year.round, averaging 24-29"C, but with somewhat more seasonal variation than in tropical forests. Plants The scattered trees found at diff~t densities in the savanna often are thorny and have smalllea\'es, an apparent adaptation to the re!ati\'ely dry conditions. Fires are common in the dry season, and the dominant plant species are fire-adapted and tolerant of seasonal drought. Grasses and forbs, which make up most of the ground cover.

grow rapidly in response to sea· sonal rains and are tolerant of grazing b)' large mammals and other herbivores. Animals large plant·eating mammals. such as wildebeests and bison. and predators, includ· ing lions and hyenas, are common inhabitants. However, the dominant herbivores are actually insects, especially termites. Our· ing seasonal droughts. grating mammals often migrate to parts ofthe savanna with more forage and scattered watering holes. Human Impact There is evi· denee that the earliest humans

.....

A savaTla 11 Kenya

lived in savannas. Fires set b}' hu· mans may help maintain this biome. Calde ranching and overhunting hm'!ed to declines in large-mammal populations.

Chaparral Distribution This biome occurs in midlatitude coastal regions on several continents, and its many names reflect its farflung distribution: chaparral in North America, matorral in Spain and Chile, ganglle and maquis in southern France, and fynbos in South Africa. Precipitation Precipitation is highly seasonal, with rainy winters and long, dry summers. Annual precipitation generally falls within the range of30-50cm. Temperature Fall, winter, and spring are cooL with average temperatures in the range of Io-ITC. Average summer temperature can reach 3O"C, and daytime maximum temperature can exceed 40"<:. Plants 01aparraI isdominated b)' shrubs and smaU trees, along with a man)' kinds of grasses and herbs. Pbnt ~t)' is high, ...ith nuny species confmed 10 a specifIC.

relatively small geographic area. Adaptations to drought include the tough evergreen leaves ofwoody plants, which reduce ",,-ater loss. Adaptations to fire are also prominent. Some ofthe shrubs produce seeds that will genninate only after a hot fire; food reserves stOTed in their fire-resistant roots enable them to resprout quickly and use nutrients released by the fire. Animals Native mammals include browsers. such as deer and goats, that feed on twigs and buds of woody vegetation, and a high diversity of small mammals. Chaparral areas also support many species of amphibians, birds and other reptiles, and insects.

Human Impact Chaparral areas have been heavily settled and reduced through conversion to agriculture and urbaniution. Humans contribute to the fires that sweep across the chaparraL (H.unl f."Y_TWO

Continued on next page An Introduction to Ecology and the Biosphere

1169


• Figure 52.21 (continued)

••

• Terrestrial Biomes Temperate Grassland Distribution The veldts of South Africa, the puSZla of Hungary, the pampas of Argentina and Uruguay, the steppes of Russia, and the plains and prairies of central North America are all temperate grasslands.

have adaptations that help them survive periodic, protracted droughts and fire: For example, grasses can sprout qUickly following fire. Grazing by large mammals helps prevent establishment of woody shrubs and trees.

Precipitation Precipitation is often highly seasonal, with relatively dry winters and wet summers. Annual precipitation generally averages between 30 and 100 em. Periodic drought is common.

Animals Native mammals indude large grazers such as bison and wild horses. Temperate grasslands are also inhabited by a wide variety of burrowing mammals, such as prairie dogs in North America.

Temperature Winters are generally cold, with average temJ}Cratures frequently falling well below Summers, with average temJ}Cratures often approaching 3Q'e, are hot.

Human Impact Deep, fertile soils make temperate grasslands ideal places for agriculture, especially for growing grains. As a consequence, most grassland in North America and much of Eurasia has been converted 10 farmland. In some drier grasslands, cattle and other grazers have helped change parts of the biome into desert.

-W·e.

Plants The dominant plants are grasses and forbs, which vary in height from a few centimeters to 2 m in lallgrass prairie. Many

Northern Coniferous Forest Distribution Extending in a broad band across northern North America and Eurasia to the edge of the arctic tundra, the northern coniferous forest, or taiga, is the largest terrestrial biome on Earth. Precipitation Annual precipitation generally ranges from 30 to 70 em, and periodic droughts are common. However, some coastal coniferous forests of the U.S. Pacific Northwest are temperate rain forests that may receive over 3()() em of annual precipitation. Temperature Winters are usually cold and long; summers may be hot. Some areas of coniferous forest in Siberia typically range in temperature from -50'C in winter to over 20'C in summer. Plants Cone-bearing trees, such as pine, spruce, fir, and hemlock,

1170

U"IT EIGHT

Ecology

dominate northern coniferous forests. The conical shape of many conifers prevents too much snow from accumulating and breaking their branches. The diversity of plants in the shrub and herb layers of these forests is lower than in temperate broadleafforests. Animals While many migratory birds nest in northern coniferous foft'sts, other species reside there year-round. The mammals of this biome, which include moose, brown bears, and Siberian tigers, are diverse. Periodic outbreaks of insecIs thai feed on the dominant trees can kill vast tracts oftrees. Human Impact Although they have not been heavily settled by human populations, northern coniferous fore~ts aft' being logged at an alarming rate, and the oldgrowth stands of these trees may soon disappear.


Temperate Broadleaf Forest Distribution

Found mainly at

midlatiludes in the Northern

Hemisphere, with smaller areas in New Zealand and Australia. Precipttation Precipitation can a\-erage from about 70 IOO\"er200 em annually. Significant amounts faU during aU seuons. including summer r1lin and. in some foresu. ....i nter snow.

Temperature Winter temperatures average around O'c. Summers, with maximum tempt'r2tures near 35"<:. a~ hot and

humid. Plants

A mature tempcl'llltc

broadleafforesl has distinct '"eTticallarers. including a dosed

canopy, one or tv.·o strata of understor)'trtes, a shrub larer, and an herbaceous stratum. There are few epiphytes. The dominant plants in the Northern Hemi-

sphere are de<:iduous tTee$,

which drop their leaves before winter, when low temperatures would reduce photosynthesis and

make 'A'ilter uptake from frozen soil difficult. In Austr.dia, evergreen eucalyptus dominate these forests.

Animals In the Northern Hemisphere, many nu.mnu.ls hibernate in winter, ....'hile man}' bird species migrate to .....armer dimates, The mammills, birds, ilnd insects make use of ill verti· allarers ofthe focest.

Human Impact TempeT'ilte broildleafforest hilS been heilvily settled on illl continents, Logging ilnd land clearing for agriculture ilnd urban development destrored virtuilly :ill the original deciduous forests in North AmeriCll. Ho.....ever, owing to their Cllpildty for recovery, these forests are returning over much of their former range.

Tundra Distribution Tundra covers expansive areas of the Arctic, amounting to 20% of Earth's land surface. High winds and low temperatures create similar planl communities, called alpine tundra, on very high mountaintops at all latitudes, including the tropics. Precipitation Precipitation averages from 20 to 60 cm annually in arctic tundra but may exceed 100 cm in alpine tundra. Temperature Winters are long and cold, with averages in some areas below - 3O'C. Summen are short with low temperatures, generally averaging less than Hrc.

Plants The vegetation oftundra is mostly hemaceous, consisting of a mixture of mosses, grasses, and forbs, along with some dwarf shrubs and trees and lichens. A permanently frozen layer of soil called permafrost restricts the gro'Nth of plant roots. Animals large grazing musk oxen are resident, while caribou and reindeer are migratory. Predators include bears, wolves, and foxes. Many bird species mi· grate to the tundra for summer nesting. Human Impact Tundra is sparsely settled but has become the focus of signiflCllllt mineral and oil extraction in recent rears.

(H ... 'T£I f.'TY·TWO

An Introduction to Ecology and the Biosphere

1171


-£.Ii ]f.- Go to the Study Area at www.masteringbio.com for 6ioFlix 3-D Animations, MP3 Tutors, Videos. Practice Tests, an eBook. and more.

- i l i l i " - 52.3 Aquatic biomes are diverse and dynamic systems that cover most of Earth (pp. 1159-1166)

SUMMARY OF KEY CONCEPTS

52.1

_i,liiiii_

Ecology integrates all areas of biological research and informs environmental decision making (pp. 1148-1151)

.. Stratification of Aquatic Biomes Aquatic biomes account for the largest part of the biosphere in terms of area and are generally stratified (layered) with regard to light penetration, temperature, and community structure. Marine biomes have a higher salt concentration than freshwater biomes.

-$1401',· Activity Aquatic Biomes

.. Linking Ecology and Evolutionary Biology Events that occur in ecological time affect life in evolutionary time. .. Ecology and Environmental Issues Ecologists distinguish hetween the science of ecology and environmental advocacy. Ecology provides a scientific basis for solving environmental problems, but policymakers must also balance social, economic, and political factors in reaching their decisions. ACllvity Science. Te<:hnology, and Society: DDT

52.2

_i,liiiii-

Interactions between organisms and the environment limit the distribution of species (pp. 1151-1159)

- i liliii- 52.4 The structure and distribution of terrestrial biomes are controlled by climate and disturbance (pp. 1166-1171) .. Climate and Terrestrial Biomes Climographs show that temperature and precipitation are correlated with biomes, but because biomes overlap, other abiotic factors playa role in biome location. .. Ceneral Features ofTerreslrial Biomes and the Role of Disturbance Terrestrial biomes are often named for major physical or climatic factors and for their predominant vegetation. Vertical layering is an important feature of terrestrial biomes. Disturbance, both natural and human induced, influences the type of vegetation found in biomes.

-$1401',·

Why iSlp('oes X arn.ent from an area?

+ Does di'>Pffi<ll

~m't

Activity Terrestrial Biomes

,ts dlstnbution?

TESTING YOUR KNOWLEDGE

Does behavior ~ml1l11 distnbution?

Do biotic filCtors (other spe<:ie<;) I,mlt It5 distribution)

SElF·QUIZ

",

--"''--'•• Habitat selection

Yes

~ Predat'on. paraSItism. compe1ltion, disease

I. Which of the following areas of study focuses on the exchange of energy, organisms, and materials between ecosystems? a. population ecology d. ecosystem ecology e. community ecology b. organismal ecology c. landscape ecology 2.

ChemICal filCtors

Do abiotic filCtorsliml1,ts dimibullOn)

-C f'hyslCal factors

Water. oXJ'gen. saI,nlty, pH, soil nutnents, etc. Temperature. light, soil struaure, lire. mOisture. etc.

.. Climate Global climate patterns are largely determined by the input of solar energy and Earth's revolution around the sun. Bodies of water, mountains, and the changing angle of the sun over the year exert regional, local, and seasonal effects on climate. Fine-scale differences in abiotic factors determine microclimates. AClivity Adaptations to Biotic and Abiotic Factor!; Innstlgatlon How Do Abiotic Factors Affe<:t Distribution of Organisms?

1172

UNIT EIGHT

Ecology

-'w,nIM

If Earth's axis of rotation suddenly became perpendicular to the plane ofits orbit, the most predictable effect would be a. no more night and day. b. a big change in the length of the year. c. a cooling of the equator. d. a loss of seasonal variation at high latitudes. e. the elimination of ocean currents.

3. When climbing a mountain, we can observe transitions in biological communities that are analogous to the changes a. in biomes at different latitudes. b. at different depths in the ocean. c. in a community through different seasons. d. in an ecosystem as it evolves over time. e. across the United States from east to west.


4. The oceans affect the biosphere in all of the following ways

that shows how otter density depends on kelp abundance, using the data shown below. Then formulate a hypothesis to explain the pattern you observed.

except a. b. c. d.

producing a substantial amount of the biosphere's oxygen. removing carbon dioxide from the atmosphere. moderating the climate of terrestrial biomes. regulating the pH of freshwater biomes and terrestrial groundwater. e. being the source of most of Earth's rainfalL

5. Which lake zone would be absent in a very shallow lake? a. benthic zone d. littoral zone b. aphotic zone e. limnetic zone c. pelagic zone 6. Which of the following is true with respect to oligotrophic lakes and eutrophic lakes? a. Oligotrophic lakes are more subject to oxygen depletion. b. Rates of photosynthesis are lower in eutrophic lakes. c. Eutrophic lake water contains lower concentrations of nutrients. d. Eutrophic lakes are richer in nutrients. e. Sediments in oligotrophic lakes contain larger amounts of decomposable organic matter. 7. Which of the following is characteristic of most terrestrial biomes? a. annual average rainfall in excess of250 cm b. a distribution predicted almost entirely by rock and soil patterns c. clear boundaries between adjacent biomes d. vegetation demonstrating stratification e. cold winter months

Kelp Abundance (% cover)

(# sightings per day)

75

98

2

15

18

3

60

85

4

25

36

Site

Otter Density

For Self-Quiz Q"swers, see Appe"dix A.

MM#,jf._ Visit the Study Area al www.masteringbio.com for a Practice Test.

EVOLUTION CONNECTION II. Discuss how the concept of timI' applies to ecological situations and evolutionary changes. Do ecological time and evolutionary time ever overlap? If so, what are some examples?

SCIENTIFIC INQUIRY 12. lens Clausen and colleagues, at the Carnegie Institution ofWashington, studied howthe size of yarrow plants (Achillea lanulosa) growing on the slopes ofthe Sierra Nevada varied with elevation. They found that plants from low elevations were generally taller than plants from high elevations, as shown below:

8. \Vhich ofthe following biomes is correctly paired with the description ofits climate? a. savanna-low temperature, precipitation uniform during the year b. tundra-long summers, mild winters c. temperate broadleaf forest-relatively short growing season, mild winters d. temperate grasslands-relatively warm winters, most rainfall in summer e. tropical forests-nearly constant day length and temperature

100

0

:g

9. Suppose that the number of bird species is determined mainly by the number of vertical strata found in the environment. If so, in which of the following biomes would you find the greatest number of bird species? a. tropical rain forest d. temperate broadleaf forest b. savanna e. temperate grassland c. desert 10. 1.@N'iI After reading the experiment ofW J. Fletcher described in Figure 52.8, you decide to study feeding relationships among sea otters, sea urchins, and kelp on your own. You know that sea otters prey on sea urchins and that urchins eat kelp. At four coastal sites, you measure kelp abundance. Then you spend one day at each site and mark whether otters are present or absent every 5 minutes during daylight hours. Make a graph

,•

3,000 2,000

Sierra Nevada

~

;;•

1,000

Great 8asin Plateau

0 Seed collection sites

Source' J Clausen et .11. Experimental studies on the nature of species. III. Environmental responses of climatic races of Achillea, Carnegie Institution of Washington Publication No. 581 (1948).

dausen and colleagues proposed two hypotheses to explain this v.uiation within a species: (I) There are genetic differences bem'een populations of plants found at different elev<1tions. (2) The species has developmentall1exibility and can assume tall or short growth forms, depending on local abiotic factors. If you had seeds from yarrow plants found at low and high elevations, what experiments would you perform to test these hypotheses?

C HfJ,PTER fifTY· TWO

An Introduction to Ecology and the Biosphere

1173


Pop latlo Ecology KEY

CONCEPTS

53.1 Dynamic biological processes influence population density, dispersion, and demographics 53.2 life history traits are products of natural selection 53.3 The exponential model describes population

growth in an idealized, unlimited environment 53.4 The logistic model describes how a population grows more slowly as it ncars its carrying capacity 53.5 Many factors that regulate population growth are density dependent

53.6 The human population is no longer growing exponentially but is still increasing rapidly r,ijji"'i~'.

Counting Sheep

"

the rugged Scottish island of Hirta, ecologists have been studying a population of sheep for more than 50 years (Figure 53.1). What makes these ani路 mals worth studying for such a long time? Soay sheep are a rare and ancient breed, the closest living relative of the domesticated sheep that lived in Europe thousands of years ago. To help preserve the breed, in 1932 conservationists captured sheep on Soay Island, at the time the animals' only home, and released them on nearby Hirta. There, the sheep became valuable for a second reason: They provided an ideal opportunity to study how an isolated population of animals changes in size when food is plentiful and predators are ab~ sent. Surprisingly, ecologists found that the number of sheep on Hirta swung dramatically under these conditions, sometimes changing by more than 50% from one year to the next. Why do populations ofsome species fluctuate greatly while populations of other species do not? To answer this question, we turn to the field of population ecology, the study of pop路 ulations in relation to their environment. Population ecology

0

1174

.... Figure 53.1 What causes a sheep population to fluctuate in size?

explores how biotic and abiotic factors influence the density, distribution, size, and age structure of populations. Our earlier study of populations in Chapter 23 emphasized the relationship between population genetics-the structure and dynamics of gene pools-and evolution. Evolution remains a central theme as we now view populations in the con路 text of ecology. in this chapter, we will first examine some of the structural and dynamic aspects of populations. We will then explore the tools and models ecologists use to analyze populations and the factors that regulate the abundance of organisms. Finally, we will apply these basic concepts as we examine recent trends in the size and makeup of the human population.

~~~4:~:~~~~gical processes

influence population density, dispersion, and demographics

A population is a group of individuals of a single species living in the same general area. Members of a population rely on the same resources, are influenced by similar environmental factors, and are likely to interact and breed with one another. Populations can evolve as natural selection acts on heritable variations among individuals and changes the frequencies of various traits over time (see Chapter 23). Three fundamental characteristics of a population are its density, dispersion, and demographics.

Density and Dispersion At any given moment, a population has specific boundaries and a specific size (the number of individuals living within those boundaries). Ecologists usually begin investigating a population by defining boundaries appropriate to the organism under


study and to the questions being asked. A population's boundaries may be natural ones, as in the case of the sheep on Hirta Island, or they may be arbitrarily defined by an investigator-for example, the oak trees within a specific county in Minnesota. Once defined, the population can be described in terms of its density and dispersion. Density is the number of individuals per unit area or volume: the number ofoak trees per square kilometer in the Minnesota county or the number of Escherichia coli bacteria per milliliter in a test tube. Dispersion is the pattern of spacing among individuals within the boundaries of the population.

Density: A Dynamic Perspective In rare cases, population size and density can be determined by counting all individuals within the boundaries ofthe population. We could count all the Soay sheep on Hirta Island or all the sea stars in a tide pool, for example. Large mammals that live in herds, such as buffalo or elephants, can sometimes be counted accurately from airplanes. In most cases, however, it is impractical or impossible to count all individuals in a population. Instead, ecologists use a variety of sampling techniques to estimate densities and total population sizes. For example, they might count the number of oak trees in several randomly located 100 x 100 m plots (samples), calculate the average density in the samples, and then extrapolate to estimate the population size in the entire area. Such estimates are most accurate when there are many sample plots and when the habitat is fairly homogeneous. In other cases, instead of counting individual organisms, popula· tion ecologists estimate density from an index (indicator) of population size, such as the number of nests, burrows, tracks, or fecal droppings. Ecologists also use the mark-recapture method to estimate the size of wildlife populations (figure 53.2). Density is not a static property but changes as individuals are added to or removed from a population (figure 53.3).

Fl~53.:z

Research Method

Determining Population Size Using the Mark-Recapture Method APPLICATION Ecologists cannot count all the indio viduals in a population if the organisms move too quickly or are hidden from view. In such cases, researchers often use the mark-recapture method to estimate population size. Andrew Gormley and colleagues at the University of Otago applied this methocl to a population of endangered Hector's dolphins (Cephalorhynchu5 heaori) near Banks Peninsula, in Ne-w Zealand, TeCHNIque SCientists typically begin by capturing a random sample of individuals in a population, They tag, or "mark," each individual and then release it. With some species. researchers can Identify Individuals without physically capturing them For example, Gormley and colleagues identified 180 Hector's dolphins by photoHector's dolphins graphing their distinctive dorsal fins from boats, After waiting for the marked or otheMiise identified individuals to mi~ back into the population, usually a few days or weeks, scientists capture or sample a second set of individuals, At Banks Peninsula. Gormley's team encountered 44 dolphins in their second sampling. 7 of which they had photographed before The number of marked animals recaptured in the second sampling (x) divided by the total number of individuals recaptured (n) should equal the number of individuals marked and released in the first sampling (m) divided by the estimated population size (N): Of,

solving for population size.

,

N= mn

The methocl assumes that marked and unmarked individuals have the same prolxlbility of being captured or sampled, that the marked organisms have mi~ed completely back into the population, and that no individuals are born, die, immigrate, or emigrate during the resampling interval. Based on these initial data, the estimated population size of Hector's dolphins at Banks Peninsula would be 180 x 44f7 = 1.131 individuals Repeated sampling by Gormley and colleagues suggested a true population size closer to 1,100.

RESULTS

SOURce

A M Gormley et al , Capwre·recaplure estimates of Hector's dolphin abundance at Banks Perunsula, New Zealillld, Marine MammiJl Science Zl :204-216 (20051,

Births

Deaths and emigration remove Individuals from a population.

Binhs and immigration add Individuals to a population,

Immigration

Emigration

.. figure 53.3 Population dynamics. CHAPTE~ f1flY·TH~EE

Population Ecology

1175


Additions occur through birth (which we will define in this context to include all forms of reproduction) and immigration, the influx of new individuals from other areas. The factors that remove individuals from a population are death (mortality) and emigration, the movement of individuals out ofa population. While birth and death rates influence the density ofall populations, immigration and emigration also alter the density of many populations. For example, long-term studies of Belding's ground squirrels (Spermophilus beldingi) in the vicinity of Tioga Pass, in the Sierra Nevada of California, show that some of the squirrels move nearly 2 km from where they are born, making them immigrants to other populations. Paul Sherman and Martin Morton, then at Cornell University and Occidental College, respectively, estimated that immigrants made up 1-8% ofthe males and 0.7-6% ofthe females in the study population. Although these immigrant percentages may seem small, they represent biologically significant exchanges between populations over time.

...•.

.....·.. (a) Clumped. Many animals, such as these sea stars, group together where food is abundant

Patterns of Dispersion Within a population's geographic range, local densities may vary substantially. Variations in local density are among the most important characteristics for a population ecologist to study, since they provide insight into the environmental associations and social interactions of individuals in the population. Environmental differences-even at a local level-contribute to variation in population density; some habitat patches are simply more suitable for a species than are others. Social interactions betv·:een members of the population, which may maintain patterns of spacing between individuals, can also contribute to variation in population density. The most common pattern ofdispersion is dumped, with the individuals aggregated in patches. Plants and fungi are often dumped where soil conditions and other environmental factors favor germination and growth. Mushrooms, for instance, may be clumped \\ithin and on top of a rotting log. Insects and salamanders may be clumped under the same log because of the higher humidity there. Oumping ofanimals may also be associated with mating behavior. Mayflies, which survive only a day or two as mating adults, often swarm in great numbers, a behavior that increases their chance ofmating. Sea stars group together in tide pools, where food is readily available and where they can breed successfully (Figure 53.4a). Forming groups may also increase the effectiveness ofcertain predators; for example, a wolf pack is more likely than a single wolfto subdue a moose or other large prey animaL A uniform, or evenly spaced, pattern ofdispersion may result from direct interactions between individuals in the population. For exanlple, some plants se<:rete chemicals that inhibit the germination and growth of nearby individuals that could compete for resources. Animals often exhibit uniform dispersion as a result of antagonistic social interactions, such as territorialitythe defense of a bounded physical space against encroachment 1176

U"IT EIG~T

Ecology

(b) Uniform. Birds nesting on small islands. such as these king

penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.

·. (c) Random. Many plants, such as these dandelions, grow

from windblown seeds that land at random and later germinate

.... Figure 53.4 Patterns of dispersion within a population's geographic range. .'I:U'•• Patterns of dispersion somefimes depend on rhe scale of the observation. How might the dispersion of the penguins look to you if you were flying in an airplane over the ocean?

by other individuals (Figure 53.4b). Uniform patterns are not as common in populations as clumped patterns are, In random dispersion (unpredictable spacing), the position of each individual is independent ofother individuals. This pattern


occurs in the absence of strong attractions or repulsions among individuals of a population or where key physical or chemical factors are relatively homogeneous across the study area. For example, plants established by windblown seeds, such as dandelions, may be randomly distributed in a fairly consistent habitat (Figure 53Ac), Random patterns are not as common in nature as one might expect; most populations show at least a tendency toward a clumped distribution.

Demographics The factors that influence population density and dispersion patterns-e<ological needs of a species, structure of the environment, and interactions between individuals within the population-also influence other characteristics of populations. Demography is the study of the vital statistics of populations and how they change over time. Of particular interest to demographers are birth rates and how they vary among individuals (specifically among females, as you'll read shortly) and death rates. A useful way of summarizing some of the vital statistics of a population is with a life table.

Life Tables About a century ago, when life insurance first became available, insurance companies began to estimate how long, on average, individuals of a given age could be expected to live. To do this,

demographers developed life tables, age-spedfic summaries of the survival pattern of a population. Population e<ologists adapted this approach to the study of nonhuman populations. The best way to construct a life table is to follow the fate of a cohort, a group of individuals ofthe same age, from birth until all are dead, To build the life table, we need to determine the number of individuals that die in each age-group and calculate the proportion ofthe cohort surviving from one age to the next. Sherman and Morton's studies of the Belding's ground squirrels near Tioga Pass produced the life table in Table 53.1. The table reveals many things about the population. For instance, the third and eighth columns list, respe<tively, the proportions offemales and males in the cohort that are still alive at each age. A comparison of the fifth and tenth columns reveals that males have higher death rates than females.

Survivorship Curves A graphic method of representing the data in a life table is a survivorship curve, a plot of the proportion or numbers in a cohort still alive at each age. As an example, let's use the data for Belding's ground squirrels in Table 53.1 to construct a survivorship curve for this population. Generally, a survivorship curve is constructed beginning with a cohort of a specified size-say, 1,000 individuals. We can do this for the Belding's ground squirrel population by multiplying the proportion

MI. 53.1 Life Table for Belding's Ground Squirrels (Spermophilus beldingi) at Tioga Pass,

in the Sierra Nevada of California* FEMALES

Age (years)

MALES

Average Number Proportion Number Additional Alive at of Deaths life Alive at Start of During Death Expectancy Start of Rate f (years) Year Year Year

Number Alive at Start of Year

Average Proportion Number Additional Alive at of Deaths Life Start of During Death Expectancy Rate! Year Year (years)

0-1

337

1.000

207

0.61

1.33

349

1.000

227

0.65

1.07

1-2

252 tt

0.386

125

0.50

1.56

248 11

0.350

140

0.56

1.12

2-3

127

0.197

60

0.47

1.60

108

0.152

74

0.69

0.93

3-4

67

0.106

32

0.48

1.59

34

0.Q48

23

0.68

0.89

4-5

35

0.054

16

0.46

1.59

II

0.015

9

0.82

0.68

5-6

19

0.029

10

0.53

1.50

2

0.003

a

1.00

0.50

6-7

9

0.014

4

0.44

1.61

0

7-8

5

0.008

I

0.20

1.50

8-9

4

0.006

3

0.75

0.75

9-10

I

0.002

I

1.00

0.50

• Female~ and males have different nlOrlalily schedules, so they are tallied separately. 'Tile death rate is the proportion of individual, d~ing during the specific time interval. "Include, 122 females and 126 male!; first captured as I·year·old, and therefore not included in the rount of squirrcls age 0-1, Sourc~ P.

W, Sherman and M. L. Morton. Demography ofBdding's ground squirrel, frology 65,1617-1628 (l984).

CHAPTER flfTY·THREE

Population Ecology

1177


1,000 1,000

:ii

0;

••v

"•<> v

<>

g 100 .,<0 < , •

0

~

< 0 >

,

-~

10

0

•,

",

~

1ll~

10

~

E

E

Z

z 1 0

2

4

6

8

10

Age (years) .. Figure 53,5 Survivorship curves for male and female Belding's ground squirrels, The logarithmic scale on the y-axis allows the number of survivors to be visible across the entire range (2-1,000 individuals) on the graph.

alive at the start of each year (the third and eighth columns of Table 53.1) by 1,000 (the hypothetical beginning cohort). The result is the number alive at the start of each year. Plotting these numbers versus age for female and male Belding's ground squirrels yields Figure 53.5. The relatively straight lines of the plots indicate relatively constant rates of death; however, male Belding's ground squirrels have a lower survival rate overall than females have. Figure 53.5 represents just one of many patterns of survivorship exhibited by natural populations. Though diverse, survivorship curves can be classified into three general types (Figure 53,6). A Type I curve is flat at the start, reflecting low death rates during early and middle life, and then drops steeply as death rates increase among older age-groups. Many large mammals, including humans, that produce few offspring but provide them with good care exhibit this kind of curve. In contrast, a Type III curve drops sharply at the start, reflecting very high death rates for the young, but flattens out as death rates decline for those few individuals that survive the early period of die-off. This type of curve is usually associated with organisms that produce very large numbers of offspring but provide little or no care, such as long-lived plants, many fishes, and most marine invertebrates. An oyster, for example, may release millions ofeggs, but most offspring die in the larval stage from predation or other causes. Those few that survive long enough to attach to a suitable substrate and begin growing a hard shell tend to survive for a relatively long time. Type II curves are intermediate, with a constant death rate over the organism's life span. This kind of survivorship occurs in Belding's ground squirrels (see Figure 53.5) and some other rodents, various invertebrates, some lizards, and some annual plants. Many species fall somewhere between these basic types of survivorship or show more complex patterns. In birds, for ex· ample, mortality is often high among the youngest individuals

1178

100

U"IT EIG~T

Ecology

1

SO

0

100

Percentage of maximum life spiln .. Figure 53.6 Idealized survivorship curves: Types I, II, and III. The y-axis is logarithmic and the x-axis is on a relative scale, so that

species with widely varying life spans can be presented together on the same graph.

(as in a Type 1lI curve) but fairly constant among adults (as in a Type II curve). Some invertebrates, such as crabs, may show a Ustair-stepped" curve, with brief periods ofincreased mortality during molts, followed by periods of lower mortality when their protective exoskeleton is hard. In populations not experiencing immigration or emigration, survivorship is one of the two key factors determining changes in population size. The other key factor determining population trends in such populations is reproductive rate.

Reproductive Rates Demographers who study sexually reproducing species gen· erally ignore the males and concentrate on the females in a population because only females produce offspring. Therefore, demographers view populations in terms of females giving rise to new females. The simplest way to describe the reproductive pattern of a population is to ask how reproductive output varies with the ages of females. A reproductive table, or fertility schedule, is an agespecific summary ofthe reproductive rates in a population. It is constructed by measuring the reproductive output of a cohort from birth until death. For a sexual species, the reproductive table tallies the number of female offspring produced by each age-group. Table 53,2 illustrates a reproductive table for Belding's ground squirrels. Reproductive output for sexual organisms such as birds and mammals is the product of the proportion of females of a given age that are breeding and the number of female offspring of those breeding females. Multiplying these numbers gives the average number of female offspring for each female in a given age-group (the last column in Table 53.2). For Belding's ground squirrels, which begin to reproduce at age 1 year, reproductive output rises to a peak at 4 years of age and then falls off in older females.


1."53.2

Reproductive Table for Belding's Ground Squirrels at Tioga Pass

Mean Mean Proportion Size of Number Average of Females litters of Number Age Weaning (Males + Females of Female (years) a litter Females) in a litter Offspring* 0-1

0.00

0.00

0.00

0.00

1-2

0.65

3.30

1.65

1.07

2-3

0.92

4.05

2.03

1.87

3-4

0.90

4.90

2.45

2.21

4-5

0.95

5.45

2.73

2.59

5-6

1.00

4.15

2.08

2.08

6-7

1.00

3M

1.70

1.70

7-8

1.00

3.85

1.93

1.93

8-9

1.00

3.85

1.93

1.93

9-10

1.00

3.15

1.58

1.58

'The a"eragc number offcmalc offsl""ing is the proportion weaning a litter multiplied by the mean numl>croffcmales in a litter, Source: P, W. Sherman and M. L. Morton, Demography ofBclding's ground squirrel, £cofogy65:1617-1628 (198")'

Natural selection favors traits that improve an organism's chances ofsurvival and reproductive success. In every species, there are trade-offs between survival and traits such as frequency of reproduction, number of offspring produced (number of seeds produced by plants; litter or clutch size for animals), and investment in parental care. The traits that affect an organism's schedule of reproduction and survival (from birth through reproduction to death) make up its life history. A life history entails three basic variables: when reproduction begins (the age at first reproduction or age at maturity), how often the organism reproduces, and how many offspring are produced during each reproductive episode. \'1ith the important exception of humans, which we will consider later in the chapter, organisms do not choose con· sciously when to reproduce or how many offspring to have. Rather, organisms' life history traits are evolutionary outcomes reflected in their development, physiology, and behavior.

Evolution and Life History Diversity Reproductive tables vary greatly, depending on the species. Squirrels have a litter of m'o to six young once a year for less than a decade, whereas oak trees drop thousands of acorns each year for tens or hundreds ofyears. Mussels and other invertebrates may release hundreds of thousands of eggs in a spawning cycle. Why a particular type of reproductive pattern evolves in a particular population-one of many questions at the interface of population ecology and evolutionary biology-is the subject ofHfe history studies, the topic of the next section.

CONCEPT

CHECK

The fundamental idea that evolution accounts for the diversity oflife is manifest in a broad range ofHfe histories found in nature. Pacific salmon, for example, hatch in the headwaters ofa stream and then migrate to the open ocean, where they require one to four years to mature. The salmon eventually return to the fresh· water stream to spawn, producing thousands of eggs in a single reproductive opportunity before they die. This "one-shot" pattern of big-bang reproduction, or semelparity (from the Latin semel, once, and parere, to beget), also occurs in some plants, such as the agave, or "century plan( (figure 53.7).

53.1

I. One spedes of forest bird is highly territorial, while a second lives in flocks. Predict each species' likely pattern of dispersion, and explain. 2.••!;t.W"1 Each female of a particular fish species produces millions of eggs per year. Draw and label the most likely survivorship curve for this spedes, and explain your choice. 3. _1,II:O'ly As noted in Figure 53.2, an important assumption of the mark-recapture method is that marked individuals have the same probability of being recaptured as unmarked individuals. Describe a situation where this assumption might not be valid, and explain how the estimate of population size would be affected.

For suggesled answers, see Appendix A.

... Figure 53.7 An agave (Agave americana), an

example of big-bang reproduction. The lea~es of the plant are ~isible at the base of the giant flowering stal~, which is produced only at the end of the agave's life,

CHAPTE~ f1flY·TH~EE

Population Ecology

1179


Agaves generally grow in arid climates with unpredictable rainfall and poor soils. An agave grows for years, accumulating nutrients in its tissues, until there is an unusually wet year. It then sends up a large flowering stalk, produces seeds, and dies. This life history is an adaptation to the agave's harsh desert environment. In contrast to semel parity is iteroparity (from the Latin iterare, to repeat), or repeated reproduction. Some lizards, for example, produce a few large eggs during their second year of life and then reproduce annually for several years. What factors contribute to the evolution of semelparity versus iteroparity? A current hypothesis suggests that there are two critical factors: the survival rate of the offspring and the likelihood that the adult will survive to reproduce again. Where the survival rate of offspring is low, typically in highly variable or unpredictable environments, the prediction is that big-bang reproduction (semelparity) will be favored. Adults are also less likely to survive in such environments, so producing large numbers of offspring should increase the probability that at least some of those offspring will survive. Repeated reproduction (iteroparity) may be favored in more dependable environments where adults are more likely to survive to breed again and where competition for resources may be intense. In such cases, a few relatively large, well-provisioned offspring should have a better chance ofsurviving until they are capable of reproducing. Nature abounds with life histories that are intermediate between the two extremes of semelparity and iteroparity. Oak trees and sea urchins are examples of organisms that can live a long time but repeatedly produce relatively large numbers of offspring.

"Trade·offs" and Life Histories Natural selection cannot maximize all reproductive variables simultaneously. We might imagine an organism that could produce as many offspring as a semelparous species, provision them well like an iteroparous species, and do so repeatedly, but such organisms do not exist. Time, energy, and nutrients limit the reproductive capabilities of all organisms. In the broadest sense, there is a trade·offbetween reproduction and survival. A study of red deer in Scotland showed that females that reproduced in a given summer were more likely to die during the next winter than females that did not reproduce. A study of European kestrels also demonstrated the survival cost to parents of caring for young (Figure 53.8). Selective pressures influence the trade-off between the number and size ofoffspring. Plants and animals whose young are subject to high mortality rates often produce large numbers of relatively small offspring. Plants that colonize disturbed environments, for example, usually produce many small seeds, only a few of which may reach a suitable habitat. Small size may also increase the chance of seedling establish1180

U"IT EIG~T

Ecology

• FI

53.8

How does caring for offspring affect parental survival in kestrels? EXPERIMENT Cor Dijkstra and colleagues in the Netherlands studied the effects of parental caregiving in European kestrels o~er fi~e years, The researchers transferred chicks among nests to produce reduced broods (three or four chicks). normal broods (fi~e or six), and enlarged broods (se~en or eight). They then measured the percentage of male and female parent birds that survi~ed the follOWIng winter (Both males and females pro~ide care for chicks) RESULTS

100

"-m

., ., c

80

~

c

0

2 •

'," ~

60

40

c

:~

,

20

0

• • E

• "

CONClUSION The lower survi~al rates of kestrels with larger broods indicate that caring for more offspring negati~ely affects survi~al of the parents, SOURCE C. OljkmJ et ai" Brood Slze manlpulationl in the kestrel (Fako tinnunculus): effects 0/1 offspring and parent suMllal, joumal of Animal Ecology 59:269-285 (1990),

_1,lIIfn!.

The males of many bird species pro~ide no parental care. If this were true for the European kestrel, how would the experimental results differ from those shown above?

ment by enabling the seeds to be carried longer distances to a broader range of habitats (Figure 53.9a). Animals that suffer high predation rates, such as quail, sardines, and mice, also tend to produce large numbers of offspring. In other organisms, extra investment on the part of the parent greatly increases the offspring's chances of survival. Walnut trees and coconut palms both provision large seeds with energy and nutrients that help the seedlings become established (Figure 53.9b). In animals, parental investment in offspring does not always end after incubation or gestation. For instance, primates generally bear only one or two offspring at a time. Parental care and an extended period oflearning in the first several years of life are very important to offspring fitness in these species.


r;~:j::;~:;t~al

model describes population growth in an idealized, unlimited environment

Cal Most weedy plants, such as this dandelion, grow Quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves

Cb) Some plants, such as this coconut palm, produce a moderate number of very large seeds. Each seed's large endosperm provides nutrients for the embryo. an adaptation that helps ensure the success of a relatively large fradion of offspring.

... Figure 53,9 Variation in the size of seed crops in plants,

CONCEPT

CHECK

53.2

I. Consider two rivers: One is spring fed and has a constant water volume and temperature year-round; the other drains a desert landscape and floods and dries out at unpredictable intervals. Which river would you predict is more likely to support many species of iteroparous animals? Why? 2. In the fish called the peacock wrasse (Symphodus tinca), females disperse some of their eggs widely and lay other eggs in a nest. Only the latter receive parental care. Explain the trade-offs in reproduction that this behavior illustrates. Mice that cannot find enough food or 3, that experience other forms of stress will sometimes abandon their young. Explain how this behavior might have evolved in the context of reproductive trade-offs and life history.

-waUl.

For suggested answers, see Appendix A.

Populations of all species, regardless of their life histories, have the potential to expand greatly when resources are abundant. To appreciate the potential for population in路 crease, consider a bacterium that can reproduce by fission every 20 minutes under ideal laboratory conditions. There would be 2 bacteria after 20 minutes, 4 after 40 minutes, and 8 after 60 minutes. If reproduction continued at this rate, with no mortality, for only a day and a half, there would be enough bacteria to form a layer a foot deep over the entire globe. At the other life history extreme, an elephant may produce only 6 offspring in a loo-year life span. Still, Charles Darwin once estimated that the descendants of a single pair of mating elephants would number 19 million within only 750 years. Darwin's estimate may not have been precisely correct, but such analyses led him to recognize the tremendous capacity for growth in all populations. Although unlimited growth does not occur for long in nature, studying population growth in an idealized, unlimited environment reveals the capacity of species for increase and the conditions under which that capacity may be expressed.

Per Capita Rate of Increase Imagine a population consisting ofa few individuals living in an ideal, unlimited environment. Under these conditions, there are no restrictions on the abilities of individuals to harvest en路 ergy, grow, and reproduce, aside from the inherent biological limitations of their life history traits. The population will increase in size with every birth and with the immigration of individuals from other populations, and it will decrease in size with every death and with the emigration of individuals out of the population. We can thus define a change in population size during a fixed time interval with the following verbal equation:

0>,""

i" (.rth Immi,m"") ( h ,mi,rum,)

population .. SIze dUring = .

time interval

Bl

s

.

g

d' urm

lime

. I mterva

+

entering . population ..

dUring time interval

-

Deat s d . UTm . g

+

leaVing . population

time..

, I mterva

dunng tune interval

For simplicity here, we will ignore the effects of immigration and emigration, although a more complex formulation would certainly include these factors. We can also use math路 ematical notation to express this simplified relationship more concisely. If N represents population size and t represents time, then t1,N is the change in population size and t1,t is the time interval (appropriate to the life span or generation time of the species) over which we are evaluating population growth. (The Greek letter delta, t1" indicates change, CHAPTE~ f1flY路TH~EE

Population Ecology

1181


such as change in time.) We can now rewrite the verbal equation as

aN -=B-D

include immigration or emigration. Most ecologists prefer to use differential calculus to express population growth instantaneously, as growth rate at a particular instant in time:

at

where B is the number of births in the population during the time interval and D is the number of deaths. Next, we can convert this simple model into one in which births and deaths are expressed as the average number of births and deaths per individual (per capita) during the specified time interval. The per capita birth rate is the number of offspring produced per unit time by an average member of the popula~ tion. If, for example, there are 34 births per year in a population of 1,000 individuals, the annual per capita birth rate is 34/1,000, orO.034. Ifwe know the annual per capita birth rate (symbolized by b), we can use the formulaB = bNto calculate the expected number ofbirths per year in a population ofany size. For example, if the annual per capita birth rate is 0.034 and the population size is 500,

B=bN B = 0.034 X 500 B = 17 per year Similarly, the per capita death rale (symbolized by d) allows us to calculate the expected number of deaths per unit time in a population ofany size, using the formula D = dN. Ifd = 0.016 per year, we would expect 16 deaths per year in a population of 1,000 individuals. For natural populations or those in the labo~ ratory, the per capita birth and death rates can be calculated from estimates of population size and data in life tables and re~ productive tables (for example, Tables 53.1 and 53.2). Now we can revise the population growth equation again, this time using per capita birth and death rates rather than the numbers of births and deaths:

In this case r;nst is simply the instantaneous per capita rate of increase. Ifyou have not yet studied calculus, don't be intimidated by the form of the last equation; it is similar to the previous one, are very short and are exexcept that the time intervals pressed in the equation as dt.ln fact, as becomes shorter, the discrete rapproaches the instantaneous riml in value.

at

at

Exponential Growth Earlier we described a population whose members all have access to abundant food and are free to reproduce at their physiological capacity. Population increase under these ideal conditions is called exponential population growth, also known as geometric population growth. Under these conditions, the per capita rate of increase may assume the maximum rate for the species, denoted as r'M"" The equation for exponential population growth is

dN

dt = r"",xN The size of a population that is growing exponentially in~ creases at a constant rate, resulting eventually in a J-shaped growth curve when population size is plotted over time (Figure 53.10). Although the maximum rate of increase is constant, the population accumulates more new individuals per unit of time when it is large than when it is small; thus, the

2.000

aN =bN-dN

at

One final simplification is in order. Population ecologists are most interested in the difference between the per capita birth rate and per capita death rate. This difference is the per capita rate a/increase, or r:

r=b-d The value of r indicates whether a given population is growing (r> 0) or declining (r < 0). Zero population growth (ZPG) occurs when the per capita birth and death rates are equal (r = 0). Births and deaths still occur in such a population, of course, but they balance each other exactly. Using the per capita rate of increase, we can now rewrite the equation for change in population size as

t1.N =rN

at

Remember that this equation is for a discrete, or fixed, time interval (often one year, as in the previous example) and does not 1182

U"IT EIG~T

Ecology

~ 1.500

dN=0,5N

,~ c ,Q

"

dt 1,000

"5

ÂŁ 500

o -I---o:::;."""",~=-,---~o 5 10 15 Number of generations â&#x20AC;˘ Figure 53.10 Population growth predicted by the exponential model. This graph compares growth in two populations with different ~alues of I"""" Increasing the ~alue from 0.5 to 1.0 increases the rate of rise in population size o~er time, as reflected by the relative slopes of the CUtves at any gi~en population size,


r;~:~:~:t~:~~del

8,000

describes how a population grows more slowly as it nears its carrying capacity

c

6,000

0

â&#x20AC;˘ "5

~

-â&#x20AC;˘ 0

~

4,000

c

~

~

" w

2,000

0 1900

1920

1940 Year

1960

1980

.. Figure 53. l' Exponential growth in the African elephant population of Kruger National Park. 50uth Africa. curves in Figure 53.10 get progressively steeper over time. This occurs because population growth depends on N as well as r",ax' and larger populations experience more births (and deaths) than small ones growing at the same per capita rate. It is also clear from Figure 53.10 that a population with a higher maximum rate of increase (dN/dt = 1.0N) will grow faster than one with a lower rate of increase (dNldt = 0.5N). The J-shaped curve ofexponential growth is characteristic of some populations that are introduced into a new environment or whose numbers have been drastically reduced by a catastrophic event and are rebounding. For example, the population of elephants in Kruger National Park, South Africa, grew exponentially for approximately 60 years after they were first protected from hunting (figure 53.11). The increasingly large number ofelephants eventually caused enough damage to vegetation in the park that a collapse in their food supply was likely. To protect other species and the park ecosystem before that happened, park managers began limiting the elephant population by using birth control and exporting elephants to other countries. CONCEPT

CHECK

53.3

1. Explain why a constant rate of increase (r",..x) for a population produces a growth graph that is J-shaped rather than a straight line. 2. \'(fhere is exponential gro\\1h by a plant population more likely-on a newly formed volcanic island or in a mature, undisturbed rain forest? Why? 3, -','!:tU1jM In 2006, the United States had a population ofabout 300 million people. If there were 14 births and 8 deaths per 1,000 people, what was the country's net population growth that year (ignoring immigration and emigration, which are substantial)? Do you think the United States is currently experiencing exponential population growth? Explain.

For suggested answers, see Appendix A.

The exponential growth model assumes that resources are unlimited, which is rarely the case in the real world. As population density increases, each individual has access to fewer resources. Ultimately, there is a limit to the number of individuals that can occupy a habitat. Ecologists define carrying capacity, symbolized as K, as the maximum population size that a particular environment can sustain. Carrying capacity varies over space and time with the abundance of limiting resources. Energy. shelter. refuge from predators, nutrient availability, water, and suitable nesting sites can all be limiting factors. For example, the carrying capacity for bats may be high in a habitat with abundant flying insects and roosting sites, but lower where there is abundant food but fewer suitable shelters. Crowding and resource limitation can have a profound effect on population growth rate. If individuals cannot obtain sufficient resources to reproduce, the per capita birth rate (b) will decline. If they cannot consume enough energy to maintain themselves, or if disease or parasitism increases with density, the per capita death rate (d) may increase. A decrease in b or an increase in d results in a lower per capita rate of increase (r).

The logistic Growth Model We can modify our mathematical model to incorporate changes in growth rate as the population size nears the carrying capacity. In the logistic population growth model, the per capita rate of increase approaches zero as the carrying capacity is reached. To construct the logistic model, we start with the exponential population growth model and add an expression that reduces the per capita rate of increase as N increases. If the maximum sustainable population size (carrying capacity) is K, then K - N is the number of additional individuals the environment can support, and (K - N)I K is the fraction of K that is still available for population growth. By multiplying the exponential rate of increase r",axNby (K - N)I K. we modify the change in population size as N increases:

dN

(K -N)

----::it = r",..x N - K -

\'(fhen N is small compared to K, the term (K - N)I K is large, and the per capita rate of increase, r",,,,,(K - N)IK, is close to the maximum rate of increase. But when N is large and resources are limiting, then (K - N)IKis small, and so is the per capita rate of increase. When N equals K. the population stops CHAPTE~ f1flY¡TH~EE

Population Ecology

1183


_....

Logistic Growth of a Hypothetical Population

Exponential

2,000

= 1.500)

(K

Popu- Intrinsic lation Rate of Size Increase

Per Capita Rate of Increase:

K-N (K - N) K '- K

Population Growth Rate:' ,~,.N

(K-K- N)

(N)

(r~Jf)

25

1.0

0.98

0.98

100

1.0

0.93

0.93

+93

250

1.0

0.83

0.83

+208

500

1.0

0.67

0.67

+333

+25

750

1.0

0.50

0.50

+375

1,000

1.0

0.33

0.33

+333

1,500

1.0

0.00

0.00

0

'Rounded tu the

ne~rei\

whole number.

growing. Table 53.3 shows cakulations of population growth rate for a hypothetical population growing according to the logistic model, with T......., "" 1.0 per individual per year. Notice that the overall population gro....rth rate is highest, +375 individuals per year, when the population size is 750, or half the carrying capacity. At a population size of 750, the per capita rate of increase remains relati\'e1y high (one-half the maximum rate), but there are more reproducing individuals (NJ in the population than at lower population sizes. The logistic model of population growth produces a sigmoid (S-shaped) growth curve when N is plotted over time (the red line in Figure 53,12). New individuals are added to the population most rapidly at intermediate population sizes, when there is not only a breeding population of substantial size, but also lots of available space and other resources in the environment. The population growth rate slows dramatically as N approaches K. Note that we haven't said anything yet about why the population growth rate slows as N approaches K For a population's growth rate to decrease, either the birth rate b must decrease, the death rate d must increase, or both. Later in the chapter, we will consider some of the factors affecting these rates.

The logistic Model and Real Populations The growth of laboratory populations of some small animals, such as beetles and crustaceans, and of some microorganisms, such as paramecia, yeasts, and bacteria, fits an S·shaped curve fairly well under conditions of limited resources (Figure S3.13a). These populations are grown in a constant environment lacking predators and competing species that may reduce growth of the populations, conditions that rarely occur in nature. 1184

UNIT !IGHT

Ecology

~

growth dN;10N d'

1,500 +-K::-~~I~,5OO=---+---:::""'------:

~

J c

•• 1,000

logIStIC growth dN~ION(',500-N) dr 1,500

500

o.j---:..,---~---~­ o 15 5 '0 Number of generations

• Figure 53.12 Population growth predicted by the logistic model. The rate of populatIOn growth slows as population size lM approaches the wrying capaCity (K) of the environment. The red line shows logistic growth In a populatIOn where rnYl( == 1.0 and K .. 1,500 individuak. for comparison, the blue line illustrates a populattOO contlnuing to grow exponentially wrth the same r",.,..

Some of the basic assumptions built into the logistic model clearly do not apply to aU populations. The logistic model assumes that populations adjust instantaneously to growth and approach carrying capacity smoothly. In reality, there is often a lag time before the negative effects of an increasing population are realized.lffood becomes limiting for a population, for instance. reproduction will decline eventually, but females may use their energy reserves to continue reproducing for a short time. This may cause the population to overshoot its carrying capacity temporarily, as shown for the water fleas in Figure S3.13b. If the population then drops below carrying capacity, there will be a delay in population growth until the increased number of offspring are actually born. Still other populations fluctuate greatly, making it difficult even to define carrying capacity. We will examine some possible reasons for such fluctuations later in the chapter. The logistic model also incorporates the idea that regardless of population density, each individual added to a population has the same negative effect on population growth rate. However, some populations show an Allee effect (named after W. C. Allee, of the University of Chicago, who first described it), in which individuals may have a more difficult time surviving or reproducing if the population size is too small. For example, a single plant may be damaged by excessive wind ifit is standing alone, but it would be protected in a clump of individuals. The logistic model is a useful starting point for thinking about how populations grow and for constructing more complex models. The model is also important in conservation


~

i

..

1,000

E180 ~

â&#x20AC;˘

800

E 0 ~

600

"w

400

,E

~ 60

200

z

!

.'

c

'. ':"':.-:.,: .,

120

..

well do these populations fit the logistic growth model?

'0 90

D

z

.. Figure 53.13 How

150

"@

'0

'.' .'

a

,E

a

30 O~'-r-~-~~-~~~-~-

5

10 Time (days)

(a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the researcher maintains a constant environment

o

15

20

40

60

80 100 Time (days)

120

140

160

(b) A Daphnia population in the lab. The growth of a population of water fleas (Daphnia) in a smalilaboralory culture

(black dots) does not correspond well to the logistic model (red curve), This population overshoots the carrying capacity of its anificial environment before it settles down to an

approximately stable population size.

biology for predicting how rapidly a particular population might increase in numbers after it has been reduced to a small size and for estimating sustainable harvest rates for fish and wildlife populations. Conservation biologists can use the model to estimate the critical size below which populations of certain organisms, such as the northern subspecies of the white rhinoceros (Ceralotllerium simum), may become extinct (figure 53.14). Like any good starting hypothesis, the logistic model has stimulated research that has led to a better understanding of the factors affecting population growth.

The Logistic Model and Life Histories The logistic model predicts different per capita growth rates for populations of low or high density relative to the carrying

... Figure 53.14 White rhinoceros mother and calf. The two animals pictured here are members of the southern subspecies, which has a population of more than 10.000 individuals. The northern subspecies is critically endangered. with a population of fewer than 25 individuals.

capacity of the environment. At high densities, each individual has few resources available, and the population grows slowly. At low densities, per capita resources are relatively abundant, and the population grows rapidly. Different life history features are favored under each condition. At high population density, selection favors adaptations that enable organisms to survive and reproduce with few resources. Competitive ability and efficient use of resources should be favored in populations that are at or near their carrying capacity. (Note that these are the traits we associated earlier with iteroparity.) At low population density, adaptations that promote rapid reproduction, such as the production of numerous, small offspring, should be favored. Ecologists have attempted to connect these differences in favored traits at different population densities with the logistic growth model. Selection for life history traits that are sensitive to population density is known as K-selection, or densitydependent selection. In contrast, selection for life history traits that maximize reproductive success in uncrowded environments (low densities) is called r-selection, or densityindependent selection. These names follow from the variables ofthe logistic equation. K-selection is said to operate in populations living at a density near the limit imposed by their resources (the carrying capacity, Kj, where competition among individuals is relatively strong. Mature trees growing in an old-growth forest are an example of K-selected organisms. In contrast, r-selection is said to maximize r, the per capita rate of increase, and occurs in environments in which population densities are well below carrying capacity or individuals face little competition. Such conditions are often found in disturbed habitats. Like the concepts ofsemelparity and iteroparity, the concepts of K- and r-selection represent two extremes in a range ofactual life histories. The framework of K- and r-selection, grounded in the idea of carrying capacity, has helped ecologists to propose CHAPTE~ f1flY¡TH~EE

Population Ecology

1185


alternative hypotheses oflife history evolution. These alternative hypotheses, in turn, have stimulated more thorough smdy of how factors such as disturbance, stress, and the frequency ofopportunities for successful reproduction affect the evolution oflife histories. They have also forced ecologists to address the importantquestion we alluded to earlier: \Vhydoes population gro\\1:h rate decrease as population size approaches carrying capacity? Answering this question is the focus of the next section. CONCEPT

CHECK

5J.4

Population regulation is an area of ecology that has many practical applications. In agriculture, a farmer may want to reduce the abundance ofinsect pests or stop the growth ofan invasive weed that is spreading rapidly. Conservation ecologists need to know what environmental factors create favorable feeding or breeding habitats for endangered species, such as the white rhinoceros and the whooping crane. Management programs based on population-regulating factors have helped prevent the extinction of many endangered species.

Population Change and Population Density

1. Explain why a population that fits the logistic growth

model increases more rapidly at intermediate size than at relatively small or large sizes. 2. When a farmer abandons a field, it is quickly colonized by fast-growing weeds. Are these species more likely to be K-selected or ,-selected species? Explain. 3. _'MUI 4 Add rows to Table 53.3 for three cases where N > K: N = 1,600, 1,750, and 2,000. What is the population growth rate in each case? In which portion of Figure 53.13b is the Daphnia population changing in a way that corresponds to the values you calculated? For suggested answers, see Appendix A,

r:i~:;~:c~~;~hat

regulate population growth are density dependent

In this section, we will apply biology's unifying theme of feedback reguiLltion (see Chapter 1) to populations, \Vhat environmental factors keep populations from growing indefinitely? Why are some populations fairly stable in size, while others, such as the Soay sheep on Hirta Island, are not (see Figure 53.1)?

To understand why a population stops growing, it is helpful to study how the rates of birth, death, immigration, and emigration change as population density rises. If immigration and emigration offset each other, then a population grows when the birth rate exceeds the death rate and declines when the death rate exceeds the birth rate. A birth rate or death rate that does not change with population density is said to be density independent. In a classic study of population regulation, Andrew Watkinson and John Harper, of the University of Wales, found that the mortality of dune fescue grass (Vulpia membranacea) is mainly due to physical factors that kill similar proportions of a local population, regardless of its density. For example, drought stress that arises when the roots of the grass are uncovered by shifting sands is a density-independent factor. In contrast, a death rate that rises as population density rises is said to be density dependent, as is a birth rate that falls with rising density. Watkinson and Harper found that reproduction by dune fescue declines as population density increases, in part because water or nutrients become more scarce. Thus, in this grass population, the key factors regulating birth rate are density dependent, while death rate is largely regulated by densityindependent factors. Figure 53.15 models how a population may stop increasing and reach equilibrium as a result ofvarious combinations of density-dependent and density-independent regulation.

DenSity-dependent birth rate Densityindependent death rate

Equilibrium density

Equilibrium density

Population denSity (a) Both birth rate and death rate change with population density.

Population density_

(b) Birth rate changes with population density while death rate is constant.

(c) Death rate changes with population density while birth rate is constant.

.. Figure 53.15 Determining equilibrium for population density. This simple model considers

UNIT EIGHT

Ecology

Equilibrium density

Population density_

onbj birth and death rates (immlgration and emigration rates are assumed to be either zero or equaQ,

1186

Densityindependent birth rate


1l 100

~

g'

,

'w 80

] ~

j!

60

C

[

40

<;

1:-

~

j

20

o+---~--~---'--"'-~--~200

300

400

500

600

Population size

(a) Cheetahs stake out their territories with a chemical marker 10 urine

... Figure 53.16 Decreased reproduction at high population densities. Reproduction by juvenile Soay sheep on Hirta Island drops dramatically as population size increases.

Density-Dependent Population Regulation \Vithout some type of negative feedback between population density and the vital rates of birth and death, a population would never stop growing. Density-dependent regulation provides that feedback, operating through mechanisms that help to reduce birth rates and increase death rates, halting population growth.

Competition for Resources In a crowded population, increasing population density intensifies competition for declining nutrients and other resources, resulting in a lower birth rate. Crowding can reduce reproduction by plants, as discussed earlier for dune fescue. Many animal populations also experience internal competition for food and other resources. On Hirta Island, ecologists have closely monitored the relationship between Soay sheep density and reproduction for many years. Their results show that the effects of increasing density on birth rate are strongest for the youngest sheep that reproduce, typically I-year-old juveniles (Figure 53.16).

(b) Gannets nest virtually a peck apart and defend their territories by calling and pecking at one another. ... Figure 53.17 Territoriality. In some animals, defense olterritories provides negative leedback on population density, nest site, but beyond that threshold, few additional birds breed successfully. Birds that cannot obtain a nesting spot do not reproduce. The presence of surplus, or nonbreeding, individuals is a good indication that territoriality is restricting population growth, as it does in many bird populations.

Disease Territoriality In many vertebrates and some invertebrates, territoriality can limit population density. In this case, territory space becomes the resource for which individuals compete. Cheetahs, for example, are highly territorial, using chemical communication to warn other cheetahs of their territorial boundaries (Figure 53.17a). Maintaining a territory increases the likelihood that a cheetah will capture enough food to reproduce. Oceanic birds, such as gannets, often nest on rocky islands to avoid predators (Figure 53.17b). Up to a certain population density, most gannets can find a suitable

Population density can also influence the health and thus the survival of organisms. If the transmission rate of a particular disease depends on a certain level of crowding in a population, the disease's impact may be density dependent. Among plants, the severity of infection by fungal pathogens is often greater in locations where the density of the host plant population is higher. Animals, too, can experience an increased rate of infection by pathogens at high population densities. Steven Kohler and Wade Hoiland, of the Illinois Natural History Survey, showed that in caddis flies (Brachycentrus americanus, a stream路dwelling insect), peaks in disease路related mortality CHAPTE~ f1flY路TH~EE

Population Ecology

1187


followed years of high insect abundance, leading to cyclic fluctuations in the density of the caddis fly population. In humans, the lung disease tuberculosis, which is caused by bacteria that spread through the air when an infe<ted person sneezes or coughs, strikes a greater percentage of people living in densely populated cities than those in rural areas.

Predation Predation may be an important cause of density-dependent mortality if a predator encounters and captures more food as the population density of the prey increases. As a prey population builds up, predators may feed preferentially on that species, consuming a higher percentage of individuals. For example, trout may concentrate for a few days on a particular species of insect that is emerging from its aquatic larval stage and then switch to eating another insect species as it becomes more abundant.

Toxic Wastes The accumulation of toxic wastes can contribute to densitydependent regulation of population size. In laboratory cultures of microorganisms, metabolic by-products accumulate as the populations grow, poisoning the organisms within this limited, artificial environment. For example, ethanol accumulates as a by-product of yeast fermentation. The alcohol content of wine is usually less than 13% be<ause that is the maximum concentration ofethanol that most wine-producing yeast cells can tolerate.

Intrinsic Factors For some animal species, intrinsic (physiological) factors, rather than the extrinsic (environmental) factors we've just discussed, appear to regulate population size. For instance, white-footed mice in a small field enclosure will multiply, but eventually their reproductive rate will decline until the population ceases to grow. This drop in reproduction is associated with aggressive interactions that increase with population density, and it occurs even when food and shelter are provided in abundance. High population densities in mice can induce a stress syndrome in which hormonal changes delay sexual maturation, cause reproductive organs to shrink, and depress the immune system. In this case, high densities cause an increase in mortality and a decrease in birth rates. Similar effects of crowding occur in other wild rodent populations. These various examples of population regulation by negative feedback show how increased densities cause population growth rates to decline by affecting reproduction, growth, and survivorship. But although negative feedback helps explain why populations stop growing, it does not address why some populations fluctuate dramatically while others remain relatively stable. This is the topic we address next.

1188

U"IT EIG~T

Ecology

Population Dynamics All populations for which we have long-term data show some fluctuation in numbers. These fluctuations from year to year or place to place influence the seasonal or annual harvest of fish and other commercially important species. They also give e<ologists insight into what controls population size. The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in the size of populations.

Stability and Fluctuation Populations of[arge mammals were once thought to remain relatively stable over time, but long-term studies have challenged that idea. The numbers ofSoay sheep on Hirta Island fluctuate greatly, rising or falling by more than half from one year to the next (Figure 53.18). \Vhat causes the size of this population to change so dramatically? The most important factor appears to be the weather. Harsh weather, particularly cold, wet winters, weakens the sheep and decreases food availability, leading to a decrease in the size ofthe population. \Vhen sheep numbers are high, other factors, such as an increase in the density of parasites, also cause the population to shrink. Conversely, when sheep numbers are low and the weather is mild, food is readily available and the population grows quickly. Like the Soay sheep population on Hirta, the moose population on Isle Royale in Lake Superior also fluctuates over time. In the case ofthe moose, predation is an additional factor that regulates the population. Moose from the mainland colonized the island around 1900 by walking across the frozen lake. Wolves, which rely on moose for most of their food, followed around 1950. Because the lake has not frozen over in recent years, both populations have been isolated from immigration and emigration. Despite this isolation, the moose population

2,100 1,900 1,700 ~

••

1,500

~

"0 1.300

•,

n

E 1,100

z

900 700 500 01"" 1955

1965

1975

1985

1995

Year

.. Figure 53.18 Variation in size of the Soay sheep population on Hirta Island. 1955-2002.

2005


so

2500 Moose

Wolves

:D 40

2,000

2

~

•"0 0

E

30

1,500 '0

E 20

1,000 E

'0

"

", z

~

~

,

z 10

500

0+--~-~--~-~---1-0

1955

1965

1975

1985

1995

2005

.. Figure 53.19 Fluctuations in moose and wolf populations on Isle Royale, 1959-2006. n The first several moose reached Isle Royale in the early 19005, and . . by 1925 the population on the island had grown to 2,000. Why do you think it was able to grow 50 quickly? What growth model best describes this initial growth?

Snowshoe hare

160

Year

•"

:;;<i;

x

~<i;

120

-~

~~

_c

Lynx

~~ 80 ~~ E,z 40

X"

6

• 0

which increased the energy needs of the animals and made it harder for the moose to find food under the deep snow.

Population Cycles: Scientific Inquiry While many populations fluctuate at unpredictable intervals, others undergo regular boom-and-bust cycles. Some small herbivorous mammals, such as voles and lemmings, tend to have 3- to 4-year cycles, and some birds, such as ruffed grouse and ptarmigans, have 9- to II-year cycles. One striking example of population cycles is the lO-year cycling of snowshoe hares (Lepus americanus) and lynx (Lynx canadensis) in the far northern forests of Canada and Alaska. Lynx are predators that spedalize in preying on snowshoe hares, so it is not surprising that lynx numbers rise and fall with the numbers of hares (figure 53.20). But why do hare numbers rise and fall in lO-yearcycles? Three main hypotheses have been proposed. First, the cycles may be caused by food shortage during winter. Hares eat the terminal twigs ofsmall shrubs such as willow and birch in winter, although why this food supply might cycle in lO-year intervals is uncertain. Second, the cycles may be due to predator-prey interactions. Many predators other than lynx eat hares, and they may overexploit their prey. Third, the size ofthe hare population may vary with sunspot activity, which also undergoes cyclic changes. When sunspot activity is low, slightly less atmospheric ozone is produced, and slightly more UV radiation reaches Eartll's surface. In response, plants produce more UV-blocking chemicals and fewer chemicals tllat deter herbivores, increasing the quality ofthe hares' food.

~o

,= E~

z 3

0

0

experienced two major increases and collapses during the last 45 years (Figure 53.19). The first collapse coincided with a peak in the numbers of wolves from 1975 to 1980. The second collapse, around 1995, coincided with harsh winter weather,

.,

9 - c 0,

1850

1875

1900

1925

Year

... figure 53.20 Population cycles in the snowshoe hare and lynx. Population counts are based on the number of pelts sold by trappers to the Hudson Bay Company.

n What do you observe about the relative timing of the peaks in lynx . . numbers and hare numbers) What might explain this observation)

Let's consider the evidence for the hypotheses. lfhare cycles are due to winter food shortage, then they should stop if extra food is provided to a field population. Researchers have conducted such experiments in the Yukon for 20 years-over two hare cycles. They have found that hare populations in the areas with extra food have increased about threefold in density but have continued to cycle in the same way as the unfed control populations. Thus, food supplies alone do not cause the hare cycle shown in Figure 53.20, so we can reject the first hypothesis. Using radio collars, ecologists have tracked individual hares to determine why they died. Predators killed almost 90% ofthe hares in such studies, and none of the hares appeared to have died of starvation. These data support the second hypothesis. When ecologists excluded predators from one area with electric fences and excluded predators and provided food in another area, they found that the hare cycle is driven largely by excessive predation but that food availability also plays an important role, particularly in the winter. Perhaps better-fed hares are more likely to escape from predators. To test the third hypothesis, ecologists compared the timing of hare cycles and sunspot activity. As predicted, periods oflow sunspot activity were followed by peaks in the hare population. The results of all of these experiments suggest that both predation and sunspot activity may regulate the cycling of hare numbers and that food availability plays a less important role. CHAPTE~ f1flY·TH~EE

Population Ecology

1189


The availability of prey is the major factor influencing population changes for predators such as lynx, great-horned owls, and weasels, each of which depends heavily on a single prey spe<ies. \'1hen prey become scarce, predators often turn on one another. Coyotes kill both foxes and lynx, and great-horned owls kill smaller birds ofprey as well as weasels, accelerating the collapse of the predator populations. Long-term experimental studies help to unravel the causes of such population cycles.

Immigration, Emigration, and Metapopulations So far our discussion of population dynamics has focused mainly on the contributions of births and deaths. However, immigration and emigration also influence populations, particularly when a number of local populations are linked, forming a metapopulation. For example, immigration and emigration link the Belding's ground squirrel population we discussed earlier to other populations of the species, all of which make up a metapopulation. Local populations in a metapopulation can be thought of as occupying discrete patches of suitable habitat in a sea of unsuitable habitat. The patches vary in size, quality, and isolation from other patches, factors that influence how many individuals move among the populations. Patches with many individuals, for instance, can supply more emigrants to other patches. Ifone population becomes extinct, the patch it occupied can be re<:olonized by immigrants from another population. The Glanville fritillary (Melitaea cinxia) illustrates the movement of individuals benwen populations (Figure 53.21). This

butterfly is found in about 500 meadows across the Aland Islands of Finland, but its potential habitat in the islands is much larger, approximately 4,000 suitable patches. New populations of the butterfly regularly appear and existing populations become extinct, constantly shifting the locations of the 500 colonized patches. The species persists in a balance ofextinctions and recolonizations. The metapopulation concept underscores the significance of immigration and emigration in the butterfly populations. It also helps ecologists understand population dynamics and gene flow in patchy habitats, providing a framework for the conservation ofspecies living in a network ofhabitat fragments and reserves. CONCEPT

CHECl(

53.5

I. Identify three density-dependent factors that limit population size, and explain how each exerts negative feedback. 2. Describe three attributes of habitat patches that could affect population density and rates of immigration and emigration. 3. -Mn'li- If you were studying an endangered species that, like the snowshoe hare, has a lO-year population cycle, how long would you need to study the species to determine if its population size is declining? Explain. For suggested answers, see Appendix A.

r;~:t~::a~3;~pulation

is no longer growing exponentially but is still increasing rapidly

In the last few centuries, the human population has grown at an unprecedented rate, more like the elephant population in Kruger National Park (see Figure 53.11) than the fluctuating populations weconsidered in Concept 53.5. No population can grow indefinitely, however, and humans are no exception. In this last section ofthe chapter, we'll apply the concepts of population dynamics to the specific case of the human population.

a â&#x20AC;˘ Occupied patch â&#x20AC;˘ Unoccupied patch

.... Figure 53.21 The Glanville fritillary: a meta population. On the Aland Islands. local populations of this butterfly (filled circles) are found in only a fraction of the suitable habitat patches (open circles) at any gi~en time. Indi~iduals can mo~e between local populations and colonize unoccupied patches,

1190

U"IT EIG~T

Ecology

The Global Human Population TIle exponential growth model in Figure 53.10 approximates the human population explosion since 1650. Ours is a singular case; it is unlikely that any other population of large animals has ever sustained so much growth for so long (Figure 53.22). TIle human population increased relatively slowly until about 1650, at which time approximately 500 million people inhabited Earth. Our population doubled to 1 billion within the next m'o centuries, doubled again to 2 billion between 1850 and 1930, and


7

2.2

"'

2.0

B

18

6 c

5

~ c

4

~,

3

0

~ ~

c 2 E r

,•

The Plague

*•

0

.~

8000

BCE

'000 BCE

3000

BCE.

2000 BCE

'000 BCE

0

'000

CE.

1,4

~ 12

•a.

1,0

2

0,8

<

2000

So far we have described changes in the global population, but population dynamics vary widely from region to region. In a stable regional human population, birth rate equals death rate (disregarding the effects of immigration and emigration). Two possible configurations for a stable population are Zero population growth = High birth rate - High death rate

Zero population growth = Low birth rate - Low death rate

,

.. .....

",

"

".

0.2 oL,--~--~--~--~­

1950

has slowed in recent decades. mainly as a result of decreased birth rales throughout the world.

Regional Patterns of Population Change

" , ./projected .. data

04

... Figure 53.22 Human population growth (data as of 2006). The global human population has grown almost continuously throughout history, but it skyrocketed after the Industrial Revolution. Though it is not apparent at this scale, the rate of population growth

a city the size of Amarillo, Texas, or Madison, \Visconsin. It takes only about four years for world population growth to add the equivalent of another United States. Population ecologists predict a population of7.8-1O.8 billion people on Earth by the year 2050. Though the global population is still growing, the rate of growth began to slow during the 1960s (Figure 53.23). The an~ nual rate of increase in the global population peaked at 2.2% in 1962; by 2005, it had dedi ned to 1.15%. Current models project a continued decline in the annual growth rate to just over 0.4% by 2050, a rate that would still add 36 million more people per year if the population climbs to a projected 9 billion. The reduction in growth rate over the past four decades shows that the human population has departed from true exponential growth, which assumes a constant rate. This departure is the result of fundamental changes in population dynamics due to diseases, including AIDS, and to voluntary population control.

"

0.6

CE

doubled still again by 1975 to more than 4 billion. The global population is now more than 6.6 billion people and is increasing by about 75 million each year. The population grows byapproximately 200,000 people each day, the equivalent ofadding

2005

E

~ 0

1.6

1975

2000 Year

2025

2050

... Figure 53.23 Annual percent increase in the global human population (data as of 2005). The sharp dip in the 1960s is due mainly to a famine in China in which about 60 million people died. 50

~ 30

~ •

••,

-5 20

-1i

~ t

10

Sweden Birth rate Death rate

Mexico Birth rate Death rate

.....

~",.

.....,

•"

~ ~ ~

~ ~ ~~

.' ~

.'

O+--~-~-~--~-~-~

1750

lBOO

1850

1900

1950

2000 2050

Year ... Figure 53.24 Demographic transition in Sweden and Mexico, 1750-2050 (data as of 2005). The movement toward the second configuration is called the demographic transition. Figure 53.24 compares the demographic transition in one of the most industrialized countries, Sweden, and in a less industrialized country, Mexico. The demographic transition in Sweden took about 150 years, from 1810 to 1960, when birth rates finally approached death rates; in Mexico, the changes are projected to continue until sometime after 2050, almost the same length of time as they took in Sweden. Demographic transition is associated with an increase in the quality of health care and sanitation as well as improved access to education, especially for women. CHAPTE~ f1flY·TH~EE

Population Ecology

1191


aspirations of women in many cultures encourage women to delay marriage and postpone reproduction. Delayed reproduction helps to decrease population growth rates and to move a society toward zero population growth under conditions oflow birth rates and low death rates. However, there is a great deal ofdisagreement among world leaders as to how much support should be provided for global family planning efforts.

After 1950, death rates declined rapidly in most developing countries, but birth rates have declined in a more variable manner. The fall in birth rate has been most dramatic in China. In 1970, the Chinese birth rate predicted an average of 5.9 children per woman per lifetime (total fertility rate); by 2004, largely because of the government's strict one-child policy, the expected total fertility rate was 1.7 children. In some countries of Africa, the transition to lower birth rates has also been rapid, though birth rates remain high in most of subSaharan Africa. In India, birth rates have fallen more slowly. How do such variable birth rates affect the growth of the world's population? In industrialized nations, populations are near equilibrium (growth rate about 0.1% per year), with reproductive rates near the replacement level (total fertility rate = 2.1 children per female). In many industrialized countries, including Canada, Germany, Japan, and the United Kingdom, total reproductive rates are in fact be/vw replacement. These populations will eventually decline if there is no immigration and if the birth rate does not change. In fact, the population is already declining in many eastern and central European coun· tries. Most of the current global population growth (L 15% per year) is concentrated in less industrialized countries, where about 80% of the world's people now live. A unique feature of human population growth is our potential ability to control it with family planning and voluntary contraception. Reduced family size is the key to the demographic transition. Social change and the rising educational and career Rapid growth Afghanistan Male Female

Age Structure One important demographic variable in present and future growth trends is a country's age structure, the relative number of individuals of each age in the population. Age structure is commonly graphed as "pyramids" like those in Figure 53.25. For Afghanistan, the pyramid is bottom-heavy, skewed toward young individuals who will grow up and may sustain the explosive growth with their own reproduction. The age structure for the United States is relatively even until the older, postreproductive ages, except for a bulge that corresponds to the "baby boom~ that lasted for about two decades after the end of World War II. Even though couples born during those years have had an average of fewer than two children, the nation's overall birth rate still exceeds the death rate because so many "boomers~ and their offspring are still of reproductive age. Moreover, although the current total reproductive rate in the United States is 2.1 children per woman-approximately replacement rate-the population is projected to grow slowly

Slow growth United States Male Female

Age

Age

85. 80-84

1

F

.• I [

10

8

•-.

, 6

, 4 2 0 2 4 Percent of population

6

• iI 8

,

Q-4

8

6

U"IT EIG~T

Ecology

8Q-84 75-79 70-74 65-69 60-64 55-59

,

r

r

SO-54 45-49 40-44 35-39 30-34 25-29 20-24 15-19 10-14 5-9

, Q-4

L

4 2 0 2 4 Percent of population

.. Figure 53.25 Age-structure pyramids for the human population of three countries (data as of 2005). As of 2007, the annual rate of population growth was approximately 2.6% in Afghanistan, 0.9% in the United States, and 0.0% in Italy.

1192

r' =J,

85.

75-79 70-74 65-69 60-64 55-59 SO-54 45-49 40-44 35-39 30-34 25-29 20-24 15-19 10-14 5-9

10

No growth Italy Male Female

6

8

, 8

6

2 0 2 4 4 Percent of population

6

8


through 2050 as a result of immigration. For Italy, the pyramid has a small base, indicating that individuals younger than reproductive age are relatively underrepresented in the population. This situation contributes to the projection ofa population decrease in Italy. Age-structure diagrams not only predict a population's gro\\1h trends but can also illuminate social conditions. Based on the diagrams in Figure 53.25, we can predict, for instance, that employment and education opportunities will continue to be a significant problem for Afghanistan in the foreseeable future. The large number of young entering the Afghan population could also be a source of continuing social and political unrest, particularly if their needs and aspirations are not met. In Italy and the United States, a decreasing proportion of younger working-age people will soon be supporting an increasing population of retired "boomers.n In the United States, this demographic feature has made the future of Social Security and Medicare a major political issue. Understanding age structures can help us plan for the future.

Infant Mortality and Life Expectancy

Infant mortality, the number of infant deaths per I,M live births, and life expectancy at birth, the predicted average length of life at birth, vary widely among different human populations. These differences reflect the quality of life faced by children at birth and influence the reproductive choices parents make. Ifinfant mortality is high, then parents are likely to have more children to ensure that some ofthem reach adulthood. figure 53.26 contrasts average infant mortality and life expectancy in the industrialized and less industrialized countries of the world in 2005. \Vhile these averages are markedly different, they do not capture the broad range ofthe human condition. In 2005, for example, the infant mortality rate was 163 (16.3%) in Afghanistan

"'

60

°°o.

50

80

~

t

~

"••

"w 40

b

~

~ c

~

"" w

60

~w 40

30

~

•x

.~

"E 20

"' ~

E

0 Industrialized countries

Less mdustrialized countries

No ecological question is more important than the future size of the human population. The projected worldwide population size depends on assumptions about future changes in birth and death rates. As we noted earlier, population ecologists project a global population of approximately 7.8-10.8 billion people in 2050. In other words, without some catastrophe, an estimated 1-4 billion people will be added to the population in the next four decades because of the momentum of population growth. But just how many humans can the biosphere support? Will the world be overpopulated in 2050? Is it already overpopulated?

Estimates of Carrying Capacity For over three centuries, scientists have attempted to estimate the human carrying capacity of Earth. The first known estimate, 13.4 billion, was made in 1679 by Anton van LeeUVt'enhoek, the discoverer of protists (see Chapter 28). Since then, estimates have varied from less than 1 billion to more than 1,000 billion (1 trillion), with an average of 10-15 billion. Carrying capacity is difficult to estimate, and the scientists who provide these estimates use different methods to get their answers. Some current researchers use curves like that produced by the logistic equation (see Figure 53.12) to predict the future maximum ofthe human population. Others generalize from existing "maximum" population density and multiply this number by the area of habitable land. Still others base their estimates on a single limiting factor, such as food, and consider many variables, including the amount of available farmland, the average yield of crops, the prevalent diet-vegetarian or meat-basedand the number of calories needed per person per day.

Limits on Human Population Size

£

0

Global Carrying Capacity

20

C 10

"

but only 3 (0.3%) in Japan, while life expectancy at birth was 43 years in Afghanistan and 81 years in Japan. Although global life expectancy has been increasing since about 1950, more recently it has dropped in a number ofregions, including countries ofthe former Soviet Union and in sub-Saharan Africa. In these regions, the combination ofsocial upheaval, decaying infrastructure, and infectious diseases such as AIDS and tuberculosis is reducing life expectancy. In the South AfricancountryofAngola, for instance, life expectancy in 2005 was approximately 39 years, about half that in Japan, Sweden, Italy, and Spain.

Industrialized countries

less industrialized countries

... Figure 53.26 Infant mortality and life expectancy at birth in industrialized and less industrialized countries (data as of 2005).

A more comprehensive approach to estimating the carrying capacity of Earth is to recognize that humans have multiple constraints: We need food, water, fuel, building materials, and other resources, such as clothing and transportation. The ecological footprint concept summarizes the aggregate land and water area required by each person, city, or nation to produce all the resources it consumes and to absorb all the waste it generates. CHAPTER flfTY·THREE

Population Ecology

1193


One way to estimate the ecological footprint of the entire human population is to add up all the ecologically productive land on the planet and divide by the population. This calculation yields approximately 2 hectares (ha) per person (I ha = 2.47 acres). Reserving some land for parks and conservation means reducing this allotment to 1.7 ha per person-the benchmark for comparing actual ecological footprints. Anyone who con路 sumes resources that require more than 1.7 ha to produce is said to be using an unsustainable share of Earth's resources. A typical ecological footprint for a person in the United States is about 10 ha. Ecologists sometimes calculate ecological footprints using other currencies besides land area. For instance, the amount of photosynthesis that occurs on Earth is finite, constrained by the amount of land and sea area and by the sun's radiation. Scientists recently studied the extent to which people around the world consume seven types of photosynthetic products: plant foods, wood for building and fuel, paper, fiber, meat, milk, and eggs (the last three based on estimates of how much plant mao terial goes into their production). Figure 53,27 shows that areas with high population densities, such as China and India, have high consumption rates. However, areas of much lower population density but higher per capita consumption, such as parts of the United States and Europe, have equally high rates, as much as 400 times the rate at which photosynthetic products are produced locally. The combination of population density and resource use per person determines our global ecological footprint. We can only speculate about Earth's ultimate carrying capacity for the human population or about what factors will eventually limit our growth. Perhaps food will be the main fac-

tor. Malnutrition and famine are common in some regions, but they result mainly from unequal distribution of food rather than inadequate production. So far, technological improvements in agriculture have allowed food supplies to keep up with global population growth. However, the principles of energy flow through ecosystems (explained in Chapter 55) tell us that environments can support a larger number of herbivores than carnivores. Ifeveryone ate as much meat as the wealthiest peo路 pie in the world, less than half of the present world population could be fed by current food harvests. Perhaps we humans will eventually be limited by suitable space, like the gannets on oceanic islands. Certainly, as our population grows, the conflict over how space is utilized will intensify, and agricultural land will be developed for housing. There seem to be few limits, however, on how closely humans can be crowded together, as long as adequate food and water are provided to them and space is available to dispose of their waste. Humans could also run out of nonrenewable resources, such as certain metals and fossil fuels. The demands of many populations have already far exceeded the local and even regional supplies of one renewable resource-fresh water. More than 1 billion people do not have access to sufficient water to meet their basic sanitation needs. It is also possible that the human population will eventually be limited by the capacity of the environment to absorb its wastes. In such cases, Earth's current human occupants could lower the planet's long-term carrying capacity for future generations. Some optimists have suggested that because of our ability to develop technology, human population growth has no practical limits. Technology has undoubtedly increased Earth's

log (g carbon/year) 13.4 9.8 5.8

CJ Not analyzed ... Figure 53.27 The amount of photosynthetic products that humans use around the world. The unit of measurement is the logarithm of the number of grams of photosynthetic products consumed each year. The greatest usage is in places where population density is high or where people consume the most resources indi~idually (high per capita consumption). 1194

U"IT EIG~T

Ecology


carrying capacity for humans, but as we have emphasized, no population can continue to grow indefinitely. After reading this chapter, you should realize that there is no single carrying capacity for the human population on Earth. How many people our planet can sustain depends on the quality of life each of us enjoys and the distribution of wealth across people and nations, topics of great concern and political debate. Unlike other organisms, we can decide whether zero population growth will be attained through social changes based on human choices or through increased mortality due to resource limitation, plagues, war, and environmental degradation.

-Mi',If.• Go to the Study Area at www.masteringbio.com for BioFliK 3-D Anim<ltions, MP3 Tutors, Videos, Pr<lcllCe Tests, an eBook, and more,

SUMMARY OF KEY CONCEPTS

Dynamic biological processes influence population density, dispersion, and demographics (pp. 1174-1179) ... Density and Dispersion Population density-the number of individuals per unit area or volume-results from the interplay of births, deaths, immigration, and emigration. Environmental and social factors influence the dispersion of individuals. Patterns of dispersion ~-~ ~-~

..: .. .:.. .. .. ., ...,.

......

'

"

.. .. .'

CHECK

53.6

I. How does a human population's age structure affect its growth rate? 2. How has the growth of Earth's human population changed in recent decades? Give your answer in terms of growth rate and the number of people added each year. 3. -·,1M"IA What choices can you make that influence your own ecological footprint? For suggested answers, see AppendiK A.

... "Trade-offs" and life Histories Ufe history traits such as brood size, age at maturity, and parental caregiving represent trade-offs between conflicting demands for time, energy, and nutrients.

_"Iili"_ 53.3

53.1

_',li'iI"_

CONCEPT

The exponential model describes population growth in an idealized, unlimited environment (pp. 1181-1183) ... Per Capita Rate of Increase Ifimmigration and emigration are ignored, a population's growth rate (the per capita rate of increase) equals birth rate minus death rate. ... Exponential Growth The exponential growth equation dNldt == r",axN represents a population's potential growth in an unlimited environment, where r""'" is the maximum per capita, or intrinsic, rate of increase and N is the number of individuals in the population.

.'

,

.'

Clumped

Uniform

Random

... Demographics Populations increase from births and immi· gration and decrease from deaths and emigration. Life tables, survivorship curves, and reproductive tables summarize specific demographic trends. Act""ty Tedniques for Estimating Population Innsity and Size Acti.ity Investigating Survivorship Curves

_il,lili"_

53.2

Life history traits are products of natural selection (pp.1179-1181) ... Ufe history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism. ... Evolution and life History Diversity Big-bang, or semelparous, organisms reproduce once and die. Iteroparous organisms produce offspring repeatedly.

Number of generations

_"Ii""_ 53.4

The logistic model describes how a population grows more slowly as it nears its carrying capacity (pp.1183-1186) ... Exponential growth cannot be sustained for long in any population. A more realistic population model limits growth by incorporating carrying capacity (K), the maximum population size the environment can support.

CHAPTER fifTY·THREE

Population Ecology

1195


.. The Logistic Growth Model According to the logistic equation dN/dt = r m"",N(K - N)/ K, growth levels off as population size approaches the carrying capacity.

S

.~

•c

K "" carrying capacity

-----------;;------

:§, o ~

Number of generations

.. The Logistic Model and Real Populations The logistic model fits few real populations perfectly, but it is useful for estimating possible growth. .. The Logistic Model and life Histories Two hypothetical but controversial life history patterns are K-selection, or density-dependent selection, and r-selection, or densityindependent selection.

-'.11"'1- 53.5 Many factors that regulate population growth are density dependent (pp. 1186-1190) .. Population Change and Population Density In densitydependent population regulation, death rates rise and birth rates fall with increasing density. In density-independent population regulation, birth and death rates do not change with increasing density. .. Density-Dependent Population Regulation Densitydependent changes in birth and death rates curb population increase through negative feedback and can eventually stabilize a population near its carrying capacity. Density-dependent limiting factors include intraspecific competition for limited food or space, increased predation, disease. stress due to crowding. and buildup of toxic substances. .. Population Dynamics Because changing environmental conditions periodically disrupt them, all populations exhibit some size fluctuations. Many populations undergo regular boom-and-bust cycles that are influenced by complex interactions between biotic and abiotic factors. A metapopulation is a group of populations linked by immigration and emigration.

_@.if.M Biology Labs On-Line Popul.tionEcologyLab

- •.11""- 53.6 The human population is no longer growing exponentially but is still increasing rapidly (pp. 1190-1195) .. The Global Human Population Since about 1650, the global human population has grown exponentially, but within the last 40 years, the rate ofgrowth has fallen by nearly 50%. Differences in age structure show that while some nations' populations are growing rapidly. those ofothers are stable or declining in size. Inf,mt mortality rates and life expectancy at birth differ markedly between industrialized and less industrialized countries.

UNIT EIGHT

_S'joIf.M Architr Hum.n Popul.tion Growth Activity An.IFing Ag~·Struclur~ Pyr.mids Graphil! Ag~ Pyramids and Population Growth Biology Lab. On-Line DemographyLab

o

11%

.. Global Carrying Capacity The carrying capacity of Earth for humans is uncertain. Ecological footprint is the aggregate land and water area needed to produce all the resources a person or group of people consume and to absorb all of their waste. It is one measure of how close we are to the carrying capacity of Earth. \Vith a world population of more than 6.6 billion people, we are already using many resources in an unsustainable manner.

Ecology

TESTING YOUR KNOWLEDGE

SELF·QUIZ t. The observation that members of a population are uniformly distributed suggests that a. the size of the area occupied by the population is increasing. b. resources are distributed unevenly. c. the members of the population are competing for access to a resource. d. the members of the population are neither attracted to nor repelled by one another. e. the density of the population is low. 2. Population ecologists follow the fate of same-age cohorts to a. determine a population's carrying capacity. b. determine if a population is regulated by densitydependent processes. c. determine the birth rate and death rate of each group in a population. d. determine the factors that regulate the size of a population. e. determine if a population's growth is cydic. 3. According to the logistic growth equation

dN

dt =

(K -N) r",uN -K--

a. the number of individuals added per unit time is greatest when N is close to zero. b. the per capita growth rate (r) increases as N approaches K. c. population growth is zero when N equals K. d. the population grows exponentially when K is small. e. the birth rate (b) approaches zero as N approaches K. 4. A population's carrying capacity a. can be accurately calculated using the logistic growth model. b. generally remains constant over time. c. increases as the per capita growth rate (r) decreases. d. may change as environmental conditions change. e. can never be exceeded.


5. Which pair ofterms most accurately describes life history traits for a stable population of wolves? a. semelparous; r-selected b. semelparous; K-selected c. iteroparous; r-selected d. iteroparous; K-selected e. iteroparous; N-selected 6. During exponential growth, a population always a. grows by thousands of individuals. b. grows at its maximum per capita rate. c. quickly reaches its carrying capacity. d. cycles through time. e. loses some individuals to emigration. 7. Scientific study of the population cycles of the snowshoe hare and its predator, the lynx, has revealed that a. the prey population is controlled by predators alone. b. hares and lynx are so mutually dependent that each species cannot survive without the other. c. multiple biotic and abiotic factors contribute to the cycling of the hare and lynx populations. d. both hare and lynx populations are regulated mainly by abiotic factors. e. the hare population is r-selected and the lynx population is K-selected. 8. Based on current growth rates, Earth's human population in 2010 will be closest to a. 2 million. d. 7 billion. b. 3 billion. e. lO billion. c. 4 billion. 9. Which of the following statements about human population in industrialized countries is incorrect? a. Average family size is relatively small. b. The population has undergone the demographic transition. c. Life history is r-selected. d. The survivorship curve is Type I. e. Age distribution is relatively uniform. 10. A recent study of ecological footprints concluded that a. Earth's carrying capacity for humans is about lO billion. b. Earth's carrying capacity would increase if per capita meat consumption increased. c. current demand by industrialized countries for resources is much smaller than the ecological footprint ofthose countries. d. the ecological footprint of the United States is large because per capita resource use is high. e. it is not possible for technological improvements to increase Earth's carrying capacity for humans.

II. "P.W'"

To estimate which age cohort in a population of females produces the most female offspring, you need information about the number of offspring produced per capita within that cohort and the number of individuals alive in the cohort. Make this estimate for Belding's ground squirrels by multiplying the number of females alive at the start of the year {column 2 in Table 53.1) by the average number offemale offspring produced per female {column 5 in Table 53.2). Draw a bar graph with female age in years on the x-axis (0-1, 1-2, and so on) and total number of female offspring produced for each age cohort on the y-axis. \Vhich cohort of female Belding's ground squirrels produces the most female young?

For Self-Quiz answers, see Appendix A.

-51401". Visit the Study Area at www.milsteringbio.com for a Practice Test.

EVOLUTION CONNECTION 12. Write a paragraph contrasting the conditions that favor the evolution ofsemelparous (one-time) reproduction versus iteroparous (repeated) reproduction.

SCIENTIFIC INQUIRY 13. You are testing the hypothesis that the population density of a particular plant species influences the rate at which a pathogenic fungus infects the plant. Because the fungus causes visible scars on the leaves, you can easily determine whether a plant is infected. Design an experiment to test your hypothesis. Include your experimental treatments and control, the data you will collect, and the results expected if your hypothesis is correct.

SCIENCE. TECHNOLOGY. ANO SOCIETY 14. Many people regard the rapid population growth of less industrialized countries as our most serious environmental problem. Others think that the population growth in industrialized countries, though smaller, is actually a greater environmental threat. \Xfhat problems result from population growth in (a) less industrialized countries and (b) industrialized nations? \Xfhich do you think is a greater threat, and why?

CHAPTE~ f1flY路TH~EE

Population Ecology

1197


Co mun EcoiD KEY

CONCEPTS

54,1 Community interactions are classified by whether they help, harm, or have no effect on the species involved 54.2 Dominant and keystone species exert strong controls on community structure 54.3 Disturbance influences species diversity and composition 54.4 Biogeographic factors affect community biodiversity 54.5 Community ecology is useful for understanding pathogen life cycles and controlling human disease

"

your next walk through a park or in the woods, or even across campus, look for evidence of interactions between different species. You may observe birds using trees as nesting sites, bees pollinating flowers, spiders trapping insects in their webs, or ferns growing in shade provided by trees-a tiny sample of the many interactions between species that exist in any ecological theater. Some ecological interactions are more obvious than others. At first glance, Figure 54.1 depicts a simple interaction be路 tween an herbivore, a hornworm caterpillar, and its preferred food, a tomato plant. But the white objects on the caterpillar's back are telltale signs of an interaction between the caterpillar and a third species, a parasitic wasp. The wasp lays its eggs inside the caterpillar, and the larvae that emerge from the eggs feed on the caterpillar's tissues. The larvae then develop into adult wasps inside the white cocoons on their host's back. This interaction will eventually kill the caterpillar. In Chapter 53, you learned how individuals within a population can affect other individuals of the same species. This

0

1198

J. Figure 54.1 How many interactions between species are occurring in this scene?

chapter will examine ecological interactions between populations of different species. A group of populations of different species living dose enough to interact is called a biological community. Ecologists define the boundaries of a particular community to fit their research questions: They might study the community of decomposers and other organisms living on a rotting log, the benthic community in Lake Superior, or the community of trees and shrubs in Shenandoah National Park. We begin this chapter by exploring the kinds of interactions that occur between species in a community. We then consider several of the factors that are most significant in structuring a community-in determining how many species there are, which particular species are present, and the relative abundance of these species. Finally, we will apply some of the principles of community ecology to the study of human disease.

~'::::n~~i~teractions

are classified by whether they help, harm, or have no effect on the species involved

Some key relationships in the life of an organism are its interactions with individuals ofother species in the community. These interspecific interactions include competition, predation, herbivory, and symbiosis (including parasitism, mutualism, and commensalism). In this section, we ""ill define and describe each of these interactions, recognizing that ecologists do not always agree on the precise boundaries ofeach type of interaction. We will use the symbols + and - to indicate how each interspeciflc interaction affects the survival and reproduction of the two species engaged in the interaction. For example,


predation is a +1- interaction, with a positive effect on the survival and reproduction of the predator population and a negative effect on that of the prey population. Mutualism is a +1+ interaction because the survival and reproduction of each species is increased in the presence of the other. A 0 indicates that a population is not affected by the interaction in any known way. Historically, most ecological research has focused on interactions that have a negative effect on at least one species, such as competition and predation. However, positive interactions are ubiquitous, and their contributions to community structure are the subject of considerable study today.

Competition Interspecific competition is a -1- interaction that occurs when individuals of different species compete for a resource that limits their growth and survival. For instance, weeds growing in a garden compete with garden plants for soil nutrients and water. Grasshoppers and bison in the Great Plains compete for the grass they both eat. Lynx and foxes in the northern forests of Alaska and Canada compete for prey such as snowshoe hares. In contrast, some resources, such as oxygen, are rarely in short supply; thus, although most species use this resource, they do not usually compete for it.

plain the niche concept: If an organism's habitat is its "address;' the niche is the organism's "profession:' Put another way, an organism's niche is its ecological role-how it "fits into" an ecosystem. For example, the niche ofa tropical tree lizard consists of, among many components, the temperature range it tolerates, the size of branches on which it perches, the time of day when it is active, and the sizes and kinds of insects it eats. We can use the niche concept to restate the principle of competitive exclusion: Two species cannot coexist permanently in a community if their niches are identical. However, ecologically similar species can coexist in a community if there are one or more significant differences in their niches. When competition between species with identical niches does not lead to local extinction ofeither species, it is generally because one species' niche becomes modified. In other words, evolution by natural selection can result in one of the species using a different set of resources. The differentiation of niches that enables similar species to coexist in a community is called resource partitioning (Figure 54.2). You can think of resource partitioning in a community as "the ghost of competition past" -the indirect evidence of earlier interspecific competition resolved by the evolution ofniche differentiation. As a result of competition, a species' fundamental niche, which is the niche potentially occupied by that species, is often different from its realized niche, the portion ofits fundamental niche that it actually occupies in a particular environment.

Competitive Exclusion What happens in a community over time when two species directly compete for limited resources? In 1934, the Russian ecologist G. F. Gause studied this question in laboratory experiments with m'o closely related species of ciliated protists, Paramecium aurelia and Paramecium caudatum. He cultured the species under stable conditions, adding a constant amount of food every day. When Gause grew the two species in separate cultures, each population grew rapidly and then leveled off at what was apparently the carrying capacity of the culture (see Figure 53.13a for an illustration of the logistic growth of P aurelia). But when Gause cultured the two species together, P caudatum was driven to extinction in the culture. Gause inferred that P. aurelia had a competitive edge in obtaining food, and he concluded that two species competing for the same limiting resources cannot coexist in the same place. In the absence of disturbance, one species will use the resources more efficiently and thus reproduce more rapidly than the other. Even a slight reproductive advantage will eventually lead to local elimination of the inferior competitor, an outcome called competitive exclusion.

Ecological Niches The sum ofa species' use ofthe biotic and abiotic resources in its environment is called the species' ecological niche. American ecologist Eugene Odum used the following analogy to ex-

A. distichus perches on fence

posts and other sunny surfaces,

A. insolitus usually perches on shady branches,

... Figure 54.2 Resource partitioning among Dominican Republic lizards. Se~en species of Anolis lizards li~e in close proximity, and all feed on insects and other small arthropods, Howe~er, competition for food is reduced because each lizard species has a different preferred perch, thus occupying a distinct niche.

CHAPTE~ FIFTY路FOU~

Community Ecology

1199


Ecologists can identify the fundamental niche of a species by testing the range of conditions in which it grows and reproduces in the absence of competitors. They can also test whether a potential competitor limits a species' realized niche by removing the competitor and seeing if the first spe<ies expands into the newly available space (Figure 54.3). The classic

54.3

l'

Can a species' niche be influenced by interspecific competition? EXPERIMENT Ecologist JOSE'ph ConnE'1i studied two barnaclE' speciE's- CIltllamalus stellatus and Balanus balanoides-that have a stratifiE'd distribution on rocks along the coast of Scotland. Chfhama/us IS usually found higher on the rocks than Balanus To detf'fmine whE'ther thE' distribution of Chthamalus is the rE'sult of interspE'cific competition with Balanus, ConnE'1i rE'moved Balanus from thE' rocks at seYE'ral sitE's.

High tide

~ Chthamalus

+

I) Balanus

t

experiment depicted in the figure clearly showed that competition from one barnacle spe<ies kept a second barnacle species from occupying part of its fundamental niche.

Character Displacement Closely related species whose populations are sometimes allopatric (geographically separate; see Chapter 24) and sometimes sympatric (geographically overlapping) provide more evidence for the importance of competition in structuring communities. In some cases, the allopatric populations of such species are morphologically similar and use similar resources. By contrast, sympatric populations, which would potentially compete for resources, show differences in body structures and in the resources they use. This tendency for characteristics to diverge more in sympatric populations of two species than in allopatric populations of the same two species is called character displacement. An example of character displacement is the variation in beak size between different populations of the Galapagos finches Geospiza ju/iginosa and Geospizajortis (Figure 54.4).

Chthamalus rE'ahzed niche

t

G fuliginosa

G. fortis

Balanus ----reahzE'd niche Ocean

I

BE'ak dE'pth

Low tide 60

RESULTS Chrhama/us sprE'ad into thE' rE'gion formerly occupiE'd by Balanus.

G, fuliginosa. allopatric

• 20

;

u

OL~_,J-''''''''',....,~~-~-~-~-.,J

~

High tidE'

los HE'rmanos

~ 40 u

.=

60 ~ 40

Daphne

,

n

,

:2 20

~ 0 '0 ~

~

~

OCE'an

low tide

~•

60 40 20

0

G, fortis. allopatrlc

Sympatric populations

Santa Marla. San Cristobal

b 8

J' 10

12

14

16

Beak depth (mm) CONCLUSION IntE'rspE'cific compE'tition makE's thE' realizE'd nichE' of Chthamalus much smallE'r than its fundamE'ntal nichE', SOURCE

J. H, Connell, The influence of in1~rweofic compell11on and olh~f faclors on the dlstnbution of lh~ bamad~ ChrlMmalus srel~M. Ecology42:71o-n3 (1961).

.'@il i•

OthE'r observalions showed that Balanus cannot survivE' high on thE' rocks bE'cause it driE'S out during low tides, How would Balanus's rE'alizE'd nichE' compare with its fundamental niche?

1200

U"'T EIG~T

Ecology

.. Figure 54.4 Character displacement: indirect evidence of past competition. Allopatric populations of Geospiza fuliginosa and Geosplza fortis on Los Hermanos and Daphne Islands havE' similar beak morphologiE's (top two graphs) and presumably eat similarly sized SE'E'ds. However, whE're the two speciE'S are sympatric on Santa Maria and San Cristobal. G, fuliginosa has a shallower, smallE'f bE'ak and G. fortis a dE'E'per. larger one (bottom graph). adaptations that favor eating differE'nt sizE'S of seE'ds n Suppose that the symparric populations of both finch spe<ies . . colonized a new island that contained seeds of only one size. What would you expect to happen to tile differences in beak size over time? Explain your reasoning.


Predation Predation refers to a +1- interaction between species in which one species, the predator, kills and eats the other, the prey. Though the term predation generally elicits such images

as a lion attacking and eating an antelope, it applies to a wide range of interactions. An animal that kills a plant by eating the plant's tissues can also be considered a predator. Because eating and avoiding being eaten are prerequisite to reproductive success, the adaptations of both predators and prey tend to be refined through natural selection. Many important feeding adaptations of predators are both obvious and familiar. Most predators have acute senses that enable them to locate and identify potential prey. In addition, many predators have adaptations such as claws, teeth, fangs, stingers, or poison that help them catch and subdue the or... Figure 54.5 Examples of defensive coloration in animals. (a) Cryptic coloration .. Canyon tree frog

(b) Aposematic coloration .. Poison dart frog

(cl Batesian mimicry: A harmless species mimics a harmful one.

ganisms on which they feed. Rattlesnakes and other pit vipers, for example, find their prey with a pair of heat-sensing organs located between their eyes and nostrils (see Figure 5O.5a), and they kill small birds and mammals by injecting them with toxins through their fangs. Predators that pursue their prey are generally fast and agile, whereas those that lie in ambush are often disguised in their environments. Just as predators possess adaptations for capturing prey, prey animals have adaptations that help them avoid being eaten. Some common behavioral defenses are hiding, fleeing, and forming herds or schools. Active self-defense is less common, though some large grazing mammals vigorously defend their young from predators such as lions. Other behavioral defenses include alarm calls that summon many individuals of the prey species, which then mob the predator. Animals also display a variety of morphological and physiological defensive adaptations. For example, cryptic coloration, or camouflage, makes prey difficult to spot (Figure 54.Sa). Other animals have mechanical or chemical defenses. For example, most predators are strongly discouraged by the familiar defenses of porcupines and skunks. Some animals, such as the European fire salamander, can synthesize toxins, whereas others passively acquire a chemical defense by accumulating toxins from the plants they eat. Animals with effective chemical defenses often exhibit bright aposematic coloration, or warning coloration, such as that of the poison dart frog (Figure 54.Sb). Aposematic coloration seems to be adaptive: There is evidence that predators are particularly cautious in dealing with potential prey having bright color patterns (see Chapter 1). Some prey spe<ies gain significant protection by mimicking the appearance of other species. In Satesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model. For example, the larva of the hawkmoth Hemeroplanes ornatus puffs up its head and thorax when disturbed, looking like the head of a small poisonous snake (Figure 54.5c).ln this case, the mimicry even involves behavior; the larva weaves its head back and forth and hisses like a snake. In Mullerian mimicry, two or more unpalatable species, such as the cuckoo bee and yellow jacket, resemble each other (Figure 54.5d). Presumably, each species gains an additional (d) Mullerian mimicry: Two unpalatable species mimic each other. .. Cuckoo bee ... Yellow Jacket

CHAPTE~ FIFTY路FOU~

Community Ecology

1201


advantage because the more unpalatable prey there are, the more quickly and effectively predators adapt, avoiding any prey with that particular appearance. The shared appearance thus be~ comes a kind ofaposematic coloration. In an example of conver~ gent evolution, unpalatable animals in several different taxa have similar patterns of coloration: Black and yellow or red stripes characterize unpalatable animals as diverse as yellow jackets and coral snakes (see Figure 1.25). Predators also use mimicry. For example, some snapping turtles have tongues that resemble a wriggling worm, thus luring small fish. Any fish that tries to eat the "bait" is itself quickly consumed as the turtle's strong jaws snap closed. Anglerfish also lure prey with their own bait, in this case a modified bone of the dorsal fin that luminesces in some species.

Herbivory Ecologists use the term herbivory to refer to a +1- interaction in which an organism eats parts of a plant or alga. While large mammalian herbivores such as cattle, sheep, and water buffalo may be most familiar, most herbivores are actually invertebrates, such as grasshoppers and beetles. In the ocean, herbivores include snails, sea urchins, some tropical fishes, and certain mammals, such as the manatee (Figure 54.6). Like predators, herbivores have many specialized adapta~ tions. Many herbivorous insects have chemical sensors on their feet that enable them to distinguish bety,'een toxic and nontoxic plants as well as between more nutritious and less nutritious plants. Some mammalian herbivores, such as goats, use their sense ofsmell to examine plants, rejecting some and eating others. They may also eat just a specific part of a plant, such as the flowers. Many herbivores also have specialized teeth or digestive systems adapted for processing vegetation (see Chapter41). Unlike prey animals, plants cannot run away to avoid being eaten. Instead, a plant's arsenal against herbivores may feature chemical toxins or structures such as spines and thorns.

.... Figure 54.6 A West Indies manatee (Trichechus manatus) in Florida. The animal in this photo is feeding on water hyacinth, an introduced species. 1202

UNIT EIGHT

Ecology

Among the plant compounds that serve as chemical weapons are the poison strychnine, produced by the tropical vine Strychnos taxifera; nicotine, from the tobacco plant; and tannins, from a variety of plant species. Plants in the genus As· traga/us accumulate selenium toxins; they are known as "locoweeds" because the cattle and sheep that eat them wander aimlessly in circles and may even die. Compounds that are not toxic to humans but may be distasteful to many herbivores are responsible for the familiar flavors of cinnamon, cloves, and peppermint. Certain plants produce chemicals that cause abnormal development in some insects that eat them.

Symbiosis When individuals oftwo or more species live in direct and in· timate contact with one another, their relationship is called symbiosis. This text adopts a general definition of symbiosis that includes all such interactions, whether harmful, helpful, or neutra1. Some biologists define symbiosis more narrowly as a synonym for mutualism, in which both species benefit.

Parasitism Parasitism is a +1- symbiotic interaction in which one or· ganism, the parasitc, derives its nourishment from another or· ganism, its host, which is harmed in the process. Parasites that live within the body oftheir host, such as tapeworms, are called cndoparasites; parasites that feed on the external surface of a host, such as ticks and lice, are called ectoparasites. In one particular type of parasitism, parasitoid insects-usually small wasps-lay eggs on or in living hosts (see Figure 54.1). The larvae then feed on the body of the host, eventually killing it. Some ecologists have estimated that at least one-third of all species on Earth are parasites. Many parasites have complex life cycles involving multiple hosts. For instance, the life cycle ofthe blood fluke, which cur· rently infects approximately 200 million people around the world, involves two hosts: humans and freshwater snails (see Figure 33.11). Some parasites change the behavior of their hosts in a way that increases the probability of the parasite being transferred from one host to another. For instance, the presence of parasitic acanthocephalan (spiny-headed) worms leads their crustacean hosts to engage in a variety of atypical behaviors, including leaving protective cover and moving into the open. As a result of their modified behavior, the crus~ taceans have a greater chance of being eaten by the birds that are the second host in the parasitic worm's life cycle. Parasites can significantly affect the survival, reproduction, and density of their host population, either directly or indirectly. For example, ticks that live as ectoparasites on moose weaken their hosts by withdrawing blood and causing hair breakage and loss, increasing the chance that the moose will die from cold stress or predation by wolves. Some of the declines of the moose population on Isle Royale, Michigan, have been attributed to tick outbreaks (see Figure 53.19).


Mutualism Mutualistic symbiosis, or mutualism, is an interspecific interaction that benefits both species (+ I +). We have described many examples of mutualism in previous chapters: nitrogen fixation by bacteria in the root nodules of legumes; the digestion ofcellulose by microorganisms in the digestive systems of termites and ruminant mammals; the exchange of nutrients in mycorrhizae, associations of fungi and the roots of plants; and photosynthesis by unicellular algae in corals. The interaction between termites and the microorganisms in their digestive system is an example of obligate mutua/ism, in which at least one spedes has lost the ability to survive without its partner. In facultative mutualism, as in the acacia-ant example shown in Figure 54.7, both species can survive alone. Mutualistic relationships sometimes involve the evolution of related adaptations in both species, with changes in either species likely to affect the survival and reproduction of the

(al Certain species of acacia trees in Central and South America ha~e hollow thorns that house stinging ants of the genus Pseudomyrmex. The ants feed on nectar produced by the tree and on protein-rich swellings (orange in the photograph) at the tips of leaflets.

other. For example, most flowering plants have adaptations such as nectar or fruit that attract animals that function in pollination or seed dispersal (see Chapter 38). In turn, many animals have adaptations that help them find and consume nectar.

Commensalism An interaction between species that benefits one of the spedes but neither harms nor helps the other (+ /0) is called commensalism. Commensal interactions are difficult to document in nature because any close association between species likely affects both species, even ifonly slightly. For instance, ~hitchhiking~ species, such as algae that live on the shells of aquatic turtles or barnacles that attach to whales, are sometimes considered commensal. The hitchhikers gain a place to grow while having seemingly little effect on their ride. However, the hitchhikers may in fact slightly decrease the reproductive success of their hosts by reducing the hosts' efficiency of movement in searching for food or escaping from predators. Conversely, the hitchhikers may provide a benefit in the form of camouflage. Some associations that are possibly commensal involve one species obtaining food that is inadvertently exposed by another. For instance, cowbirds and cattle egrets feed on insects flushed out of the grass by grazing bison, cattle, horses, and other herbivores. Because the birds increase their feeding rates when following the herbivores, they clearly benefit from the association. Much of the time, the herbivores may be un路 affected by the relationship (Figure 54.8). However, they, too, may sometimes derive some benefit; the birds tend to be opportunistic feeders that occasionally remove and eat ticks and other ectoparasites from the herbivores. They may also give warning to the herbivores of a predator's approach. All four types of interactions that we have discussed so farcompetition, predation, herbivory, and symbiosis-strongly influence the structure of communities. You will see other examples of these interactions throughout this chapter.

(b) The acacia benefits because the pugnacious ants. whICh attack anything that touches the tree, remo~e fungal spores, small herbi~ores, and debris, and clip ~egetation that grows close to the acacia. .. Figure 54.7 Mutualism between acacia trees and ants.

... Figure 54.8 A possible example of commensalism between cattle egrets and water buffalo. CHAPTE~ FIFTY路FOU~

Community Ecology

1203


CONCEPT

CHECI(

54.1

1. Explain how interspecific competition, predation, and mutualism differ in their effects on the interacting

populations of two species. 2. According to the principle of competitive exclusion, what outcome is expected when two species with identical niches compete for a resource? Why? Suppose you live in an agricultural area. 3, What examples of the four types of community interactions (competition, predation, herbivory, and symbiosis) might you see in the growing or use of food?

-',11M".

For suggested answers. see Appendix A.

Community 1 A: 25% B: 25% C: 25% D: 25%

r~::~~:n~:~: keystone species exert strong controls on community structure

Although the interactions of many species influence biological communities, sometimes a few species exert strong control on a community's structure. particularly on the composition. relative abundance, and diversity of its species. Before examining the effects ofthese particularly influential species. we first need to consider two fundamental features of community structure: species diversity and feeding relationships.

Species Diversity The species diversity ofa community-the variety ofdifferent kinds of organisms that make up the community-has two components. One is species richness, the number of different species in the community. The other is the relative abundance ofthe different species, the proportion each species represents of all individuals in the community. For example, imagine two small forest communities, each with 100 individuals distributed among four tree species (A, B. C, and D) as follows: Community 1: 25A, 25B, 25C, 250 Community 2: 80A, 5B, 5C, 100 The species richness is the same for both communities because they both contain four species of trees, but the relative abundance is very different (Figure 54.9). You would easily notice the four types of trees in community 1, but without looking carefully, you might see only the abundant species A in the second forest. Most observers would intuitively describe community 1 as the more diverse of the two communities. Ecologists use many tools to quantitatively compare the diversity of different communities across time and space. They often calculate an index of diversity based on species richness 1204

U"IT EIG~T

Ecology

Community 2 A: 80% B: 5% C: 5% D: 10%

.... Figure 54.9 Which forest is more diverse? Ecologists would ~y that community 1 has greater species diversity. a measure that includes both species richness and relative abundance.

and relative abundance. One widely used index is the Shannon diversity (H):

H = -[(PA InpA)

+ (PB In Pu) + (Pc In pel +... J

where A, B, C ... are the species in the community, P is the reI路 ative abundance ofeach species, and In is the natural logarithm. Let's use this equation to calculate the Shannon diversity ofthe two communities in Figure 54.9. For community 1, P = 0.25 for each community, soH = -4 x (0.25 In 0.25) = 1.39. For community 2, H = - [(0.8 In 0.8) + (0.05 In 0.05) + (0.05 In 0.05) + (0.1 In O.1)J = 0.71. These calculations confirm our intuitive description of community 1 as more diverse. Determining the number and relative abundance ofspecies in a community is easier said than done. Many sampling techniques can be used, but since most species in a community are relatively rare, it may be hard to obtain a sample size large enough to be representative. It is also difficult to census the highly mobile or less visible members ofcommunities, such as mites, nematodes, and microorganisms. The small size of microorganisms makes them particularly difficult to sample. so ecologists now use molecular tools to help determine microbial diversity (Figure 54.10). Although measuring species diversity is often challenging. it is essential not only for understanding community structure but for conserving biodiversity, as you will read in Chapter 56.


FI~

54.10

Quaternary consumers

Determining Microbial Diversity Using Molecular Tools APPLICATION Ecologists are Increasingly uSing molecular techniques. such as the analysis of restriction fragment length polymorphisms (RFlPs), to determine microbial diversity and richness in environmental samples. As used in this application, RFlP analysis produces a DNA fingerprint for microbial taxa based on sequence variations in the DNA that encodes the small subunit of ribosomal RNA. Noah Fierer and Rob Jackson, of Duke University, used this method to compare the diversity of soil baderia in 98 habitats across North and South America to help identify environmental variables associated with high bacterial diversity.

TEcHNIque Researchers first extract and purify DNA from the microbial community in each sample, They use the polymerase chain reaction {PCR} to amplify the ribosomal DNA and label the DNA with a fluorescent dye (see Chapter 20). Restridion enzymes then cut the amplified. labeled DNA into fragments of different lengths, which are separated by gel electrophoresis. The number and abundance of these fragments characterize the DNA fingerprint of the sample, Based on their RFlP analYSiS, Fierer and Jackson calculated the Shannon diversity (H) of each sample, They then looked for a correlation between H and several environmental variables, including vegetation type, mean annual temperature and rainfall, and acidity and quality of the soil at each site,

Carnivore Tertiary consumers Carnivore

Carnivore

+ Secondary consumers

Carnivore

+ Primary consumers Herbivore

Zooplankton

+ Primary producers

RESULTS

The diversity of bacterial communities in soils across North and South America was related almost exclusively to soil pH, with the Shannon diversity being highest 10 neutral soils and lowest in acidic SOils. Amazonian rain forests, which have extremely high plant and animal diversity, had the most acidic soils and the lowest bacterial diversity of the samples tested,

..,.....

, .fa.

3.6

• • ••

••

....

• ••

• •

SOURCE

4

A terrestrial food chain

A marine food chain

... Figure 54.11 Examples of terrestrial and marine food chains. The arrows trace energy and nutrients that pass through the trophic levels of a community when organisms feed on one another, Decomposers, which "feed" on organisms from all trophic levels, are not shown here,

Food Webs

2.2

3

Phytoplankton

The structure and dynamics ofa community depend to a large extent on the feeding relationships between organisms-the trophic structure of the community. The transfer of food energy up the trophic levels from its source in plants and other autotrophic organisms (primary producers) through herbivores (primary consumers) to carnivores (secondary, tertiary, and quaternary consumers) and eventually to decomposers is referred to as a food chain (figure 54.11).

24

~

Trophic 51ructure

• 2.6

Plant

f

•,~"""-'!.i..

3.4

.<.~

.;~~

5

6 Soil pH

7

8

N Fierer and R 8 ladson, The diversity and

b'09W9raphyof soil b<tcteri<ll communlt'es. f'nXeed''''l> of Ihe Nalional Academy of Science; USA 103626-631 (2006).

9

In the 19205, Oxford University biologist Charles Elton recognized that food chains are not isolated units but are linked together in food webs. An ecologist can summarize the trophic relationships of a community by diagramming a food web with arrows linking species according to who eats whom. In an antarctic pelagic community, for example, the primary producers are phytoplankton, which serve as food for the dominant

CHAPTER FIFTY·FOUR

Community Ecology

1205


Smaller toothed whales

Baleen whales

Sperm whales

Leopard seals

r

- Fishes

.. - Birds

Euphausids (krill)

....

"j Zooplankton Squids 1!CII:~~

-----

Copepods

Phytoplankton

.. Figure 54.12 An antarctic marine food web. Arrows follow the transfer of food from the producers (phytoplankton) up through the trophic levels For simplicity. this diagram omits decomposers grazing zooplankton, especially euphausids (krill) and copepods, both ofwhich are crustaceans (Figure 54.12). These zooplankton species are in turn eaten by various carnivores, including other plankton, penguins, seals, fishes, and baleen whales. Squids, which are carnivores that feed on fishes as well as zooplankton, are another important link in these food webs, as they are in turn eaten bysealsand toothed whales. During the time when whales were commonly hunted for food, humans became the top predator in this food web. Having hunted many whale species to low numbers, humans are now harvesting at lower trophic levels, catching krill as well as fishes for food. How are food chains linked into food webs? First, a given species may weave into the web at more than one trophic level. For example, in the food web shown in Figure 54.12, euphausids feed on phytoplankton as well as on other grazing zooplankton, such as copepods. Such "nonexclusive" consumers are also found in terrestrial communities. For instance, foxes are omni1206

U"IT EIG~T

Ecology

Juvenile striped bass

\).1

t Crab-eater seals

Sea nettle

.. Figure 54.13 Partial food web for the Chesapeake Bay estuary on the U.S. Atlantic coast. The sea nettle (Chrysaora quinquecirrha) and juvenile striped bass (Morone saxarilis) are the main predators of fish larvae (bay anchovy and several other species). Note that sea nettles are secondary consumers (black arrows) when they eat zooplankton. but tertiary consumers (red arrows) when they eat fish larvae. which are themselves secondary consumers of zooplankton, vores whose diet includes berries and other plant materials, herbivores such as mice, and other predators, such as weasels. Humans are among the most versatile of omnivores. Food webs can be very complicated, but we can simplify them for easier study in two ways. First, we can group species with similar trophic relationships in a given community into broad functional groups. For example, in Figure 54.12, more than 100 phytoplankton spe<ies are grouped as the primary producers in the food web. A second way to simplify a food web for closer study is to isolate a portion of the web that interacts very little with the rest ofthe community. Figure 54.13 illustrates a partial food web for sea nettles (a type of cnidarian) and juvenile striped bass in Chesapeake Bay.

Limits on Food Chain Length Each food chain within a food web is usually only a few links long. In the antarctic web of Figure 54.12, there are rarely more than seven links from the producers to any top-level predator, and most chains in this web have fewer links. In fact, most food webs studied to date have chains consisting offive or fewer links. Why are food chains relatively short? There are two main hypotheses. One, the energetic hypothesis, suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain. As you will read in Chapter 55, only about 10% of the energy stored in the organic matter of each trophic level is converted to organic matter at the next trophic level. Thus, a producer level consisting of 100 kg of plant material can support about 10 kg of herbivore biomass (the total mass of all individuals in a population) and I kg of carnivore


biomass. The energetic hypothesis predicts that food chains should be relatively longer in habitats ofhigher photosynthetic production, since the starting amount ofenergy is greater than in habitats with lower photosynthetic production. A second hypothesis, the dynamic stability hypothesis, proposes that long food chains are less stable than shortchains. Population fluctuations at lower trophic levels are magnified at higher levels, potentially causing the local extinction of top predators. In a variable environment, top predators must be able to recover from environmental shocks (such as extreme winters) that can reduce the food supply all the way up the food chain. The longer a food chain is, the more slowly top predators can recover from environmental setbacks. This hypothesis predicts that food chains should be shorter in unpredictable environments. Most of the data available support the energetic hypothesis. Forexample, ecologists have used tree-hole communities in tropical forests as experimental models to test the energetic hypothesis. Many trees have small branch scars that rot, forming holes in the tree trunk. The tree holes hold water and provide a habitat for tiny communities consisting of microorganisms and insects that feed on leaflitter, as well as predatory insects. figure 54.14 shows the results ofexperiments in which researchers manipulated productivity (leaflitter falling into the tree holes). As predicted by the energetic hypothesis, holes with the most leaf litter, and hence the greatest total food supply at the producer level, supported the longest food chains. Another factor that may limit food chain length is that carnivores in a food chain tend to be larger at successive trophic levels. The size of a carnivore and its feeding mechanism put some upper limit on the size of food it can take into its mouth. And except in a few cases, large carnivores cannot live on very small food items because they cannot procure enough food in a given time to meet their metabolic needs. Among the ex-

High (control): natural rate of litter fall

Medium: 1/'0 natural rate

Low: '/100 natural rate

ProdUdiVlly ... figure 54.14 Test of the energetic hypothesis for the restriction of food chain length. Researchers manipulateclthe productivity of tree·hole communities in Queensland. Australia. by providing leaf litter input at three levels. Reducing energy input reduced food chain length, a result consistent with the energetic hypothesis. E'I According to the dynamic stability hypothesis, which productivity

. . treatment should have the most stable food cham? Explain.

ceptions are baleen whales, huge suspension feeders with adaptations that enable them to consume enormous quanti· ties of krill and other small organisms (see Figure 41.6).

Species with a large Impact Certain species have an especially large impact on the structure of entire communities either because they are highly abundant or because they playa pivotal role in community dy· namics. The impact of these species can occur either through their trophic interactions or through their influences on the physical environment.

Dominant Species Dominant species are those species in a community that are the most abundant or that collectively have the highest biomass. As a result, dominant species exert a powerful control over the occurrence and distribution of other species. For example, the abundance of sugar maples, the dominant plant species in many eastern North American forest communities, has a major impact on abiotic factors such as shading and soil, which in turn affect which other species live there. There is no single explanation for why a species becomes dominant in a community. One hypothesis suggests that dominant species are competitively superior in exploiting limited resources such as water or nutrients. Another explanation is that dominant spe<ies are most successful at avoiding preda· tion or the impact ofdisease. This latter idea could explain the high biomass attained in some environments by invasive species, organisms (typically introduced by humans) that take hold outside their native range. Such species may not face the natural predators and agents of disease that would otherwise hold their populations in check. One way to discover the impact of a dominant species is to remove it from the community. This type of experiment has been carried out many times by accident. The American chestnut was a dominant tree in deciduous forests of eastern North America before 1910, making up more than 40% of mature trees. Then humans accidentally introduced the fungal disease chestnut blight to New York City via nursery stock imported from Asia. Between 1910 and 1950, this fungus killed all the chestnut trees in eastern North America. In this case, removing the dominant species had a relatively small impact on some species but severe effects on others. Oaks, hickories, beeches, and red maples that were already present in the forest increased in abundance and replaced the chestnuts. No mammals or birds seemed to have been harmed by the loss of the chestnut, but seven species of moths and butterflies that fed on the tree became extinct. The American chestnut story is only one example ofa community response to the loss of a dominant species. More research is needed before we can generalize about the overall effects of such losses. CHAPTER FlfTY·FOUR

Community Ecology

1207


â&#x20AC;˘ F 15 Pisaster ochraceus a keystone predator? EXPERIMENT

In rocky intertidal communities of western

North America. the relatively uncommon sea star Pi5ilster ochraceus preys on mussels such as Mytilu5 califormanus, a dominant spl'cies and strong competitor for space. Robert Paine, of the University of Washington, removed Pisasrer from an area in the intertidal zone and examined the effect on species richness.

100 ~c

80

~

8

60

,E O?:.

40

t

~o

c , d

20

a

\

(al Sea otter abundance

t

400 :;;'" 300 ~E

E

200

\3 0

100

lt1

.N

a

\

â&#x20AC;˘

t

(b) Sea urchin biomass 10

\

8 6 4

2 RESULTS

In the absence of Pisaster, species richness de~ (lined as mussels monopolized the rock face and eliminated most other invertebrates and algae. In a control area where Pisaster was not removed, species richness changed very little.

With Pisaster (control)

1963'64 '65 '66 '67 '68 '69 '70 '71 '72 '73 Year

CONCLUSION Pisaster acts as a keystone species, exerting an influence on the community that is not reflected in its abundance. SOURCE

R, T. P~lne. food web compleXIty ~nd $pe<:1e$ dlver>lty, (1966)

Ame,ic~n N~ru'alisI1()():6S-7S

Mi,iI:f.\lljI Suppose that an invasive fungus killed most individuals of Mytilus at these sites What do you think would happen to species richness if Pisaster were then removed?

oL,-~\-~

1972

1985

1989 Year

1993 1997

Food chain

(c) Total kelp density .. Figure 54.16 Sea otters as keystone predators in the North Pacific. The graphs correlate changes over time in sea oller abundance (a) With changes In sea urchin biomass (b) and changes in kelp density (c) in kelp forests al Adak Island (part of the Aleutian Island chain). The vertical diagram on the right represents Ihe food chain afler orcas (top) entered the chain.

keystone species in maintaining the diversity of an intertidal community. The sea otter, a keystone predator in the North Pacific, offers another example (figure 54.16). Sea otters feed on sea urchins, and sea urchins feed mainly on kelp. In areas where sea otters are abundant, sea urchins are rare and kelp forests are well developed. \xrhere sea otters are rare, sea urchins are common and kelp is almost absent. Over the last 20 years, orcas have been preying on sea otters as the whales' usual prey has dedined. As a result, sea otter populations have declined predpitously in large areas off the coast of western Alaska, sometimes at rates as high as 25% per year. The loss ofthis keystone species has allowed sea urchin populations to increase, resulting in the loss of kelp forests.

Keystone Species In contrast to dominant species, keystone species are not necessarily abundant in a community. They exert strong control on community structure not by numerical might but by their pivotal ecological roles, or niches. One way to identify keystone species is by removal experiments like the one described in Figure 54.15, which highlights the importance ofa 1208

U"'T EIG~T

Ecology

Foundation Species (Ecosystem "Engineers") Some organisms exert their influence on a community not through their trophic interactions but by causing physical changes in the environment. Such organisms may alter the environment through their behavior or their large collective biomass.


Species that dramatically alter their physical environment on a large scale are called ecosystem ~engineers" or, to avoid implying conscious intent, ~foundation species:' A familiar foundation species is the beaver (Figure 54.17), which, through tree felling and dam building, can transform landscapes. The effects of foundation species on other species can be positive or negative, depending on the needs ofthe other species. By altering the structure or dynamics of the environment, foundation species sometimes act as facilitators: They have positive effects on the survival and reproduction of other species in the community. For example, by modifying soils, the black rush /uncus gerardi increases the species richness in some zones of New England salt marshes./uncus helps prevent salt buildup in the soil by shading the soil surface, which reduces evaporation (Figure 54.18a)./uncus also prevents the salt marsh soils from becoming oxygen depleted as it transports oxygen to its belowground tissues. Sally Hacker and Mark Bertness, of Brown University, uncovered some of /uncus's facilitation effects by removing /uncus from study plots. Their experiment suggested that without/uncus, the up-

... Figure 54.17 Beavers as ecosystem "engineers:' By felling trees. building dams. and creating ponds. beavers can transform large areas of forest into flooded wetlands.

••••

8

v

" ~

6

c

~

'0

•,

4

~

E 2

z (a) Salt marsh with Juncus (foreground)

o (bl

With Juncus

Without Juncus

... Figure 54.18 Facilitation by black rush (Juncus gerard;) in New England salt marshes. Black rush facilitates the occupation of the middle upper zone of the marsh. which increases local plant speCies richness.

per middle intertidal zone would support 50% fewer plant species (Figure 54.18b).

Bottom-Up and Top-Down Controls Simplified models based on relationships between adjacent trophic levels are useful for discussing community organization. For example, let's consider the three possible relationships between plants (Vfor vegetation) and herbivores (H):

V-->H

V<-H

The arrows indicate that a change in the biomass of one trophic level causes a change in the other trophic level. V ~ H means that an increase in vegetation will increase the numbers or biomass of herbivores, but not vice versa. In this situation, herbivores are limited by vegetation, but vegetation is not limited by herbivory. In contrast, V f- H means that an increase in herbivore biomass will decrease the abundance of vegetation, but not vice versa. A double-headed arrow indicates that feedback flows in both directions, with each trophic level sensitive to changes in the biomass of the other. Two models of community organization are common: the bottom-up model and the top-down model. The V -7 H linkage suggests a bottom-up model, which postulates a unidirectional influence from lower to higher trophic levels. In this case, the presence or absence of mineral nutrients (N) controls plant (V) numbers, which control herbivore (H) numbers, which in turn control predator (P) numbers. The simplified bottom-up model is thus N -7 V ~ H ~ P. To change the community structure of a bottom-up community, you need to alter biomass at the lower trophic levels, allowing those changes to propagate up through the food web. For example, ifyou add mineral nutrients to stimulate growth ofvegetation, then the higher trophic levels should also increase in biomass. If you add predators to or remove predators from a bottom-up community, however, the effect should not extend down to the lower trophic levels. In contrast, the top-down model postulates the opposite: Predation mainly controls community organization because predators limit herbivores, herbivores limit plants, and plants limit nutrient levels through their uptake of nutrients during growth and reproduction. The simplified top-down model, N f- V f- H f- P, is also called the trophic cascade modeL For example, in a lake community with four trophic levels, the model predicts that removing the top carnivores will increase the abundance of primary carnivores, in turn decreasing the number of herbivores, increasing phytoplankton abundance, and decreasing concentrations of mineral nutrients. If there were only three trophic levels in a lake, removing primary carnivores would increase the number of herbivores and decrease phytoplankton abundance, causing nutrient levels to rise. The effects of any manipulation thus move down the trophic structure as alternating +/- effects. CHAPTE~ FIFTY·FOU~

Community Ecology

1209


Diana Wall (see interview on pages 1146-1147) and Ross Virginia investigated whether bottom-up or top-down factors are more important in a community of soil nematodes in the deserts of Antarctica. They chose this extreme environment because its nematode community contains only two or three species and is therefore easier to manipulate and study than other more speciesrich communities. Their experiment, described in Figure 54.19, showed that top-down factors appear to control the organization of this simple community. The top-down model has practical applications. For example, ecologists have applied the top-down model to improve water quality in polluted lakes. This approach, called biomanipulation, attempts to prevent algal blooms and eutrophication by altering the density of higher-level consumers in lakes instead of using chemical treatments. In lakes with three trophic levels, for example, removing fish should improve water quality by increasing zooplankton and thereby decreasing algal populations. In lakes with four trophic levels, adding top predators should have the same effect. We can summarize this scenario with the following diagram:

Fish

Polluted State

Restored State

Abundant

Rare

• 'I

54.19

Are soil nematode communities in Antarctica controlled by bottom-up or top-down factors? EXPERIMENT

Previous research in the deserts of Antardica had shown that the predatory nematode Eudorylaimus antareticus becomes I~s abundant in drier soils, but its prey speci~. the nematode Smttnema lindsayae, does not. To determine whether bottom-up or top-down factors control interactions in these communities, Diana Wall and Ross Virginia, then both of Colorado State University, decreased the abundance of E antareticus in seleded plots by warming and drying the soil. They placed clear plastic chambers over the ground for a year to trap the heat from sunlight and warm the soil by SoC, RESULTS The density of E. antareticus in the warmed plots dropped to one-quarter of the density in control plots. In contrast. the density of S. Iindsayae increased by one·siKth.

Control plots Warmed plots

n

a "-------:cJ..-'....--~:--'--'--Zooplankton

Rare

Abundant

Algae

Abundant

Rare

Ecologists used biomanipulation on a large scale in Lake Vesijarvi in southern Finland. Lake Vesijarvi is a large (llO km2 l, shallow lake that was polluted with city sewage and industrial wastewater until 1976. After pollution controls reduced these inputs, the water quality of the lake began to improve. By 1986, however, massive blooms of cyanobacteria started to occur in the lake. These blooms coincided with a dense population of roach, a fish that had benefited from the mineral nutrients that the pollution provided over many years. Roach eat zooplankton, which otherwise keep the cyanobacteria and algae in check. To reverse these changes, ecologists removed nearly a million kilograms of fish from Lake Vesijarvi between 1989 and 1993, reducing roach to about 20% of their former abundance. At the same time, the ecologists stocked the lake with pike perch, a predatory fish that eats roach. This added a fourth trophic level to the lake, which kept down the population of roach. Biomanipulation was a success in Lake Vesijarvi. TIle water became dear, and the last cyanobacterial bloom was in 1989. The lake remains dear even though roach removal ended in 1993. As these examples show, communities vary in their degree of bottom-up and top-down control. To manage agricultural landscapes, parks, reservoirs, and fisheries, we need to understand each particular community's dynamics. 1210

U"IT EIG~T

Ecology

5 Imdsayae

E antarC(lCUS

CONCLUSiON The prey species' increase in density as the predator density declined suggests that this soil nematode community is controlled by top-down factors. SOURCE

D

W~II-Frl.'Ckm~n ~nd

R. A.

Virg,n,~, Low·d'ver~ty

Antar(ll( wil nematode <ommUml,es. distr,butoon and response to d,sturbance, Ecology 78:363-369 (1997),

_','11° 11, . Suppose a second predatory species eKISted

In this community and that its abundance was unaffected by soil warming, How would you eKpect the density of S. lindsayae to change if the eKperiment were repeated under these conditions? hplain.

CONCEPT

CHECK

54.2

L What two components contribute to species diversity? Explain how two communities that contain the same number of species can differ in species diversity. 2. Describe two hypotheses that explain why food chains are usually short, and state a key prediction of each hypothesis. 3. _w:ru1fM Consider a grassland with five trophic levels: plants, grasshoppers, snakes, raccoons, and bobcats. If you released additional bobcats into the grassland, how would plant biomass change ifthe bottom-up model applied? If the top-down model applied? For suggested answers. see AppendiK A.


Decades ago, most ecologists favored the traditional view that biological communities are in a state of equilibrium, a more or less stable balance, unless seriously disturbed by human activities. The "balance of nature" view focused on interspecific competition as a key factor determining community composition and maintaining stability in communities. Stability in this context refers to a community's tendency to reach and maintain a relatively constant composition of species. One of the earliest proponents of this view, F. E. Clements, ofthe Carnegie Institution ofWashington, argued in the early 1900s that the community of plants at a site had only one state of equilibrium, controlled solely by climate. According to Clements, biotic interactions caused the species in this climax community to function as an integrated unit-in effect, as a superorganism. His argument was based on the observation that certain species of plants are consistently found together, such as the oaks, maples, birches, and beeches in deciduous forests of the northeastern United States. Other ecologists questioned whether most communities were at equilibrium or functioned as integrated units. A. G. Tansley, of Oxford University, challenged the concept of a climax community, arguing that differences in soils, topography, and other factors created many potential communities that were stable within a region. H. A. Gleason, ofthe University of Chicago, saw communities not as superorganisms but more as chance assemblages of species found in the same area simply because they happen to have similar abiotic requirementsfor example, for temperature, rainfall, and soil type. Gleason and other ecologists also realized that disturbance keeps many communities from reaching a state of equilibrium in species diversity or composition. A disturbance is an event, such as a storm, fire, flood, drought, overgrazing, or human activity, that changes a community by removing organisms from it or altering resource availability. This recent emphasis on change has produced the nonequilibrium model, which describes most communities as constantly changing after being affected by disturbances. Even where relatively stable communities do exist, they can be rapidly transformed into nonequilibrium communities. Let's now take a look at the ways disturbances influence community structure and composition.

munitieSi in fact, chaparral and some grassland biomes require regular burning to maintain their structure and species composition. Freezing is a frequent occurrence in many rivers, lakes, and ponds, and many streams and ponds are disturbed by spring flooding and seasonal drying. A high level of distur· bance is generally the result of a high intensity and high frequency of disturbance, while low disturbance levels can result from either a low intensity or low frequency of disturbance. The intermediate disturbance hypothesis states that moderate levels of disturbance can create conditions that foster greater species diversity than low or high levels of disturbance. High levels of disturbance reduce species diversity by creating environmental stresses that exceed the tolerances of many species or by subjecting the community to such a high frequency of disturbance that slow-growing or slow-colonizing species are excluded. At the other extreme, low levels ofdisturbance can reduce species diversity by allowing competitively dominant species to exclude less competitive species. Meanwhile, intermediate levels ofdisturbance can foster greater species diversity by opening up habitats for occupation by less competitive species. Such intermediate disturbance levels rarely create conditions so severe that they exceed the environmental tolerances of or rate of recovery by potential community members. The intermediate disturbance hypothesis is supported by many terrestrial and aquatic studies. In one such study, ecologists in New Zealand compared the richness of invertebrate taxa living in the beds of streams exposed to different frequencies and intensities of flooding (Figure 54.20). When floods occurred either very frequently or rarely, invertebrate richness was low. Frequent floods made it difficult for some species to become established in the streambed, while rare floods resulted in species being displaced by superior competitors. Invertebrate richness peaked in streams that had an intermediate frequency or intensity of flooding, as predicted by the intermediate disturbance hypothesis.

35

,•

30

x

'0

~

,E z

25

20 15

• •

::.•• ••..••

• ••

'.

1O-I--~~~-~~~~~-~~~-

09 10 1 1 12 1.3 1.4 1.5 1.6 1.7 18 1.9 20 Log intensity of disturbance

Characterizing Disturbance The types of disturbances and their frequency and severity vary from community to community. Storms disturb almost all communities, even those in the oceans, through the action of waves. Fire is a significant disturbance in most terrestrial com-

... Figure $4.20 Testing the intermediate disturbance hypothesis. Researchers identified the taxa (species or genera) of in~ertebrates at two locations in each of 27 New Zealand streams. They assessed the intensity of flooding at each location using an index of streambed disturbance. The number of in~ertebrate taxa peaked when the intensity of flooding was at Intermediate levels

CHAPTE~ FIFTY·FOU~

Community Ecology

1211


Although moderate levels of disturbance appear to maximize species diversity, small and large disturbances can have important effects on community structure. Small-scale disturbances can create patches of different habitats across a landscape, which can be a key to maintaining diversity in a community. Large-scale disturbances are also a natural part of many communities. Much of Yellowstone National Park, for example, is dominated by lodgepole pine, a tree that requires the rejuvenating influence of periodic fires. Lodgepole cones remain closed until exposed to intense heat. \Vhen a forest fire burns the trees, the cones open and the seeds are released. The new generation of lodgepole pines can then thrive on nutrients released from the burned trees and in the sunlight that is no longer blocked by taller trees. In the summer of 1988, extensive areas of Yellowstone burned during a severe drought. By 1989, burned areas in the park were largely covered with new vegetation, suggesting that the species in this community are adapted to rapid recovery after fire (Figure 54.21). In fact, large-scale fires have periodically swept through the lodgepole pine forests of Yellowstone and other northern areas for thousands of years. In contrast, more southerly pine forests were historically affected by frequent but low-intensity fires. In these forests, a century ofhuman intervention to suppress small fires has allowed an unnatural buildup of fuels and elevated the risk of large, severe fires to which the species are not adapted. Studies of the Yellowstone forest community and many others indicate that they are nonequilibrium communities, changing continually because of natural disturbances and the internal processes of growth and reproduction. Mounting evidence suggests that nonequilibrium conditions resulting from disturbance are in fact the norm for most communities.

(3) Soon after fire. The burn left a patchy landscape, Note the unburned trees in the far distance,

Ecological Succession Changes in the composition and structure of terrestrial communities are most apparent after some severe disturbance, such as a volcanic eruption or a glacier, strips away all the existing vegetation. The disturbed area may be colonized by a variety of species, which are gradually replaced by other species, which are in turn replaced by still other species-a process called ecological succession. When this process begins in a virtually lifeless area where soil has not yet formed, such as on a new volcanic island or on the rubble (moraine) left by a retreating glacier, it is called primary succession. Often the only life-forms initially present are autotrophic prokaryotes and heterotrophic prokaryotes and protists. Lichens and mosses, which grow from windblown spores, are commonly the first macroscopic photosynthesizers to colonize such areas. Soil develops gradually as rocks weather and organic matter accumulates from the decomposed remains of the early colonizers. Once soil is present, the lichens and mosses are usually overgrown by grasses, shrubs, and trees that sprout from seeds blown in from nearby areas or carried in by animals. Eventually, an area is colonized by plants that become the commwlity's prevalent form of vegetation. Producing such a commWlity through primary succession may take hundreds or thousands of years. Secondary succes....ion occurs when an existing community has been cleared by some disturbance that leaves the soil intact, as in Yellowstone following the 1988 fires (see Figure 54.21). Sometimes the area begins to return to something like its original state. For instance, in a forested area that has been cleared for farming and later abandoned, the earliest plants to recolonize are often herbaceous species that grow from windblown or animal-borne seeds. If the area has not been burned or

(b) One year after fire. The community has begun to recover, A variety of herbaceous plants, different from those in the former forest, cover the ground,

.... Figure 54.21 Recovery following a large-scale disturbance. The 1988 Yellowstone National Park fires burned large areas of forests dominated by lodgepole pines,

1212

U"IT EIG~T

Ecology


heavily grazed, woody shrubs may in time replace most of the herbaceous species, and forest trees may eventually replace most of the shrubs. Early arrivals and later-arriving species may be linked in one of three key processes. The early arrivals may facilitate the appearance of the later species by making the environment more favorable-for example, by increasing the fertility ofthe soil. Alternatively, the early species may inhibit establishment of the later species, so that successful colonization by later species occurs in spite of, rather than because of, the activities of the early species. Finally, the early species may be completely independent of the later species, which tolerate conditions created early in succession but are neither helped nor hindered by early species. Let's look at how these various processes contribute to primary succession on glacial moraines. Ecologists have conducted the most extensive research on moraine succession at Glacier Bay in southeastern Alaska, where glaciers have retreated more than 100 km since 1760 (Figure 54.22). By studying the communities on moraines at different distances from the mouth of the bay, ecologists can examine different stages in succession. 0 The exposed moraine is colonized first by pioneering spedes that include liverworts, mosses, fireweed, scat-

tered Dryas (a mat-forming shrub), willows, and cottonwood. After about three decades, Dryas dominates the plant community. A few decades later, the area is invaded by alder, which forms dense thickets up to 9 m tall. In the next two centuries, these alder stands are overgrown first by Sitka spruce and later by a combination ofwestern hemlock and mountain hemlock. In areas of poor drainage, the forest floor of this spruce-hemlock forest is invaded by sphagnum moss, which holds large amounts of water and acidifies the soil, eventually killing the trees. Thus, by about 300 years after glacial retreat, the vegetation consists ofsphagnum bogs on the poorlydrained flat areas and spruce-hemlock forest on the well-drained slopes. How is succession on glacial moraines related to the environmental changes caused by transitions in the vegetation? The bare soil exposed as the glacier retreats is quite basic, with a pH of8.0-8.4 due to the carbonate compounds in the parent rocks. The soil pH falls rapidly as vegetation develops. Decomposition ofacidic spruce needles in particular reduces the pH of the soil from 7.0 to approximately 4.0. The soil concentrations of mineral nutrients also change with time. Because the bare soil after glacial retreat is low in nitrogen content, almost all the pioneer plant species begin succession with poor

o

e

o

I

I

I

S

10

IS

f.) Dryas stage

K IQrn.w,

o Spruce stage

(} Alder stage

... Figure 54.22 Glacial retreat and primary succession at Glacier Bay, Alaska. The different shades of blue on the map show retreat of the glacier since 1760. based on historical descriptions, CHAPTE~ FIFTY路FOU~

Community Ecology

1213


60

50 N

~

40

1

30

c

'0 20 ~

10

0"--"_"-_ _ Pioneer

Dryas

Alder

Spruce

Successional stage ... Figure 54.23 Changes in soil nitrogen content during

succession at Glacier Bay.

growth and yellow leaves due to inadequate nitrogen supply. The exceptions are Dryas and, particularly, alder; these species

.... Figure 54.24 Disturbance of the ocean floor by trawling. These photos show the seafloor off northwestern Australia before (top) and after (bonom) deep'sea trawlers have passed.

have symbiotic bacteria that fix atmospheric nitrogen (see

Chapter 37). Soil nitrogen content increases rapidly during the alder stage of succession and continues to increase during the spruce stage (Figure 54.23). By altering soil properties, pioneer plant species permit new plant species to grow, and the

new plants in turn alter the environment in different ways, contributing to succession.

Human Disturbance Ecological succession is a response to disturbance of the environment, and one of the strongest agents of disturbance today is human activity. Of all animals, humans have the greatest impact on biological communities worldwide. Agricultural development has disrupted what were once the vast grasslands ofthe North American prairie. Logging and clearing for urban development, mining, and farming have reduced large tracts of forests to small patches ofdisconnected woodlots in many parts of the United States and throughout Europe. After forests are cleaHut, weedy and shrubby vegetation often colonizes the area and dominates it for many years. This type ofvegetation is also found in agricultural fields that are no longer under culti· vation and in vacant lots and construction sites. Human disturbance of communities is by no means limited to the United States and Europe; nor is it a recent problem. Tropical rain forests are quickly disappearing as a result of clear-cutting for lumber, cattle grazing, and farmland. Centuries of overgrazing and agricultural disturbance have contributed to famine in parts of Africa by turning seasonal grasslands into vast barren areas. Humans disturb marine ecosystems just as extensively as terrestrial ones. The effects ofocean trawling, where boats drag weighted nets across the seafloor, are similar to those of clear~ 1214

U"IT EIG~T

Ecology

cutting a forest or plowing a field (figure 54.24). The trawls scrape and scour corals and other life on the seafloor and in its sediments. In a typical year, ships trawl 15 million km 2 ofocean floor, an area about the size of South America and 150 times larger than the area of forests that are clear-cut annually. Because human disturbance is often severe, it reduces species diversity in many communities. In Chapter 56, we will take a closer look at how community disturbance by human activities is affecting the diversity of life. CONCEPT

CHECK

54.3

1. Why do high and low levels of disturbance usually

reduce species diversity? \Xfhy does an intermediate level of disturbance promote species diversity? 2. During succession, how might the early species facilitate the arrival of other species? 3. •ImU'liII Most prairies experience regular fires, typically every few years. How would the species di· versity of a prairie likely be affected if no burning occurred for 100 years? Explain your answer. For suggested answers, see Appendix A.

r:';~";::;~~i: factors affect community biodiversity

So far we have examined relatively small-scale or local factors that influence the diversity of communities, including the effects of species interactions, dominant species, and many


types ofdisturbances. Ecologists also recognize that large-scale biogeographic factors contribute to the tremendous range of diversity observed in biological communities. The contributions oftwo biogeographic factors in particular-the latitude of a community and the area it occupies-have been investigated for more than a century.

Area Effects In 1807, naturalist and explorer Alexander von Humboldt described one ofthe first patterns ofbiodiversity to be recognized,

·• •,•..• · .. •

160

•••• •••• • •

140

Latitudinal Gradients In the 185Os, both Charles Darwin and Alfred Wallace pointed out that plant and animal life was generally more abundant and diverse in the tropics than in other parts of the globe. Since that time, many researchers around the world have confirmed this observation. For example, one study found that a 6.6-hectare (1 ha = 10,000 m 2 ) plot in tropical Malaysia contained 711 tree species, while a 2-ha plot of deciduous forest in Michigan typically contains just 10 to 15 tree species. Moreover, in all of western Europe north of the Alps there are only 50 tree species. Many groups of animals show similar latitudinal gradients. For instance, there are more than 200 species of ants in Brazil but only 7 in Alaska. The two key factors in latitudinal gradients of species richness are probably evolutionary history and climate. Over the course of evolutionary time, species diversity may increase in a community as more speciation events occur. Tropical communities are generally older than temperate or polar com munities. This age difference stems partly from the fact that the growing season is about five times as long in tropical forests as in the tundra communities of high latitudes. In effect, biological time, and hence intervals between speciation events, run about five times as fast in the tropics as near the poles. And many polar and temperate communities have repeatedly "started over~ as a result of major disturbances in the form of glaciations. Climate is likely the primary cause of the latitudinal gradient in biodiversity. In terrestrial communities, the two main climatic factors correlated with biodiversity are solar energy input and water availability, both of which are relatively high in the tropics. These factors can be considered together by measuring a community's rate of evapotranspiration, the evaporation of water from soil plus the transpiration of water from plants. Evapotranspiration, a function of solar radiation, temperature, and water availability, is much higher in hot areas with abundant rainfall than in areas with low temperatures or low precipitation. Potential evapotranspiration, a measure of potential water loss that assumes that water is readily available, is determined by the amount of solar radiation and temperature and is highest in regions where both are plentiful. The species richness of plants and animals correlates with both measures of evapotranspiration (Figure 54.25).

::

180

.S;

~ ~

100

80

• • 40

..-'... . "

J'

20

• • • • • .' I • •• • • • • • • •

..

300 500 700 900 Actual evapotranspiration (mmlyr)

1.100

(a) Trees

-, ...

200

"

• • • 1O+-__~__~

__~

~ 500 1.000 1,500 Potential evapotranspiration (mmlyr)

o

2,000

(b) Vertebrates ... Figure 54.25 Energy, water, and species richness. (a) Species richness of NOrlh American trees increases most predictably with actual evapotranspiration, while (b) verlebrate species richness in North America increases most predictably with potential evapotranspiration. Evapotranspiration values are expressed as rainfall equivalents.

the species-area curve; All other factors being equal, the larger the geographic area ofacommunity, the more species it has. The likely explanation for this pattern is that larger areas offer a greater diversity of habitats and microhabitats than smaller areas. In conservation biology, developing species-area curves for the key taxa in acommunity helpsecotogists predict how the potentialloss of a certain area of habitat is likely to affect the communitys biodiversity. CHAPTE~ FIFTY·FOU~

Community Ecology

1215


1,000

••" 100 u

.'

~

<;" •"

~

E 0

z

10

T

1 0.1

10 100 10' 10' 10' 10' 10'

10

8

109 10 10

Area {hedaresl ... Figure 54.26 Species-area curve for North American breeding birds. 80th area and number of species are plotted on a logarithmic scale. The data points range from a 0.2-ha plot with 3species in Pennsyl~ania to the whole United States and Canada (1.9 billion hal with 625 species. Figure 54.26 is a species-area curve for North American breeding birds (birds with breeding populations in the mapped area, as opposed to migrant populations). The slope indicates the extent to which species richness increases with community area, While the slopes of different species-area curves vary, the basic concept of diversity increasing with increasing area applies in a variety of situations, from surveys of ant diversity in New Guinea to the number of plant species on islands ofdifferent sizes. In fact, island biogeography provides some of the best examples of species-area curves, as we will discuss next.

1

Island Equilibrium Model Because oftheir isolation and limited size, islands provide excellent opportunities for studying the biogeographic factors that affect the species diversity of communities. By ~islands:' we mean not only oceanic islands, but also habitat islands on land, such as lakes, mountain peaks separated by lowlands, or natural woodland fragments surrounded by areas disturbed by humans-in other words, any patch surrounded by an environment not suitable for the "island" species. In the 1960s, American ecologists Robert MacArthur and E. O. Wilson developed a general model of island biogeography identifying the key determinants of species diversity on an island with a given set of physical characteristics (figure 54.27). Consider a newly formed oceanic island that receives colonizing species from a distant mainland. Two factors that determine the number of species on the island are the rate at which new species immigrate to the island and the rate at which species become extinct on the island. At any given time, an island's immigration and extinction rates are affected by the number of species already present. As the number of species on the island increases, the immigration rate of new species decreases, because any individual reaching the island is less likely to represent a species that is not already present. At the same time, as more species inhabit an island, extinction rates on the island increase because of the greater likelihood of competitive exclusion. Two physical features of the island further affect immigration and extinction rates: its size and its distance from the mainland. Small islands generally have lower immigration rates because potential colonizers are less likely to reach a small island. For instance, birds blown out to sea by a storm

1

c

!

c Q

Q

~

-~

~ 5

~ 5

g ,

-~

~

.~

<;

<;

~Eq':"~;I~;bc,;C"=mCoC"=m:;b='C' :i------"~

~

.~

.~

Number of species on island ---+. (a) Immigration and extinction rates. The equilibrium number of species on an island represents a balance between the immigration of new species and the extinction of species already there.

sCm~'~"~;'~"~O:'d:;i:---l.~l'='·9'~iSI""Od~ Number of species on island

(b) Effect of island size. Large islands may ultimately ha~e a larger equilibrium number of species than small islands because immigration rates tend to be higher and extinction rates lower on large islands.

... figure 54.27 The equilibrium model of island biogeography. Black triangles represent equilibrium numbers of species. 1216

U"IT EIG~T

Ecology

Far island Near island Number of species on island •

---> (c)

Effect of distance from mainland. Near islands tend to ha~e larger equilibrium numbers of species than far islands because immigration rates to near islands are higher and extinction rates ICMler.


are more likely to land by chance on a larger island than on a small one. Small islands also have higher extinction rates, as they generally contain fewer resources and less diverse habitats for colonizing species to partition. Distance from the mainland is also important; for two islands of equal size, a closer island generally has a higher immigration rate than one farther away. Because of their higher immigration rates, closer islands also have lower extinction rates, as arriving colonists help sustain the presence of a species on a near island and prevent its extinction. These relationships make up MacArthur and Wilson's model ofisland biogeography (see Figure 54-.27). Immigration and extinction rates are plotted as a function of the number of species present on the island. This model is called the island equilibrium model because an equilibrium will eventually be reached where the rate of species immigration equals the rate of species extinction. The number of species at this equilibrium point is correlated with the island's size and distance from the mainland. Like any ecological equilibrium, this species equilibrium is dynamic; immigration and extinction continue, and the exact species composition may change over time. MacArthur and Wilson's studies of the diversity of plants and animals on many island chains, including the Galapagos Islands, support the prediction that species richness increases with island size, in keeping with the island equilibrium model (figure 54.28). Species counts also fit the prediction that the number of species decreases with increasing remoteness of the island. The island equilibrium model's predictions of equilibria in the species composition of communities may apply in only a limited number of cases and over relatively short periods, where colonization is the main process affecting species composition. Over longer periods, abiotic disturbances such as storms, adaptive evolutionary changes, and speciation generally alter the species composition and community structure on islands. Nonetheless, the model is widely applied in conservation biology, particularly for the design of habitat reserves and for providing a starting point for predicting the effects ofhabitat loss on species diversity. CONCEPT

CHECK

54.4

1. Describe tv.'o hypotheses that explain why species diversity is greater in tropical regions than in temperate and polar regions. 2. Describe how an island's size and distance from the mainland affect the island's species richness. 3. Based on MacArthur and \Vi.lson's model of island biogeography, how would you expect the richness of birds on islands to compare with the richness of snakes or mammals? Explain.

-w:rUla

For suggested answers, see Appendix A.

Fl~54.:Z8

In ui

How does species richness relate to area? fiELD STUDY Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galapagos Islands, which vary greatly in size, in relation to the area of each island, RESULTS

:;; 400

•w ~

01 200

g

•G

100

~

50

~

• ~

C

<i

"•,

25

'0

~

E

z

5 '0

100

103 lo-t Area of island (hectares) (log scale)

10'

CONCLUSION Plant species richness increases with island size. supporting the island equilibrium model. SOURCE R. fl, Ma<:Arthur and E, 0 Wilson, The Tlieoryof Island Biogeography, PrirKeton University Press, Princeton, Nt (1967).

_W:U'la

Four islands In this study ranging in area from about 40 to 10,000 ha all contained about 50 plant species, What does such variation tell you about the simple assumptions of the island equilibrium model?

r~::::n~~~~ology

is useful for understanding pathogen life cycles and controlling human disease

Now that we have examined several important factors that structure ecological communities, let's finish the chapter by examining community interactions involving pathogensdisease-causing microorganisms, viruses, viroids, or prions (viroids and prions are infectious RNA molecules and proteins, respectively; see Chapter 19). Scientists have recently come to appreciate how universal the effects of pathogens are in ecological communities. Pathogens can alter community structure quickly and extensively, as you saw in the discussion ofchestnut blight and the fungus that causes it (see Concept 54.2). Ecologists are also applying ecological knowledge to help track and control the pathogens that cause human diseases. (HAPTE~ FtFTY·FOU~

Community

Ecology

1217


Pathogens and Community Struclure In spite of the potential of pathogens to limit populations,

pathogens have until recently been the subject of relatively few ecological studies. This imbalance is now being addressed as dramatic events highlight the ecological importance ofdisease. Coral reef communities are increasingly susceptible to the influence of newtydiscovered pathogens. \'(!hite-band disease, caused by an unknown pathogen, has resulted in dramatic changes in the structure and composition of Caribbean reefs. The disease kills corals by causing their tissue to slough off in a band from the base to the tip ofthe branches (Figure 54.29), Because of the disease. stagham coral (Acropora cervicornis) has virtually disappeared from the Caribbean since the 1980s. In the same region, populations of elkhorn coral (Acropora

palmata) have also been decimated. Such corals provide key habitat for lobsters as well as snappers and other fish species. When the corals die, they are quickly overgrown by algae. Surgeonfish and other herbivores that feed on algae come to dominate the fish community. Eventually. the corals topple because ofdamage from storms and other disturbances. The complex, three-dimensional structure of the reefdisappears, and diversity plummets. Pathogens also influence community structure in terrestrial ecosystems. In the forests and savannas of California. trees of se\'Crai species are dying from sudden oak death (SOD). This recently discovered disease is caused by the fungus-like protist Phytoplltltora ramorum (see Chapter 28). SOD was first described in California in 1995 when hikers noticed trees dying around San Francisco Bay. By 2007. it had spread more than 650 km. During that time. it killed more than a million oaks and other trees from the central California coast to southern Oregon. The loss of these oaks led to a decrease in the abundance of at least five bird species, including the acorn woodpecker and the oak titmouse, that rely on the oaks for food and habitat. Although there is currently no cure for SOD. scientists

.. Figure 54.29 White-band disease visible on a coral.

1218

U~IT ElGIn

Ecology

recently sequenced the genome of P. ramorum in hopes of finding a way to fight the pathogen. One reason ecologists now study pathogens is that human activities are transporting pathogens around the world at unprecedented rates. Genetic analyses using simple sequence DNA (see Chapter21) suggest that the fungus that causes SOD likely came from Europe through the horticulture trade. Similarly. the pathogens that cause human diseases are spread by our global economy. A person tra\'Cling by airplane can quickly introduce a pathogen to a new location; this mechanism may ha\'e been how the West Nile virus arrived in North America in 1999. Many diseases are becoming more common. and community ecology is needed to help study and combat them.

Community Ecology and Zoonotic Diseases Three-quarters of today's emerging human diseases, including hantavirus and mad cow disease (see Chapter 19), and many historically important ones. such as malaria (see Chapter 28). are caused by 'Zoonotic pathogens. Zoonotic pathogens are defined as those that are transferred from other animals to humans, either through direct contact with an infected animal or by means ofan intermediate species, called a vector. The \MOrs that spread zoonotic diseases are often parasites, including ticks. lice, and mosquitoes. Community ecologists can help prevent zoonotic diseases by identifying key species interactions involving pathogens and their vectors and by tracking pathogen spread. Understanding parasite life cycles enables scientists to devise ways to control zoonotic diseases. The disease river blindness. for instance, is caused by a nematode transmitted by blackflies. \Vhen the World Health Organization began a global fight against river blindness. doctors had no medical treatments for the disease. Scientists focused instead on controlling the blackflies that spread the pathogenic nematodes. They llsed airplanes to spray biodegradable insecticides (which were monitored to minimize harm to aquatic communities). Ivermectin, a drug that kills the nematodes, was developed in 1987, and since then the combination ofvector control and ivermectin use has saved the sight ofan estimated 3OO.lXX) people. However, research published in 2007 suggests that the nematodes are developing resistance to ivermectin, so blackfly control remains a key part of the program to fight the disease. Ecologists also use their knowledge ofcommunity interactions to track the spread of zoonotic diseases. A timely example is ecological research on the spread of avian flu. Avian Au is caused by highly contagious viruses transmitted through the saliva and fecesofbirds (see Chapter 19). Most ofthese viruses affect wild birds mildly, but they often cause stronger symptoms in domesticated birds, the most common source of human infections. Since 2003, one particular viral strain. called HSN1. has killed hundreds of millions of poultry and more than 150 people. Millions more people are at risk of infection.


the Americas. The most likely place for infected wild birds to enter the Americas is Alaska, the entry point for ducks, geese, and shorebirds that migrate across the Bering Sea from Asia each year. Ecologists are studying the potential spread of the virus by trapping and testing migrating and resident birds in Alaska (Figure 54.30). These ecological detectives are trying to catch the first wave ofthe disease entering North America. Community ecology provides the foundation for understanding the life cycles of pathogens and their interactions with hosts. Pathogen interactions are also greatly influenced by changes in the environment. To control pathogens and the diseases they cause, scientists need an ecosystem perspective-an intimate knowledge of how the pathogens interact with other species and with their environment. Ecosystems are the subject of Chapter 55. CONCEPT

... Figure 54.30 Tracking avian flu. Graduate student Travis Booms, of the University of Alaska, Fairbanks, bands a young gyrfalcon (Fa/co rusticolus) as part of a project to monitor the spread of the disease,

3-D Animations. MP3 Tutors. Videos, Practice Tests, an eBook, and more

!.

For suggested answers, see Appendix A.

• -MHt,W Go to the Study Area at www.masteringbio.com lor BioFlix

54.5

I. \Vhat are pathogens? 2. Some parasites require contact with at least m'o host species to complete their life cycle. Why might this characteristic be important for the spread of certain zoonotic diseases? n Suppose a new zoonotic disease 3. emerges from a tropical rain forest. Doctors have no way yet to treat the disease, so preventing infections is particularly important. As a community ecologist, how might you help prevent the spread of the disease?

_',m

Control programs that quarantine domestic birds or monitor their transport may be ineffective if avian flu spreads naturally through the movements ofwild birds, From 2003 to 2006, the H5Nl strain spread rapidly from southeast Asia into Europe and Africa, but by late 2007 it had not appeared in Australia or

CHECK

Interspecific Interaction

Description

Competition (- 1-)

SUMMARY OF KEY CONCEPTS

_',llii"_ 54.1 Community interactions are classified by whether they help, harm, or have no effect on the species involved (pp.1198-1204) ... Populations are linked by interspecific interactions that affect the survival and reproduction of the species that engage in them. These interactions include competition, predation, herbivory, and symbiosis. Parasitism, mutualism, and commensalism are types of symbiotic interactions.

-Mit.• Acti'ity Interspe<:ific Interactions Biology Labs On-line PopuiationEcologyLab

Two or more species compete for a resource that is in short supply. The competitive exclu· sion principle states that two specks cannot coexist in the same community if their niches (ecological roles} are identical. Predation (+1-) One species, the predator, kills and eats the other, the prey. Predation has led to diverse adaptations, including mimicry. Herbivory (+ 1-) An herbivore eats part ofa plant or alga. Plants have various chemical and mechanical defenses against herbivory, and herbivores have spedalized adaptations for feeding. Symbiosis Individuals of two or more spedes live in dose contact with one another. Symbiosis includes parasitism, mutualism, and commensalism. Parasitism (+1-) The parasite derives its nourishml'fit from a second organism, its host, which is harmed. Mutualism (+1+) Both sp<x:ies benefit from the interaction. Commensalism (+ 10) One species benefits from the interaction, while the other is unaff<x:ted by it.

CHAPTE~ FIFTY·FOU~

Community Ecology

1219


_.,111.., , _

54.2

_. lil""_ 54.4

Dominant and keystone species exert strong controls on community structure (pp. 1204-1210)

(pp.1214-1217)

.. Species Diversity Species diversity measures the number of species in a community-its species richness-and their relative abundance. A community with similar abundances of species is more diverse than one in which one or two species are abundant and the remainder are rare.

.. latitudinal Gradients Species richness generully declines along a latitudinal gradient from the tropics to the poles. The greater age of tropical environments may account for the greater species richness of the tropics. Oimate also influences the biodiversity gradient through energy (heat and light) and water.

.. Trophic Structure Trophic structure is a key factor in community dynamics, Food chains link the trophic levels from producers to top carnivores. Branching food chains and complex trophic interactions form food webs. The energetic hypothesis suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain. The dynamic stability hypothesis proposes that long food chains are less stable than short chains.

.. Area Effects Species richness is directly related to a community's geographic size, a principle formalized in the speciesarea curve.

.. Species with a Large Impact Dominant species and keystone species exert strong controls on community structure. Dominant species are the most abundant species in a community, and their dominance is achieved by having high competitive ability. Keystone species are usually less abundant species that exert a disproportionate influence on community structure because of their ecological niche. Ecosystem "engineers; also called foundation species, exert influence on community structure through their effects on the physical environment. .. Bottom-Up and Top-Down Controls The bottom-up model proposes a unidirectional influence from lower to higher trophic levels, in which nutrients and other abiotic factors are the main determinants of community structure, including the abundance of primary producers. The top-down model proposes that control of each trophic level comes from the trophic level above, with the result that predators control herbivores, which in turn control primary producers, In\"~.tigation How Ar~ Impacts on Community Div~rsity M~asured? Activity Food Webs

_",'ili"_ 54.3 Disturbance influences species diversity and composition (pp. 1211-1214) .. Characterizing Disturbance More and more evidence suggests that disturbance and lack of quilibrium, rather than stability and equilibrium, are the norm for most communities. According to the intermediate disturbance hypothesis, moderate levels of disturbance can foster higher species diversity than can low or high levels of disturbance. .. Ecological Succession Ecological succession is the sequence of community and ecosystem changes after a disturbance. Primary succession occurs where no soil exists when succession begins; secondary succession begins in an area where soil remains after a disturbance. Mechanisms that produce community change during succession include facilitation and inhibition. .... Human Disturbance Humans are the most widespread agents of disturbance, and their effects on communities often reduce species diversity. Humans also prevent some naturally occurring disturbances, such as fire. which can be important to community structure. Acti\ity Primary Succession

1220

UNIT EIGHT

Ecology

Biogeographic factors affect community biodiversity

.. Island Equilibrium Model Species richness on islands depends on island size and distance from the mainland, The island eqUilibrium model maintains that species richness on an ecological island reaches an equilibrium where new immigrations are balanced by extinctions. This model may not apply over long periods, during which abiotic disturbances, evolutionary changes, and speciation may alter community structure.

-t,j4o!,.â&#x20AC;˘ Acti\ity Exploring Island Biogoogral'hy Graphlt! S[>"Cies¡Area Effect and lsland Biogeography

_ .. 'i'il"_ 54.5 Community ecology is useful for understanding pathogen life cycles and controlling human disease

(pp.1217-1219) .. Pathogens and Community Structure Recent work has highlighted the role that pathogens play in structuring terrestrial and marine communities. .. Community Ecology and Zoonotic Diseases Zoonotic diseases, caused by pathogens transferred from other animals to humans, are the largest class of emerging human diseases. Community ecology provides the framework for understanding the species interactions associated with such pathogens and for our ability to track and control their spread.

TESTING YOUR KNOWLEDGE

SELF¡QUIZ I. The feeding relationships among the species in a community determine the community's a. secondary succession, b. ecological niche. c. trophic structure. d. species-area curve. e. species richness. 2. The principle of competitive exclusion states that a. two species cannot coexist in the same habitat. b. competition between two species always causes extinction or emigration of one species. c. competition in a population promotes survival of the bestadapted individuals. d. two species that have exactly the same niche cannot coexist in a community, e. two species will stop reproducing until one species leaves the habitat.


3. Keystone predators can maintain species diversity in a communityifthey a. competitively exclude other predators. b. prey on the community's dominant species. c. allow immigration of other predators. d. reduce the number of disruptions in the community. e. prey only on the least abundant species in the community. 4. Food chains are sometimes short because a. only a single species of herbivore feeds on each plant species. b. local extinction of a species causes extinction of the other species in its food chain. c. most of the energy in a trophic level is lost as it passes to the next higher level. d. predator species tend to be less diverse and less abundant than prey species. e. most producers are inedible. 5. Based on the intermediate disturbance hypothesis, a community's spe<:ies diversity is a. increased by frequent massive disturbance. b. increased by stable conditions with no disturbance. c. increased by moderate levels of disturbance. d. increased when humans intervene to eliminate disturbance. e. increased by intensive disturbance by humans. 6. Which of the following could qualify as a top-down control on a gmssland community? a. limitation of plant biomass by rainfall amount b. influence of temperature on competition among plants c. influence of soil nutrients on the abundance of grasses versus wildflowers d. effect ofgrazing intensity by bison on plant species diversity e. effect of humidity on plant growth mtes 7. The most plausible hypothesis to explain why species richness is higher in tropical than in temperate regions is that a. tropical communities are younger, b. tropical regions generally have more available water and higher levels of solar radiation, c. higher temperatures cause more rapid speciation. d. biodiversity increases as evapotranspiration decreases. e. tropical regions have very high rates of immigration and very low rates of extinction. 8. According to the equilibrium model of island biogeography, spe<:ies richness would be greatest on an island that is a. small and remote. b. large and remote. c. large and close to a mainland. d. small and close to a mainland. e. environmentally homogeneous. 9. Community 1 contains 100 individuals distributed among four spe<:ies (A, B. C, and D). Community 2 contains 100 individuals distributed among three species (A, B, and C). Community 1: SA, 5B. 85C, 5D Community 2: 30A, 4OB. 30C Calculate the Shannon diversity (11) for each community. Which community is more diverse?

10.

I'P.,I.',,'

Figure 54.13 presents a partial food web for the Chesapeake Bay estuary. Another important species in Chesapeake Bay is the blue crab (Callinectcs sapidus). It is an omnivore, eating eelgrass and other primary producers as well as clams. Blue crabs are also cannibals. In turn, the crabs are a preferred food source for the endangered Kemp's Ridley sea turtle and, of course, for people. Based on this information, draw a food web that includes the blue crab. Assuming that the top-down model holds for this system, what would happen to the abundance of eelgrass if people were banned from catching crabs?

For Self-Quiz answers, see Appendix A.

-51401". Visit the Study Area at www.masteringbio.com for a Pra<tice Test.

EVOLUTION CONNECTION II. Explain why adaptations of particular organisms to interspecific competition may not necessarily represent instances of character displacement. What would a researcher have to demonstmte about two competing spe<:ies to make a convincing case for character displacement?

SCIENTIFIC INQUIRY 12. An e<:ologist studying plants in the desert performed the following experiment. She staked out two identical plots, each of which included a few sagebrush plants and numerous small annual wildflowers. She found the same five wildflower species in roughly equal numbers on both plots. She then enclosed one of the plots with a fence to keep out kangaroo rats. the most common grain-eaters of the area. After two years, four of the wildflower species were no longer present in the fenced plot. but one species had increased drastically, The control plot had not changed in spe<:ies diversity. Using the principles of community ecology, propose a hypothesis to explain her results. What additional evidence would support your hypothesis?

SCIENCE. TECHNOLOGY. AND SOCIETY 13. By 1935, hunting and trapping had eliminated wolves from the United States except for Alaska. Because wolves have since been protected as an endangered species, they have moved south from Canada and have become reestablished in the Rocky Mountains and northern Great Lakes region. Conservationists who would like to speed up wolf recovery have reintroduced wolves into Yellowstone National Park, Local ranchers are opposed to bringing back the wolves because they fear predation on their cattle and sheep. What are some reasons for reestablishing wolves in Yellowstone National Park? What effects might the reintroduction of wolves have on the ecological communities in the region? What might be done to mitigate the conflicts between ranchers and wolves?

(HAPTE~ FIFTY路FOU~

Community Ecology

1221


Ecos~~

.... Figure 55.1 What makes this ecosystem dynamic"? KEY

CONCEPTS

55.1 Physical laws govern energy flow and chemical

cycling in ecosystems 55.2 Energy and other limiting factors control

primary production in ecosystems 55.3 Energy transfer between trophic levels is

typically only 10% efficient 55.4 Biological and geochemical processes cycle nutrients between organic and inorganic parts of an ecosystem 55.5 Human activities now dominate most chemical cycles on Earth

itting beside a mountain lake, you watch the last rays of the sun reflected on its surface (Figure 55.1). While enjoying the tranquil scene, you begin to sense that the lake is much more dynamic than you first thought. Small rings form where fish snatch insects that have fallen to the lake's surface. A stream flows into the lake, delivering a bounty of mineral nutrients and organic matter. A slight breeze carries the lake's scent, shaped by microorganisms whose activities affect the composition of Earth's atmosphere. More than just a body of water, the lake is an ecosystem, the sum of all the organisms living within its boundaries and all the abiotic factors with which they interact An ecosystem can encompass a vast area, such as a forest, or a microcosm, such as the space under a fallen log or a small pool (figure 55.2). As with populations and communities, the boundaries of ecosystems sometimes are not discrete. Many e<ologists view the entire biosphere as a global ecosys~ tem, a composite of all the local ecosystems on Earth. Regardless of an ecosystem's size, its dynamics involve two processes that cannot be fully described by population or

S

1222

community phenomena: energy flow and chemical cycling. Energy enters most ecosystems as sunlight. It is converted to chemical energy by autotrophs, passed to heterotrophs in the organic compounds of food, and dissipated as heat Chemical elements, such as carbon and nitrogen, are cycled among abiotic and biotic components of the ecosystem. Photosynthetic organisms assimilate these elements in inorganic form from the air, soil, and water and incorporate them into their biomass, some of which is consumed by animals. The elements are returned in inorganic form to the environment by the metabolism of plants and animals and by other organisms, such as bacteria and fungi, that break down organic wastes and dead organisms. Both energy and matter are transformed in ecosystems through photosynthesis and feeding relationships. Unlike matter, however, energy cannot be recycled. Therefore, an ecosys路 tem must be powered by a continuous influx ofenergy from an external source-in most cases, the sun. Energy flows through ecosystems, whereas matter cycles within and through them.

.... Figure 55.Z A cave pool. This small ecosystem is home to a complex microbial commUnity.


Resources critical to human survival and welfare, ranging from the food we eat to the oxygen we breathe, are products of ecosystem processes. In this chapter, we will explore the dynamics of energy flow and chemical cycling, emphasizing the results of ecosystem experiments. One way to study ecosystem processes is to alter environmental factors, such as temperature or the abundance of nutrients, and study how ecosystems respond. We will also consider some of the impacts ofhuman activities on energy flow and chemical cycling. Those impacts are evident not just in human-dominated ecosystems, such as cities and farms, but in the most remote ecosystems on Earth.

r;~;:~:71~~;~overn energy flow and chemical cycling in ecosystems

In Unit Two, we saw how cells transform energy and matter, subject to the laws of thermodynamics. Like cell biologists, ecosystem ecologists study the transformations of energy and matter within a system and measure the amounts ofboth that cross the system's boundaries. By grouping the species in a community into trophic levels of feeding relationships (see Chapter 54), we can follow the transformations ofenergy in an ecosystem and map the movements of chemical elements.

Conservation of Energy Because ecosystem ecologists study the interactions of organisms with the physical environment, many ecosystem approaches are based on well-established laws of physics and chemistry. The first law of thermodynamics, which you studied in Chapter 8, states that energy cannot be created or destroyed but only transferred or transformed. Thus, we can potentially account for the transfer of energy through an ecosystem from its input as solar radiation to its release as heat from organisms. Plants and other photosynthetic organisms convert solar energy to chemical energy, but the total amount of energy does not change: The total amount ofenergy stored in organic molecules plus the amounts reflected and dissipated as heat must equal the total solar energy intercepted by the plant. One area of ecosystem ecology involves computing such energy budgets and tracing energy flow through ecosystems in order to understand the factors that control these energy transfers. Such transfers help determine how many organisms a habitat can support and the amount of food humans can harvest from a given site. One implication of the second law of thermodynamics, which states that every exchange of energy increases the entropy of the universe, is that energy conversions cannot be completely efficient; some energy is always lost as heat (see Chapter 8). This idea suggests that we can measure the effi-

ciency of ecological energy conversions in the same way we measure the efficiency of light bulbs and car engines. Energy flowing through ecosystems is ultimately dissipated into space as heat, so if the sun were not continuously providing energy to Earth, most ecosystems would vanish.

Conservation of Mass Matter, like energy, cannot be created or destroyed. This law of conservation of mass is as important to ecosystem ecologists as the laws of thermodynamics are. Because mass is conserved, we can determine how much of a chemical element cycles within an ecosystem or is gained or lost by that ecosystem over time. Unlike energy, chemical elements are continually recycled within ecosystems. A carbon atom in CO 2 is released from the soil by a decomposer, taken up by a grass through photosynthesis, consumed by a bison or other grazer, and returned to the soil in the bison's waste. The measurement and analysis of such chemical cycling within ecosystems and in the biosphere as a whole are an important aspect of ecosystem ecology. Although elements are not lost on a global scale, they move between ecosystems as inputs and outputs. In a forest ecosystem, for example, most mineral nutrients-the essential elements that plants obtain from soil-enter as dust or as solutes dissolved in rainwater or leached from rocks in the ground. Nitrogen is also supplied through the biological process of nitrogen fixation (see Figure 37.9). On the output side, gases return elements to the atmosphere, and water carries materials away. Like organisms, ecosystems are open systems, absorbing energy and mass and releasing heat and waste products. Most inputs and outputs are small compared to the amounts recycled within ecosystems. Still, the balance between inputs and outputs determines whether an ecosystem is a source or a sink for a given element. If a mineral nutrient's outputs exceed its inputs, it will eventually limit production in that system. Human activities often change the balance of inputs and outputs considerably, as we will see later in this chapter.

Energy, Mass, and Trophic levels As you read in Chapter 54, ecologists assign species to trophic levels on the basis of their main source of nutrition and energy. The trophic level that ultimately supports all others consists of autotrophs, also called the primary producers of the ecosystem. Most autotrophs are photosynthetic organisms that use light energy to synthesize sugars and other organic compounds, which they then use as fuel for cellular respiration and as building material for growth. Plants, algae, and photosynthetic prokaryotes are the biosphere's main autotrophs, although chemosynthetic prokaryotes are the primary producers in certain ecosystems, such as deep-sea hydrothermal vents (see Figure 52.18) and some spring-fed pools in caves (see Figure 55.2). CHAPTH flFTY路fIVE

Ecosystems

1223


Tertiary consumers Microorganisms and other detritivores

It Detritus

Primary consumers

...................;~!- __~p:rim:ary prod~C~"~'~; _____

[ .. Figure 55.3 Fungi decomposing a dead tree.

U"IT EIG~T

Ecology

..JI'"'\I~! Heat

K,y

â&#x20AC;˘

Chemical cycling

â&#x20AC;˘

Energy flow

.. Figure 55.4 An overview of energy and nutrient dynamics in an ecosystem. Energy enters. flows through. and exits an ecosystem, whereas chemICal nutrients cycle primarily within it. In this generalized scheme, energy (dark orange arrows) enters from the sun as radiation, moves as chemical energy transfers through the food web, and exits as heat radiated into space. Most transfers of nutrients (blue arrows) through the trophic levels lead eventually to detritus; the nutrients then cycle back to the primary producers.

Organisms in trophic levels above the primary producers are heterotrophs, which directly or indirectly depend on the biosynthetic output of primary producers. Herbivores, which eat plants and other primary producers, are primary consumers. Carnivores that eat herbivores are secondary consumers, and carnivores that eat other carnivores are tertiary consumers. Another important group of heterotrophs consists of the detritivores. Detritivores, or decomposers, are consumers that get their energy from detritus, which is nonliving organic material, such as the remains of dead organisms, feces, fallen leaves, and wood. Many detritivores are in turn eaten by secondary and tertiary consumers. Two important groups of detritivores are prokaryotes and fungi (figure 55.3). These organisms secrete enzymes that digest organic material; they then absorb the breakdown products, linking the consumers and primary producers in an ecosystem. In a forest, for example, birds eat earthworms that have been feeding on leaf litter and its associated prokaryotes and fungi. Even more important than this channeling of resources from producers to consumers is the role that detritivores play in recycling chemical elements back to primary producers. Detritivores convert organic materials from all trophic levels to inorganic compounds usable by primary producers, closing the loop of an ecosystem's chemical cycling. Producers can then recycle these elements into organic compounds. If decomposition stopped, all life on Earth would cease as detritus piled up and the supply of chemical ingredients for the syn1224

__

thesis of new organic matter was exhausted. Figure 55.4 summarizes the trophic relationships in an ecosystem. CONCEPT

CHECK

55.1

I. Why is the transfer of energy in an ecosystem referred to as energy flow, not energy cycling? 2. How does the second law of thermodynamics explain why an ecosystem's energy supply must be continually replenished? 3. 4#"1. You are studying nitrogen cycling on the Serengeti Plain in Africa. During your experiment, a herd of migrating wildebeests grazes through your study plot. What would you need to know to measure their effect on nitrogen balance in the plot? For suggested answers, see Appendix A.

r:~:;;;:n~5~~er limiting factors control primary production in ecosystems

The amount of light energy converted to chemical energy (organic compounds) by autotrophs during a given time period is an ecosystem's primary production. This photosynthetic


product is the starting point for studies of ecosystem metabolism and energy flow.

Ecosystem Energy Budgets Most primary producers use light energy to synthesize energyrich organic molecules, which are subsequently broken down to generate ATP (see Chapter 10). Consumers acquire their organic fuels secondhand (or even third- or fourthhand) through food webs such as those in Figures 54.12 and 54.13. Therefore, the amount of all photosynthetic production sets the spending limit for the entire e<:osystem's energy budget.

The Global Energy Budget Every day, Earth's atmosphere is bombarded by about 1022 joules ofsolar radiation (1 , = 0.239 cal). This is enough energy to supply the demands of the entire human population for approximately 25 years at 2006 consumption levels. As described in Chapter 52, the intensity of the solar energy striking Earth varies with latitude, with the tropics receiving the greatest input. Most incoming solar radiation is absorbed, scattered, or reflected by clouds and dust in the atmosphere. TIle amount of solar radiation that ultimately reaches Earth's surface limits the possible photosynthetic output of ecosystems. Furthermore, only a small fraction of the solar radiation that makes it to Earth's surface is used in photosynthesis. Much of the radiation strikes materials that don't photosynthesize, such as ice and soil. Of the radiation that does reach photosynthetic organisms, only certain wavelengths are absorbed by photosynthetic pigments; the rest is transmitted, reflected, or lost as heat. As a result, only about 1% of the visible light that strikes photosynthetic organisms is converted to chemical energy by photosynthesis. Nevertheless, Earth's primary producers collectively create about ISO billion metric tons (lSO x 10 12 kg) of organic material each year.

Gross and Net Primary Production

Net primary production can be expressed as energy per unit area per unit time (J/m 2 'yr) or as biomass (mass ofvegetation) added to the ecosystem per unit area per unit time (g/m 2·yr). (Note that biomass is usually expressed in terms of the dry mass of organic material.) An ecosystem's net primary production should not be confused with the total biomass of photosynthetic autotrophs present at a given time, a measure called the standing crop. Net primary production is the amount of new biomass added in a given period of time. Although a forest has a very large standing crop, its net primary production may actually be less than that of some grasslands, which do not accumulate much vegetation because animals consume the plants rapidly and because grasses and herbs decompose more quickly than trees do. Satellites provide a powerful tool for studying global patterns of primary production (Figure 55.5). Images produced from satellite data show that different ecosystems vary considerably in

Determining Primary Production with Satellites APPLICATION Because chlorophyll captures visible light (see Figure 10,9), photosynthetic organisms absorb more visible wavelengths (about 380-750 nm) than near-Infrared wavelengths (750-1,100 nm). Scientists use this difference in absorption to estimate the rate of photosynthesis in different regions of the globe using satellites, TECHNIQU E Most satellites determine what they "see" by comparing the ratios of wavelengths reflected back to them. Vegetation reflects much more near-infrared radiation than visible radiation. producing a reflectance pattern very different from that of snow. clouds. soil, and liquid water,

, u

Clouds

60

C

Vegetation

u

Total primary production in an ecosystem is known as that ecosystem's gross primary production (GPPl-the amount oflight energy that is converted to chemical energy by photosynthesis per unit time. Not all of this production is stored as organic material in the primary producers because they use some ofthe mole<:ules as fuel in their own cellular respiration. Net primary production (NPP) is equal to gross primary production minus the energy used by the primary producers for respiration (R); NPP = GPP - R In many ecosystems, NPP is about one-half ofGPP. To ecologists, net primary production is the key measurement because it represents the storage of chemical energy that will be available to consumers in the ecosystem.

~ 40

,•

C

Soil

~

20 liquid water

o ~~:;::::::'S--~~~~ 400

600

800

1,000

'---v._ _~"'-_ _~.~_ _.J Visible

1,200

Near-infrared Wavelength (nm)

RESULTS Scientists use the satellite data to help produce maps of primary production like that in Figure 55,6,

CHAPTE~ flFTY·fIVE

Ecosystems

1225


Iflight were the main variable limiting primary production in the ocean, we would expect production to increase along a gradient from the poles toward the equator, which receives the greatest intensity oflight. However, you can see in Figure 55.6 that there is no such gradient. Another factor must influence primary production in the ocean.

Nutrient limitation More than light, nutrients limit primary production in different geographic regions of the ocean and in lakes. A limiting nutrient is the element that must be Net primary production (kg C<lrbonlm 2.yr) added for production to increase. The nu~ trient most often limiting marine production is either nitrogen or phosphorus. o 2 3 Concentrations of these nutrients are ... Figure 55.6 Global net primary production in 2002. The map is based on data. such very low in the photic wne because they as chlorophyll activity, collected by satellites. Note that tropical areas on land have the highest rates are rapidly taken up by phytoplanktonand of production (yellow and red on the map). 1:1 Does this global map accurately reflect the importance of some highly productive habitats. because detritus tends to sink. a such as wetlands. coral reefs. and coastal zones? Explain. A5 detailed in Figure 55.7, nutrient enrichment experiments confirmed that nitrogen was limiting phytoplankton growth offthe south shore of their net primary production (Figure 55.6). Tropical rain forests Long Island, New York. Practical applications of this work in~ are among the most productive terrestrial ecosystems and con~ dude preventing algal "blooms" caused by nitrogen pollution tribute a large portion of the planet's overall net primary prothat fertilizes the phytoplankton. Eliminating phosphates from duction. Estuariesandcoral reefs also have very high net primary sewage, once thought to bethe cause ofthe problem, will not help production, but their contribution to the global total is relatively prevent algal blooms unless nitrogen pollution is also controlled. small because these ecosystems cover only about one-tenth the Several large areas of the ocean, however, have low phytoarea covered by tropical rain forests. One striking aspect of plankton densities in spite of relatively high nitrogen concenFigure 55.6 is how unproductive the oceans are per unit area comtrations. For example, the Sargasso Sea, a subtropical region of pared to tropical forests and some other ecosystems. Because of the Atlantic Ocean, has some of the dearest water in the world their vast size, however, the oceans altogether contribute as much because of its very low density of phytoplankton. A series of global net primary production as terrestrial systems do. nutrient enrichment experiments revealed that the availability What limits primary production in ecosystems? To ask this of the micronutrient iron can limit primary production there question another way, what factors could we change to in(Table 55.1). Windblown dust from the land is the main input crease or decrease primary production for a given ecosystem? of iron to the ocean, but relatively little windblown dust We'll address this question first for aquatic ecosystems. reaches the center of oceans. The finding that iron limits production in some oceanic Primary Production in Aquatic Ecosystems ecosystems encouraged marine ecologists to carry out recent In aquatic (marine and freshwater) ecosystems, both light and large-scale experiments in the Pacific Ocean. In one study, nutrients are important in controlling primary production. researchers spread low concentrations of dissolved iron over 72 km1 of ocean and then measured the change in phytoplankLight Limitation ton density over a seven-day period. A massive phytoplankton Because solar radiation drives photosynthesis, you might exbloom occurred, as indicated by increased chlorophyll concentration in the water. Adding iron stimulates growth ofcyanobacpect that light is a key variable in controlling primary production in oceans. Indeed, the depth of light penetration affects teria that fix atmospheric nitrogen (see Chapter 27), and the primary production throughout the photic zone of an ocean extra nitrogen stimulates proliferation of phytoplankton. or lake (see Figure 52.16). About half of the solar radiation is Areas ofupwelling, where nutrient-rich deep waters circulate absorbed in the first 15 m of water. Even in ~dear" water, only to the ocean surface, have exceptionally high primary produc5-10% of the radiation may reach a depth of75 m. tion, which supports the hypothesis that nutrient availability 1226

U"IT EIG~T

Ecology


"'~55.7

In ui

1'IIb1e 55.1 Nutrient Enrichment Experiment

Which nutrient limits phytoplankton production along the coast of Long Island? EXPERIMENT

Pollution from duck farms concentrated near

for Sargasso Sea Samples Nutrients Added to Experimental Culture

Relative Uptake of 14C by Cultures*

Moriches Bay adds both nitrogen and phosphorus to the coastal water off long Island, New York. To determiru" which nutrient limits phytoplankton growth in this area, John Ryther and William Dunstan, of the Woods Hole Oceanographic Insti·

None (controls)

1.00

Nitrogen (N) + phosphorus (P) only

1.10

N + P + metals (excluding iron)

1.08

tution, cultured the phytoplankton Nannochloris aramus with water collected from several sites (labeled A-G on the map be-

N + P + metals (including iron)

12.90

N+P+iron

12.00

low). They added either ammonium (P0 4 l -) to some of the cultures.

(NH4~)

or phosphate

·"c uptake by cultures measures prim.ry production, Source: D. W. Menzel and J. H. Ryther, Nutrients limiting the productiOll of phytoplankton in the Sargasso s"a, with 'pedal reference to iron, Deep Sea Researc/r 7,276-281 (1961).

E Moriches Bay Atlantic Ocean

A. ---..--

--~ RESULTS

The addition of ammonium caused heavy phy-

toplankton growth in the cultures, but the addition of phosphate did not.

• • o. • 30

Ammonium enriched

24

Phosphate enriched

~~ c~

18

Unenriched control

.

12

",::I

' S; E cWW "

~.::0

-•

~c

t.£

.c:= ~5

6

o A

B

C

D

E

F

G

determines marine primary production. Because the steady supply of nutrients stimulates growth of the phytoplankton populations that form the base ofmarine food webs, upwelling areas are prime fishing locations. The largest areas of upwelling occur in the Southern Ocean (also called the Antarctic Ocean) and the coastal waters off Peru, California, and parts of western Africa. Nutrient limitation is also common in freshwater lakes. During the 1970s, scientists showed that sewage and fertilizer runoff from farms and yards added large amounts of nutrients to lakes. Cyanobacteria and algae grow rapidly in response to these added nutrients, ultimately reducing the oxygen concentration and clarity of the water. This process, known as eutrophication (from the Greek eutrophos, well nourished), has many ecological impacts, including the eventual loss ofall but the most tolerant fish species from the lakes (see Figure 52.18). Controlling eutrophication requires knowing which polluting nutrient is responsible; nitrogen is rarely the limiting factor for primary production in lakes. A series of whole-lake experiments conducted by ecologists showed that phosphorus availability limited cyanobacterial growth. This and other research led to the use of phosphatefree detergents and other important water quality reforms.

Collection site

Primary Production in Terrestrial Ecosystems CONClUSION Since adding phosphorus, which was already in rich supply, had no effect on Nannochloris growth, whereas adding nitrogen increased phytoplankton density dramatically, the researchers concluded that nitrogen is the nutnent that limits phytoplankton growth in this ecosystem. SOURCE j H. Ryth€r and w M. Dunstan. Nitrogen, pnosphOl1JS, and eutrophication in the coastal marine envlronment, 5deoce 171.1008-1013 (1971),

M:rUIN

How would you exped the results of this experiment to change if new duck farms subSlantially increased the amount of pollution in the water? Explain your reasoning.

On a large geographic scale, temperature and moisture are the main factors controlling primary production in terrestrial ecosystems. Note again in Figure 55.6 that tropical rain forests, with their warm, wet conditions that promote plant growth, are the most productive of all terrestrial ecosystems. In contrast, low-productivity terrestrial ecosystems are generally dry-for example, deserts-or cold and dry-for example, the arctic tundra. Between these extremes lie the temperate forest and grassland ecosystems, which have moderate climates and intermediate productivity. These contrasts in climate can be represented by a measure called actual evapotranspiration, CHAPTE~ flFTY·fIVE

Ecosystems

1227


3,000

• Troplcalloresl

~

~g 2,000

i

. ~

• Temperate forest

§

1,000

• Mountain comferous forest

~

~

0",,"

• Temperate grassland

shrubland

• ArctlC tundra _ _~_ _~_ _ 500 1,000 1,500

O+-"L-_~

o

Actual evapotranspiration (mm H20lyr)

.... Figure 55.8 Relationship between net primary production and actual evapotranspiration in six terrestrial ecosystems.

which is the annual amount of water transpired by plants and evaporated from a landscape, usually measured in millimeters. Actual evapotranspiration increases with the amount of precipitation in a region and the amount of solar energy available to drive evaporation and transpiration. Figure 55.8 shows the positive relationship between net primary production and actual eV3potranspiration in selected ecosystems. On a more local scale, mineral nutrients in the soil can limit primary production in terrestrial ecosystems. As in aquatic ecosystems, nitrogen and phosphorus are most often the nutrients limiting terrestrial production. Adding a nonlimiting nutrient, even one that is scarce, will not stimulate production. Conversely, adding more of the limiting nutrient will increase production until some other nutrient becomes limiting. Studies relating nutrients to terrestrial primary production have practical applications in agriculture. Farmers maximize their crop yields by using fertilizers with the right balance of nutrients for the local soil and the type of crop. CONCEPT

CHECK

r:~:;;;7r;~f:r

between trophic levels is typically only 10% efficient

Theamount ofchemical energy in consumers' food that iscon· verted to their own nev.' biomass during a gi\'en time period is called the secondary production of the ecosystem. Consider the transfer of organic matter from primary producers to herbivores. the primary consumers. In most ecosystems, herbivores eat only a small fraction of plant material produced. Moreover, they cannot digest all the plant material that they dQ eat, as anyone who has walked through a dairy farm will attest. Thus, much of primary production is not used by consumers. Let's analyze this process of energy transfer more closely.

Production Efficiency Rrst let's examine secondary production in an individual organism-a caterpillar. \'<'hen a caterpillar feeds on a plant leaf. only about 33 Jout of2fX) J(48 cal), or one-sixth of the energy in the leaf. is used for secondary production, or growth (Figure 55.9). The caterpillar uses some of the remaining energy for cellular respiration and passes the rest in its feces. TIle energy contained in the feces remains in the ecosystem temporarily, but most of it is lost as heat after the feces are consumed by detritivores. The energy used for the caterpillar's respiration is also lost from the ecosystem as heal This is why energy is said to flow through, not cycle within, ecosystems. Only the chemical energy stored by

55.2

I. \'<'hy is only a small portion of the solar energy that

strikes Earth's atmosphere stored by primary producers? 2. How can ecologists experimentally determine the factor that limits primary production in an ecosystem? 3. elf U !. As part ofa science project, a student is trying to estimate total primary production of plants in a prairie ecosystem for a year. Once each quarter. the student cuts a plot ofgrass with a lawnmower and then collects and weighs the cuttings to estimate plant production. \'<'hat components of plant primary production is the student missing with this approach? For suggested answers. see Appendix A.

1228

UNIT !IGHT

Ecology

Cellular respiration

Growth (new biomass) .... Figure 55.9 Energy partitioning within a link of the food chain. less than 17% of the caterpillar's food ~ actually used for secondary production (growth).


herbivores as biomass (through growth or the production ofoffspring) is available as food to secondary consumers. We can measure the efficiency of animals as energy transformers using the following equation: Production efficiency =

Net secondary production X 100% Assimilation of primary production

Netsecondary production is the energy stored in biomass represented by growth and reproduction. Assimilation consists ofthe total energy taken in and used for growth, reproduction, and respiration. Production efficiency, therefore, is the percentage of energy stored in assimilated fOCK!. that is nOl used for respiration. For the caterpillar in Figure 55.9, production efficiency is 33%; 67 , ofthe 100 , ofassimilated energy is used for respiration. (Note that the energy lost as undigested material in feces does not count toward assimilation.) Birds and mammals typically have low production efficiencies, in the range of 1-3%, because they use so much energy in maintaining a constant, high body temperature. Fishes, which are ectothem1s{seeChapter40), have production efficiencies arOlmd 10%. Insects and microorganisms are even more efficient, with production efficiencies averaging 40% or more.

Trophic Efficiency and Ecological Pyramids Let's scale up now from the production efficiencies of individual consumers to the flow of energy through trophic levels. Trophic efficiency is the percentage of production transferred from one trophic level to the next. Trophic efficiencies must always be less than production efficiencies because they take into account not only the energy lost through respiration and contained in feces, but also the energy in organic material in a lower trophic level that is not consumed by the next trophic level. Trophic efficiencies are generally about 10% and range from approximately 5% to 20%, depending on the type of ecosystem. In other words, 90% of the energy available at one trophic level typically is not transferred to the next. This loss is multiplied over the length ofa food chain. For example, if 10% ofavailable energy is transferred from primary producers to primary consumers, and 10% of that energy is transferred to secondary consumers, then only 1%of net primary production is available to secondary consumers (10% of 10%). The progressive loss of energy along a food chain severely limits the abundance oftop-level carnivores that an ecosystem can support. Only about 0.1% of the chemical energy fixed by photosynthesis can flow all the way through a food web to a tertiary consumer, such as a snake or a shark. This explains why most food webs include only about four or five trophic levels (see Chapter 54). The loss of energy with each transfer in a fOCK!. chain can be represented by a pyramid ofnet production, in which the trophic levels are arranged in tiers (Figure 55.10). The width ofeach tier is proportional to the net production, expressed in joules, ofeach trophic level. The highest level, which represents top-level pred-

ators, contains relatively few individuals. Because populations of top predators are typically small and the animals may be widely spaced within their habitats, many predator species are highly susceptible to extinction (as well as to the evolutionary consequences of small population size, discussed in Chapter 23). One important ecological consequence of low trophic efficiencies is represented in a biomass pyramid, in which each tier represents the standing crop (the total dry mass ofall organisms) in one trophic level Most biomass pyramids narrow sharply from primary producers at the base to top-level carnivores at the apex because energy transfers between trophic levels are so inefficient (Figure 55.11a). Certain aquatic ecosystems, however,

Tertiary consumers

Secondary consumers

Primary consumers Primary producers

10,000 J

1,000,000 J of sunlight

... Figure 55.10 An idealized pyramid of net production. This example assumes a trophic efficiency of 10% for each link in the food chain, Notice that primary producers convert only about 1% of the energy available to them to net primary production,

Dry mass

Trophic level

(gfm 1)

Tertiary consumers Secondary consumers Primary consumers Primary producers

1.5 11 37 809

(a) Most biomass pyramids show a sharp decrease in biomass at succeSSively higher trophic levels, as Illustrated by data from a Florida bog,

Dry mass (gfm 2)

Trophic level Primary consumers (zooplankton) Primary producers (phytoplankton)

21 4

(b) In some aquatic ecosystems, such as the English Channel, a small standing crop of primary producers (phytoplankton) supports a larger standing crop of primary consumers (zooplankton),

... Figure 55.11 Pyramids of biomass (standing crop). Numbers denote the dry mass of all organisms at each trophic level.

(HAPTE~ flFTY路fIVE

Ecosystems

1229


have inverted biomass pyramids: Primary consumers outweigh the producers (figure 55.11b). Such inverted biomass pyra-

mids occur because the producers-phytoplankton-grow, reproduce, and are consumed so qUickly by the zooplankton that they never develop a large population size, or standing crop. In other words, the phytoplankton have a short turnover time, which means they have a small standing crop compared to their production:

. Turnover time =

.S=",=n=d,=ng"-c=m=,p"I,,,g/=m,-,-'l Production (glm 2 'day)

-

Because the phytoplankton continually replace their biomass at such a rapid rate, they can support a biomass of zooplankton bigger than their own biomass. Nevertheless, because phytoplankton have much higher production than zooplankton, the pyramid of prodl芦-tion for this ecosystem is still bottom~ heavy, like the one in Figure 55.10.

The dynamics of energy flow through ecosystems have im路 portant implications for the human population. Eating meat is a relatively inefficient way of tapping photosynthetic production. A person obtains far more calories by eating grains directly as a primary consumer than by eating the same amount of grain fed to an animal. Worldwide agriculture could, in fact, successfully feed many more people and require less cultivated land ifhumans all fed more efficiently-as primary consumers, eating only plant material. Consequently, estimates of Earth's human carrying capacity (see Chapter 53) depend greatly on our diet and on the amount of resources each of us consumes.

The Green World Hypothesis Earlier in this book, you learned why the world is green: Plants reflect more green light than red or blue light (see Figure 10.9). Land plants store approximately 70 x 10 10 metric tons of carbon, and global terrestrial primary production is about 6 x 10 10 metric tons per year. However, herbivores annually consume less than one~sixth the global NPP by plants (Figure 55.12).

... Figure 55.12 A green ecosystem. Most terrestrial ecosystems have large standing crops of vegetation despite the large number of reSident herbivores The green world hypothesis offers possible explanations for this observation.

1230

U"IT EIG~T

Ecology

Most of the rest is eventually consumed by detritivores. Thus, despite occasional outbreaks of pests, herbivores are generally only a minor nuisance to plants. Why do herbivores consume such a small fraction of plants' net primary production? According to the green world hypothesis, terrestrial herbivores are held in check by a vari路 ety of factors. Plant defenses, such as spines or noxious chem路 icals (see Chapter 39), limit the success of herbivores. Low nutrient concentrations in plant tissues mean that large quantities of biomass are needed to support each herbivore. Other factors also limit the number of herbivores, including abiotic pressures, such as temperature and moisture extremes; intraspecific competition, including territorial behavior; and interspecific competition, particularly from predators, parasites, and pathogens (as in the top~down model of community struc~ ture, which you learned about in Chapter 54). In the next section, we will look at how the transfer of nutrients along with energy through food webs is part of a larger picture of chemical cycling in ecosystems. CONCEPT

CHECK

55.!

1. If an insect that eats plant seeds containing 100 J of

energy uses 30 J of that energy for respiration and excretes 50 J in its feces, what is the insect's net secondary production? What is its production efficiency? 2. Tobacco leaves contain nicotine, a poisonous compound that is energetically expensive for the plant to make. What advantage might the plant gain by using some of its resources to produce nicotine? 3. E:fUIN As part of a new reaEty show on television, a group of overweight people are trying to safely lose in one month as much weight as possible. In addition to eating less, what could they do to decrease their production efficiency for the food they eat? For suggested answers, see Appendix A.


r:·t:I:·;~a~:~: geochemical

Reservoir A

Reservoir B Organic matenals unavailable

processes cycle nutrients between organic and inorganic parts of an ecosystem

as nutrients Fossilization Coal. oil. peat

Although most ecosystems receive an abundant supply of solar energy, chemical elements are available only in limited amounts. (The meteorites that occasionally strike Earth are the only extraterrestrial source of new matter.) Life on Earth therefore depends on the recycling of essential chemical elements. While an organism is alive, much of its chemical stock is replaced continuously as nutrients are assimilated and waste products released. When the organism dies, the atoms in its complex molecules are returned in simpler compounds to the atmosphere, water, or soil by the action of decomposers. Decomposition replenishes the pools of inorganic nutrients that plants and other autotrophs use to build new organic matter. Because nutrient cycles involve both biotic and abiotic components, they are called biogeochemical cycles.

Reservoir C

t!

Reservoir 0

Inorganic materials

Inorganic materials

Assimilation, photosynthesis

'"pi"tioo. decomposition. excretion

available

as nutrients Atmosphere, soil, water

Weathering, erosion

Formation of sedimentary rock

unavailable

as nutrients

Minerals in rocks

... Figure 55.13 A general model of nutrient cycling. Arrows indicate the processes that move nutrients between reservoirs. IE'II Recent evidence suggests (hat mycorrhizal fungi can release acids

Biogeochemical Cycles

. . that dissolve some minerals, including cakium phosphate. Where does (his fungal 3c1ivity fit into the model 7

An element's specific route through a biogeochemical cycle depends on the element and the trophic structure ofthe ecosystem. We can, however, recognize two general categories of biogeochemical cycles: global and local. Gaseous forms of carbon, oxygen, sulfur, and nitrogen occur in the atmosphere, and cycles of these elements are essentially global. For example, some of the carbon and oxygen atoms a plant acquires from the air as CO2 may have been released into the atmosphere by the respiration of an organism in a distant locale. Other elements, including phosphorus, potassium, and calcium, are too heavy to occur as gases at Earth's surface. In terrestrial ecosystems, these elements cycle more locally, absorbed from the soil by plant rootsand eventually returned to the soil by decomposers. In aquatic systems, however, they cycle more broadly as dissolved forms carried in currents. Before examining the details of individual cycles, let's look at a general model of nutrient cycling that includes the main reservoirs of elements and the processes that transfer elements between reservoirs (Figure 55.13). Each reservoir is defined by two characteristics: whether it contains organic or inorganic materials and whether or not the materials are directly available for use by organisms. The nutrients in living organisms themselves and in detritus (reservoir A in Figure 55.13) are available to other organisms when consumers feed and when detritivores consume nonliving organic matter. Some material moved from the living organic reservoir to the fossilized organic reservoir (reservoir B) long ago, when dead organisms were converted to coal, oil, or peat (fossil fuels). Nutrients in these deposits generally cannot be assimilated directly.

Inorganic materials (elements and compounds) that are dissolved in water or present in soil or air (reservoir C) are available for use. Organisms assimilate materials from this reservoir directly and return chemicals to it through the relatively rapid processes ofcellular respiration, excretion, and de· composition. Although most organisms cannot directly tap into the inorganic elements tied up in rocks (reservoir DJ, these nutrients may slowly become available through weathering and erosion. Similarly, unavailable organic materials move into the available reservoir of inorganic nutrients when fossil fuels are burned, releasing exhaust into the atmosphere. How have ecologists worked out the details of chemical cycling in various ecosystems? Two common methods use isotopes-either by adding tiny amounts of radioactive isotopes of specific elements and tracking their progress or by following the movement of naturally occurring, nonradioactive isotopes through the biotic and abiotic components of an ecosystem. For example, scientists have been able to trace the flow into ecosystems of radioactive carbon (,4C) released into the atmosphere during atom bomb testing in the 1950s and early 1960s. This "spike" of 14C can be used to date the age of bones and teeth, to measure the turnover rate of soil organic matter, and to foUow changes in many other carbon pools in the environment. figure 55.14, on the next two pages, provides a detailed look at the cycling ofwater, carbon, nitrogen, and phosphorus. Examine these four biogeochemical cycles closely, considering the major reservoirs of each chemical and the processes that drive the movement of each chemical through its cycle. CHAPTH flFTY·fIVE

Ecosystems

1231


• Figure 55.14

••

• Nutrient Cycles The Water Cycle

Biological importance \Vater is essential to all organisms {see Chapter 3), and its availability influences the rates of ecosystem processes, particularly primary production and decomposition in terrestrial ecosystems.

Transport over land

Forms available to life Liquid water is the primary physical phase in which water is used, though some organisms can harvest water vapor. Freezing of soil water can limit water availability to terrestrial plants. Reservoirs The oceans contain 97% of the water in the biosphere. Apprm::imately 2% is bound in glaciers and polar ice caps, and the remaining 1% is in lakes, rivers, and groundwater, with a negligible amount in the atmosphere.

Solar energy

Net mo~ement of water vapor by wind

Predpitatio over ocean

E~aporation

from ocean

Key processes The main processes driving the water cycle are

evaporation of liquid water by solar energy, condensation ofwater vapor into clouds, and precipitation. Transpiration by terrestrial plants also moves significant volumes ofwater into the atmosphere. Surface and groundwater flow can return water to the oceans, completing the water cycle. The widths ofthe arrows in the diagram reflect the relative contribution of each process to the movement of water in the biosphere.

Runoff and groundwater

The Carbon Cycle Biological importance Carbon forms the framework of the organic molecules essential to all organisms. Forms available to life Photosynthetic organisms utilize CO 2 during photosynthesis and convert the carbon to organic forms that are used by consumers. including animals, fungi. and heterotrophic protists and prokaryotes. Reservoirs The major reservoirs of carbon include fossil fuels, soils, the sediments of aquatic ecosystems, the oceans (dissolved carbon compounds), plant and animal biomass, and the atmosphere (C0 2). The largest reservoir is sedimentary rocks such as limestone; however, this pool turns over very slowly. Key processes Photosynthesis by plants and phytoplankton re-

c::::;.c.::::.J

1232

Phytoplankton

U"IT EIGHT

Ecology

/' Hlg er-Ievel Primary consumers consumers

moves substantial amounts of atmospheric CO 2 each year. This quantity is approximately equaled by CO 2 added to the atmosphere through cellular respiration by producers and consumers. Over geologic time, volcanoes are also a substantial source of CO 2• The burning of fossil fuels is adding significant amounts of additional CO 2 to the atmosphere. The widths of the arrows reflect the relative contribution of each process.


The Terrestrial Nitrogen Cycle Biological importance Nitrogen is part of amino acids, proteins, and nucleic acids and is often a limiting plant nutrient. Forms available to life Plants can use two inorganic forms of nitrogen-ammonium (NH 4 +) and nitrate (N0 3 )-and some organic forms, such as amino acids. Various bacteria can use all of these forms as well as nitrite (N0 2 ). Animals can use only organic forms of nitrogen.

Reservoirs The main reservoir of nitrogen is the atmosphere. which is 80% nitrogen gas (N 2 ). The other reservoirs are soils and the sediments oflakes, rivers, and oceans (bound nitrogen); surface water and groundwater (dissolved nitrogen); and the biomass ofliving organisms.

rl\r ~

NilrQixing bilGer..

In

foot

noctules of legumes

Decompose's

O

NitrlfY1ng

AmmonihcillIOn

bilCl€f1i1

NN,

Nilrogen·l,xmg soil b3Gerlil

NltfllYlflg

bacteria

Key processes The major pathway for nitrogen to enter an ecosystem is via nitrogenfixation. the conversion ofN 2 by bacteria to forms that can be used to synthesize nitrogenous organic compounds (see Chapter 37). Some nitrogen is also fixed by lightning. Nitrogen fertilizer, precipitation, and blowing dust can also provide substantial inputs of NH4 + and N0 3 - to ecosystems. Ammonification decomposes organic nitrogen to NH 4 +. In nitrification, NH 4 + is converted to N0 3 - by nitrifying bacteria. Under anaerobic conditions, denitrifying bacteria use N0 3 in their metabolism instead of O 2, releasing N 2 in a process known as denitrification. The widths of the arrows reflect the relative contribution of each process.

The Phosphorus Cycle Biological importance Organisms require phosphorus as a major constituent of nucleic acids. phospholipids. and ATP and other energy-storing molecules and as a mineral constituent of bones and teeth.

Forms available to life The most biologically important inorganic form of phosphorus is phosphate (P0 43 -). which plants absorb and use in the synthesis of organic compounds. Reservoirs The largest accumulations of phosphorus are in sedimentary rocks of marine origin. There are also large quantities of phosphorus in soils, in the oceans (in dissolved form), and in organisms. Because humus and soil particles bind phosphate, the recycling of phosphorus tends to be quite localized in ecosystems. 3

Key processes Weathering of rocks gradually adds P04 to soil; some leaches into groundwater and surface water and may eventually reach the sea. Phosphate taken up by producers and incorporated into biological molecules may be eaten by consumers and distributed through the food web. Phosphate is returned to soil or water through either decomposition of biomass or excretion by consumers. Because there are no signific-.mt phosphorus-containing gases. only relatively small amounts ofphosphorus move through the atmosphere, usually in the formsof dust and sea spmy. Thewidths of the arrows reflect the relative contribution ofeach process.

Decomposition Plankton

[);ssoM;od Uptake

pot 5<>1 LeacNng

C~APTE~ flFTY·fIVE

Ecosystems

1233


Decomposition and Nutrient Cycling Rates

• Fi

The diagrams in Figure 55.14 illustrate the essential role that decomposers (detritivores) play in recycling carbon, nitrogen,

How does temperature affect litter decomposition in an ecosystem?

and phosphorus. The rates at which these nutrients cycle in different ecosystems are extremely variable, mostly as a result

EXPERIMENT

of differences in rates of decomposition. Decomposition is controlled by the same factors that limit primary production in aquatic and terrestrial ecosystems (see Concept 55.2). Those factors include temperature, moisture,

55.15

Researchers with the Canadian Forest Service placed identical samples of organic material on the ground in 21 sites across Canada (marked by letters on the map below). Three years later, they returned to see how much of each sample had decomposed. Ecosystem type

and nutrient availability. Decomposers usually grow faster and decompose material more quickly in warmer ecosystems

Arctic Subarctic

(Figure 55.15). In tropical rain forests, for instance, most or-

ganic material decomposes in a few months to a few years, while in temperate forests, decomposition takes four to six years, on average. The difference is largely the result of the higher temperatures and more abundant precipitation in tropical rain forests. Because decomposition in a tropical rain forest is rapid, relatively little organic material accumulates as leaflitter on the forest floor; about 75% ofthe nutrients in the ecosystem is present in the woody trunks of trees, and about 10% is contained in the soil. Thus, the relatively low concentrations of some nutrients in the soil of tropical rain forests result from a short cycling time, not from a lack of these elements in the ecosystem. In temperate forests, where decomposition is much slower, the soil may contain as much as 50% of all the organic material in the ecosystem. The nutrients that are present in temperate forest detritus and soil may remain there for fairly long periods before plants as· similate them. Decomposition on land is also slower when conditions are either too dry for decomposers to thrive or too wet to supply them with enough oxygen. Ecosystems that are both cold and wet, such as peatlands, store large amounts of organic matter; decomposers grow poorly most of the year there, and net primary production greatly exceeds decomposition. In aquatic ecosystems, decomposition in anaerobic muds can take 50 years or more. Bottom sediments are comparable to the detritus layer in terrestrial ecosystems; however, algae and aquatic plants usually assimilate nutrients directly from the water. Thus, the sediments often constitute a nu· trient sink, and aquatic ecosystems are very productive only when there is interchange between the bottom layers of water and the surface (as in the upwelling regions described earlier).

Case Study: Nutrient Cycling in the Hubbard Brook Experimental Forest In one of the longest-running ecological research experiments in North America, ecologists Herbert Bormann, Eugene Likens, and their colleagues have been studying nutrient cycling in a 1234

U"IT EIG~T

Ecology

• •

Boreal Temperate

Grassland Mountain

RESULTS litter mass decreased four times faster in warmer ecosystems than in colder ones.

80 70

]

60

E

50

'0 40 C ~

g:

a

K.

J.

,N

30 20

I

I

T p.

5

• M

l

• H G

10

o-fo"'-~-~-~-~-~-~

-15

-10

-S 0 5 10 Mean annual temperature (OC)

15

Decomposition increases with temperature across much of Canada.

CONCLUSION

SOURCE T. R. Moore et al.,I..I1ler decomposition rate-; in Canadian forests, Globitl Change Biology 5:75-82 (1999).

_Mfn.M

What factors other than temperature might also have varied across these 21 sites? How might this variation have affected the interpretation of the results)


forest ecosystem since 1%3. Their study site, the Hubbard This study demonstrated that the amount of nutrients leavBrook Experimental Forest in the White Mountains of New ing an intact forest ecosystem is controlled mainly by the Hampshire, is a dedduous forest with several valleys, each plants. The effects ofdeforestation occur within a few months drained by a small creek that is a tributary of Hubbard Brook. and continue as long as living plants are absent. Bedrock impenetrable to water is close to the surface of the The 45 years of data from Hubbard Brook reveal some other trends. For instance, in the last half century, acid rain soil, and each valley constitutes a watershed that can drain and snow have dissolved most of the Ca2+ in the forest soil, only through its creek. The research team first determined the mineral budget for and the streams have carried it away. By the 19905, the forest each of six valleys by measuring the input and outflow of sevbiomass at Hubbard Brook had stopped increasing, appareral key nutrients. They collected rainfall at several sites to ently because of a lack ofCaH . To test this idea, ecologists at measure the amount ofwater and dissolved minerals added to Hubbard Brook began a massive experiment in 1998. They the ecosystem. To monitor the loss ofwater and minerals, they first established a control and an experimental watershed, constructed a small concrete dam with a V-shaped spillway which they monitored over two years before using a helicopter to add Ca 2 + to the experimental watershed_ By 2006, sugar across the creek at the bottom ofeach valley (Figure 55.16a). maple trees growing in the CaH -enriched location had higher About 60% of the water added to the ecosystem as rainfall and CaH concentrations in their foliage, healthier crowns, and snow exits through the stream, and the remaining 40% is lost by evapotranspiration. Preliminary studies confirmed that internal cycling within a terrestrial ecosystem conserves most of the mineral nutrients. For example, only about 0.3% more calcium (Ca2+) left a valley via its creek than was added by rainwater, and this small net loss was probably replaced by chemical decomposition of the bedrock. During most years, the forest actually registered small net gains of a few mineral nutrients, including nitrogen. (a) Concrete dams and weirs built across streams at the In one experiment, the trees in one bottom of watersheds valley were cut down and then the valenabled researchers to ley was sprayed with herbiddes for monitor the outflow of water and nutrients from three years to prevent regrowth of the ecosystem plants (Figure 55.16b). All the origi(b) One watershed was c1ear-wt to study the effects of the loss of nal plant material was left in place to vegetation on drainage and nutrient cycling. decompose. The inflow and outflow of water and minerals in this experimen80 tally altered watershed were compared 60 with those in a control watershed. .'= 40 Over the three years, water runoff o .搂:_ 20 from the altered watershed increased ~~ o E by 30-40%, apparently because there Completion of e4 o were no plants to absorb and transpire 8 3 Control t~'1U;"g water from the soil. Net losses of min2 erals from the altered watershed were .~ z huge. The concentration ofCa2+ in the creek increased 4-fold, for example, 1965 1966 1967 1968 and the concentration of K+ increased by a factor of 15. Most remarkable was (c) The concentration of nitrate in runoff from the deforested watershed was 60 limes greater than the loss of nitrate, whose concentrain a control (unloggedl watershed. tion in the creek increased 6O-fold, reaching levels considered unsafe for ... Figure 55.16 Nutrient cycling in the Hubbard Brook Experimental Forest: an drinking water (Figure 55.16c). example of long-term ecological research.

,

;~......L!.~~~,."....,~~~".......,~L~~

CHAPTE~ flFTY路fIVE

Ecosystems

1235


greater seedling establishment than those growing in the control watershed. These data suggest that sugar maple declines in the northeastern United States and southern Canada are attributable at least in part to the consequences of soil acidification. The Hubbard Brook studies, as well as many other longterm ecological research projects funded by the National Science Foundation, assess natural ecosystem processes and provide important insight into the mechanisms by which human activities affect these processes. CONCEPT

CHECK

55.4

1. • l.f.MiM For each of the four biogeochemical cycles detailed in Figure 55.14, draw a simple diagram that shows one possible path for an atom or molecule of that chemical from abiotic to biotic reservoirs and back. 2. Why does deforestation of a watershed increase the concentration of nitrates in streams draining the watershed? 3. e:t ill • Why is nutrient availability in a tropical rain forest particularly vulnerable to logging? For suggested answers, see Appendix A,

-...J

r:;:::~·a~t~:i~ies now dominate most chemical cycles on Earth

As the human population has grown rapidly in size (see Concept 53.6), our activities and technological capabilities have disrupted the trophic structure, energy flow, and chemical cycling of ecosystems. In fact, most chemical cycles are now influenced more by human activities than by natural processes.

Agriculture and Nitrogen Cycling After natural vegetation is cleared from an area, the existing reserve of nutrients in the soil is sufficient to grow crops for some time. In agricultural ecosystems, however, a substantial fraction of these nutrients is exported from the area in crop n biomass. The "free period for crop production-when there is no need to add nutrients to the soil-varies greatly. When some of the early North American prairie lands were first tilled, good crops could be produced for decades because the large store of organic materials in the soil continued to decompose and provide nutrients. By contrast, some cleared land in the tropics can be farmed for only one or two years because so little of the ecosystems' nutrient load is contained in the soil. Despite such variations, in any area under intensive agriculture, the natural store of nutrients eventually becomes exhausted. Nitrogen is the main nutrient lost through agriculture; thus, agriculture has a great impact on the nitrogen cycle. Plowing mixes the soil and speeds up decomposition of organic matter, releasing nitrogen that is then removed when crops are harvested. Applied fertilizers make up for the loss of usable nitrogen from agricultural ecosystems (Figure 55.17). In addition, as we saw in the case of Hubbard Brook, without plants to take up nitrates from the soil, the nitrates are likely to be leached from the ecosystem. Recent studies indicate that human activities have more than doubled Earth's supply of fixed nitrogen available to primary producers. Industrial fertilizers provide the largest additional nitrogen source. Fossil fuel combustion also releases nitrogen oxides, which enter the atmosphere and dissolve in rainwater; the nitrogen ultimately enters ecosystems as nitrate. Increased cultivation of legumes, with their nitrogenfixing symbionts, is a third way in which humans increase the amount of fixed nitrogen in the soil.

Nutrient Enrichment Human activity often removes nutrients from one part of the biosphere and adds them to another. On the simplest level, someone eating a piece of broccoli in Washington, DC, consumes nutrients that only days before were in the soil in Cal· ifornia; a short time later, some of these nutrients will be in the Potomac River, having passed through the person's digestive system and a local sewage treatment facility. On a larger scale, nutrients in farm soil may run off into streams and lakes, depleting nutrients in one area, increasing them in another, and altering chemical cycles in both. Furthermore, humans have added entirely novel materials-some of them toxic-to ecosystems. Humans have altered nutrient cycles so much that we can no longer understand any cycle without taking these effects into account. Let's examine a few specific examples of how humans are impacting the biosphere's chemical dynamics.

1236

U"IT EIG~T

Ecology

.... Figure 55.17 Fertilization of a corn (maize) crop. To replace the nutrients removed in crops, farmers must apply fertilizerseither organic, such as manure or mulch, or synthetic. as shown here,


... Figure 55.18 The dead zone arising from nitrogen pollution in the Mississippi basin. In these satellite images from 2004. red and orange represent high concentrations of phytoplankton and ri~er sediment in the Gulf of Mexico. This dead zone extends much farther from land in summer than in winter.

Winter

Summer

Contamination ofAquatic Ecosystems The key problem with excess nutrients is the critical load, the amount of added nutrient, usually nitrogen or phosphorus, that can be absorbed by plants without damaging ecosystem integrity. For example, nitrogenous minerals in the soil that exceed the critical load eventually leach into groundwater or run off into freshwater and marine ecosystems, contaminating water supplies and killing fish. Nitrate concentrations in groundwater are increasing in most agricultural regions, sometimes exceeding safe levels for drinking. Many rivers contaminated with nitrates and ammonium from agricultural runoff and sewage drain into the Atlantic Ocean, with the highest inputs coming from northern Europe and the central United States. The Mississippi River carries nitrogen pollution to the Gulf of Mexico, fueling a phytoplankton bloom each summer. When the phytoplankton die, their decomposition creates an extensive ~dead zone~ of low oxygen availability along the coast (Figure 55.18). Fish, shrimp, and other marine animals disappear from some of the most economically important waters in the country. To reduce the size ofthe dead zone, farmers have begun using fertilizers more efficiently, and managers are restoring wetlands in the Mississippi watershed, two changes stimulated by the results of ecosystem experiments. Nutrient runoffcan also lead to the eutrophication of lakes, as you learned in Concept 55.2. The bloom and subsequent die-off of algae and cyanobacteria and the ensuing depletion of oxygen are similar to what occurs in a marine dead zone. Such conditions threaten the survival of organisms. For example, eutrophication of Lake Erie coupled with overfishing wiped out commercially important fishes such as blue pike, whitefish, and lake trout by the 1960s. Since then, tighter regulations on waste dumping into the lake have enabled some fish populations to rebound, but many native species of fishes and invertebrates have not recovered.

Acid Precipitation The burning ofwood and of fossil fuels, including coal and oil, releases oxides of sulfur and nitrogen that react with water in the atmosphere, forming sulfuric and nitric acid, respectively.

The acids eventually fall to Earth's surface as acid precipitation-rain, snow, sleet, or fog that has a pH less than 5.2. Acid precipitation lowers the pH of streams and lakes and affects soil chemistry and nutrient availability. Although acid precipitation has been occurring since the Industrial Revolution, the emissions that cause it have increased during the past century, mainly from ore smelters and electrical generating plants. Acid precipitation is a regional problem arising from local emissions. Smelters and generating plants are built with exhaust stacks more than 300 m high that reduce pollution at ground level but export it far downwind. Sulfur and nitrogen pollutants may drift hundreds of kilometers before falling as acid precipitation. In the l%Os, ecologists determined that lake-dwelling organisms in eastern Canada were dying because ofair pollution from factories in the midwestern United States. Lakes and streams in southern Norway and Sweden were losing fish because of acid rain from pollutants generated in Great Britain and central Europe. By 1980, the pH of precipitation in large areas of North America and Europe averaged 4.0-4.5 and occasionally dropped as low as 3.0. In terrestrial ecosystems, such as the deciduous forests of New England, the change in soil pH due to acid precipitation causes calcium and other nutrients to leach from the soil (see the Hubbard Brook studies in Concept 55.4). The nutrient deficiencies affect the health of plants and limit their growth. Acid precipitation can also damage plants directly, mainly by leaching nutrients from leaves. Freshwater ecosystems are particularly sensitive to acid precipitation. The lakes in North America and northern Europe that are most readily damaged by acid precipitation are those that have a low concentration ofbicarbonate, an important buffer (see Chapter 3). Fish populations have declined in thousands of such lakes in Norway and Sweden, where the pH of the water has dropped below 5.0. In Canada, newly hatched lake trout, a keystone predator, die when the pH drops below 5.4. When the trout are replaced by acid-tolerant fish, the dynamics of food webs change dramatically. Several large ecosystem experiments have been carried out to test the feasibility ofreversing the effects ofacid precipitation. One is the ea2+ addition experiment at Hubbard Brook discussed earlier in this chapter. Another is a 17-year experiment CHAPTE~ flFTY路fIVE

Ecosystems

1237


4.5

4.4

• •

4.3 I

~

• •

4.2 4.1

40

1~196S1m197S1~Ol%51M01MS2~

Year

.... Figure 55.19 Changes in the pH of precipitation at Hubbard Brook. Although still very acidic. the precipitation in this northeastern U.S. forest has been increasing in pH for more than

three decades.

in Norway in which scientists built a glass roofover a forest and then showered the forest with precipitation from which acids had been removed. This "clean" precipitation quickly increased the pH and decreased the nitrate, ammonium, and sulfate concentrations in stream water in the forest Results from this and other experiments helped convince leaders of more than 40 European nations to sign a treaty to reduce air pollution. Environmental regulations and new industrial technologies have enabled many developed countries to reduce sulfur diox~ ide emissions during the past 40 years. In the United States, for example, sulfur dioxide emissions decreased 31% between 1993 and 2002. As a result, precipitation in the northeastern United States is gradually becoming less acidic (Figure 55.19). However, ecologists estimate that it will take decades for aquatic ecosystems in this region to recover, even ifsulfur dioxide emissions continue to decrease. Meanwhile, emissions of nitrogen oxides are increasing in the United States, and emissions of sulfur dioxide and acid precipitation continue to damage forests in central and eastern Europe.

Toxins in the Environment Humans release an immense variety of toxic chemicals, including thousands of synthetic compounds previously unknown in nature, with little regard for the ecological consequences. Organisms acquire toxic substances from the environment along with nutrients and water. Some ofthe poisons are metabolized and excreted, but others accumulate in specific tissues, especially fat. One of the reasons accumu~ lated toxins are particularly harmful is that they become more concentrated in successive trophic levels of a food web, a process called biological magnification. Magnification occurs because the biomass at any given trophic level is produced from a much larger biomass ingested from the level 1238

U"IT EIG~T

Ecology

below (see Concept 55.3). Thus, top-level carnivores tend to be the organisms most severely affected by toxic compounds in the environment. One class of industrially synthesized compounds that have demonstrated biological magnification are the chlorinated hydrocarbons, which include the industrial chemicals called PCBs (polychlorinated biphenyls) and many pesticides, such as DDT. Current research implicates many of these compounds in endocrine system disruption in a large number of animal species, including humans. Biological magnification of PCBs has been found in the food web of the Great Lakes, where the concentration of PCBs in herring gull eggs, at the top of the food web, is nearly 5,000 times that in phytoplank~ ton, at the base of the food web (Figure 55.20). An infamous case of biological magnification that harmed top-level carnivores involved DDT, a chemical used to control insects such as mosquitoes and agricultural pests. In the decade after World War II, the use afOOT grew rapidly; its ecological consequences were not yet fully understood. By the 1950s, scientists were learning that DDT persists in the environment and is transported by water to areas far from where it is applied. One of the first signs that DDT was a serious en-

Herring gull eggs 124 ppm

Smelt

?~ Zooplankton 0.123 ppm

Phytoplankton 0,025 ppm

... Figure 55.20 Biological magnification of PCBs in a Great Lakes food web.


vironmental problem was a decline in the populations of pelicans, ospreys, and eagles, birds that feed at the top of food webs. The accumulation of DDT (and DOE, a product of its breakdown) in the tissues of these birds interfered with the deposition of calcium in their eggshells. When the birds tried to incubate their eggs, the weight of the parents broke the shells ofaffected eggs, resulting in catastrophic declines in the birds' reproduction rates. Rachel Carson's book Silent Spring helped bring the problem to public attention in the 1960s (see Chapter 52), and DDT was banned in the United States in 1971. A dramatic recovery in populations of the affected bird species followed. In much of the tropics, DDT is still used to control the mosquitoes that spread malaria and other diseases. Societies there face a trade~off between saving human lives and protecting other species. The best approach seems to be to apply DDT sparingly and to couple its use with mosquito netting and other low-technology solutions. The complicated history of DDT illustrates the importance of understanding the ecological connections between diseases and communities (see Concept 54.5). Many toxins cannot be degraded by microorganisms and persist in the environment for years or even decades. In other cases, chemicals released into the environment may be relatively harmless but are converted to more toxic products by reaction with other substances, by exposure to light, or by the metabolism of microorgan390 isms. For example, mercury, a by~product ofplastic production and coal-fired power '80 generation, has been routinely expelled into rivers and the sea in an insoluble form. Bacteria in the bottom mud convert the waste to methylmercury (CH3 Hg+), an extremely toxic soluble c compound that accumulates in the tis-2 350 ~ sues of organisms, including humans c ~ who consume fish from the contamic '40 nated waters. 8

feet ecosystems. Although global warming will likely bring some benefits to people, it will also bring enormous costs to humans and to many other species on Earth.

Rising Atmospheric CO 2 Levels Since the Industrial Revolution, the concentration of CO 2 in the atmosphere has been increasing as a result of the burning offossil fuels and deforestation. Scientists estimate that the average CO 2 concentration in the atmosphere before 1850 was about 274 ppm. In 1958, a monitoring station began taking very accurate measurements on Hawaii's Mauna Loa peak, a location far from cities and high enough for the atmosphere to be well mixed. At that time, the CO 2 concentration was 316 ppm (Figure 55.21). Today, it exceeds 380 ppm, an increase of about 40% since the mid-19th century. If CO 2 emissions con· tinue to increase at the present rate, by the year 2075 the atmospheric concentration of this gas will be more than double what it was at the start of the Industrial Revolution. Increased productivity by plants is one predictable consequence of increasing CO 2 levels. In fact, when CO 2 concentrations are raised in experimental chambers such as greenhouses, most plants grow faster. Because C 3 plants are more limited than C4 plants by CO2 availability (see Chapter 10), one effect of

149 14.8 14.7 14.6

Temperature

G 145 '14,4

•,

:;

~ 14.3 E

14.2 14.1

6' u 330

"2 ~

~

• ~

14.0

Greenhouse Gases and

~

<

Global Warming

13.9

Human activities release a variety of gaseous \\'aste products. People once thought that the vast atmosphere could absorb these materials indefinitely, but we now know that such additions can cause fundamental changes to the atmosphere and to its interactions with the rest of the biosphere. In this section, we will examine how increasing atmospheric carbon dioxide concentration and global warming af-

13.8

'10

13.7 136 1960

1965

1970

1975

1980 1985 Year

1990

1995

2000

2005

... Figure 55.21 Increase in atmospheric carbon dioxide concentration at Mauna loa, Hawaii, and average global temperatures. Aside from normal seasonal fluctuations. the (0 2 concentration (blue curve) has increased steadily from 1958 to 2007. Though average global temperatures (red curve) fluctuated a great deal over the same period. there IS a clear warming trend.

CHAPTE~ flFTY·fIVE

Ecosystems

1239


increasing global CO 2 concentration may be the spread of C 3 species into terrestrial habitats that currently favor C4 plants. Such changes could influence whether corn (maize), a C4 plant and the most important grain crop in the United States, will be replaced by wheat and soybeans, C3 crops that could outproduel' corn in a CO 2-enriched environment. To predict the grad· ual and complex effects ofrising CO2 levels on productivity and species composition, scientists are turning to long-term field experiments.

How Eleyated CO 2 Leyels Affect Forest Ecology: The FACTS·' Experiment To assess how the increasing atmospheric concentration ofCO 2 might affect temperate forests, scientists at Duke University began the Forest·Atmosphere Carbon Transfer and Storage (FACTS-I) experiment in 1995. The researchers are manipulating the concentration of CO 2 to which trees are exposed. The FACTS-I experiment includes six plots in an SO-hectare (200acre) tract of loblolly pine within the university's experimental forest. Each plot consists of a circular area, approximately 30 m in diameter, ringed by 16 towers (Figure 55.22). In three of the six plots, the towers produce air containing about I~ times present-day CO 2 concentrations. Instruments on a tall tower in the center of each plot measure the direction and speed of the wind, adjusting the distribution of CO 2 to maintain a

.... Figure 55.22 Large.scale experiment on the effects of elevated Cal concentration. Rings of towers in the Duke University bperimental Forest emit enough carbon dioxide to raise and maintain CO 2 levels 200 ppm above present·day concentrations in half of the experimental plots. 1240

U"IT EIG~T

Ecology

stable CO 2 concentration. All other factors, such as temperature, precipitation, and wind speed and direction, vary normally for both experimental plots and adjacent control plots exposed to atmospheric CO 2 . The FACTS·I study is testing how elevated CO 2 levels influence tree growth, carbon concentration in soils, insect populations, soil moisture, the growth of plants in the forest understory, and other factors. After ten years, trees in the ex· perimental plots produced about 15% more wood each year than those in the control plots. This increased growth is im· portant for timber production and carbon storage but is far lower than predicted from the results of greenhouse experiments. The availability of nitrogen and other nutrients apparently limits the ability of the trees to use the extra CO 2 . Researchers at FACTS·l began removing this limitation in 2005 by fertilizing half of each plot with ammonium nitrate. In most of the world's ecosystems, nutrients limit ecosystem productivity and fertilizers are unavailable. The results of FACTS-I and other experiments suggest that increased at· mospheric CO 2 levels will increase plant production somewhat, but far less than scientists predicted even a decade ago.

The Greenhouse Effect and Climate Rising concentrations of long-lived greenhouse gases such as CO 2 are also changing Earth's heat budget. Much of the solar radiation that strikes the planet is reflected back into space. AI· though CO:b water vapor, and other greenhouse gases in the at· mosphere are transparent to visible light, they intercept and absorb much of the infrared radiation the Earth emits, rereflecting some of it back toward Earth. This process retains some ofthe solar heat.lfit were not for this greenhouse effect, the average air temperature at Earth's surface would be a frigid -18'C (-2.4'F), and most life as we know it could not exist. The marked increase in the concentration of atmospheric CO 2 over the last 150 years concerns scientists because of its link to increased global temperature. For more than a century, scientists have studied how greenhouse gases warm Earth and how fossil fuel burning could contribute to the warm· ing. Most scientists are convinced that such warming hasal· ready begun and will increase rapidly this century (see Figure 55.21). Global models predict that by the end of the 21st century, the atmospheric CO 2 concentration will more than double, increasing average global temperature by about 3'C (5'F). Supporting these models is a correlation between CO 2 levels and temperatures in prehistoric times. One way climatologists estimate past CO 2 concentrations is to measure CO 2 levels in bubbles trapped in glacial ice, some ofwhich are halfa million years old. Prehistoric temperatures are inferred by several methods, including analysis of past vegetation based on fossils and the chemical isotopes in sediments and corals. An in· crease ofonly L3'C would make the world warmer than at any time in the past 100,000 years.


The ecosystems where the largest warming has already occurred are those in the far north, particularly northern coniferous forests and tundra. As snow and ice melt and uncover darker, more absorptive surfaces, these systems reflect less radiation back to the atmosphere and warm further. Arctic sea ice in the summer of2oo7 covered the smallest area on record. Climate models suggest that there may be no summer ice there by the end of this century, decreasing habitat for polar bears, seals, and seabirds. Higher temperatures also increase the likelihood of fires. In boreal forests of western North America and Russia, fires have burned twice the usual area in recent decades. A warming trend would also alter the geographic distribution of precipitation, making major agricultural areas of the central United States much drier, for example. However, the various mathematical models disagree about the details of how climate in each region will be affected. By studying how past periods of global warming and cooling affected plant communities, ecologists are trying to predict the consequences of future temperature changes. Analysis of fossilized pollen indicates that plant communities change dramatically with changes in temperature. Past climate changes occurred gradually, though, and plant and animal populations had time to migrate into areas where abiotic conditions allowed them to survive. Many organisms, especially plants that cannot disperse rapidly over long distances, may not be able to survive the high rates of climate change projected to result from global warming. Furthermore, many habitats today are much more fragmented than they were in the past (see Chapter 56), further limiting the ability of many organisms to migrate. We will need many tools to slow global warming. Quick progress can be made in using energy more efficiently and in replacing fossil fuels with renewable solar and wind power and, more controversially, with nuclear power. Today, coal, gasoline, wood, and other organic fuels remain central to industrialized societies and cannot be burned without releasing CO2 , Stabilizing CO 2 emissions will require concerted international effort and the acceptance ofchanges in both personal lifestyles and industrial processes. Many e<ologists think that effort suffered a major setback in 200 1, when the United States pulled out of the Kyoto Protocol, a 1997 pledge by industrialized nations to reduce their CO 2 output by about 5%. Such a reduction would be a first step in the journey to stabilize atmospheric CO 2 concentrations.

Depletion of Atmospheric Ozone Life on Earth is protected from the damaging effects of ultraviolet (UV) radiation by a layer of ozone mole<ules (03 ) located in the stratosphere 17-25 km above Earth's surface. However, satellite studies of the atmosphere show that the ozone layer has been gradually thinning since the mid-1970s (Figure 55,23). The destruction of atmospheric ozone results mainly

3SO

"' 300 <:

~o o

~

"

250

~

~

200

"~

" 150 <1J

o

2 o

100

oX~~~~~~~~~ 1955 '60 '65 '70 '75 '80 '85 '90 '95 2000 '05 Year

... Figure 55.23 Thickness of the ozone layer over Antarctica in units called Dobsons.

o Chlorine from CFCs interacts with ozone (03), forming chlorine monoxide (CIO) and Chlorine atom

oxygen (Ol)'

•o

CIO

0,

Sunlight causes

CIO

ellO l to break

down into O2 and free chlorine atoms. The chlorine Sunlight atoms can begin the cycle again.

f) Two (10 molecules read. forming chlorine peroxide ((1 2°2)'

... Figure 55.24 How free chlorine in the atmosphere destroys ozone. from the accumulation of chlorofluorocarbons (CFCs), chemicals used in refrigeration and in manufacturing. When the breakdown products from these chemicals rise to the stratosphere, the chlorine they contain reacts with ozone, reducing it to mole<ular O 2 (figure 55.24). Subsequent chemical reactions liberate the chlorine, allowing it to react with other ozone molecules in a catalytic chain reaction. The thinning of the ozone layer is most apparent over Antarctica in spring, where cold, stable air allows the chain reaction to continue. The magnitude of ozone depletion and the size of the ozone hole have generally increased in recent years, and the hole sometimes extends as far as the southernmost portions ofAustralia, New Zealand, and South America CHAPTE~ flFTY·fIVE

Ecosystems

1241


(a)

September 1979

(b) September 2006

.... Figure 55.25 Erosion of Earth's ozone shield. The ozone hole over Antarctica is visible as the dark blue patch in these images based on atmospheric data.

(Figure 55.25). At the more heavilypopuJated middle latitudes, ozone levels have decreased 2-10% during the past 20 years. Decreased ozone levels in the stratosphere increase the intensity of UV rays reaching Earth's surface. The consequences of ozone depletion for life on Earth may be severe for plants, animals, and microorganisms. Some scientists expect increases in both lethal and nonlethal forms of skin cancer and in cataracts among humans, as well as unpredictable effects on crops and natural communities, especially the phytoplankton that are responsible for a large proportion of Earth's primary production. To study the consequences of ozone depletion, ecologists have conducted field experiments in which they use filters to decrease or block the UV radiation in sunlight. One such experiment, performed on a scrub ecosystem near the tip of South America, showed that when the ozone hole passed over the area, the amount of UV radiation reaching the ground increased sharply, causing more DNA damage in plants that were not protected by filters. Scientists have shown similar DNA damage and a reduction in phytoplank-

ton growth when the ozone hole opens over the Southern Ocean each year. The good news about the ozone hole is how quickly many countries have responded to it. Since 1987, approximately 190 nations, including the United States, have signed the Montreal Protocol, a treaty that regulates the use of ozone-depleting chemicals. Many nations, again including the United States, have ended the production of CFCs. As a consequence of these actions, chlorine concentrations in the stratosphere have stabilized and ozone depletion is slowing. Even if all CFCs were globally banned today, however, chlorine molecules that are already in the atmosphere would continue to influence stratospheric ozone levels for at least 50 years. The partial destruction of Earth's ozone shield is one more exampleofhow much humans have been able to disrupt the dynamics of ecosystems and the biosphere. It also highlights our ability to solve environmental problems when we set our minds to it. In this book's final chapter, we will explore how scientists in the fields ofconservation biology and restoration ecology are studying the effects of human activities on Earth's biodiversity and are using ecological knowledge to reduce those effects. CONCEPT

CHECK

55.5

1. How can the addition of excess nutrients to a lake threaten its fish population? 2. In the face of biological magnification of toxins, is it healthier to feed at a lower or higher trophic level? Explain. 3. MIUIIM There are vast stores oforganic matter in the soils of northern coniferous forests and tundra around the world. Based on what you learned about decomposition from Figure 55.15, suggest an explanation for why scientists who study global warming are closely monitoring these stores. For suggested answers. see Appendix A.

-MN·It·. Go to the Study Area at www.masteringbio.comlorBIOFlix 3-D Animations. MP3 Tuto~. Videos. Practke Tests, an e8ook. and more.

SUMMARY OF KEY CONCEPTS

• •,lllii'- 55.1

Physical laws govern energy now and chemical cycling in ecosystems (pp.1223-1224) .. Conservation of Energy An ecosystem consists of all the organisms in a community and all the abiotic factors with which

1242

U"IT EIG~T

Ecology

they interact. The laws of physics and chemistry apply to ecosystems, particularly in regard to the flow of energy. Energy is conserved but degraded to heat during ecosystem processes. .. Conservation of Mass Ecologists study how much of a chemical element enters and leaves an ecosystem and cycles within it. Inputs and outputs are generally small compared to recycled amounts, but their balance determines whether the ecosystem gains or loses an element over time.


_.,1:1..., - 55.4

... Energy, Mass, and Trophic levels

Biological and geochemical processes cycle nutrients between organic and inorganic parts of an ecosystem

(pp.1231-1236) ... Biogeochemical Cycles Organic matenals

Organic materials

ava,lable

unavaIlable

as nutrients

as nutrients Fossilization

"'........~t_...:':'~:;o Prod""":'":':"_t'~~

Key , • ChemiC<l1 cycling • Energyflow

~

He~t

_',Ii'''''_ 55.2 Energy and other limiting factors control primary production in ecosystems (pp. 1224-1228) ... Ecosystem Energy Budgets Primary production sets the spending limit for the global energy budget. Gross primary production is the total energy assimilated by an ecosystem in a given period. Net primary production, the energy accumu· lated in autotroph biomass, equals gross primary production minus the energy used by the primary producers for respiration. Only net primary production is available to consumers. ... Primary Production in Aquatic Ecosystems In marine and freshwater ecosystems, light and nutrients limit primary production. Within the photic zone, the factor that most often limits primary production is a nutrient such as nitrogen, phosphorus, or iron. ... Primary production in Terrestrial Ecosystems In terrestrial ecosystems, climatic factors such as temperature and moisture affect primary production on a large geographic scale. More locally, a soil nutrient is often the limiting factor in primary production.

-mit.• Innstillation How Do Temperature and Light Affect Primary Production?

_',Ii'''''_ 55.3

Living organisms, detritus Assimilation, photosynthesis

tI •

Respiration.,

Inorganic matenals

Inorganic materials Weathering. erosion

available

as nutrients Atmosphere, SOil, water

Formation of sedimentary rock

Graphlt! Animal Food Production Efficiency and Food Policy "11'3 Tutor Energy Flow in Ecosy1;tems

Minerals in rocks

... Case Study: Nulrient Cycling in the Hubbard Brook Experimenlal forest Nutrient cycling is strongly regulated by vegetation. The Hubbard Brook study showed that logging increases water runoff and can cause large losses of minerals. It also demonstrated the importance oflong-term ecological measurements in documenting the occurrence of and recov· ery from environmental problems.

-3!JIt··

_Mjlt.

... Decomposition and Nutrient Cycling Rates The propor· tion of a nutrient in a particular form and its cycling time in that form vary among ecosystems, largely because of differences in the rate of decomposition.

Acthity Energy Flow and Chemical Cycling

Acthity Pyramids of Production

unavaIlable

as nutrients

Water moves in a global cycle driven by solar energy. The car· bon cycle primarily reflects the reciprocal processes of photosynthesis and cellular respiration. Nitrogen enters ecosystems through atmospheric deposition and nitrogen fixation by prokaryotes, but most of the nitrogen cycling in natural ecosystems involves local cycles between organisms and soil or water. The phosphorus cycle is relatively localized.

Energy transfer between trophic levels is typically only

... The Green World Hypolhesis According to the green world hypothesis, herbivores consume only a small percentage of vegetation because predators, pathogens, competition, nutrient limitations, and other factors keep their populations in check.

Coal. oil, peat

~~r~~~~Sltion .

10% efficient (pp. 1228-1230) ... Production Efficiency The amount of energy available to each trophic level is determined by the net primary production and the efficiency with which food energy is converted to biomass at each link in the food chain. The percentage of energy transferred from one trophic level to the next, called trophic efficiency, is generally 5-20%, with 10% being the typical value. Pyramids of net production and biomass reflect low trophic efficiency.

Acthity The Carbon Cycle Acthity The Nitrogen Cycle

_i,IIli"_

55.5

Human activities now dominate most chemical cycles on Earth (pp. 1236-1242) ... Nutrient Enrichment Agriculture removes nutrients from ecosystems, so large supplements are usually required. The nutrients in fertilizer can pollute groundwater and surfacewater aquatic ecosystems, where they can stimulate excess algal growth (eutrophication). ... Acid Precipitation Burning of fossil fuels is the main cause of acid precipitation. North American and European ecosystems downwind from industrial regions have been damaged by rain and snow containing nitric acid and sulfuric acid. CHAPTE~ flFTY·fIVE

Ecosystems

1243


.. Toxins in the Environment Toxins can become concentrated in successive trophic levels of food webs. The release of toxic wastes has polluted the environment with harmful substances that often persist for long periods and become concentrated along the food chain by biological magnification. .. Greenhouse Gases and Global Warming Because of the burning of wood and fossil fuels and other human activities, the atmospheric concentration of CO 2 has been steadily increasing. The ultimate effects include significant global warming and other climate changes. .. Depletion of Atmospheric Ozone The ozone layer reduces the penetration of UV radiation through the atmosphere. Human activities. including release of chlorine-containing pollutants, are eroding the ozone layer. but government policies are helping to solve the problem.

-w".-

ACllvity Waler Pollution from Nitrales AClivity The Gr~nhouse Effect Graphlt! Almospherk CO, and Temperature Changes MP3 Tutor Global Warming

TESTING YOUR KNOWLEDGE

SELF-QUIZ I. Which of the following organisms is incorrectly paired with its trophic level? a. cyanobacterium-primary producer b. grasshopper-primary consumer c. zooplankton-primary producer d. eagle-tertiary consumer e. fungus-detritivore 2. Which of these ecosystems has the lowest net primary production per square meter? a. a salt marsh d. a grassland b. an open ocean e. a tropical rain forest c. a coral reef

c. deforestation increased water runoff. d. the nitrate concentration in waters draining the deforested area became dangerously high. e. calcium levels remained high in the soil of deforested areas. 6. \xrhich of the following is a consequence of biological magnification? a. Toxic chemicals in the environment pose greater risk to top-level predators than to primary consumers. b. Populations of top-level predators are generally smaller than populations of primary consumers. c. The biomass of producers in an ecosystem is generally higher than the biomass of primary consumers. d. Only a small portion of the energy captured by producers is transferred to consumers. e. The amount of biomass in the producer level of an ecosystem decreases if the producer turnover time increases. 7. The main cause of the increase in the amount of CO2 in Earth's atmosphere over the past ISO years is a. increased worldwide primary production. b. increased worldwide standing crop. c. an increase in the amount of infrared radiation absorbed by the atmosphere. d. the burning of larger amounts of wood and fossil fuels. e. additional respiration by the rapidly growing human population. 8. â&#x20AC;˘ ],'''''liM Using Figure 55.21 as a starting point, extend the x-axis to the year 2100. Then extend the CO 2 curve, assuming that the CO 2 concentration continues to rise as fast as it did from 1974 to 2007. What will be the approximate CO 2 concentration in 2100? What ecological factors and human decisions will influence the actual rise in CO 2 concentration? How might additional scientific data help societies predict this value? For Self-Quiz answers, see Appendix A.

3. Nitrifying bacteria participate in the nitrogen cycle mainly by a. converting nitrogen gas to ammonia. b. releasing ammonium from organic compounds, thus returning it to the soil. c. converting ammonia to nitrogen gas, which returns to the atmosphere. d. converting ammonium to nitrate, which plants absorb. e. incorporating nitrogen into amino acids and organic compounds. 4. Which of the following has the greatest effect on the rate of chemical cycling in an ecosystem? a. the ecosystem's rate of primary production b. the production efficiency of the ecosystem's consumers c. the rate of decomposition in the ecosystem d. the trophic emciency of the ecosystem e. the location of the nutrient reservoirs in the ecosystem 5. The Hubbard Brook watershed deforestation experiment yielded all of the following results except that a. most minerals were recycled within a forest ecosystem. b. the flow of minerals out of a natural watershed was offset by minerals flowing in. 1244

UNIT EIGHT

Ecology

-$1401',- Visit the Study Area at www.masteringbio.comfora Practice Test.

EVOLUTION CONNECTION 9. Some biologists have suggested that ecosystems are emergent, "living" systems capable of evolving. One manifestation ofthis idea is environmentalist James l.ovelock's Gaia hypothesis, which views Earth itself as a living, homeostatic entity-a kind of superorganism. Use the principles of evolution you have learned in this book to critique the idea that ecosystems and the biosphere can evolve. If ecosystems are capable of evolving, is this a form of Darwinian evolution? Why or why not?

SCIENTIFIC INQUIRY 10. Using two neighboring ponds in a forest as your study site, design a controlled experiment to measure the effect offa11ing leaves on net primary production in a pond. 8iologicalinquiry' A Workbook of [n,¡e.tigative Ca.u Explore how change. to the Che,apeake affect ,hellfi,hing with the case "Back to the Bay."


Can

Bioi Rest~~路

n

Ecol~1-' KEY

CONCEPTS

56.1 Human activities threaten Earth's biodiversity 56.2 Population conservation focuses on population size, genetic diversity, and critical habitat 56.3 landscape and regional conservation aim to sustain entire biotas 56.4 Restoration ecology attempts to restore degraded ecosystems to a morc natural state 56.5 Sustainable development seeks to improve the human condition while conserving biodiversity

rOI"j".".",. Striking Gold ucking its wings, a bird lands on a branch deep inside a tropical jungle. Sensing the motion, a conservation biologist scans the branch through binoculars, a glimpse of golden orange stopping her short. Staring back is a smoky honeyeater, a species that had never been described before {Figure 56.1). In 2005, a team of American, Indonesian, and Australian biologists experienced many moments like this as they spent a month cataloging the living riches hidden in a remote mountain range in Indonesia. In addition to the honeyeater, they discovered dozens of new frog, butterfly, and plant species, including five new palms. To date, scientists have described and formally named about 1.8 million species of organisms. Some biologists think that about 10 million more species currently exist; others estimate the number to be as high as 100 million. Some of the greatest concentrations ofspecies are found in the tropics. Unfortunately, tropical forests are being cleared at an alarming rate to make room for and support a burgeoning human population. Rates of deforestation in Indonesia are among the highest in the world (Figure 56.2). What will become of the smoky honeyeater and other newly discovered species in Indonesia if such deforestation continues unchecked?

T

... Figure 56.1 What will be the fate of this newly described bird species? Throughout the biosphere, human activities are altering trophic structures, energy flow, chemical cycling, and natural disturbance-ecosystem processes on which we and all other species depend (see Chapter 55). We have physically altered nearly half of Earth's land surface, and we use over half of all accessible surface fresh watet: In the oceans, stocks of most major fisheries are shrinking because of overharvesting. By some estimates, we are pushing more species toward extinction than the large asteroid or comet that triggered the mass extinctions at the close ofthe Cretaceous period 65.5 million years ago (see Figure 25.16). Biology is the science ofHfe. Thus, it is fitting that our final chapter focuses on two disciplines that seek to preserve life. Conservation biology integrates ecology, physiology, molecular biology, genetics, and evolutionary biology to conserve biological diversity at all levels. Efforts to sustain ecosystem processes and stem the loss ofbiodiversity also connect the life sciences with the social sciences, economics, and humanities.

... Figure 56.2 Tropical deforestation in West Kalimantan, an Indonesian province. 1245


Restoration ecology applies ecological principles to return ecosystems that have been disturbed by human activity to a condition as similar as possible to their natural state. In this chapter, we will take a closer look at the biodiversity crisis and examine some of the conservation and restoration strategies being adopted to slow the rate of species loss.

~:::~:~t~路i~ies threaten Earth's biodiversity

Extinction is a natural phenomenon that has been occurring since life first evolved; it is the rate of extinction that is responsible for today's biodiversity crisis (see Chapter 25). Because we can only estimate the number of species currently existing, we cannot determine the exact rate ofspecies loss. However, wedo know for certain that the extinction rate is high and that human activities threaten Earth's biodiversity at all levels.

Three levels of Biodiversity Biodiversity-short for biological diversity-can be considered at three main levels: genetic diversity, species diversity, and ecosystem diversity (Figure 56.3).

Genetic Diversity Genetic diversity comprises not only the individual genetic variation within a population, but also the genetic variation between populations that is often associated with adaptations to local conditions (see Chapter 23). Ifone population becomes extinct, then a species may have lost some of the genetic diversity that makes microevolution possible. This erosion of genetic diversity in turn reduces the adaptive prospects ofthe species. The loss of genetic diversity throughout the biosphere also affects human welfare. If we lose ",~ld populations of plants closely related to agricultural species, we lose genetic resources that could be used to improve crop qualities, such as disease reo sistance, through plant breeding. For example, plant breeders responded to devastating outbreaks ofthe grassy stunt virus in rice (Oryza sativa) by screening 7,000 populations of this species and its dose relatives for resistance to the virus. One population of a single relative, Indian rice (Oryza nivara), demonstrated resistance to the virus, and scientists succeeded in breeding the resistant trait into commercial rice varieties. Today, the original disease-resistant population has apparently become extinct in the wild.

Species Diversity Public awareness of the biodiversity crisis centers on species diversity-the variety of species in an ecosystem or throughout the biosphere (see Chapter 54). As more species are lost to extinction, species diversity decreases. The U.S. Endangered 12%

U"IT EIG~T

Ecology

Community and ecosystem diversity across the landscape of an entire region

... Figure 56.3 Three levels of biodiversity. The oversized chromosomes in the top diagram symbolize the genetic variation within the population.

Species Act (ESA) defines an endangered species as one that is "in danger of extinction throughout all or a significant portion of its range:' Also defined for protection by the ESA, threatened species are those that are considered likely to become endangered in the foreseeable future. The following are just a few statistics that illustrate the problem of species loss: ~

According to the International Union for Conservation of Nature and Natural Resources (IUCN), 12% of the nearly 10,000 known species of birds and at least 20% of the nearly 5,000 known species of mammals are threatened. ~ A survey by the Center for Plant Conservation showed that of the nearly 20,000 known plant species in the United States, 200 have become extinct since such records have been kept, and 730 are endangered or threatened.


.. About 20% of the known species of freshwater fishes in the world have either become extinct during historical times or are seriously threatened. In North America, 123 freshwater animal species have become extinct since 1900, and hundreds more species are threatened. The extinction rate for North American freshwater fauna is about five times as high as that for terrestrial animals. ... According to a 2004 report in the journal Science that was based on a global assessment of amphibians by more than 500 scientists, 32% of all known amphibian species are either very near extinction or endangered. Extinction of species may be local; for example, a species may be lost in one river system but survive in an adjacent one. Global extinction of a species means that it is lost from all the ecosystems in which it lived, leaving them permanently impoverished (Figure 56.4). (al Philippine eagle

Ecosystem Diversity The variety of the biosphere's ecosystems is a third level ofbiological diversity. Because of the network of community interactions among populations of different species within an ecosystem, the local extinction of one species can have a negative impact on the overall species richness of the community n (see Figure 54.15). For instance, bats called ~f1ying foxes are important pollinators and seed dispersers in the Pacific Islands, where they have been subject to increasing pressure from hunters, who sell them as luxury foods (Figure 56.5). Conservation biologists fear that the extinction of flying foxes would also harm the native plants of the Samoan islands, where more than 79% of the trees depend on flying foxes for pollination or seed dispersal. Some ecosystems have already been heavily impacted by humans, and others are being altered at a rapid pace. For example, since European colonization, more than 50% of wetlands in the contiguous United States have been drained and converted to other ecosystems, primarily agricultural ones. In California, Arizona, and New Mexico, approximately 90% of native riparian communities have been affected by overgrazing, flood control, water diversions, lowering of water tables, and invasion by non-native plants.

Biodiversity and Human Welfare \Vhy should we care about the loss of biodiversity? Perhaps the purest reason is what E. O. Wilson calls bivphilia, our sense of connection to nature and other forms of life. The belief that other species are entitled to life is a pervasive theme of many religions and the basis ofa moral argument that we should protect biodiversity. There is also a concern for future human generations: Is it fair to deprive them of Earth's species richness? Paraphrasing an old proverb, G. H. Brundtland, a former prime minister ofNorway, said: "We must consider our planet to be on

... Figure 56.4 A hundred heartbeats from extinction. These are just three of the members of what Harvard biologist E. O. Wilson grimly calls the Hundred Heartbeat Club. species with fewer than 100 individuals remaining on Earth. The Yangtze River dolphin has not been seen since 2004 and may already be eKlinct. To document thaI a species has actually become extincl, what spatial and temporal factors would you need to consider)

D

... Figure 56.5 The endangered Marianas "flying fox" bat (Pteropus mariannus), an important pollinator.

CHAPTER FlfTY路SIX

Conservation Biology and Restoration Ecology

1247


loan from our children, rather than being a gift from our ancestors:' In addition to such philosophical and moral justifications, species and genetic diversity bring us many practical benefits.

Benefits of Species and Genetic Diversity Many species that are threatened could potentially provide crops, fibers, and medicines for human use, making biodiversity a crucial natural resource. In the United States, about 25% ofthe prescriptions dispensed from pharmacies contain substances originally derived from plants. In the 1970s, researchers discovered that the rosy periwinkle, which grows on the island of Madagascar, off the coast of Africa, contains alkaloids that in路 hibit cancer cell growth (Figure 56.6). This discovery led to treatments for rn'o deadly forms of cancer, Hodgkin's disease and a form of childhood leukemia, resulting in remission in most cases. Madagascar is also home to five other species of periwinkles, one of which is approaching extinction. The loss of these species would mean the loss of any possible medicinal benefits they might offer. Each loss of a species means the loss of unique genes, some of which may code for enormously useful proteins. Consider the example of Taq polymerase, a DNA polymerase first extracted from the bacterium Thermus aquaticus in hot springs at Yellowstone National Park. This enzyme is an essential part of the polymerase chain reaction (PCR) because it is stable at the high temperatures required for PCR (see Figure 20.8). DNA from many other species of prokaryotes in a variety of environments is used in the mass production of proteins for new medicines, foods, petroleum substitutes, industrial chemicals, and other products. However, because millions of species may become extinct before we even know about them, we stand to lose irretrievably the valuable genetic potential held in their unique libraries of genes.

Ecosystem Services The benefits that individual species provide to humans are often substantial, but saving individual species is only part ofthe

(catharanthus

rose us), a plant that saves lives.

UNIT EIGHT

Three Threats to Biodiversity Many different human activities threaten biodiversity on local, regional, and global scales. The threats posed by these activities are of three major types: habitat loss, introduced species, and overexploitation.

Habitat Loss Human alteration ofhabitat is the single greatest threat to biodiversity throughout the biosphere. Habitat loss has been brought about by agriculture, urban development, forestry, mining, and pollution. Global warming is already altering habitats today and will have an even larger effect later this century (see Chapter 55). When no alternative habitat is available or a species is unable to move, habitat loss may mean extinction. The IUCN implicates destruction of physical habitat for 73% of the species that have become extinct, endangered, vulnerable, or rare in the last few hundred years. Habitat loss and fragmentation may occur over immense regions. For instance, approximately 98% of the tropical dry forests ofCentral America and Mexico have been cleared (cut down). Clearing of tropical rain forest in the state of Veracruz,

,. Figure 56.6 The rosy periwinkle

1248

rationale for saving ecosystems. Humans evolved in Earth's ecosystems, and we rely on these systems and their inhabitants for our survival. Ecosystem services encompass all the processes through which natural ecosystems help sustain human life on Earth. Ecosystems purify our air and water. They detoxify and decompose our wastes and reduce the impacts of extreme weather and flooding. The organisms in ecosystems pollinate our crops, control pests, and create and preserve our soils. Moreover, ecosystems provide all these services and countless others for free. Perhaps because we don't attach a monetary value to the services ofnatural ecosystems, we generally undervalue them. In a controversial 1997 article, ecologist Robert Costanza and his colleagues estimated the value of Earth's ecosystem services at $33 trillion per year, nearly twice the gross national product of all the countries on Earth at that time ($18 trillion). It may be more realistic, and more meaningful, to do the accounting on a smaller scale. In 1996, New York City invested more than $1 billion to buy land and restore habitat in the Catskill Mountains, the source of much of the city's fresh water. This investment was spurred by increasing pollution ofthe water by sewage, pesticides, and fertilizers. By harnessing ecosystem services to purify its water naturally, the city saved $8 billion it would have otherwise spent to build a new filtration plant and $300 million a year to run the plant. There is growing evidence that the functioning of ecosystems, and hence their capacity to perform services, is linked to biodiversity. As human activities reduce biodiversity, we are reducing the capacity of the planet's ecosystems to perform processes critical to our own survival.

Ecology


Introduced Species

... Figure 56.7 Habitat fragmentation in the foothills of Los Angeles. Development in the valleys may confine the organisms that inhabit the narrow strips of hillside.

Introduced species, also called non-native or exotic species, are those that humans move, either intentionally or accidentally, from the species' native locations to new geographic regions. Rapid human travel by ship and airplane has accelerated the transplant of species. Free from the predators, parasites, and pathogens that limit their populations in their native habitats, such transplanted species may spread very rapidly through a new region. Some introduced species that gain a foothold disrupt their adopted community, often by preying on native organisms or outcompeting them for resources. The brown tree snake was accidentally introduced to the island of Guam as a "stowaway" in military cargo after World War II (Figure 56.Sa). Since then, 12 species of birds and 6 species oflizards on which the snakes prey have become extinct on Guam. The devastating zebra mussel was introduced into the Great Lakes of North America in 1988, most likely in the ballast water of ships arriving from Europe. Efficient suspension-feeding molluscs that form dense colonies, zebra mussels have extensively disrupted freshwater ecosystems, threatening native aquatic species. Zebra mussels have also dogged water intake structures, disrupting domestic and industrial water supplies and causing billions of dollars in damage. Humans have deliberately introduced many species with good intentions but disastrous effects. For example, an Asian plant called kudzu, which the U.S. Department of Agriculture once introduced in the southern United States to help control erosion, has taken over large areas of the landscape there (Figure 56.8b). The European starling, brought intentionally into New York's Central Park in 1890 by a citizens' group intent on introducing all the plants and animals mentioned in Shakespeare's plays, quickly spread across North America, increasing to a population of more than 100 million and displacing many native songbirds. Introduced species are a worldwide problem, contributing to approximately 40% of the extinctions recorded since 1750

Mexico, mostly for cattle ranching, has resulted in the loss of approximately 91% ofthe original forest, leaving a fragmented archipelago ofsmall forest islands. Other natural habitats have also been fragmented by human activities (figure 56.7). In almost all cases, habitat fragmentation leads to species toss, since the smaller populations in habitat fragments have a higher probability of local extinction. The prairies of North America are an example: Prairie covered about 800,000 hectares of southern \Visconsin when Europeans first arrived, but now occupies less than 0.1% of its original area. Plant diversity surveys of 54 \Visconsin prairie remnants were conducted in 1948-1954 and then repeated in 1987-1988. During the three decades between the surveys, the various prairie fragments lost between 8% and 60% of their plant species. Though most studies have focused on terrestrial ecosystems, habitat loss is also a major threat to aquatic biodiversity, especially along continental coasts and around coral reefs. About 93% of coral reefs, among Earth's most species-rich aquatic communities, have been damaged by human activities. At the current rate of destruction, 40-50% of the reefs, home to one-third ofmarine fish species, could disappear in the next 30 to 40 years. Freshwater habitats are also being lost, often as a result of the dams, reservoirs, channel modification, and flow regulation now affecting most of the world's rivers. For example, the more than 30 dams and locks built along the Mobile River basin, in the southeastern United States, changed (a) Brown tree snake, Introduced to (b) Introduced kudzu thriving in South Carolina river depth and flow and thereby helped Guam in cargo drive more than 40 species of endemic mussels and snails to extinction. ... figure 56.8 Two introduced species. CHAPTER FlfTY路SIX

Conservation Biology and Restoration Ecology

1249


and costing billions of dollars annually in damage and control efforts. There are more than 50,000 introduced species in the United States alone.

OverexpJoitation The term overexploitntion refers generally to the human harvesting ofwild organisms at rates exceeding the ability ofpopulations of those species to rebound. Species with restricted habitats, such as small islands, are particularly vulnerable to overexploitation. One such species was the great auk, a large, flightless seabird found on islands in the North Atlantic Ocean. By the 1840s, humans had hunted the great auk to extinction to satisfy demand for its feathers, eggs, and meat. Also susceptible to overexploitation are large organisms with low intrinsic reproductive rates, such as elephants, whales, and rhinoceroses. The decline of Earth's largest extant terrestrial animals, the African elephants, is a classic example of the impact of overhunting. Largely because of the trade in ivory, elephant populations have been declining in most of Africa during the last 50 years. An international ban on the sale of new ivory resulted in increased poaching (illegal hunting), so the ban had little effect in much of central and eastern Africa. Only in South Africa, where once-decimated herds have been well protected for nearly a century, have elephant populations been stable or increasing (see Chapter 53). Conservation biologists increasingly use the tools of molecular genetics to track the origins of tissues harvested from threatened and endangered species. For instance, Samuel Wasser and colleagues, at the University of Washington, created a DNA reference map for the African elephant using DNA isolated from elephant dung. By comparing this reference map with DNA isolated from a small sample ofivory harvested either legally or by poachers, they can determine where the elephant was killed to within a few hundred kilometers. Similarly, biologists using phylogenetic analyses of mitochondrial DNA (mtDNA) showed that some whale meat sold in Japanese fish markets came from illegally harvested species, including fin and humpback whales, which are endangered (see Figure 26.6). Many populations of commercially important marine fishes, once thought to be inexhaustible, have been dramatically reduced by overfishing. The exploding human population's increasing demand for protein, coupled with new harvesting technologies, such as long-line fishing and modern trawlers, have reduced these fish populations to levels that cannot sustain further exploitation. The fate of the North Atlantic bluefin tuna is just one example. Until the past few decades, this big tuna was considered a sport fish of little commercial value-just a few cents per pound for cat food. Then, in the 1980s, wholesalers began airfreighting fresh, iced bluefin to Japan for sushi and sashimi.ln that market, the fish now brings up to $100 per pound (Figure 56.9). With the in1250

U"IT EIG~T

Ecology

.. Figure 56.9 Overexploitation. North Atlantic blue/in tuna are auctioned In a Japanese fish market.

creased harvesting spurred by such high prices, it took just ten years to reduce the western North Atlantic bluefin population to less than 20% of its 1980 size. The collapse of the northern cod fishery off Newfoundland in the 19905 is a more recent example of how it is possible to overharvest what was formerly a very common species. CONCEPT

CHECk

56.1

1. Explain why it is too narrow to define the biodiversity crisis as simply a loss of species. 2. Identify the three main threats to biodiversity and explain how each damages diversity. Imagine two populations of a fish 3. species, one in the Mediterranean Sea and one in the Caribbean Sea. Now imagine two scenarios: (1) The populations breed separately, and (2) adults of both populations migrate to the North Atlantic to interbreed. Which scenario would result in a greater loss of genetic diversity if the Mediterranean population were harvested to extinction? Explain your answer.

N'mu'l.

For suggested answers, see Appendix A.

r;:;:~:~o~~~~servation focuses on population size, genetic diversity, and critical habitat

Biologists focusing on conservation at the population and species levels follow m'o main approaches: the small-population approach and the declining-population approach.


Small-Population Approach A species is designated as endangered when its populations are very small. Small populations are particularly vulnerable to

overexploitation, habitat loss, and the other threats to biodiversity that you read about in Concept 56.1. After such factors have taken their toll on population size, a population's smallness itself can drive it to extinction. Conservation biologists who adopt the small-population approach study the processes that cause extinctions once population sizes have been severely reduced.

The Extinction Vortex A small population is prone to positive-feedback loops of inbreeding and genetic drift that draw the population down an extinction vortex toward smaller and smaller population size until no individuals exist (Figure 56.10). One key factor driving the extinction vortex is the loss of the genetic variation necessary to enable evolutionary responses to environmental change, such as the appearance of new strains of pathogens. Both inbreeding and genetic drift can cause a loss of genetic variation (see Chapter 23), and the effects of both processes become more significant as a population shrinks. Inbreeding often reduces fitness because offspring are more likely to be homozygous for harmful recessive traits. Not all small populations are doomed by low genetic diversity, and low genetic variability does not automatically lead to permanently small populations. For instance, over-

Small population

hunting of northern elephant seals in the 1890s reduced the species to only 20 individuals-clearly a bottleneck with reduced genetic variation. Since that time, however, the northern elephant seal populations have rebounded to about 150,000 individuals today, though their genetic variation remains relatively low. Furthermore, a number of plant species seem to have inherently low genetic variability. For example, many populations of cord grass (Spartina anglica), which thrives in salt marshes, are genetically uniform at many loci. S. anglica arose from a few parent plants only about a century ago by hybridization and allopolyploidy (see Figure 24.11). Having spread by doning, this species now dominates large areas of tidal mudflats in Europe and Asia. Thus, in rare cases, low genetic diversity has not impeded population growth.

Case Study: The Greater Prairie Chicken and the Extinction Vortex When Europeans arrived in North America, the greater prairie chicken (Tympanuchus cupido) was common from New England to Virginia and all across the western prairies of the continent. As you read in Chapter 23, land cultivation for agriculture fragmented the populations of this species, and its abundance decreased drastically. In Illinois, there were millions of greater prairie chickens in the 19th century, but fewer than 50 were left by 1993. Researchers found that the decline in the Illinois population was associated with a decrease in fertility. As a test of the extinction vortex hypothesis, the scientists imported genetic variation by transplanting 271 birds from larger populations elsewhere (figure 56.11, on the next page). TIle Illinois population rebounded, confirming that it had been on its way down the extinction vortex until rescued by the transfusion of genetic variation.

Minimum Viable Population Size

Reduction in individual fitness and population adaptability

... Figure 56.10 Processes culminating in an extinction vortex.

How small does a population have to be before it starts down an extinction vortex? The answer depends on the type of organism and other factors. For example, large predators that feed high on the food chain usually require very large individ路 ual ranges, resulting in very low population densities. Therefore, not all rare species concern conservation biologists. All populations, however, require some minimum size in order to remain viable. TIle minimal population size at which a species is able to sustain its numbers and survive is known as the minimum viable population (MVP). MVP is usually estimated for a given species using computer models that integrate many factors. The calculation may include, for example, an estimate of how many individuals in a small population are likely to be killed by some natural catastrophe such as a storm. Once in the extinction vortex, two or three years in a row ofbad weather could finish offa population that is already below MVP.

CHAPTER FlfTY路SIX

Conservation Biology and Restoration Ecology

1251


Effective Population Size

What caused the drastic decline of the Illinois greater prairie chicken population? EXPERIMENT

Researchers had observed that the population collapse of the greater prairie chicken was mirrored in a redudion in fertility, as measured by the hatching rate of eggs. Comparison of DNA samples lrom the Jasper County, Illinois, population with DNA from leathers in museum specimens showed that genetic variation had declined in the study population (see Figure 23,10), In 1992. Ronald Westemeier, Jeffrey Brawn. and colleagues began transplanting prairie chickens from Minnesota, Kansas, and Nebraska in an attempt to increase genetic variation,

Genetic variation is the key issue in the small-population approach. The total size of a population may be misleading because only certain members of the population breed successfully and pass their alleles on to offspring. Therefore, a meaningful estimate ofMVP requires the researcher to determine the effective population size, which is based on the breeding potential of the population. The following formula incorporates the sex ratio of breeding individuals into the estimate of effective population size, abbreviated Ne ;

RESULTS Alter translocation (blue arrow), the viability of eggs rapidly Increased. and the population rebounded.

l'

:0 150

"• E

'0 100

•,

Translocation

~

E

50 0 1970

1975

1980

1985 Year

1990

1995

(a) Population dynamks

100 90

'{;

80

~

~

~

~

~ ~

w

70 60

50 40 30

1970-'74 '75-'79 '80-'84 '85-'89 Years (b) Hatching rate

'90

'93-'97

CONCLUSION Reduced genetic variation had started the Jasper County population of prairie chickens down the extlndion vortex. SOURCE

R, L Westeme,er et al. Trackln9the long·term ded,ne and rl!CCM!ry of an isolated population, SciMc" 282:1695-1698 (1998),

InqUiry ActiOfl Read and analyze the original paper in Inquiry in Action' Interpreting Scientific Papers

Mlil:f.\lljI Given the success of using transplanted birds as a tool lor increasing the percentage 01 hatched eggs in Illinois, why wouldn't you transplant additional birds immediately to Illinois)

1252

e

N.t

200

z

N = 4NrN",

U"IT EIG~T

Ecology

~+N",

where and N", are, respectively, the number of females and the number of males that successfully breed. If we apply this formula to an idealized population whose total size is 1,000 individuals, Ne will also be 1,000 if every individual breeds and the sex ratio is 500 females to 500 males. In this case, Ne = (4 X 500 X 500)/(500 + 500) = 1,000. Any deviation from these conditions (not all individuals breed or there is not a 1;1 sex ratio) reducesNe. For instance, if the total population size is 1,000 but only 400 females and 400 males breed, then Ne = (4 X 400 X 400)/(400 + 400) = 800, or 80% of the total population size. Numerous life history traits can influence Ne, and alternative formulas for estimating Ne take into account family size, age at maturation, genetic relatedness among population members, the effects of gene flow between geographically separated populations, and popula· tion fluctuations. In actual study populations, Ne is always some fraction of the total population. Thus, simply determining the total num· ber of individuals in a small population does not provide a good measure of whether the population is large enough to avoid extinction. Whenever possible, conservation programs attempt to sustain total population sizes that include at least the minimum viable number of reproductively active individuals. The conservation goal of sustaining effective population size (Ne) above MVP stems from the concern that populations retain enough genetic diversity to adapt as their environment changes. The MVP of a population is often used in population via· bility analysis. The objective ofthis analysis is to predict a pop· ulation's chances for survival, usually expressed as a specific probability of survival (for example, a 95% chance) over a particular time interval (for instance, 100 years). Such modeling approaches allow conservation biologists to explore the potential consequences of alternative management plans. Because modeling depends on reliable information about the populations under study, conservation biology is most robust when theoretical modeling is combined with field studies of the managed populations.


Case Study: Analysis of Grizzly Bear Populations One of the first population viability analyses was conducted in 1978 by Mark Shaffer, of Duke University, as part of a longterm study of grizzly bears in Yellowstone National Park and its surrounding areas (Figure 56.12). A threatened species in the United States, the grizzly bear (Ursus arctos horribilis) is currently found in only 4 of the 48 contiguous states. Its populations in those states have been drastically reduced and fragmented: In 1800, an estimated 100,000 grizzlies ranged over about 500 million hectares of mostly continuous habitat, while today only about 1,000 individuals in six relatively isolated populations range over less than 5 million hectares. Shaffer attempted to determine viable sizes for the Yellowstone grizzly populations. Using life history data obtained for individual Yellowstone bears over a 12-year period, he simulated the effects of environmental factors on survival and reproduction. His models predicted that, given a suitable habitat, a Yellowstone grizzly bear population of70 to 90 individuals would have about a 95% chance of surviving for 100 years, whereas a population of 100 bears would have a 95% chance of surviving for twice as long, about 200 years. How does the actual size of the Yellowstone grizzly population compare with Shaffer's estimates of MVP? A current estimate puts the total grizzly bear population in the greater Yellowstone ecosystem at about 400 individuals. The relationship ofthis estimate to the effective population size, Nt:' depends on several factors. Usually, only a few dominant males breed, and it may be difficult for them to locate females, since individuals inhabit such extensive areas. Moreover, females may reproduce only when there is abundant food. As a result, N e is only about 25% of the total population size, or about 100 bears. Because small populations tend to lose genetic variation over time, a number of research teams have analyzed proteins,

mtDNA, and short tandem repeats (see Chapter 21) to assess genetic variability in the Yellowstone grizzly bear population. All results to date indicate that the Yellowstone population has less genetic variability than other grizzly bear populations in North America. However, the isolation and decline in genetic variability in the Yellowstone grizzly bear population were gradual during the 20th century and not as severe as feared: Museum specimens collected in the early 1900s demonstrate that genetic variability among the Yellowstone grizzly bears was low even then. How might conservation biologists increase the effective size and genetic variation ofthe Yellowstone grizzly bear population? Migration between isolated populations of grizzlies could increase both effective and total population sizes. Computer models predict that introducing only two unre路 lated bears each decade into a population of 100 individuals would reduce the loss of genetic variation by about half. For the grizzly bear, and probably for many other species whose populations are very small, finding ways to promote dispersal among populations may be one of the most urgent conservation needs. This case study and that of the greater prairie chicken bridge small-population models to practical applications in conservation. Next, we look at an alternative approach to understanding the biology of extinction.

Declining-Population Approach The declining-population approach focuses on threatened and endangered populations that show a downward trend, even if the population is far above MVP. The distinction between a declining population (which is not always small) and a small population (which is not always declining) is less important than the different priorities of the two basic conser路 vation approaches. The small-population approach emphasizes smallness itselfas an ultimate cause ofa population's extinction, especially through loss ofgenetic diversity. In contrast, the declining-population approach emphasizes the environmental factors that caused a population decline in the first place. If, for example, an area is deforested, then species that depend on trees will decline in number and become locally extinct, whether or not they retain genetic variation.

Steps for Analysis and Intervention

... Figure 56.12 Long-term monitoring of a grizzly bear population. The ecologist is fitting this tranquilized bear with a radio collar so that the bear's movements can be compared with those of other individuals in the Yellowstone National Park population.

The declining-population approach requires that population declines be evaluated on a case-by-case basis, with researchers carefully dissecting the causes of a decline before taking steps to correct it. If, for example, the biological magnification of a toxic pollutant is harming some top-level consumer such as a predatory bird (see Chapter 55), then managers need to reduce or eliminate the pollutant in the environment to restore vulnerable populations of the bird. Although most situations are

CHAPTER FlfTY路SIX

Conservation Biology and Restoration Ecology

1253


more complex, we can use the following steps for analyzing declining populations: I. Confirm, using population data, that the species is presently in decline or that it was formerly more widely distributed or more abundant. 2. Study the natural history of this and related species, including reviewing the research literature, to determine the species' environmental requirements. 3. Develop hypotheses for all possible causes ofthe decline, including human activities and natural events, and list the predictions of each hypothesis. 4. Because many factors may be correlated with the decline, test the most likely hypothesis first. For example, remove the suspected agent of decline to see if the experimental population rebounds compared to a control population. 5. Apply the results of the diagnosis to manage the threatened species and monitor recovery.

The following case study is an example of how the decliningpopulation approach has been applied in recent years to one endangered species.

Case Study: Decline of the Red-Cockaded Woodpecker The red-cockaded woodpecker (Picoides borealis) is an endangered species endemic to the southeastern United States. This species requires mature pine forests, preferably ones dominated by the longleaf pine, for its habitat. Most woodpeckers nest in dead trees, but the red-cockaded woodpecker

(a) Forests that can sustain red-cockaded woodpeckers ha~e

low undergrowth.

drills its nest holes in mature, living pine trees. Red-cockaded woodpeckers also drill small holes around the entrance to their nest cavity, which causes resin from the tree to ooze down the trunk. The resin seems to repel certain predators, such as corn snakes, that eat bird eggs and nestlings. Another critical habitat factor for this woodpecker species is that the understory of plants around the pine trunks must be low (Figure 56.13a). Breeding birds tend to abandon nests when vegetation among the pines is thick and higher than about 4.5 m (Figure 56.Bb). Apparently, the birds require a clear flight path between their home trees and the neighboring feeding grounds. Periodic fires have historically swept through longleaf pine forests, keeping the undergrowth low. One factor leading to decline of the red-cockaded woodpeder is the destruction or fragmentation of suitable habitats by logging and agriculture. By recognizing key habitat factors, protecting some longleaf pine forests, and using controlled fires to reduce forest undergrowth, conservation managers have helped restore habitat that can support viable populations. However, designing a recovery program was complicated by the birds' social organization. Red-cockaded woodpeckers live in groups ofone breeding pair and up to four uhelpers,~ mostly males (an example of altruism; see Chapter 51). Helpers are offspring that do not disperse and breed but remain behind to help incubate eggs and feed nestlings. They may eventually attain breeding status within the flock when older birds die, but the wait may take years, and even then, helpers must compete to breed. Young birds that do disperse as members of new groups also have a tough path to reproductive success. New groups usually occupy abandoned territories or start at a new site and excavate nesting cavities, which

(b) Forests that cannot sustain red-cockaded woodpeders have high.

dense undergrowth that impacts the woodpeckers' access to feeding grounds.

.... Figure 56.13 Habitat requirements of the red-cockaded woodpecker. How is habitat disturbance absolutely necessary for the long-term survival of the woodpecker?

D

1254

U"IT EIG~T

Ecology


can take several years. Individuals generally have a better chance of reproducing by remaining behind than by dispersing and expending the effort to excavate homes in new territories. To test the hypothesis that this social behavior contributes to the decline of the reckockaded woodpecker, Carole Copeyon, Jeffrey Walters, and Jay Carter, of North Carolina State University, constructed cavities in pine trees at 20 sites. The results were dramatic: Cavities in 18 of the 20 sites were colonized by red-cockaded woodpeckers, and new breeding groups formed only in these sites. The experiment supported the hypothesis that this woodpecker species had been leaving much suitable habitat unoccupied because ofa lack ofbreeding cavities. Based on this experiment, conservationists initiated a habitat maintenance program that included controlled burning and excavation of new breeding cavities, enabling this endangered species to begin to recover.

Weighing Conflicting Demands Determining population numbers and habitat needs is only part of the effort to save species. Scientists also need to weigh a species' biological and ecological needs against other conflicting demands. Conservation biology often highlights the relationship between science, technology, and society. For example, an ongoing, sometimes bitter debate in the U.S. Pacific Northwest pits habitat preservation for northern spotted owl, timber wolf, grizzly bear, and bull trout populations against job opportunities in the timber, mining, and other resource extraction industries. Programs to restock wolves in Yellowstone National Park were opposed by some recreationists concerned for human safety and by many ranchers concerned with potentialloss of livestock. Large, high-profile vertebrates are not always the focal point in such conflicts, but habitat use is almost always the issue. Should work proceed on a new highway bridge if it destroys the only remaining habitat of a species of freshwater mussel? If you were the owner of a coffee plantation growing varieties that thrive in bright sunlight, would you be willing to change to shade-tolerant varieties that produce less coffee per area but can grow beneath trees that support large numbers of songbirds? Another important consideration is the ecological role of a species. Because we will not be able to save every endangered species, we must determine which species are most important for conserving biodiversity as a whole. Identifying keystone species and finding ways to sustain their populations can be central to maintaining communities and ecosystems. Management aimed at conserving a single species carries with it the possibility of negatively affecting populations of other species. For example, management of open pine forests for the red-cockaded woodpecker might impact migratory birds that use later-successional broadleaf forests. To test for such impacts, ecologists compared bird communities near

clusters of nest cavities in managed pine forests with communities in forests not managed for the woodpeckers. Contrary to expectations, the managed sites supported higher numbers and a higher diversity of other birds than the control forests. In this case, managing for one bird species increased the diversity of an entire community of birds. In most situations, conservation must look beyond single species and consider the whole community and ecosystem as an important unit of biodiversity. CONCEPT

CHECK

56.2

1. Why does the reduced genetic diversity of small populations make them more vulnerable to extinction? 2. Consider a hypothetical population of 100 greater prairie chickens, a species in which females choose a mate from a group of displaying males. What is the effective population size if35 females and 10 males of this species breed? 3. _i,'!:tUI$l In 2005, at least ten grizzly bears in the greater Yellowstone ecosystem were killed through contact with people. Three things caused most of these deaths: collisions with automobiles, hunters (not of grizzly bears) shooting when charged by a female with cubs nearby, and conservation managers killing bears that attacked livestock repeatedly. If you were a conservation manager, what steps might you take to minimize such encounters in Yellowstone? For suggested answers, see Appendix A.

r~:~~:;:p:~~: regional

conservation aim to sustain entire biotas

Preservation efforts historically focused on saving individual species, but efforts today often aim to sustain the biodiversity of entire communities, ecosystems, and landscapes. Such a broad view requires understanding and applying the princi路 pIes of community, ecosystem, and landscape ecology as well as the principles of human population dynamics and economics. The goals of landscape ecology (see Chapter 52), of which ecosystem management is a part, include understanding past, present, and future patterns of landscape use and making biodiversity conservation part ofland-use planning.

landscape Structure and Biodiversity The biodiversity ofa given landscape is in large part a function of the structure of the landscape. Understanding landscape structure is critically important in conservation because many

CHAPTER FlfTY路SIX

Conservation Biology and Restoration Ecology

1255


species use more than one kind of ecosystem, and many live on the borders between ecosystems.

Fragmentation and Edges The boundaries, or edges, between ecosystems-such as between a lake and the surrounding forest or between cropland and suburban housing tracts-are defining features of landscapes (figure 56.14). An edge has its own set of physical conditions, which differ from those on either side of it. The soil surface of an edge between a forest patch and a burned area receives more sunlight and is usually hotter and drier than the forest interior, but it is cooler and wetter than the soil surface in the burned area.

Some organisms thrive in edge communities because they gain resources from both adjacent areas. The ruffed grouse (Bonasa umbe//atus) is a bird that needs forest habitat for nesting, winter food, and shelter, but it also needs forest openings with dense shrubs and herbs for summer food. Whitetailed deer also thrive in edge habitats, where they can browse on woody shrubs; deer populations often expand when forests are logged and more edges are exposed. The proliferation of edge species can have positive or negative effects on biodiversity. A 1997 study in Cameroon comparing edge and interior populations of the little greenbul (a tropical rain forest bird) suggested that forest edges may be important sites of speciation. On the other hand, communities in which edges arise from human alterations often have reduced biodiversity because the relatively large percentage of edge habitat leads to a preponderance of edge-adapted species. For example, the brown-headed cowbird (M%thrus ater) is an edge-adapted species that lays its eggs in the nests of other birds, particularly migratory songbirds. Cowbirds need forests, where they can parasitize the nests of other birds, and also open fields, where they forage on insects. Thus, their populations are growing where forests are being cut and fragmented, creating more edge habitat and open land. Increasing cowbird parasitism and loss of habitat are correlated with declining populations of several of the cowbird's host species. The influence of fragmentation on the structure ofcommunities has been explored since 1979 in the long-term Biological Dynamics of Forest Fragments Project. Located in the heart of the Amazon River basin, the study area consists of isolated fragments of forest separated from surrounding continuous tropical rain forest by distances of 80-1,000 m (figure 56.15). Researchers from all over the world have clearly documented

(a) Natural edges. Grasslands give way 10 foresl ewsyslems in Yellowstone National Park.

(b) Edges created by human activity. Pronounced edges (roads) surround c1ear-culS in this photograph of a heavily logged rain forest in Malaysia. ... figure 56.14 Edges between ecosystems. 1256

U"IT EIG~T

Ecology

... Figure 56.15 Amazon rain forest fragments created as part of the Biological Dynamics of Forest Fragments Project.


the physical and biological effects of this fragmentation in taxa ranging from bryophytes and beetles to birds. They have consistently found that species adapted to forest interiors show the greatest dedines in the smallest fragments, suggesting that landscapes dominated by small fragments will support fewer species, mainly due to a loss of species adapted to the interior.

Corridors That Connect Habitat Fragments In fragmented habitats, the presence ofa movement corridor, a narrow strip or series of small clumps of habitat connecting otherwise isolated patches, can be extremely important for conserving biodiversity. Streamside habitats often serve as corridors, and in some nations, government policy prohibits altering these riparian areas. In areas of heavy human use, artificial corridors are sometimes constructed. Bridges or tunnels, for instance, can reduce the number of animals killed trying to cross highways (Figure 56.16). Movement corridors also can promote dispersal and reduce inbreeding in declining populations. Corridors have

been shown to increase the exchange of individuals in many organisms, including butterflies, voles, and aquatic plants. Corridors are especially important to species that migrate be· tween different habitats seasonally. However, a corridor can also be harmful-as, for example, in the spread ofdisease. In a 2003 study, Agustin Estrada-Pena, of the University of Zaragoza, Spain, showed that habitat corridors facilitate the movement of disease-carrying ticks among forest patches in northern Spain. All the effects of corridors are not yet understood, and their impact is an area of active research in conservation biology and restoration ecology.

Establishing Protected Areas Conservation biologists are applying their understanding of community, ecosystem, and landscape dynamics in establishing protected areas to slow biodiversity loss. Currently, governments have set aside about 7% of the world's land in various forms of reserves. Choosing where to place and how to design nature reserves poses many challenges. Should the reserve be managed to minimize the risks of fire and predation to a threatened species? Or should the reserve be left as natural as possible, with such processes as fires ignited by lightning allowed to play out on their own? This is just one of the debates that arise among people who share an interest in the health of national parks and other protected areas. In deciding which areas are of highest conservation priority, biologists often focus on hot spots of biological diversity.

Finding Biodiversity Hot Spots

.. Figure 56.16 An artificial corridor. This bridge in Banff National Park, Canada, helps animals cross a human-created barrier.

A biodiversity hot spot is a relatively small area with an exceptional concentration of endemic species and a large number of endangered and threatened species (Figure 56.17). Nearly 30% ofall bird species are confined to only about 2% of Earth's land area. Approximately 50,000 plant species, or about one-sixth of all known plant species, inhabit just 18 hot

Terrestrial biodiversity hot spots • Marine biodiversity hot spots

... Figure 56.17 Earth's terrestrial and marine biodiversity hot spots.

CHAPTER FlfTY·SIX

Conservation Biology and Restoration Ecology

1257


spots covering 0.5% of the global land surface. Together, the "hottest" ofthe terrestrial biodiversity hot spots total less than 1.5% of Earth's land but are home to more than a third of all species of plants, amphibians, reptiles (including birds), and mammals. Hot spots also include aquatic ecosystems, such as coral reefs and certain river systems. Biodiversity hot spots are obviously good choices for nature reserves, but identifying them is not always simple. One problem is that a hot spot for one taxonomic group, such as butterflies, may not be a hot spot for some other taxonomic group, such as birds. Designating an area as a biodiversity hot spot is often biased toward saving vertebrates and plants, with less attention paid to invertebrates and microorganisms. Some biologists are also concerned that the hot-spot strategy places too much emphasis on such a small fraction of Earth's surface.

a

50

100

Kilometers .,' '.--_._'-~

, ,• ,• ••• • ••

,, ,

Yellowstone National Park

MONTANA IpAHO

••

•• •, , s• '?<}.f,

.... ('>1' '

..'" o

Philosophy of Nature Reserves Nature reserves are biodiversity islands in a sea of habitat degraded to varying degrees by human activity. Protected "is_ lands~ are not isolated from their surroundings, however, and the nonequilibrium model we described in Chapter 54 applies to nature reserves as well as to the larger landscapes in which they are embedded. An earlier policy-that protected areas should be set aside to remain unchanged forever-was based on the concept that ecosystems are balanced, self-regulating units. As we saw in Chapter 54, however, disturbance is a functional component of all ecosystems, and management policies that ignore natu~ ral disturbances or attempt to prevent them have generally failed. For instance, setting aside an area of a fire-dependent community, such as a portion of a tallgrass prairie, chaparral, or dry pine forest, with the intention of saving it is unrealistic if periodic burning is excluded. \Vi.thout the dominant disturbance, the fire-adapted species are usually outcompeted and biodiversity is reduced. Because human disturbance and fragmentation are increasingly common landscape features, the dynamics of disturbances, populations, edges, and corridors are all important for designing and managing protected areas. An important con~ servation question is whether to create fewer large reserves or more numerous small reserves. One argument for extensive re~ serves is that large, far-ranging animals with low-density populations, such as the grizzly bear, require extensive habitats. More extensive areas also have proportionately smaller perimeters than smaller areas and are therefore less affected by edges. As conservation biologists learn more about the requirements for achieving minimum viable populations for endangered species, they realize that most national parks and other reserves are far too small. For example, the area needed for the long-term survival of the Yellowstone grizzly bear population is more than ten times the combined area of Yellowstone and Grand Teton National Parks (Figure 56.18). Given 1258

U"IT EIG~T

Ecology

~~~~

.1(,;,.; ~~~~ O~toneR, ..,

fie

~

R.

I

MONTANA WYOMING

:

• •• • ,• • ,• ,,

Grand Teton ./ National Par:••• Biotic boundary for ____ ,.--.~ short-term survival; ~ MVP is 50 individuals.

:J:

~

~ ~ $:

Biotic boundary for long-term survival; MVP is 500 individuals.

.... Figure 56.18 Biotic boundaries for grizzly bears in Yellowstone and Grand Teton National Parks. The biotic boundaries (solid and dashed red lines) surround Ihe areas needed to support minimum viable populations of 50 and 500 bears, Even the smaller of these areas is larger than the two parks.

political and economic realities, many existing parks will not be enlarged, and most newly created reserves will also be too small. Areas of private and public land surrounding reserves will likely have to contribute to biodiversity conservation. On the other side of the argument, smaller, unconnected reserves may slow the spread of disease throughout a population. In practical terms, land use by humans may outweigh all other considerations and ultimately dictate the size and shape of protected areas. Much of the land left for conservation efforts is useless for exploitation by agriculture or forestry. But in some cases, as when reserve land is surrounded by commercially viable property, the use of land for agriculture or forestry must be integrated into conservation strategies.

Zoned Reserves Several nations have adopted a wned reserve approach to landscape management. A zoned reserve is an extensive region that includes areas relatively undisturbed by humans surrounded by areas that have been changed by human activity and are used for economic gain. The key challenge of the zoned reserve approach is to develop a social and economic climate in the sur~ rounding lands that is compatible with the long-term viability of the protected core. These surrounding areas continue to be used to support the human population, but with regulations that prevent the types of extensive alterations likely to impact the protected area. As a result, the surrounding habitats serve as buffer zones against further intrusion into the undisturbed area.


The small Central American nation ofCosta Rica has become a world leader in establishing zoned reserves (Figure 56.19). An agreement initiated in 1987 reduced Costa Rica's international debt in return for land preservation there. The agreement resulted in eight zoned resen'es, called "conservation areas;' that contain designated national park land. Costa Rica is making progress toward managing its zoned reserves, and the buffer zones provide a steady, lasting supply of forest products, water, and hydroelectric power and also support sustainable agriculture and tourism. An important goal is providing a stable economic base for people living there. AJ;, University ofPennsylvania ecologist Daniel Janzen, a leader in tropical conservation, has said, "The likelihood of long-term survival of a conserved wildland area is directly proportional to the economic health andstability of the society in which that wildland is embedded:' Nicaragua

~i)Jr---;1i

o o

CARIBBEAN SEA

National park land Buffer zone

PACIFIC OCEAN

Destructive practices that are not compatible with long-term ecosystem conservation and from which there is often little local profit, such as massive logging, large-scale single-crop agriculture, and extensive mining, are ideally confined to the outermost fringes of the buffer zones and are gradually being discouraged. Costa Rica relies on its zoned reserve system to maintain at least8O% onts native species, but the system is notwithoutproblems. A 2003 analysis of land cover change between 1960 and 1997 showed negligible deforestation within Costa Rica's national parks and a gain in forest cover in the l-km buffer around the parks. However, significant losses in forest cover were discovered in the ID-km buffer zones around all national parks, which threaten to turn the parks into isolated habitat islands. Although marine ecosystems have also been heavily impacted by human exploitation, reserves in the ocean are far less common than reserves on land. Many fish populations around the world have collapsed as increasingly sophisticated equipment puts nearly all potential fishing grounds within human reach. In response, Fiona Gell and Callum Roberts, ofthe University of York, England, have proposed establishing marine reserves around the world that would be offlimits to fishing. Gell and Roberts present strong evidence that a patchwork of marine reserves can serve as a means of both increasing fish populations within the reserves and improving fishing success in nearby areas. Their proposed system is a modern application of a centuries-old practice in the Fiji Islands in which some areas have historically remained closed to fishing-a traditional example of the zoned reserve concept. The United States adopted such a system in establishing the Rorida Keys National Marine Sanctuary in 1990 (Figure 56.20). Populations of marine organisms, including fishes and lobsters, recovered quickly after harvests were banned in the 9,500_km 2 reserve. Larger and more abundant fish now produce larvae that

(a) Boundaries of the zoned reserves are indicated by black outlines. GULF OF MEXICO

(b) Local schoolchildren marvel at the diversity of life in one of Costa Rica's reserves

... Figure 56.19 Zoned reserves in Costa Rica.

... Figure 56.20 A diver measuring coral in the Florida Keys National Marine Sanctuary.

CHAPTER FlfTY路SIX

Conservation Biology and Restoration Ecology

1259


help repopulate reefs and improve fishing outside the sanctuary. The increased marine life within the sanctuary also makes it a favorite for recreational divers, increasing the economic value of this zoned reserve. CONCEPT

CHECI(

56.3

L What is a biodiversity hot spot? 2. How do zoned reserves provide economic incentives for long-term conservation of protected areas? 3. -'MUI 4 Suppose a developer proposes to clearcut a forest that serves as a corridor between two parks. To compensate, the developer also proposes to add the same area of forest to one of the parks. As a professional ecologist, how might you argue for retaining the corridor? For suggested answers, see Appendix A.

r::;:;~~i:n6;~IOgy attempts to

restore degraded ecosystems to a more natural state

Given enough time, biological communities can recover naturally from most disturbances through the stages of ecological succession that we discussed in Chapter 54. Sometimes that recovery takes centuries, though, particularly when lmmans degrade the environment. Degraded habitats are increasing in area because the natural rate of recovery by successional processes is often slower than the rate of degradation by human activities. The soils of many tropical areas quickly become unproductive and are soon abandoned after

(a) In 1991, before restoration

being cleared for farming. Mining activities may last for several decades, but the lands are then abandoned in a degraded state. Many ecosystems are also damaged inadvertently by the dumping of toxic chemicals or such mishaps as oil spills. Restoration ecology seeks to initiate or speed up the recovery of degraded ecosystems. One of its basic assumptions is that environmental damage is at least partly reversible. This optimistic view must be balanced by a second assumptionthat ecosystems are not infinitely resilient. Restoration ecologists therefore work to identify and manipulate the processes that most limit the speed of recovery of ecosystems from disturbances. Where disturbance is so severe that restoring all of a habitat is impractical, ecologists try to reclaim as much of a habitat or ecological process as possible, within the limits of the time and money available to them. In extreme cases, the structure of a site may first need to be restored before biological restoration can occur. Ifa stream was straightened to channel water quickly through a suburb, restoration ecologists may reconstruct a meandering channel to slow down the flow ofwater eroding the stream bank. To restore an open-pit mine, engineers may first grade the site with heavy equipment to reestablish a gentle slope, spreading topsoil when the slope is in place (Figure 56.21). Once such physical reconstruction is complete-or when it is not needed-biological restoration is the next step. Two key strategies in restoration ecology are bioremediation and biological augmentation.

Bioremediation The use oforganisms, usually prokaryotes, fungi, or plants, to detoxify polluted ecosystems is known as bioremediation (see Chapter 27). Some plants adapted to soils containing heavy metals can accumulate high concentrations of potentially toxic metals such as zinc, nickel, lead, and cadmium in

(b) In 2000, near the completion of restoration

... Figure 56.21 A gravel and clay mine site in New Jersey before and after restoration.

1260

U"IT EIG~T

Ecology


6

....

5 •..:. ... '

4 ]

.l.' \:

.:...•. :.: ....

2

. ,..•,". .... ••'

..... . . ..

.,. • ."\ II '.'

. .' .., ..

;'

'" 1·/:..·.. • ••••••'

'.

.·.t. ' ...

••••::-••:.;.:-.';'•• I'" '.1'" , ,- ." ;'

.--

'

o+-~-~~-~~-~~~-

a

(a) Wastes containing uranium were dumped in these lour unlined pits for more than 30 years. contaminating soils and groundwater,

50

100

150 200 250 300 Days after adding ethanol

350

400

(b) After ethanol was added. microbial activity decreased the concentration of soluble uranium in groundwater near the pits.

.. Figure 56.22 Bioremediation of groundwater contaminated with uranium at Oak Ridge National laboratory. Tennessee. their tissues. Restoration ecologists can use such plants to revegetate sites polluted by mining and other human activities and then harvest the plants to remove the metals from the ecosystem. Researchers in the United Kingdom discovered a lichen species that grows on soil polluted with uranium dust left over from mining. The lichen concentrates uranium in a dark pigment, making it useful as a biological monitor and potentially as a remediator. Ecologists are examining the abilities of many prokaryotes to carry out bioremediation of soils and water. Scientists have sequenced the genomes of at least seven prokaryotic species specifically for their bioremediation potential. One of the species, the bacterium Shewane/la oneidensis, appears particularly promising. It can metabolize more than ten elements under aerobic and anaerobic conditions to generate its energy. For instance, it converts soluble uranium, chromium, and nitrogen to insoluble forms that are less likely to leach into streams or groundwater. Wei-Min Wu and colleagues, at Oak Ridge National Laboratory, in Tennessee, stimulated the growth of Shewanella and other uranium-reducing bacteria by adding ethanol to groundwater contaminated with uranium; over five months, the concentration of soluble uranium dropped by 80% (figure 56.22). In the future, genetic engineering may become increasingly useful as a tool for improving the performance of prokaryotes and other organisms as bioremediators.

Biological Augmentation In contrast to bioremediation, which is a strategy for removing harmful substances, biological augmentation uses organisms to add essential materials to a degraded ecosystem. Augmenting ecosystem processes requires determining what

factors, such as chemical nutrients, have been removed from an area and thus limit its rate of recovery. Encouraging the growth of plants that thrive in nutrient-poor soils often speeds up the rate of successional changes that can lead to recovery of damaged sites. In alpine ecosystems of the western United States, nitrogen-fixing herbs such as lupines are often planted to bolster nitrogen concentrations in soils disturbed by mining and other uses. Once these nitrogen-fixing plants become established, other native species are better able to obtain enough nitrogen from the soil to survive. In other systems where the soil has been severely disturbed or where topsoil is missing entirely, plant roots may lack the mycorrhizal symbionts that help them meet their nutritional needs (see Chapter 31). Ecologists restoring a tallgrass prairie in Minnesota recognized this limitation and significantly accelerated the recovery of native species by adding mycorrhizal symbionts to the soil they seeded.

Exploring Restoration Because restoration ecology is a relatively new discipline and because ecosystems are complex, restoration ecologists generally learn as they go. Many restoration ecologists advocate adaptive management: experimenting with several promising types of management to learn what works best. The long-term objective of restoration is to speed the reestablishment of an ecosystem as similar as possible to the predisturbance ecosystem. figure 56.23, on the next two pages, identifies several ambitious and successful restoration projects around the world. The great number ofsuch projects, the dedication of the people engaged in them, and the successes that have been achieved suggest that restoration ecology will continue to grow as a discipline for many years.

CHAPTER FlfTY·SIX

Conservation Biology and Restoration Ecology

1261


• Figure 56.23

••

• Restoration Ecology Worldwide

The examples highlighted on these pages are just a few of the many restoration e<:ology projects taking place around the world. The color-coded dots on the map indicate the locations of the proje<:ts.

Equator

• Truckcc Rivcr, Ncvada. Damming and water diversions during the 20th century reduced flow in the Truckee River, Nevada, leading to declines in riparian forests. Restoration ecologists worked with water managers to ensure that sufficient water would be released during the short season of seed release by the native cottonwood and willow trees for seedlings to become established. Nine years of controlledflow release led to the result shown here: a dramatic recovery of cottonwood-willow riparian forest .

• Kissimmee River. Florida. The Kissimmee River was converted from a meandering river to a 9O-km canal, threatening many fish and wetland bird populations. Kissimmee River restoration has filled 12 km of drainage canal and reestablished 24 km of the original 167 km of natural river channel. Pictured here is a section of the Kissimmee canal that has been plugged (wide, light strip on the right side of the photo). diverting flow into remnant river channels in the center of the photo. The project will also restore the natural flow regime, which will foster self-sustaining populations of wetland birds and fishes.

1262

U"IT EIGHT

Ecology

• Tropical dry forest. Costa Rica. Clearing for agriculture. mainly for livestock grazing, eliminated approximately 98% of tropical dry forest in Central America and Mexico. Reversing this trend, tropical dry forest restoration in Costa Rica has used domestic livestock to disperse the seeds of native trees into open grasslands. The photo shows one of the first trees (right center). dispersed as seed by livestock, to colonize former pastureland. This project is a model for joining restoration ecology with the local economy and educational institutions.


Rhine River, Europe. Centuries of dredging and channeling for navigation (see the barges in the wide, main channel on the right side of the photo) have straightened the once-meandering Rhine River and disconnected it from its floodplain and associated wetlands. The countries along the Rhine, particularly France, Germany, Luxembourg, the Netherlands, and Switzerland, are cooperating to reconnect the river to side channels, such as the one shown on the left side of the photo. Such side channels increase the diversity of habitats available to aquatic biota, improve water quality, and provide flood protection.

â&#x20AC;˘ Succulent Karoo, South Africa. In this desert region of southern Africa, as in many arid regions, overgrazing by livestock has damaged vast areas. Reversing this trend, private landowners and government agencies in South Africa are restoring large areas of this unique region, revegetating the land and employing more sustainable resource management. The photo shows a small sample of the exceptional plant diversity of the Succulent Karoo; its 5,000 plant species include the highest diversity ofsucculent plants in the world.

â&#x20AC;˘ Coastal Japan. Seaweed and seagrass beds are important nursery grounds for a wide variety of fishes and shellfish. Once extensive but now reduced by development, these beds are being restored in the coastal areas ofJapan. Techniques include constructing suitable seafloor habitat, transplanting from natural beds using artificial substrates, and hand seeding (shown in this photograph).

Maungatautari, New Zealand. Weasels, rats, pigs, and other introduced species pose a serious threat to New Zealand's native plants and animals, including the kiwi, a flightless, ground-dwelling bird. The goal of the Maungatautari restoration project is to exclude all exotic mammals from a 3,400-ha reserve located on a forested volcanic cone. A specialized fence around the resen'e eliminates the need to continue setting traps and using poisons that can harm native wildlife. In 2006, a pair of critically endangered takahe (a species of flightless rail) were released into the reserve in hopes of reestablishing a breeding population of this colorful bird on New Zealand's north island.

CHAPTER FlfTY¡SIX

Conservation Biology and Restoration Ecology

1263


CONCEPT

CHECI(

56.4

1. What are the goals of restoration ecology? 2. How do bioremediation and biological augmentation differ? 3, _"ll:f.jjl. In what way is the Kissimmee River project a more complete ecological restoration than the Maungatautari project (see Figure 56.23)? For suggested answers. see Appendix A.

r::~"t:~~;b~e6d~velopment seeks

to improve the human condition while conserving biodiversity

With the increasing loss and fragmentation of habitats, we face difficult trade-offs in how to manage Earth's resources. Preserving all habitat patches isn't feasible, so biologists must help societies set conservation priorities by identifying which habitat patches are most crucial. Ideally, implementing these priorities should also improve the quality of life for local people. Ecologists use the concept of sustainability as a tool to establish long-term conservation priorities.

Sustainable Biosphere Initiative We must understand the complex interconnections of the biosphere to protect species from extinction and to improve the quality of human life. To this end, many nations, scientific societies, and other groups have embraced the concept of sustainable development, development that meets the needs of people today without limiting the ability of future generations to meet their needs. The forward·looking Eco· logical Society of America, the world's largest organization of professional ecologists, endorses a research agenda called the Sustainable Biosphere Initiative. The goal ofthis initiative is to define and acquire the basic ecological information needed to develop, manage, and conserve Earth's resources as responsibly as possible. The research agenda includes studies of global change, including interactions between climate and ecological processes; biological diversity and its role in maintaining ecological processes; and the ways in which the productivity of natural and artificial ecosystems can be sustained. This initiative requires a strong commitmentofhuman and economic resources. Achieving sustainable development is an ambitious goal. To sustain ecosystem processes and stem the loss ofbiodiver· sity, we must connect life science with the social sciences, economics, and humanities. \Y/e must also reassess our personal values. Those of us living in wealthier nations have a larger ecological footprint than do people living in developing na-

1264

U"IT EIG~T

Ecology

tions (see Chapter 53). By reducing our orientation toward short-term gain, we can learn to value the natural processes that sustain us. The following case study illustrates how the combination of scientific and personal efforts can make a significant difference in creating a truly sustainable world.

Case Study: Sustainable Development in Costa Rica The success of conservation in Costa Rica that we discussed in Concept 56.3 has involved an essential partnership between the national government, nongovernment organiza· tions (NGOs), and private citizens. Many nature reserves established by individuals have been recognized by the gov· ernment as national wildlife reserves and given significant tax benefits. However, conservation and restoration ofbiodiversity make up only one facet of sustainable development; the other key facet is improving the human condition. How have the living conditions of the Costa Rican people changed as the country has pursued its conservation goals? As we discussed in Chapter 53, two of the most fundamental indicators of living conditions are infant mortality rate and life expectancy. From 1930 to 2007, the infant mortality rate in Costa Rica declined from 170 to 9 per 1,000 live births; over the same period, life expectancy increased from about 43 years to 77 years (Figure 56.24). Another indicator of living conditions is literacy rate. The 2004 literacy rate in Costa Rica was 96%, compared to 97% in the United States. Such statistics show that living conditions in Costa Rica have improved greatly over the period in which the country has dedicated itself to conservation and restoration. While this result does not prove that conservation causes an increase in human welfare, we can say with certainty that development in Costa Rica has attended to both nature and people.

200

80

-life expectancy _Infant mortality

40 0~-----~----~~30

1900

1950

2000

Year

... Figure 56.24 Infant mortality and life expectancy at birth in Costa Rica.


Despite the successes in Costa Rica, many problems remain. One of the challenges that the country faces is maintaining its commitment to conservation while its population grows. Costa Rica is in the middle of a rapid demographic transition (see Chapter 53), and even though birth rates are dropping rapidly, its population is growing at about 1.5% annually (compared to 0.9% gro\\1h in the United States). Costa Rica's population, which is currently about 4 million, is predicted to continue to grow until the middle of this century, when it is projected to level off at approximately 6 million. If recent success is any guide, the people of Costa Rica will overcome the challenge of population growth in their quest for sustainable development.

The Future of the Biosphere Our modern lives are very different from those of early humans, who hunted and gathered to survive. Their reverence for the natural world is evident in the early murals of wildlife they painted on cave walls (Figure 56.25a) and in the stylized visions of life they sculpted from bone and ivory (Figure 56.25b). (a) Detail of animals in a 36,OOO·year-old cave painting, lascaux, France

Our lives reflect remnants of our ancestral attachment to nature and the diversity of life-the concept of biophilia that we introduced early in this chapter. We evolved in natural environments rich in biodiversity, and we still have an affinity for such settings (Figure 56.25c). E. O. \Vilson makes the case that our biophilia is innate, an evolutionary product of natural selection acting on a brainy species whose survival depended on a close connection to the environment and a practical appreciation of plants and animals. Our appreciation of life guides the field of biology today. We celebrate life by deciphering the genetic code that makes each species unique. We embrace life by using fossils and DNA to chronicle the march of evolution through time. We preserve life through our efforts to classify and protect the millions ofspecies on Earth. We respect life by using nature responsibly and reverently to improve human welfare. Biology is the scientific expression ofour desire to know nature. We are most likely to protect what we appreciate, and we are most likely to appreciate what we understand. By learning about the processes and diversity ofHfe, we also become more aware of ourselves and our place in the biosphere. We hope this book serves yOll well in this lifelong adventure. CONCEPT

CHECK

56.5

1. What is meant by the term sustainable development? 2. How might biophilia influence us to conserve species and restore ecosystems? 3. • i,IlIfu!£i Suppose a new fishery is discovered, and you are put in charge of developing it sustainably. What ecological data might you want on the fish population? What criteria would you apply for the fishery's development? For suggested answers, see Appendix A.

(b) A 30,OOO·year-old ivory carving of a water bird, found In Germany

(c) Biologist Carlos RIVera Gonzales examining a tiny tree frog in Peru

... Figure 56.25 Biophilia. past and present. CHAPTER FlfTY·SIX

Conservation Biology and Restoration Ecology

1265


(,;:) 1.I1!&I~l~.I~'路.1I Go 10 the Study Area at www.masteringbio.comforBioFlix -./

~

3-D AJlImiltions, MP3 Tutors, Videos, Practice Tests, iln eBook, ilJld more.

SUMMARY OF KEY CONCEPTS Mi,lili"_

~ Weighing Conflicting Demands Conserving species often

56.1

requires resolving conflicts between the habitat needs of endangered species and human demands.

Human activities threaten Earth's biodiversity (pp.124(""1250)

w' li"I'. 56.3

.. Three Levels of Biodiversity

landscape and regional conservation aim to sustain entire biotas (pp. 1255-12(0) ~

Genetic diversity: source of

Declining-Population Approach The declining-population approach focuses on the environmental factors that cause decline, regardless of absolute population size. It follows a stepby-step proactive conservation strategy.

~arlatlons

landscape Structure and Biodiversity The structure of a landscape can strongly influence biodiversity. As habitat fragmentation increases and edges become more extensive, biodiversity tends to decrease. Movement corridors can promote dispersal and help sustain populations.

~ Establishing Protected Areas Biodiversity hot spots are also

that enable

hot spots of extinction and thus prime candidates for protection. Sustaining biodiversity in parks and reserves requires management to ensure that human activities in the surrounding landscape do not harm the protected habitats. The zoned reserve model recognizes that conservation efforts often involve working in landscapes that are greatly affected by human activity.

populations to adapt to environmental changes

w' li"I'. 56.4 Restoration ecology attempts to restore degraded ecosystems to a more natural state (pp. 12&0-12&4) ~ Bioremediation Restoration ecologists harness organisms to

detoxify polluted ecosystems. ~ Biological Augmentation Ecologists also use organisms to

add essential materials to ecosystems. ~

..... 1

~E;"~"fs'to';;~:;:;',versity: Provide life-sustaining services

J

Exploring Restoration The newness and complexity of restoration ecology require scientists to consider alternative solutions and adjust approaches based on experience.

as nutrient cycling and waste dewmposil'tio""'''''. . . .

Inn.ligation How Are Potentiall'rairie Re.toration Site< Analyzed?

w, 11111'- 56.5 Sustainable development seeks to improve the human condition while conserving biodiversity (pp.1264-1265)

Mi,lili"_

56.2

Population conservation focuses on population size, genetic diversity, and critical habitat (pp. 1250-1255) .. Small路 Population Approach When a population drops beIowa minimum viable population (MVP) size, its loss of genetic variation due to nonrandom mating and genetic drift can trap it in an extinction vortex.

1266

U"IT EIG~T

Ecology

~

Sustainable Biosphere Initiative The goal of the Sustainable Biosphere Initiative is to acquire the ecological information needed for the development. management. and conservation of Earth's resources.

~

Case Study: Sustainable Development in Costa Rica Costa Rica's success in conserving tropical biodiversity has involved partnerships between the government. other organizations, and private citizens. Human living conditions in Costa Rica have improved along with ecological conservation.

~ The Future of the Biosphere By learning about biological

processes and the diversity of life, we become more aware of our close connection to the environment and the value of other organisms in it.


-51401"·

l\cthity Conservation Biology Review Graphlt! Glob.l Freshw.ter Re>ources Graphlt! Pro~pects for Renew.ble Energy

TESTING YOUR KNOWLEDGE

SELF·QUIZ 1. Ecologists conclude there is a biodiversity crisis because a. biophilia causes humans to feel ethically responsible for protecting other species. b. scientists have at last discovered and counted most of Earth's species and can now accurately calculate the current extinction rate. c. current extinction rates are very high and many spedes are threatened or endangered. d. many potential life-saving medicines are being lost as species evolve. e. there are too few biodiversity hot spots. 2. Which of the following would be considered an example of bioremediation? a. adding nitrogen-fixing microorganisms to a degraded ecosystem to increase nitrogen aV'dilability b. using a bulldozer to regrade a strip mine c. identifying a new biodiversity hot spot d. reconfiguring the channel of a river e. adding seeds of a chromium-accumulating plant to soil contaminated by chromium 3. What is the effective population size (N,) of a population of SO strictly monogamous swans (40 males and 10 females) if every female breeds successfully? b. 40 c. 30 d. 20 e. 10 a. SO 4. One characteristic that distinguishes a population in an extinction vortex from most other populations is that a. its habitat is fragmented. b. it is a rare, top-level predator. c. its effective population size is much lower than its total population size. d. its genetic diversity is very low. e. it is not well adapted to edge conditions. 5. The discipline that applies ecological principles to returning degraded ecosystems to more natural states is known as a. population viability analysis. b. landscape ecology. c. conservation ecology. d. restoration ecology. e. resource conservation. 6. \'Vhat is the single greatest threat to biodiversity? a. overexploitation of commercially important species b. introduced species that compete with or prey on native species c. pollution of Earth's air, water, and soil

d. disruption of trophic relationships as more and more prey species become extinct e. habitat alteration, fragmentation, and destruction 7. Which of the following strategies would most rapidly increase the genetic diversity of a population in an extinction vortex? a. Capture all remaining individuals in the population for captive breeding followed by reintroduction to the wild. b. Establish a reserve that protects the population's habitat. c. Introduce new individuals transported from other populations of the same species. d. Sterilize the least fit individuals in the population. e. Control populations of the endangered population's predators and competitors. 8. Of the following statements about protected areas that have been established to preserve biodiversity, which one is not correct? a. About 25% of Earth's land area is now protected. b. National parks are one of many types of protected area. c. Most protected areas are too small to protect species. d. Management of a protected area should be coordinated with management of the land surrounding the area. e. It is especially important to protect biodiversity hot spots. For Self-Quiz ll"$Wers, see Appe"dix A.

-51401". Visit the Study Area at www.masteringbio.com for a Practice Test.

EVOLUTION CONNECTION 9. One FJctor Favoring rapid population growth by an introducecl species is the absence of the predators. parasites, and pathogens that controlled its population in the region where it evolvecl. Over the long term, how should evolution by natural selection influence the rate at which the native predators, parasites, and pathogens in a region ofintroduction attack an introducecl species?

SCIENTIFIC INQUIRY 10. ••Ijl.W"1 Suppose that you are in charge of planning a forest reserve, and one of your goals is to help sustain local populations of woodland birds suffering from parasitism by the brownheaded cowbird. Reading research reports, you note that female cowbirds are usually reluctant to penetrate more than about 100 m into a forest and that nest parasitism is reduced for woodland birds nesting in denser, more central forest regions. The forested area you have to work with extends about 6,000 m from east to west and 1,000 m from north to south. Intact forest surrounds the reserve everywhere but on the ....'est side, where the reserve borders deforested pastureland, and in the southwest m. Your corner, where it borders an agricultural field for plan must include space for a small maintenance building, which you estimate to take up about 100 m l . It will also be necessary to build a road, 10 m by 1,000 m, from the north to the south side of the reserve. Draw a map of the reserve, showing where you would construct the road and the building to minimize cowbird intrusion along edges. Explain your reasoning.

sao

CHAPTER FtfTY·SIX

Conservation Biology and Restoration Ecology

1267


Answers Concept Check 1.3

CHAPTER 1 Figure Queslions Figure 1.3 Of the properties sho¥.'Jl in this figure, the 1.a....'Jl!llO\\l'!" dlo....l1 only order. regulation.andenergyproces.sing. Fig...-e 1.6 Thearnngemenloflingers and opposabIlo thwnb in the human hand. combined ....i th fingernails and ~ complex S)'5tem of nen'\'$ and muscles, allows the hand 10 grasp and manipuiale ob;ects .....ithgreat:dexterity. Figure 1.13 Substance B.....Ollld bernadfocontinuousIy and ....~ aceumu1ale in Iargt amounts. Neither C new D ouId be made. Figure 1.27 The percent:aseoCbro,m artDdaI snakes aruacked 'OllId probably be higher than the perctnbge 0{ artifICial kin:gsnakcs attrled in aD areas (....M her or 001 inhabited byconlsnakrs). Figure 1.28 The hok-...tJuldaJb,o,· some mixing of blood bttv.ftn the 1";0 \'01tridcs. As ~ resull. some of the blood ~ from tht left. \'011ric1e to the bodywould noI have recei\'l:'d I.H}-gtn in the lmgs, and some of tilt blood pumped to the lungs ...tJUId already any oxygen.

Concept Ched 1.1 1. Examples: A molecule consists 0{ /ltoms bonded togethlor. Each organelko has an orderly arrangement of nwheu/n. Photosynthctjc plant «Us contain organtlks called chloroplaslS. A ti~ consists of a group ofsimilar uUs. Or· gans such as the heart a~ conSlructed from st\'erallissues.. A complex mul· ticeUular organism, s.uch as a plant. has severallypCS of organs, such as 1e:l\'t"S and rooIS. A population is a SCI of OIfanisms of !he same species. A communilyconsists of popul/ltionsofttK> \'lIrious species inhabiting a specifIC area. An ccosyslem consists of a biological communiry along ....ith the nonliving factors important to lire. such as air. soil, and ....'lIter. The biosphere is made upo{ all orEarth'swn,sums. 2. (a) StTUctureand function ~recorrelated. (b) Cells are an organism's oosic units. /lnd the continuity or lire is baSl'd. on heritable information in thl' rorm or DNA. (c) Organisms interact wi!h their environments, l'xchanging mauer ~nd energy. 3. Some possible ans.....ers: Evolution: All planlS have chloroplaslS, indicating their dcscl'nt rrom a common ancestor. Emergenl properlies: The ability of a human heart to pump blood requires an intact heart; it is not a capability of any of the heart's tis· sues or cells working alonl'. Excha/lge ofmarrerand e/lerg)' with the elll'ironment; A moult' eats food, then uses the nutrients ror growth and the generation orl'nergy for its activities; some orthe rood material is expelled in urine and feces, and some of the energy returns to thc environment as heat. Structure a/ld fU/lctiuIJ: The strong, sharp teeth of a wolf arc well suited to grasping and dismembering its prey. Cells; The digestion of food is made possible by chemicals (chiefly enzymes) made by cells o( the digestive tract. DNA: Human eye color is determined by the combination of genes inherited from the two parents. Feedback regulaliol1: When your stomach is full, it sig· nals your brain to decrease your appetite.

1. Inductive reasooing derives generalizations from specific cases; deducti\l' reasoning predicts specific outcomes from gcncr.a.l premises.. 2. Compared to a hypothesis. a sdmtiflC lhtory is U5W1JIy more general and substantiated by a much greater amount of t'Yickntt. Natur.Jol scIeclion is an explanatory idea that IppUes to all kinds of orpnisrns and is supported by vast: amounts of evidena of various kinch. 3. Based on the results dloY.T1 in FIgUl'l' 127, )'OU might predict that the coIocfuJ artificial ~ ....~ be anaded mort' often than the bro¥>TI ones, simply ~ thc-y an' a5KT to SC'e. This prcdjction assumes thai the area in \'if!inia ....-hen-)'OU an' ",orting has predators thai attack ~kes but no poisonous snakes that resemble the: coIocfuJ artificial snakes.

Self-Quiz 1. b 2. d 3. a 4. c 5. c 6. c 7. c 8. d 9. b 10. c 11. Your figuruhould show: (I) For the biosphere, the Earth ¥dth an arrow coming oul of a tropical ocean; (2) for the e«lS)'Stem, a distant view of a coral M; (3) for tM community, a colkction of reef animals and algae. with conls, fish, some sea.... ud. and any other organisms you Gin think of; (4) for the population. a group of fish of the same sp!'CK-s; (5) for the organism, one ftsh from your popubtion; (6) for the organ, the fish's stomach. and for the organ system, the whole dlgesth'c traci (sec Chapter 41 for help); (7)

for a tissue, a group of similar cdls from lile 5tOmach; (8) for ~ cdl, one «II from the tissue, shovoing its nucleusand ~ fewOlherorganelles; (9) foran organelle, ttK> nucleus, ....'here most ofttK> cetrs DNA is located; and (10) for a molecule, ~ DNA double helix. Your skctchcs can be ,try rough!

CHAPTER 2 Figure Questions Figure 2.2 The most significant dif(eren~ in thl' results .....ould be that the two CtdT"l'la saplings inside each gardl'n would sh()\l,' similar amounts o( dy· ing leaf tissue becauS(' a poisonous chemical released rrom!he Duroia trees would presumably reach the saplings via the air or soil and would not be blocked by the insect barrier. The Cedrela saplings planted outside the gar· dens .....ould not show damage unless Duroia trees were nearby. Also, any ants present on the unprotected Cedrela saplings inside the gardens would probably not be observed making injections into the leaves. However, formic acid would likely still be found in the ants' glands, as for most speeies of ants. Figure 2.9 Atomic number = 12; 12 protons, 12 electrons; three electron shells; 2 electrons in the valence shell Figure 2.16

Concept Check 1.2 1. An address pinpoints a location by tracking from broader to nano....·er categories-a state, dty, zip. street, and building number. This is analogous to the groups·subordinate-to-groups structure of biological taxonomy. 2. Natural selection staTU with the naturally occurring heritable variation in a population and thl'n "edits" the population as individuals with heritable traits better suiled to theenvironmenl survive and reproduu more successfully than Olhers.

,.

r------"....

Figure 2.19 The plant is submergN in .....ater (H:z<), in ....tlich the COz is dissoh'ed. l1Kosun's l"nergy is used to make sugar. which is found in theplant and can act as food for the pbnl itse!r, as .....ell as (or animals that eat the plant. The oxygl'n (Oz) is present in the bubbles.

Concept Check 2.1 L . . - - A.....

A·I

1_ Table salt is INdt up of sodium and chlorine. We are able to tat tht compound. sho¥.ing!hat it hasdifftrmt properties from those of a metal and a poi. sonousps. 2. Yes, becauseanorganism requirestral:edements,l'\'l"I1 though


only in small amounts. 3. A person with an iron defICiency will probably show effects oflow oxygen in the blood, such as fatigue. (The condition is called anemia andean also result from too few red. blood cclIsor abnormal hemoglobin.) Concept Check 2.2 1.7 2. '~N 3. gelectrons;twoclectronshells;ls,2s,2p(threeorbitals); I electron is needed to fill the valence shell. 4. The elements in a row all have the same number of electron shells. In a column, all the elements have the same number of electrons in their vaknce shells. Concept Check 2.3 1. Each carbon atom has only three covalent bonds instead of the required four. 2. The attractions bctween oppositely charged ions form ionic bonds. 3. If researchers can synthesize molecules that mimic these shapes, they may be able to treat diseases or conditions caused by the inability of affected. individuals to synthesize such molecules. Concept Check 2.4

1.

:O:tI

H: H H :H

H

0::0

-7 :O:H H

".

O.

JHolp

2. At equilibrium, the forward and reverse reactions occur at the same rate. 3. C;H 120 6 + 6 O 2 ---> 6 CO2 + 6 H 20 + Energy. Glucose and oxygen react to fonn carbon dioxidc, water, and energy. We breathe in oxygcn because we need it for this reaction to occur, and we breathe out carbon dioxide because it is a by-product of this reaction. By the way, this reaction is called. cellular respiration, and you will learn more about it in Chapter 9.

Self.quiz 1. a 2. b

9.

3. b 4. e S. b 6. a 7. b 8. b

k-' ~: J.l Th....tn..tfun &tsn'l T'II4ke oen~ btcau,t. -!he \Icl,IVlu, S/ltlt rf c.ub:n it int~m,'mj ~ CAll &-rJI 4lx:rlJ\.

..

l'l.

b.

90"

•. ~ ~ .• "ThiS 5tTlldu.te. fi'\l.1:tHt:n~ bust.

H :~:~: c:: 9. I--l

,.

H

(lll'IAltlU91ell.s are~ 411tt4U ~ N.~t.ik~ r.:.ur«r,f~.

~~gyticlurt.cWrltfll1lo:t.Wtst.

H:~:ft.C::~

~ ).IhlslW~ It.J~lowt, it UIYrl ftn, bnIs wi#! 'l4ibm5.

H

:>/I

d. "Thi!. !Jtllthlft ~rft ~~ i.-sa'trlJ rt.\UIl.s: .F' 1'ht v~lt~t .htll rf Cl/~¥'l it. inumpleltj :0: 0l~1Il Cll.I1 Wm 2..borrh.

H:~)'UH ~ IJ'II~ I ~'!&.sh1tt,~jj.I'.aIlI\ltFw",. rWole~,

would freeze. The krill could not survive. Figure 3.7 Heating the solution would cause the water to evaporate faster than it is evaporating at room temperature. AI a certain point, there wouldn't be enough water molecules to solubilizc the salt ions. The salt would start coming out of solution and re-forming crystals. Ewntually, all the ",ater would evaporate, leaving behind a pile of salt Iikc the original pile. Figure 3.11 Given that Ca2+ and C032 - must in· teract to form CaC03 , you would pred.iet that (Ca 2 +1 would also have an ef· fect on the calcification rate, and this result is observed in the current study. Under natural conditions in the oceans, the [CaHJ remains relatively constant, so the [COJ2-J has a much more important effect on calcification rate. Concept Check 3.1 1. Electronegativity is the attraction of an alom for the electrons of a covalent bond. Because oxygen is more ekctronl:gative than hydrogen, the oxy· gen atom in H2 0 pulls electrons toward itself, resulting in a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. Oppositely charged ends of water molecules are attracted. to each other, forming a hydrogen bond. 2. The hydrogen atoms of one molecule, with their partial positive charges. would repel the hydrogen atoms of the adjacent molecule. 3. Water molecules would not be polar, and they would not form hydrogm bonds with each other. Concept Check 3.2 1. Hydrogen bonds hold neighboring water molecules together. This cohl> sion helps the molecules resist gravity. Adhesion between water molecules and the walls of water-conducting cells also counters gravity. As water C'Vaporates from leaves, the chain of water molecules in water-conducting cells moves upward. 2. High humidity hampers cooling by suppressing the evaporation of sweat. 3. As waterfreacs. it expands becauscwater molecules move farther apart in forming icc crystals. When there is water in a crevice of a boulder, expansion due to freezing may crack the rock. 4. The molecular massofNaCI is 58.5 daltons. A mole would hal'e a massof585 g, so you would measure out 0.5 mol, or 29.3 g.. of NaCI and gradually add water, stirring until it is dissolved.. You would add ",ater to bring the final volume to 1 L. 5. The hydrophobic substance repels water, perhaps helping to keep the ends of the legs from becoming coatl'd with water and breaking through the surface. If thc legs wcre coated with a hydrophilic substance. water would be drawn up them, possibly making it more difficult for the water strider to walk on water. Concepl Check 3.3 1. Hr, or 100,000 2. [WI = M = 10- 2 M, so pH = 2 3. CH3COOH ~ CH 3COO- + W. CH 3COOH is the acid (the H+ donor) and CH 3COO- is the base (the H+ acceptor). 4. The pH of the water should go from 7 to about 2; the pH of the acetic acid solution will only decrease a smaJl amount, lx'Cause the reaction shown for question 3 will shift to the left, with CH 3COO accepting the influx of H + and bt'Coming CH 3COOH molecules.

om

Self-Quiz

1. d 2. c 3. b 4. c 5. c 6. d 7. c 8. c

~ °b:"6~"'~~ 0

,

Ni~ LISlo1/Iq I'Mm ... ~ 3 b.n4 'Rdots n.th.t.w tJIctIjl deetnN.., 1lII!:t. 2..51~~, rNI~

II

~ lzM,aIliCl>/7lpldt if.s 'ollJ0\q'..911'11.

CHAPTER 3 Figure Questions Figure 3.6 Without h)"drogen bonds, water would behave like other small molecules, and the solid phase (icc) would be denser than liquid water. The ice would sink to the bottom, and because it would no longer insulate the whole body of water, it could freeze. Freezing would take a longer time be· cause the Antarctic is an ocean (the Southern Ocean), not a pond or lake, but the average annual temperature at the South Pole is -5O'C, so ewntualJy it

CHAPTER 4 Figure Questions Figure 4.2 Because the concentration of the reactants influences the equilibrium (as discussed in Chapter 2), there might be more HCN relative to CH 20, since there would be a higher concentration of the reactant gas that contains nitrogen. Figure 4.4

Na:

.p.

·s: Answers

A-2

~

•~

.. c


Figure 5.18

Figure 4.7

peptide¢'" ..,..,

H

H-C-H

~

~

I

Oll

1

H-C-C -C-H

HH-C-H I H

,

H Figure 4.10 Molecule b, because there are not only Ihe two electronegative oxygms of the carboxyl group, but also an oxygen on the next (carbonyl) carbon. All of theS(' oxygcns hdp make the bond belween the 0 and H of the -OH group more polar, thus making the dissociation of H- more likely. ~

Concept Check 4. t

..

gases ofthe primitive aunosphereon Earth demonstrated that Iife's molecules cook! initially have been synthesized from nonliving molecules. 2. The spark provides

•~ C

1. Amino acids are essential molecules for living organisms. Their synthesis from

energy needed for the inorganic molecules in the atmosphere to reacl with roch other. (You] learn more about encrg)' and cnemicalrt:aclions in Chaptcr8.}

Concept Check 4.2 1. H H /

,

H'"

'\-I

Concept Check 4.3 1. II has both an amino group (-NH 2), which makes it an amine, and a carboxyl group (-COO H), which makes it a carboxylic acid. 2. The ATP molecule loses a phosphate, becoming ADP. 3. 0 H 0 A chemical group that can act as a base has ~ I "y been replaced wilh a group that can act as an C- C-C acid, increasing the acidic properties of Ihe I \ \ molecule. The shape of the molecule would HO H OH also change.likdychanging the molecules with which it can interact.

Self-Quiz

·~i·

~A

~A

~

&""3

..

~(DUf

Figure 5.25 The green spiral is an

7. d

Si has four valence electrom, the.same number ascarbon. Therefore, silicon would be able to form long chains, induding branches, that could act as skektons for organic molecules. It would dearly do this much better than neon (with no valence electrons} or aluminum (with three valence electrons).

Figure Questions Figure 5.4

helix.

lipids, and nucleic acids 2. Nine, with one water required to hydrolyze each connected pair of monomers 3. The amino acids in the green bean protein are released in hydrolysis reactions and incorporated into other proteins in dehydration reactions.

Concept Check 5.2

1. Both have a g1rcerol molecule attached to fatty acids. The glrcerol of a fat has Ihree fatty acids attached, whcreas the glycerol of a phospholipid is attached to two fatty acids and one phosphate group. 2. Human sex hor· mones are steroids. a type of hydrophobic compound. 3. The oil droplet membrane could consiSI of a single layer of phospholipids rather than a bilayer, because an arrangement in which the hydrophobic tails of Ihe membrane pbospholipids were in contact with the hydrocarbon regions of the oil molccules would be more stable.

Concept Check 5.4 1. Thc function of a protein is a consequence of its specific shape, which is lost when a protein becomes denatured. 2. Secondary structure involves hydrogen bonds between atoms of the polypeptide backbone. Tertiary structure involves bonding between aloms of the R groups of the amino acid subunits. 3. Primary structure, the amino acid sequence, affecls the secondary structure, which affects Ihe tertiary structure, which affects the quaternary structure (if any). In short, Ihe amino acid s<'{juence affects the shape of the protein. Because the function of a protein depends on its shape, a change in primary structure can destroy a protein's function.

Concept Check 5.5 1.

5' end

Unear Form

"

,I

H-C-OH I IC=O ,I

HO-C-H ,I

H-C-DH

1 I-j..!C-OU 1

Hi-O"

"

Four carbons are in the fructose ring. and tv.:o arc not (The latter two carbons are hanging off carbom 2 and 5, which are in the ring.) This form differs from glucose, which has fIve carbons in the ring and one that is not. (Note that the orientation of this fructose molecule is flipped relative 10 Ihe one in Figure 5.5b.) Appendix A

0:

Concept Check 5.1 1. Proteins, carbohydrates,

CHAPTERS

A-3

'

A

Concept Check 5.3

wrsity in Ihe atoms. It can't fonn structural isomers lx'Causdhere is only onl' way for thr'l.'C carbons to attach to each othl'f (in a line). There arc no double bonds, so geometric isomers are not possible. Each carbon has at least tv.·o hydrogcm attached to it. so the molecule is symmetrical and cannot haveenantiometic isomers.

1

3. The absence of these prokaryotes would hamper the cow's ability to obtain energy from food and could lead to weight loss and possibly dealh.

C=c

1. b 2. d 3. a 4. b 5. b 6. a

SH

CH,

CHI

1. C:JH 60 3 2. C'2HUO"

2. The forms of C 4 H,o in (b) are structural isomers. as are the butenes in (c). 3. Both comist largely o£hydrocarbon chains. 4. No. There is nol enough di-

8.

AmJrIt> ~(tIIlP

CHI

OH

3' end


Concept Check b.l

2. S'·TAGGCCf-3' 3'-ATCCGGA-5'

5'-T ArilG C CT-3'

1. Stains used for light microsc:opy are colored moIemles that bind to cell components, affecting the light passing through, while stains used for electron microscopy involve hl'3vy metals that affect the !'x.'ams of ek-etrons passing through. 2. (a) Light microscope, (b) scanningeh:tron microscope, (c) transmission ele<:· tmn microscope

3/-A T\0C

Concepl Check b.l

3. (3)

Mi5P'1O..+c.h

C;

6 A-Sf

1. See Figure 6.9. (b)

2.

Jol

J

3'-A T T C G GA-5' Self-Quiz 1. d 2. c 3. a 4. b 5. a 6. d 7. b 8. 1'.>I':j1'lV ~,.

~I'\t"'tn

.,-c... f""'U>k

''''''

Iu¥f"",10t

~.nO~IDtS

~" """'""

Tj""D?lln~~ ~,·d.'('.II~l:A.1£S

Ufj~

'Fa~ tltj~l

T~:"ejl~~tWI.

frolrinl:

Itfl\l~G.el~$

/b{jpcp-kde~

P~pHde bo~,"

Nu.dto;,deS

1b1'll"/.4clttlhik5

~inter 11.t.. ~

~ld~

ILcicl,

g.

OIl

j ,

OIl

Or~in4J

Ij"~l.S

,

S'

strAllO

E!.tv

S'

I

C,mpl"',"",y &tr4J1d

CHAPTER 6 Figure Questions Figure 6.7 A phospholipid is

<I

lipid. consisting of a glycerol molecule

joined to two fatty acids and one phosphate group. Together, the glycerol and phosphate end of the phospholipid form is the "head," which is hydrophilic.

while the hydrocarbon chains on the fatty acids form hydrophobic "tails." Tbe presence in a single molecule of both a hydrophilic and a hydrophobic region makes the molecule ideal as the main building block of a membrane. Figure 6.22 Each centriole has 9 sets of3 microtubulcs, so the entire centrosome has 54. Each microtubule consists of a helical array of tubulin dimers (as shown in Table 6.1).

This cell would havc the same volume as the cells in columns 2 and 3 but proportionally more surface area than thaI in column 2 and less than that in column 3. Thus, the surface-to-volume ratio should be greater than 1.2 but less than 6. To obtain the surface area, you'd have to add the area of the six sides (the top, bottom, sides, and ends): 125 + 125 + 125 + 125 + 1 + 1 = 502. Thc surface-to-volume ratio equals 502 dividcd by a volume of 125, or 4.0.

Concept Check 6.3 1. Ribosomes in the cytoplasm translate the genetic message, carried from the DNA in the nucleus by mRNA, into a polypeptide chain. 2. Nucleoli consist of DNA and the ribosomal RNA (rRNA) made according to its instructions, as well as proteins imported from the cytoplasm. Together, the rRNA and proteins are assembled into large and small ribosomal subunits. (These are exported through nuclear pores to the cytoplasm, wheretheywill participate in polypeptide synthesis.) 3. The information in a gene (on a chromosome in thc nucleus) is used to synthesize an mRNA that is then transported through a nuclear pore to the cytoplasm. There it is translated into protein, which is transported back through a nuclear pore into the nucleus, where it joins other proteins and DNA, forming chromatin.

Concepl Check 6.4 1. The primary distinction between rough and smooth ER is the presence of bound ribosomes on the rough ER. \'(!hile both types of ER make phospholipids, membranl' proteins and secretory proteins arc all produced on the rio bosomes of the rough ER. Thl' smooth ER also functions in detoxification, carbohydrate metabolism, and storage of calcium ions. 2. Transport vesicles move membranes and substances they enclose between other components of the endomembrane system. 3. The mRNA is synthesized in the nucleus and then passes out through a nuclear pore to be translated on a bound ribosome, attached to the rough ER. The protein is synthesized into thl' lumen of the ER and perhaps modified there. A transport vesicle carries the protein to the Golgi apparatus. After further modification in the Golgi, another transport vesicle carries it back to the ER, where it will perform its cellular function.

Concepl Check 6.5 1. Both organelles are involved in encrgytransfonnation, mitochondria in cellular respiration and chloroplasts in photosynthesis. They both have multiple membranes that separate their interiors into compartments. In both organeUes, the innermost membranes-cristae, or infoldings of the irmer membrane, in mito· chondria, and the thylakoid membranes in chloroplasts-have large surface areas with embedded enzymes that carry out their main functions. 2. Mitochondria, chloroplasts, and peroxisomes are not derived from the ER, nor arc they connected physically or via transport vesicles to organelles of the endomembrane system. Mitochondria and chloroplasts are structurally quite different from vesicles derived from the ER, which are bounded by a single membrane.

Concept Check b.6

Figure 6.29 The microtubules would reorient. and based on the earlier results, the cellulose synthase proteins would also change their path, orienting along the repositioned microtubules. (This is, in fact, what was observed.)

1. Both systems of movement involve long filaments that are moved in relation to each other by motor proteins that grip, release, and grip again adjacl.'Jlt polymers. 2. Dynein arms, powered by ATP, move neighboring doublets of microtubules relative toone another. Because they are anchored within the organelle and with respect to each other. the doublets bend instead of sliding past one another. 3. Such individuals have defects in the microtubule-based movement of cilia and flagella. Thus, the sperm can't move because of malfunctioning flageUa; the airways ar(' compromised; and signaling events during embryogenesis do not occurcorn'Ctly due to malfunctioning cilia.

Answers

A-4

~

•~

.. C


Concept Che<:k &.7 1. The most obvious difference is the presence of direct cytDplasmic oonnecoons between cells of plants (plasmodesmata) and animals (gap junctions). These 0011ncctions r'l'SUIt iI1 the cytoplasm Ixing continuous bctwcrn adjacent ceUs. 2. The cell would not lx, able to function properly and would probably soon die, as the cell wall or ECM must be penneable to allow the exchange of matter bctv.·een the cell and its external environment. Molecules involved with energy production and use must be aDoo,ed entry, as well as those that provide information about the cell's environment. Othcr molecules, such as products synthesized by the cell forexportand the by-products of ceUuIar respiration, must be aDowed to exit.

taken up at a particular time; pinocytosis takes up substances in a nonspecific manner. Self-Quiz

1. b 2. c 3. a 4. d S. b 6. a.

-Cell---l;--0.03 M ;oxrose

Self-Quiz

,02 M glucose

1. c 2. b 3. d 4. d 5. b 6. c 7. e 8. a

",,

9. See Figure 6.9. ~

•~

.. C

En....ronment om M \.lK1"O\.e om Mglucole om Mfrudole

,

CHAPTER 7

'1-1:...0

Figure Queslions Figure 7.6 You couldn't rule out mOl'('ment of proteins within the cell membrane of the same spedes. You might speculate that the membrane lipids and proteins from one species weren't able to mingle with those from the other species because of some incompatibility. Figure 7.9 A transmembrane protein like the intcgrin dimer iI1 (f) might chang., its shape upon binding to a particular ECM molecule. The new shape might enable the iI1terior portion of the protein to bind to a second, cytoplasmic protein that would relay the message to the inside of the cell, as shown in (c). Figure 7.12 The orange solute would be evenly distributed throughout the solution on both sides of the membrane. The solution levels would not be affeded because the orange solute can diffuse through the membrane and l"qualizc its concentration. Thus, no additional osmosis of water would take place in either direction.

Concept Check 7.1 1. They arc on the inner side of the transport wsielc ml'TTlbrane. 2. Plants adapted to cold environments would be expected to hal'(' more unsaturated fatty acids in their membranes because those remain fluid at 10000'er temperatures. Plants adapted to hot elwironments would beexpected to have more saturated fatty acids, ....n ich would aDO\'" the fatty acids to ·stack" more eloscly, making the membranes less fluid and therefore helping them to stay intact at higher tempcrawres.

Concept Che<:k 7.2 1. O 2 and CO 2 are both small nonpolar molecules that can easily pass through the hydrophobic core of a mcmbrane. 2. Water is a polar mokcule. so it cannot pass very rapidly through the hydrophobic region in the middle of a phospholipid bilayer. 3. The hydronium ion is charged, while glycerol is not. Charge is probably more significant than size as a basis forexelusion by the aquaporin channel.

Concept Che<:k 7.3 1. CO 2 is a small nonpolar molecule that can diffuse through the plasma membrane. As long as it diffuses away so the concentration remains low outside the cell, it will continu.' to exit the cdl in this way. (This is the opposite ofth., case for 02' described in this section.) 2. The water is hypotonic to the plant cells, so they take up water and the cells of the vegetable remain turgid. rather than plasmolyzing. The vegetable (for example, lettuce or spinach) remains crisp and not wilted. 3. The activity of the Paramecium's contractile vacuole will decrease. The vacuole pumps out excess water that flows into the cell; this flow occurs only in a hypotonic environment.

Concept Check 7.4 1. The pump uses ATP. To establish a voltage, ions have to be pumped against their gradients, which rcquires energy. 2. Each ion is being transported against its electrochemical gradient. If either ion were flOWing down its electrochemical gradient, this WQuld be considered cotransport. 3. Even if proton pumps were still using ATP and moving protons, no proton gradient would become established. This would have serious consequences forthecells, because processes Uke the cotranspon of sucrose (as wellassynthcsis of ATP) dep<'nd on establishment of a proton gradient.

Concept Check 7.S 1. Exocytosis. When a transport vesicle fuses with the plasma membrane, the vesicle membrane becomes part of the plasma membrane. 2. Receptormediated endocytosis. In this case, one specific kind of molecule needs to be

A-S

Appendix A

b. The solution outside is hypotonic. It has less sucrose, which is a nonpenetrating solute. e. See answer for (a). d. Th., artificial cell will become more turgid. e. Eventually, the two solutions will have the same solute concentrations. Even though sucrose can't move through the membrane, water flow (osmosis} will lead to isotonic conditions.

CHAPTER 8 Figure Questions Figure 8.14

Figure 8.18

~

~..... r I

:@@ @@J

t

~I"'''''~to,.,...

,I $h--.-4..,....,---':>-'..,....,-~

3~~~1&'1lI

.~

Figure 8.21 Because the affinity of the caspase for the inhibitor is very low (as is expected of an allosterically iI1hibited enzyme), the inhibitor is likely to diffuse away. &'Cause no additional source of the inhibitory compoUJ1d is present (the concentration of inhibitor is very10\','), the iI1hibitor is unlikely to biI1d again to the enzyme once the covalent linkage is broken. Thus, normal activity of the enzyme would most likely not be affected. (This test was performed by the researchers, and enzyme activity was observed to be normal upon release of the inhibitor.)

Concept Ched 8.1 1. The second law is the trend toward randomness. Equal concentrations of a substance on both sides of a membrane is a more random distribution than unequal concentrations. Diffusion of a substance to a region where it is initially less conccntrated increascs entropy, as described by thc second law. 2. The apple has potential energy in its position hanging on the tree, and the sugars and other nutrients it contains have chemical energy. The apple has kinetic energy as it fans from the tree to the ground. Finally, when the apple is digested and its molecules broken down, some of the chemical energy is used to do work, and the rest is lost as thermal energy. 3. The sugar crystals become less ordered (entropy increases} as they dissolw and become randomly spread out in the water, Over time, the water evaporates, and the crystals form again because the water volume is insufficient to keep them in solution. While the reappearance of sugar crystals may represent a "spontaneous" increase in order (decrease in entropy), it is balanced by the decrease in order (increase in entropy} of the water molecules, which changed from a relatively compact arrangemcnt in liquid water to a much more dispersed and disordered form in water vapor.

Concept Cheel< 8.2 1. Cenular respiration is a spontaneous and exergonic p = Theenergy released from glucose is used todo work in thecell or is lost as heat. 2. Hydrogen ions can


petform wor{.; only iftheirconcentrntions on each side ofa membrane differ. When the H+ concentrations are the same, the ~tem is at equilibrium and can do 110 wor{.;. 3. The reaction is exergonic because it releases energy-in thiscase, in the form ofUght. (This is a chmlical wrsion of the bioluminescence St.'t.'T1 in Figure RI.}

Conn'pl Check 8.3 1. ATr transfers energy to endergooic processes by phosphorylating (adding phosphalC groups to) other moh:ules. (Exergonic processes phosphorylate ADr to regenerate ATP.) 2. A set of coopled reactions can transform the first combination into the second. Since, overall, this is an exergonic process, JlG is negative and the first group must have more free energy. (See Figure RIO.} Conn'pl Check 8.4 1. A spontaneous reaction is a reaction that is exergonic. However, if it has a high activation energy that is rarely attained, the rate of the reaction may be low. 2. Only the spedflc substrate(s) wiU fit properly into the active site of an en~yme, the part of the enzyme that carries out catalysis. 3. Increase the con路 centration of the normal substrate (succinate} and see whether the rate of reaction increases. If it does, malonate is a competitive inhibitor. Concepl Check 8.5 1. The activator binds in such a way that it stab~i1.CS the active form ofan enzyme, whereasthe inhibitor stabilizes the inactive form. 2. You might choose Ioscreen chemicalcompounds that bind allosterically to tbe enzyme, because allosteric reg. ulatory sill'S are less likely to share similarity lJctv,'l'Cn diffl'fCnt enzyml"S. Self-Quiz

1. b 2. c 3. b 4. a 5.

I'

6. c 7. c

Scientific Inquiry 9. t ~ A: The substrate molecules are entering the cells, ~ so no product is made yet. 1-~ D B: There is sufficient substrate, so the reaction is &. B proceeding at a maximum rate. <ti;: C: As the substrate is used up, the rate falls. A "Tt""f. --+ D: The line is flat because no new substrate remains and thus no new product appears.

r

CHAPTER 9 Figure Questions Figure 9.7 Because an enzyme is catalyzing this reaction and tbere is no ex路 ternal source of energy, it must be C};ergonic, and the reactants must be at a higher energy Icvc1 than the products. Figure 9.9 It would probably stop glycolysis, or at least slow it down, since this would tend to push the equilibrium for this step toward the left. If less (or no) glyceraldehyde-3-phosphate were made, step 6 would slow down (or be unable to occur). Figure 9.15 Rotation in the opposite direction (blue bars} would be C};pccted to hydrol)'"ll' some of the ATr present, lowering ATr concentration below the background kvel. Thus, the blue bars would be expected to be lower than the gray bars, which is not what the researchers observed. (A possible explanation: In the article, the researchers explained that when they endosed ATP synthases in a chamber for this assay, a number o( the complexes adhered to the chamber ceiling instead of the nickel plate. The enzymes adhering to the chamber ceiling would be expl'Cted to spin in an opposite dirl'Ction to those on the nickel plate on the floor. When the floor-based enzymes produce AT!' during a particular spin ()'ellow bars), those on the ceiling would be expected to consume ATP, which would make the yellow bars lower than they would have be<-n if all enzymes werefloor路based. Theopposite isalso true: When thefloor路based enzymes hydrol)'"ll' ATr (blue bars), the ceiling-based enzymes would be synthesizing ATP, which would make the blue bars higher than if all enzymes w<'re floor-based. Evidmceofthis phenomenon is shown in the graph: Th<'spins npected to hydrolyze ATP (blue bars) result in higher AT!' levels than those with no rotation (gray bars), suggesting that there are probably some "upsidedown" ceiling-based complexes generntingATr while the rest are floor-based and are hydrolyzing ATP.) Figure 9.16 At first, some ATr could be made, since ekctron transport could procel-d as far as compk'X 111, and a small H' gradient could be built up. Soon, how<'Ver, no more electrons could lx, p3SSl-d to compk'X III because it could not be reoxidized by passing its electrons to complex IV.

Concept Check 9.1 1. Both processes include glycolysis, the citric acid cycle, and oxidative phosphorylation. In aerobic respiration, the final electron acceptor is molecular oxygen (Oz), whereas in anaerobic respiration, the final electron aca:plor is a different substance. 2. C1 H6 0 5 would be oxidized and NAD+ would be reduced. Concepl Check 9.2 1. NAD+ actsas the oxidizing agent in step6, accepting electrons from glyceraldehyde-3-phosphate, which thus acts as the redUcing agent. 2. Since the o....erall process of glycolysis results in net production of ATr, it would make sense for the process toslow dm'irl ....-hen ATr revels have increased substantially. Thus we would expect A11' to allosterically inhibit phosphofructokinase. Concept Check 9.3 1. NADH and FADH2l they will donate electrons to the electron transport chain. 2. CO 2 is released from the pyruvate that is formed during glycolysis, and CO2 is also released during the citric acid cyele. 3. In both cases, the precursor molecule loses a CO 2 molecule and then donates electrons to an electron carrier in an oxidation step. Also, the product has been activated due to the attachment of a CoA group. Concept Check 9.4 1. Oxidative phosphorylation would stop entirely, resulting in no ATr production by this process. Without oxygen to "pull" electrons down the electron transport chain, H' would not be pumped into the mitochondrion's intermembrane space and chemiosmosis would not occur. 2. Decreasing the pH is the addition of H+.tt would establish a proton gradient even without the (unction of the electron transport chain, and we would expect ATP synthase to function and synthesize ATr. (In fact, it was experiments like this that provided support (orchemiosmosis as an energy-coupling mechanism.) Concept Check 9.5 1. A derivative of pyruvate-such as acetaldehyde during alcohol fermentation-or pyruvate itself during lactic acid fermentation; oxygen. 2. The cell would need to consume glucose at a rate about 19 times the consumption rate in the aerobic environment (2 ATP are generated by fermentation versus up to 38 ATr by cellular respiration). Concept Check 9.6 1. The fat is much more reduced; it has many -CH 2- units, and in all these bonds the electrons are equally shared. The electrons present in a carbohydrate molecule are already somewhat oxidized (shared unequally in bonds), as quite a few of them are bound to oxygen. 2. When we consume more food than necl'SSary for mclaboUc processes, our body synthesizes fat as a way of storing energy for later use. 3. AMI' will accumulate, stimulating phosphofructokinase, which increases the ratcof glycolysis. Since oxygen is not present, the cen will convert pyruvate to lactate in lactic acid (ermentation, providing a supply ofATP. Self-Quiz

1. b 2. d 3. c 4. c

5. a

6. a

7. d 8. b 9. b

10.

Time_

CHAPTER 10 Figure Questions Figure 10.9 Red, but not violet-blue, wavelengths would pass through the mter. so the bacteria would not congregate where the violet-blue light normally comes through. Therefore, the left "peak" of bacteria would not be present, but the right peak would be observed because the red wavelengths passing through the filter would be used for photosynthesis. Figure 10.11 In the leaf, most of the chlorophyll electrons excited by photon absorption arc USl-d to po....w the reactions of photosynthesis.

Answers

A-6


The AT? would end up outside the thylakoid. The chloroplasts were able to make ATP in the dark because the researchers set up an artiticial proton concentration gradient across the thylakoid membr.me: thus, the light reactions ".iere not necessary to establish the H' gradicnt requirt.xt for ATPsynthesis by ATP synthase.

Figure 10.18

CHAPTER 11

~

•~

.. c

Three carbon atoms enter the cycle, one by one, as individual CO 2 molecules, and leave thecyc1c in one three-<:arbon mokculc (G31') pcrthrt.'C turns ofthecyc1e. Concept Check 10.1 1. CO 2 enters leaves via stomata, and water enters via roots and is carried to leaves through veins. 2. Using lBO, a h("avy isotope of oxygen, as a labd, van Niel was able to show that the oxygen produced during photosynthesis originates in water, not in carbon dioxide. 3. The light reactions could not keep producingNADI'H and ATP without the NADI'+, ADP,and that the Calvin cycle generates. The two cycles are interdependent.

®;

Concept Che<:k 10.2 1. Green, because green light is mostly transmitted and reflected-not absorbed-by photosynthetic pigments 2. In chloroplasts, light.excited clc<:trons arc trapped by a primary electron acceptor, which prevents them from dropping back to the ground state. In isolated chlorophyll, then' is no elc<:tron acceptor, so the photoexcited electrons immediately drop back down to the ground state, with the emission of light and heat. 3. Water (H 2 0} is the initial electron donor; NADI'+ accepts electrons at the end of the electron transport chain, becoming reduced to NADI'H. 4. In this experiment, the rate of AT? synthesis would slow and eventually stop. Because the added compound would not allow a proton gradient to build up across the membrane, ATP synthase could not catalyze ATP production,

Concept Che<:k 10.3 1. 6, 18, 12 2. The more potential energy a molecule stores, the more energy and rNucing power is required for the formation of that molewle. Glucose is a valuable energy source because it is highly reduced, storing lots of potential energy in its electrons. To reduce CO 2 to glucose, much energy and reducing power are fC{juircd in the fonn oflarge numbersofAT? and NADPH molecules, reslX'Ctivdy. 3. The light reoctions require ADP and NADP', which would not lx, formed in suffICient quantities from ATP and NADPH if the Calvin cyclestopped.

Concept Che<:k 10.4 1. I'hotorespiration decreases photosynthctic output by adding oxygen, instead of carbon dioxide, to the Calvin cycle. As a result, no sugar is generated (no carbon is fixed), and O 2 is used rather than generated. 2. Without I'S II, no O 2 is generated in bundle-sheath cells. This avoids the problem of O 2 competing with CO 2 for binding to cubisco in these cells. 3, C 4 and CAM spc<:ics would replace many of the C:! species. Self-Quiz 1. d 2. b

3. b

4. c

5, d

6. d

7. c

Figure Questions Figure 11,6 Epinephrine is a signaling molecule: presumably it binds to a cell· surface receptor protein. Figure 11.8 The testosterone molecule is hydrophobicandcan therdore pass dircctJythrough the lipid bUayl'Tofthe plasma ml'fllbfanc into the cell. (Hydrophilic molecules cannot do this.} Figure 11.9 The active form of protein kinase 2 Figure 11.10 The signaling molecule (cAMP) would remain in its active fonn and would continue to Signal. Figure 11,16 In the model. the directionality of gro....t h isdetermined by the association ofFus3 ",ith the membrane ncar thl' site of rt.'Ceptor activation. Thus, the devdopm('T1t of stmlOOS would be 8l.'vercly compromi8lxt, and the affl'Cted ceO would likely resemble the ~Fus3 and Monnin cells.

Concept Check 11,1 1. The two cells of opposite mating type (ill and al each secrete a certain signal. ing molecule, which can only be bound by receptors carried on cells of the orr posite mating type. Thus, the a mating factor cannot bind to another a cell and cause it to grow toward the first a cell Only an a cell can 'receive" the signaling moh:ule and respond by direcllxt gro....t h (see Figure 11.16 for more information}. 2, The secretion of neurotransmitter molc<:ules at a synapse is an exampleoflocal signaling. Theelectrical signal that travels along a very long nerve cell and is p<lssed to the next nerve cell can be considered an example of longdistance signaling. (Note, however, that local signaling at the synapse between two cells is necessary for the signal to pass from one cell to the next.) 3. GlucoseI-phosphate is not gl'fK'l'allxt, lx'Cause the activation of the enzyme requires an intact cell, ....i th an intact receptor in the ml'fllbrane and an intact signal transduction pathway. The enzyme cannot be activated directly by interaction with the signaling molecule in the test tube. 4. Glycogen phosphorylase acts in the third stage, the response to epinephrine signaling.

Concept Check 11.2 1. The water-soluble NGF molecule cannot p<lss through the lipid membrane to reach intracellular receptors, as steroid hormones can. Therefore, you'd expect the NGF receptor to be in the plasma membrane-which is, in fact, the case. 2. The cell ....i th the faulty recl'ptor would not be able to respond appropriately to the signaling mol~ule when it was present. This .....ould most likely have dire consequences for the cell, since regulation of the cell's activities by this r~eptor would not occur appropriately.

Concept Check 11,3 1. A protein kinase is an enzyme that transfers a phosphate group from ATP toa protein, usually activating that protein (often a second type of protein kinase). Many signal transduction pathways include a series of such interac· tions, in which each phosphorylated protl'in kinase in turn phosphorylates the next protein kinase in the series. Such phosphorylation cascades carry a signal from outside the cell to the cellular protein(s) that will carry out the response. 2. Protein phosphatases reverse the effects of the kinases. 3. Information is transduced by way of sequential protein· protein interactions that change protein shapes, causing them to function in a way that passes the signal along. 4, The IP 3 -gated channel opens, allowing calcium ions to flow out of the ER, which raises the cytosolic Ca H concentration.

Scientific Inquiry

Concept Check 11,4

9.

1. Ateach step in acascade ofsequential activations, one molecule orion may activate numerous molecules functioning in the next step, 2. Scaffolding proteins hold molecular components of signaling p<lthways in a complex with each other. Different scaffolding proteins would assemble different collections of proteins, leading to different cellular responses in the two cells.

~-----

Concept Check 11.5 1. In formation of the hand or paw in mammals, cens in the regions between the digits are programmed to undergo apoptosis. This serves to shape the digitsofthe hand orp<lw so that they arc not wl-bblxt. 2. If a Tl'CCplor protein for a death·signaling molecule was defective so that it was activated for signaling even in the absence of the death signal, this would lead to apoptosis when it

A-7

Appendix A


wouldn't normally occur. Similar defects in any of the proteins in the signaling pathway, which would activate these relay or response proteins in the absence of interaction with the previous protein or second messenger in the pathway, would have the same effect. Conversely, if any protein in the path,,-ay were defectil'e in its ability to respond to an interaction with an early prOlein or other molecule or ion, apoptosis would not occur when it nonnally should. For example. a receptor protein for a death-signaling ligand might not be able to be activated, even when ligand was bound. This would stop the signal from being transduced into the cell. Sclf-Quiz 1. c 2. d 3. a 4. c 5. c 6. b 7. a 8. d 9. This is one possible drawing of the pathway. (Similar drawings would also be correct.)

would not have occurred until the S and

G~

phases had been completed.

Figure 12.15 The cell would divide under conditions where it was inappropriate to do so. If the daughter cells and their descendants also ignored the checkpoint and divided, there would soon b.' an abnormal mass of cells. (This type of inappropriate cell division can contribute to the development of cancer.) Figure 12.16 Given that control experiments showed that the aie2 protein kinase was the primary source of kinase activity detected in this experiment, there would be virtually no kinase activity. The percentage of cells dividing would be zero because the cells would be unable to undergo mitosis without thc ede2 kinase. Figure 12.18 Th., cells in the vessel with PDGF would not be able to respond to the growth factor sig· nal and thus would not divide. The culture would resemble that without the added PDGF. Concept Check T2.1 1. 32 cells 2. 2 3. 39; 39; 78

~

Concept Check 12.2 1. 6; 12 2. Cytokinesis results in genetically identical daughter cells in both plant cells and animal cells, but the mechanism ofdi\~ding the cytoplasm is different in animals and plants. In an animal cell, cytokinesis occurs by cleavage, which divides the parent cell in two with a contractile ring of actin filaments. [n a plant cell, a cell plate forms in the middle of theeI'll and grows until its membrane fuses with the plasma membrane of the parent celL A new cell wall gro",; inside the cell plate. 3. They elongate the cell during anaphase. 4. Sample answer. Each type of chromosome consists of a single molecule of DNA with attached proteins. If stretched out, the molcrules of DNA would be many timl'S longer than the cells in which they reside. Ouringcell division, the two copies of each type of chromosome actively move apart, and one copy ends up in each of the two daughter cells. Chromosome movement in both types of cells may involve similar cytoskeletal proteins. 5. During eukaryotic cell division, tubulin is involved in spindle formation and chromosome movement, while actin functions during cytokinesis. [n bacterial binary fission, it's theopposite: Tubulin-Iike molecules ar.' thought to act in daughter cell separation, and actin· like molecules are thought 10 move the daughter bacterial chromosomes to opposite ends of the cell. 6. From the end ofS phase in interphase through the end of metaphase in mitosis

""0

CHArnR12 Figure Questions Figure 12.4

Circling the other chromatid instead would also be corre<:t. The chromosome has four arms. Figure 12.6 12: 2; 2; I Figure 12.7

Concept Check 12.3 1. The nucleus on the right was originally in theG, phase; therefore, it had not yet duplicated its chromosome. The nucleus on the left was in the M phase, so it had already duplicated its chromosome. 2. A sufficient amount of MPF has to build up for a cell to pass the Gz checkpoint. 3. Most body cells are in a nondividing state called Go> 4. Both types of tumors consist of abnormalcells. A benign tumor stays at the original site and can usually be surgically removed. Cancer cells from a malignant tumor spread from the original site by metastasis and may impair the functions of one or more organs. 5. The cells might divide even in th., absence of PDGf, in which case they would not stop when the surface of the culture vessel was oover.'d; they .....ould continue to divide, piling on top of one another. Self.Quiz

1. b 2. a 3. a 4. c 5. c 6.

I'

7. a 8. b

9. See Figure 12.6 for a description of major events.

".k

"''""<::'''Mt~

"'-'~-",­ flIG'It"tol-

Figure 12.8 The mark would have moved toward the nearer pole. The lengths of fluorescent microtubules between that pole and the mark would have decreased, while the lengths between the chromosomes and the mark would have remained the same. Figure 12.131n both cascs, the G 1 nucleus would have remained in G 1 until the time it normally would have en· tered the S phase. Chromosome condensation and spindle formation

Answers

A-S

•~

.. c


10.

~

th"~'{S) N""w

'€tl~l~

r..tup~

PrQphASe

genome (in the absence of mutation}. 3. She should done it. Breeding it would generate offspring that have additional variation, which she no longer desires now that she has obtained her ideal orchid.

Concepl Ched;: 13.2 1. A female has two Xchromosomes: a male has an X and a Y. 2. In meiosis, the chromosome COllIlt is reduced from diploid to haploid; the union of two haploid gao mctes in fcrrilization r'l'Storl'S the diploid chromosome count. 3. The haploid numbcr(n) is 7; the diploid numbcr(2n) is 14. 4. Thisorganism hasthe lifecyck: sho.....n in Rgure 13.6c. Therefore, it must be a fungus or a protist, perhaps an alga. Concept Ched< 13.3 1. The chromosomes arc similar in that each is composed oft.....o sisterchro· matids, and the individual chromosomes are positioned similarly on the metaphase plate. The chromosomes differ in that in a mitotically dividing cell. sister chromatids of each chromosome are genetically identical, but in a meiotically dividing cdl, sister chromatids arc genetically distinct because of crossing over in meiosis 1. Moreover, the chromosomes in metaphase of mitosis can be a diploid set or a haploid set, but the chromosomes in metaphase of meiosis 11 always consist of a haploid set. 2. If crossing over did not occur, the two homologs would not be associated in anyway. This might result in incorrect arrangement of homo logs during metaphase I and ultimately in formation of gametes with an abnormal number of chromosomes.

~

•~

.. C

Concepl Ched;: 13.4

1. Mutations in a gene lead to the different versions (alleles) ofthat gene. 2. Without crossing over, independent assortment of chrornosoml'S during

CHAPTER 13 Figure Queslions Figure 13.4 The haploid number, n, is 3. A set is alwa~ haploid. Figure 13.7 A short strand of DNA is shown here for simplicity, but each chromosome or

chromatid contains a very long coiled and folded DNA mok·culc.

meiosis I theoretically can generate 2" possible haploid gametes, and random fertilization can produce 2" x 2" possible diploid zygotes. Because the haploid number (n) of grasshoppers is 23 and that of fruit flies is 4, two grasshoppers would be expected to produce a greater variety of zygotes than would two fruit flies. 3. If the segments of the maternal and paternal chromatids that un· dergo crossing over arl' genetically identical and thus have the saml' two alleles for every gene, then the recombinant chromosomes will be gendically l'quivalent to the p3rental chromosomes. Crossing over contributes to genetic varia· tion only when it involves the rearrangement of different aUeles. Self.Quiz

1. a 2. d 3. b 4. a 5. d 6. c 7. d 8. This cdl must be undergoing meiosis because homologous chromosomes are associated with each other; this does not occur in mitosis. 9. Metaphase I

10.

S·s/u a.rt;mIHJs (.r.tJIloe CDIIT) ~ . t tu chromatiJ lyj'alw

k'sndcchorn; Non!.I5ttr~~nc15:

(o:l;/fUtfltCd~(I;)

Chrart1~ (repll'wed

~"'l61~!>flIir at Figure 13.9 Yes, Each of the chromosomes shown in telophase [ has one nonrecombinant chromatid and one recombinant chromatid. Therefore. eight possible sets of chromosomes can be generated for the cell on the left

and eight for the cell on the right.

"P.1.irrJ

hDm6('T

eI\r~ll1o.k1fl1tS tfcrc calif Il'\1U. u.pa. hD..p1c:id ~ All ~ arJ b1ut. ~ -tn,t+nu flIIl1t up It d<pad set.

1ht

Figure 13.10 The chromosomes aris-

ing from chromatids oflhe unlabeled chromosome would be expected to be-

have exactly like thoS<' of the labdcd chromosome. Therefore, the graph would look identical to the one shown in the figure.

CHAPTER 14

Concept Check 13.1

Figure Questions Figure 14.3 All offspring would have purple flowers. (Thl' ratio would be one purple to zero white.) The I' generation plants are true· breeding, so mat· ing two purple-flowered plants produces the same result as self-po11ination: All the offspring have the same trait.

1. Pan:nts pass genes to their offspring; the genes program cells to make specific enzymes and other proteins, whose cumulative action produces an individual's inherited traits. 2. Such organisms reproduce by mitosis, which generates offspring whose genomes are exact copies of the parent's

A-9

Appendix A


The majority of individuals have intermediate phenotypes (skin color in the middle range), while fewer individuals have phenotypes at either end (very dati; orvery light skin). (As you may know, this is called a "bell curve" and represents a "normal distribution.") Figure 14.16 In the l'unnett square, two ofth.· three individuals with normal coloration arc carriers, so the probability is7\.

Concepl Check 14,1 1. A cross of Ii x ii would yield offspring with a genotypic ratio of Iii: 1 ii (2:2 is an equivalent ans....-er) and a plK"T1otypic ratio of 1 inllatcd: 1 constricted (2:2 is equivalent).

If i r\:lepeMfIlh:llSart mtll+:

y

~~:r

1,</@

x

ffr

y

ii

Sl"'m iTbm it pllUlt

o

Q

Ii

Ii

"

"

CD

1'A("fIIls Ylj~r

Ii

Ge~pil.

U.:Z

k;

~

•~

.. c

rtJJ-ilJ 1 L·; 1 u:

e.tt¥IV41tf1t)

Sp"m /Tom

1~l'e~~pi' roth·o 1 ifttWW : I Cilnstridtd

Y!:JIlf" plu7+-

(U. is etu.ivoJtn+)

1/,(9 1,</0 1/,(ij

(0/ YlRr I GJ 'rye(

Rr

[:ire

I

1".tj jtJk>lll-fDund

: 1/4.j€llDw-uril1lJed: 1JI ~ret:Jl-r~ : IN tj'W'- uri'li'.leJ 1

1

'Phtnoi'jpit:-

1 mti ()

1

Yes, this cross would also have allowed Mendel to make different predictions for the two hypotheses, thereby allowing him to distinguish the correct one. Figure 14.10 Your elassmate would probably point out that the F) generation hybrids show an intermediate phenotype between those of the homozygous parents, which supports the blending hypothesis. You could respond that crossing the F, hybrids results in the reappearance of the white phenotype, rather than identical pink offspring, which fails to support the idea of blending traits during inheritanee. Figure 14.11 Both the t'- and I B al· leles arc dominant to the i allele, which results in no attached carbohydrate. The ,'" and alleles arc codominant; both arc expressed in the phenotype of ''''I Bheterozygotes, who have type AB blood. Figure 14.13

2. According to the law of independent assortment' 25 plants (y,. of the offspring) ar<' predick'd to be aatt, or recessive for both characters. The actual result is likely to differ slightly fnHll this value.

'fh.renb

!ta. Tt-)(.

y

S~fTl h-olll

Ems {rom ~,n

I'",t

,B

AiT+

IfI.Tfplant

®®@9

® AIITT

MTI

1lA7T

IJ"Tt

@J

MH

fIA/f

A.T+

At/f

@

Ij"Tr

A,T/

"TT "T+

@>

I+I..T+

Ad/

aa.T+

,,/I-

3. The plant could make eight different gametes (YR', YRi, Yr!, Yri, yRI, yRi, yr!, and yri). To fit all the possible gametes in a self-pollination, a Punnett square would need 8 rows and 8 columns. It would have spaces for the 64 possible unions of gametes in the offspring.

"'M

Concept Check T4.2 1. ~ homozygous dominant (CC j, 0 homozygous recessive (ee), and ~ heterozygous (Ce) 2. 'I. BBDD; 'Ii BbDD; 'I. BBDd; 'Ii BbDd 3. The geno· types that fulfill this condition arc ppyyli, ppYyii, Ppyyii, ppITii, and ppyyii. Use the multiplication rule to find the probability of getting each genotype. and then use the addition rule to find the overall probability of meeting the conditions of this problem:

I~ I~

pp~~ r; ~ Ir""LW'IJ<f/l'ldl; Ill" l> Iii) = +. ffYjii Y;Llpfh.l1l.(YJh(~(ii) -=hPf~~i"

ffYlfL'~'. ff!1~j.l.

V1(PpJ.d~(~:f.X%.(iiJ

!J1CfFx v.,o(yy)", 'h.fii) Y.z.(pp)'>C V'+(~~))C VuiiJ

h-a.cfion prtdided. -tv h4>1e 'tt Iet1.ST jw, 'lctM;lIe +rOods

-= h, =k '=

=-

*

It or % Answers

A- JO


Concept Che<:k 14.3 1. Incomplete dominance describes the relationship between two alleles of a single gene, whereas epistasis relates to the genetic relationship between two g<>nes (and the respl'Ctive alleles of each). 2. Half of the children would be cxpl'Cted to have type A blood and half type B blood. 3. The black and white alleles are incompletely dominant, with heterozygotes being gray in color. A cross between a gray rOOSler and a black hen should yield approximately equal number.; of gray and black offspring.

Concept Che<:k 14.4 1. :0 (Since cystic fibrosis is caused bya recessive allele, Beth and Tom's siblings who have CF must be homozygous recessive. Therefore, each parent must be a carrier ofthe rc<:essive allele. Since neither Beth nor Tom has CF, this means they each have a lo: chance ofbeinga carrier.lfthq' are both carriers, there is al(chance that they will have a child with CF. Xx Xx)( = 禄); 0 (Both Beth and Tom would have to be carriers 10 produce a child with the disease.) 2. Joan's genotype is Dd. Because the allele for polydactyly (D) is dominant to the allele for five digits per appendage (d), the trait is expressed in people with either the DD or Dd genotype. But becausc Joan's father doc'S not have polydactyly, his genotype must hedd, which means Joan inherited adaUde from him. Therefore Joan, who docs have the trait, must be heterozygous. 3. Since polydactyly is a dominant trait, one of the parents of an affeded individual should show the trait. Therefore, this must be an extremely rare casc of a mutation that occurred during formation of one of the gametes involved in the fertilization that created Peter.

Gem'tics Problems 1. Gene, I. Allele, e. Character, g. Trait, b. Dominant allele, j. Recessive allele, a. Genotype, k. Phenotype, h. Homozygous, c. Heterozygous, f. Testcross, i. Monohybrid cross, d.

2.

@J

6&U 66f' G6Ii

GGtii

G-JII G-iI' Ggli

Gjii

6.lIJ:

G:9li

!JJIl 9jli

GgIi

G!1 ii

jJIi

gJit'

'fjrten.inHxteJ: 35Wl-tcnrlridtti:

.3 jeUOll).lrfl~:

1(1!l1~1ll- Ctnstrj~

3. Parental cross is MCIlC Il xaaCwCW. F[ genotype isAaCIl C W, phenotype

is all axial-pink. F2 genotypes are I AAC II C ll : 2 AACIlC W : I AACwC w : 2 AaCIlC Il :4AaC II C w : 2AaC wC w : laaCIIC II : 2aaC II C w : laaCwCW: F2 phenotypcsarc3axial-RxI; 6axial-pink : 3axial-white: 1 terminal-red: 2 tenninal-pink : I tenninal-white. 4. a. ',. b. '..

c. " d. '" 5. Albino (b) is a recessive trait; black (8) is dominant. First cross: parents BB x bb; gametes Band b; offspring all Bb (black coat}. Second cross: parents Bb x bb; gametes Y, Band Y, b (heterozygous parent) and b; offspring Y, Bb and \'i bb. 6. a. PPLl x ppu. PPLl x PpLl, or PPLl x ppLl.

b. ppLl x ppLl. c. PPLL x any ofthl' 9 possible genotypes or pp{l x ppLL. d. PpLi x Ppll. e. PpLi x PpU.

A-II

Appendix A

f\

7. Man

\\"oman fJi; child ii. Other genotypes for children are Y,

r,B,

y,ri,Y,fi. 8. a.Y,x'Y.x%="" b.I-')'"=",,

c. Y,xv.xY,

,.

= '..

d.l-Y"=",,

, ',.

b. L

d.

'" ',. '"

e. '",

10.

,.

b '" e. " d.

"

11. " 12. Matings of the original mutant cat with true-brl-eding noncurl cats will produce both curl and noncurl F] offspring if the curl allele is dominant, but only noncurl offspring if the curl allele is recessive. You would obtain some true-breeding offspring homozygous for the curl allele from matings between the F1 cats resulting from the original curl x noncurl crosses whether the curl trait is dominant or recessive. You know that cats are true-breeding when curl x curl matings produce onlycurl offspring. As it turns out, the allele that causes curled cars is dominant. 13. '.. 14. 25% will he cross-eyed; all of the cross-e)'ed offspring will also be white. 15. The dominant allele' is epista路 tic to the Pip locus, and thus the genotypic ratio for the F, generation will be 9 1]_ (colorless): 3Cpp{colorless): 3iiP_(purple): I iipp {red}. Overall, the phenotypic ratio is 12 colorless: 3 purple: I red 16. Rcu'SSivc. All affc<:te<! individuals (Arlene, Tom, Wilma, and Carla} arc homozygous R'U'SSivc aa. George is Aa, sincesome ofhis children with Arlene are affected. Sam, Ann, Daniel, and Alan are each All, since they are an unaffected children \\"ith one affected parent. Michael also isAa, since he has an affected child (Carla) with his heterozygous wife Ann. Sandra, Tma, and Christophercan each have theAA or Aa genotype. 17. " 18.', 19. 9 B_A_ (agouti): 3B_aa (black): 3bbA_ (white): I bbaa (white}. Ovl'l'all, 9 agouti: 3 black: 4 white.

CHAPTER 15

Figure Questions Figure 15.2 The ratio would be I yellow-round: 1green-round: I yellowwrinkled: I green-wrinkled. Figure 15.4 About Y. of the F2 offspring would have red eyes and about 'I. would have white eyes. About half of the white-eyed flies would be female and half would be male; about half of the red-eyed flies would he female. Figure 15.7 All the males would becolorblind, and all the females would be carriers. Figure 15.9 The two largest classes would still be the parental-type offspring, but now they would be gray-vestigial and black-normal bc<:ause those were the spc<:if1c allele combinations in the P generation. Figure 15.10 The two chromosomes below, left are like the two chromosomes inherited by the Fj female, one from each P generation ny. They arc passed by the F] female intact to the offspring and thus could be called "parental" chromosomes. The other two chromosomes re路 suIt from crossing over during meiosis in the F] female. Because they have combinations of alleles not seen in either of the F, female'schromosomes, they can be called "recombinant" chromosomes.

88~@ '--_-,y

Po..renta.1 c.hromoSoll\~S

J

l

v

Ru:.ombino.t'lt

)

c.hromoSoll\~S

Concepl Check 15.1 1. The law of segregation relates to the inheritance of alleles for a single character. The law of independent assortment of alleles relates to the inheritance of alleles for t\\"o characters. 2. The physical basis for the law of


segregation is the separation of homologs in anaphase L The physical basis for the law of independent assortment is the alternative arrangements of homologous chromosome pairs in metaphase 1. 3. To show the mutant phenotype, a mak needs to possess only one mutant allek. If this gene had been on a pair of autosomes, two mutant alleles would have had to be present for an individual to show the mutant phenotype, a much less probable situation.

6.

'.:~b:':"9 b' '(j

Concept Check 15.2 1. Because the gene for this eye-color character is located on the X chro· mosome, all female offspring will be red·eyed and heterozygous wt W (X X ); all male offspring will inherit a Ychromosome from the father and be white-eyed (Xwy). 2.)1,; ~ chance that the child will inherit a Y chro· mosome from the father and be male x Y, chance that he will inherit the X carrying the diseaSl.' allek from his mother. If the child is a boy, there is a \Ii chance he will have the disease; a female would have zero chance (but \Ii chance of being a carrier). 3. The cells in the eye responsible for color vision must come from multiple cells in the early embryo. The descendants of half of those cells express the allele for normal color vision and half the allele for color blindness. Having half the number of mature eye cells expressing the normal anele must be sufficient for normal color vision.

Concept Check 15.3 1. Crossing over during meiosis 1in the heterozygous parent produces some gametes with re<:ombinant genotypes for the two genes. Offspring with a recombinant phenotype arise from fertilization of the recombinant gametes by homozygous recessive gametes from the double-mutant parent. 2. In each case, the alleles contributed by the female parent determine the pheno· type of the offspring because the male contributes only recessive alleles in this cross. 3. No. Theordercould beA-C-B or C-A-B. Todetermine which pOSSibility is correct, you need to know the recombination fr({juency between Band C.

Concept Check 15.4 1. At some point during development. one of the embryo's cells may have failed to carry out mitosis after duplicating its chromosomes. Subsequent nor· mal cell cyek'S would produce genetic copies of this tetraploid cell. 2. In meiosis, a combined 14-21 chromosome will behave as one chromosome. If a gamete receives the combined 14-21 chromosome and a normal copy of chromosome 21, trisomy 21 will result when this gamete combines with a normal gamete during fertilization. 3. No. The child can be either JAr'i or could result from nondisjunction in the faii. A sperm of genotype ther during meiosis [I, while an egg with the genotype ii could result from nondisjunction in the mother during either meiosis I or meiosis 11.

,A

,At'

Concept Check 15.5 1. Inactivation of an X chromosome in females and genomic imprinting. Because of X inactivation, the effective dose of genes on the X chromosome is the same in males and females. As a result of genomic imprinting, only one allele of certain genes is phenotypically expressed. 2. The genes for leaf coloration arc located in plastids within the cytoplasm. Normally, only the maternal parent transmits plastid genes to offspring. Since varkgated offspring are produced only when the female parent is of the B variety, we can conclude that variety B contains both the wild-type and mutant alleles of pigment genes, producing variegated leaves. 3. The situation is similar to that for chloroplasts. Each cell contains numerous mitochondria, and in affe<:ted in· dividuals, most cells contain a variable mixture of normal and mutant mitochondria. The normal mitochondria carry out mough cellular respiration for survival.

Genetics Problems 1, 0; \Ii,)I" 2. Recessive; if the disorder were dominant, it would affl'Ct at least one parent of a child born with the disorder. The disorder's inheritance is sex·linked because it is seen only in boys. For a girl to have the disorder, she would have to inherit recessive alleles from both parents. This would be very rare, since males with the recessive allele on their X chromosome die in their early teens. 3, Y. for each daughter (Yi chance that child will be female x \Ii chance of a homozygous recessive genotype); \Ii for first son. 4, 17% 5. 6%. Wild typl' (heterozygous for normal wings and red eyes) x recessive homozygote with vestigial wings and purple eyes

b+

v~

• "l

,

b "1'

.1

F

IJ

<~

-£6

b

<.

~

•<'

• "9

d. 41.5% gray hody, vestigial wings 41.5% black body, normal wings 8.5% gray body, normal wings 8.5% black body, vestigial wings 7. The disorder would always be inherited from the mother. 8. The inactivation of two X chromosomes in XXX women would leave them with one genetically active X, as in women wilh the normal number of chromosomes. Microscopy should reveal two Barr hodies in XXX women. 9. D-A-B-C 10. Fifty percent of the offspring would show phenotypes that resulted from crossovers. These results would be the same as those from a cross where A and B were not linked. Further crosses involving other genes on the same chromosome would reveal the linkage and map distances. 11. Between T and A, 12%; between A and 5, 5% 12. Bern'een T and 5, 18%: sequence of genes is T-A-5 13. 450each of blue-oval and white-round (parentals) and 50 each of blue-round and white-oval (recombinants) 14. About onethird of the distance from the vestigial-wing locus to the brown·eye locus 15. Because bananas are triploid, homologous pairs cannot line up during meiosis. Therefore, it is not possible to generate gametes that can fuse to produce a zygote with th(' triploid number of chromosomes.

CHAPTER 16 Figure Questions Figure 16.2 The living S cells found in the blood sample ""ere able to reproduce to yield more S cells, illdicating that the S trait is a permanent, heritable change, rather than just a one-time use of the dead Scells' capsules. Figure 16.4 The radioactivity would have been found in the pellet when proteins ""ere labeled (batch I) because proteins would have had tocntcrthe bacterial cells to program them with genetic ins01.lCtions.lt"s hard for us to imagine now, but the DNA might have played a structural role that allowed some of the proteins to be injected while it remained outside the bact('rial cell (thus no radioactivity in the pellet in batch 2}. Figure 16.11 Thetube from the first replication would look the S3JTll', with a middle band of hybrid 1'N_ 14 N DNA, hut the second tube ""(II-lid not have the upper band oftwo light blue strands. Instead it would have a bottom band of two dad blue strands, ~ke the bottonl band in the result predicted afterone replication ill the COIlservative model. Figure 16.12 In the bubble at the top in (b), arrows should be dral'Tl pointing left and right to indicate the rn"O replication fods. Figure 16.14 Looking at any of the DNA strands, ,,-c sec that one end is calk-d the 5' end and the other the 3' end If we proceed from the 5' end to the 3' end on the left·most strand, for example. we list the components in this order: phosphate group > 5' C of the sugar .3' C • phosphate • S' C • 3' C. Going in the opposite direction on the same strand. the componrnts proceed in the reverse order: 3' C > 5' C • phosphate. Thus, thern'o dircctionsarl'distinguishable, which is what we mean when we say that the strands have diR'Ctionality. (Review Figure 165 if necessary.) Figure 16.22 The cens in the mutant would probably have the same defects in meiosis that were seen in this experiment, such as the failureofconderu;in to beconcentrated in a small region in the nucleus. Thc defect in the rn'o mutants is essentially the same: In the mutant described ill the experiment, the kinase doesn't function PTOpl-rly; ill the n,'wly diSCOVl1'ed mutant, the kinase could not phosphorylate the correct amino add because that amino add is missing.

Answers

A-12

~

•~

.. C


Concept Ch~k 16.1 1. Chargaffs rules state that in DNA, the percentages of A and T and of G and Care l'SS<'ntially the same. and the fly data are consistent with those rules. (Slight V;lriations arc most likely due to limitations of analytical tc<hnique.) 2. In the Watson·Crick model, each A hydrogen.bonds to a T, so in a DNA dooble helill:, their numbeTs are equal; the same is true for G and C. 3. The mouse injccted with the mixture ofheat-killed S cells and liVing Rcells woold have survived, since neither type of cell alone could have killed the

Concept Check 17.1 1. A polypeptide made up of 10 Gly (glycine) amino acids

2. Tet'llrlAtl! Stt(u.l!oct. (fnmprllblt:.nV:

"-TTCI6Tc6T-S'

Nontt!lllpktte ~UlU·

0'- ANSTCAGO.-3'

mRNA sequer'IU:

5' -A1'rI6IJCA6CA -3'

~~.

Concept Check 16.2 1. Complementary base pairing ensun:'S lhat me tv.u daughter molecules are exact copies of the parent molecult-. WNon the tv.u str.mds of the parent rnoIecult- separ1lk, each serves as a tl'mpialC' 00 which nucleotides are arranged. by the base-pairing rules. into new complemrotarystrands. 2. DNA pol III c0valently adds nuclwtides to new DNA strands and proofreads each added nucleotide lOr correct base pairing. 3. S)nthesisofthe leading strand is initialed by an RNA primer, ...itidl must bercmOlled and replaced ...ith DNA,a task that cooJd not be: performed if the- cdl's DNA poll ....ere nonfunctional In the (J\·crv~.... box in figure 16.17. just to the- Irft ofthe- top origin of reptica.tion, a ru~l DNA poll ....ould. rtpbtt the RNA primer of the leading strand (WM'Tl in red) ...ith DNA nudeoddes (blue). Concrpt Ched: 16.3 1. A nucleosome is made up of right histone proteins, t....o each offour dif· fermt types, around ....hich DNA is ....ound. Unker DNA runs from one nudeosome to the next. 2. Euchromatin is chromatin that becomes less compacted dUring interphase and is w::essible to the cel.Iular machinery reo sponsible fOf gene activity. Heterochromatin, on the other hand, remains quite condensed during interphase and contains gene'S that are largdy inac· cessible to this machinery. 3. Uke histones, the: £. roti proteins ....ould be expected to cOlltain many basic (positi\'dy charged) amino acids, such iII5 lysine and arginine, ...-hich can form ....eak bonds ....ith the negatiwly charged phosphate groups on the sugar· phosphate backbone of the DNA molecule.

self-Quiz 1. c 2. d 3. b 4. c 5. b 6. d 7. a 8. c 10.

JlUI!ntallU

str~ (~~

S1,Ji~

cJu"p

J::M.

IllIlL

Sl~·str.wl bi,:;j,~~

birtdimlf rtpl;utfll1

CHAPTER 17 Figure Qucstions Figure 17.2 The previously presumed path....ay ....ould have been ....rong. The ne.... results ....ould support this path....ay: precursor • citrulline • ornithine ' arginine.1bcy ....(MI1d also indica~ thai class I mutants have a defect in the sec· ond step and class II mutanlS have a defect in the first step. Figure 17.8 The RNA polymerase ....ould bind direcdy to the promoter, rather than de· pending on the previous binding of other factors. Figure 17.24 The mRNA on the right (the longest one) started transcription first. The ribosome at the top, doscst to the DNA. started tnnslating firs! and thus has me longest polypeptide.

A·I)

AppendiX A

The nontemplate and mRNA base sequences are the same, except there is T in the nontemplate str1lnd of DNA ....hel"('\"t'r there is U in the mRNA.

3. It!"mplde'E£quen£e'''(~ rlotlttmplah:!B{uetlle i" prlIbfem, writttn 3' -+ S"}. 3'-AC&ACT6-AA-S" 6'-l/6CltGACUU-3'

(Remember that the mRNA is antiparallet to the DNA strand) A protein tr1lnstated from the nontemplate sequence .... ouk! ha\"t' a completdy different amino acid sequence and ....ould su~1y be nonfunctiorulL (It ...'OUki also be shorter be<auseofthe- stop signal shown in the mRNA sequenceabo\'e-and pouibly othen earlier in the mRNA sequence.) Concept Ched: 17.2 1. 80th assemble nucleic acid chains from Il"IOf"IOf\l('! nudecJtides ...-hose ordeT is dctcnnined by complcmentary base pairing to a template strand. Both S)"Il' thesizc in the 5' • 3' direction. antipamlet to the template. DNA polymer;Ise requires a primer. but RNA poIymcnsc can start a nudcotide chain from scratch. DNA poIymense uses nucleotides ....ith the sugardeoxyribose and the base T, ...-hereas RNA poIymens.e uses nuclcotides IOith me sugarribose and the base U. 2. The promoter is the region of DNA to ....hich RNA polymerase binds to begin tr1lnscription. and it is at me upsueam end of the gme (tran· scription unit). 3. In a bacterial cel.I, RNA polymerase ~ the gene's promoter and binds to illn a cuk.aryotic cell, tnnseription factors mediate the binding ofRNA poIymer1lse to the promoter. 4. The transcription factor that recognizes the TATA sequence ....oold be unable to bind. so RNA polymerase could not bind and tr1lnscription of that gene probably ",'ould not occur. Concept Check 17.3 1. The 5' cap and poly.A tail facilitate mRNA export from the nucleus, prevent the mRNA from beingdegraded by hydrolytic enzymes, and facilitate ribosome attachment. 2. In editing a video, Sl'gments arc cut out and discarded (like introns), and the remaining segments are joined together (like nons) so that the regions of joining ("splicing") are not noticeable. 3. Six different forms could be made b«ausc altern~tivesplicing could generate six different ml{NAs (two possibilities for non 4 x three possibilities forexon 7). Concept Check 17.4 1. First, each aminoacyt.tRNA synthetase specifically recognizes a single amino acid and attaches it only to an appropriate tRNA. Seoond, a tRNA charged with its specifIC aminoacid bindsonlytoan mRNA codon for that amino acid. 2. Polyri· bosomcscnablc the cdl t.o produce multiple copies of a poIypcptidcvcryquiddy. 3. Asignal peptide on the leading end of the polypeptide being synthesiled is rec· ognized bya signal.m:ognition particle that brings the ribosome to the ER memo br.mc. There the ribosome attaches and continues t.o synthesize me polypeptide, depositing it in me ER lumen. 4. The structure and function of the rixJsome sa'TTl t.o depend more on the rRNAs than on the ribosomal protI.'ins. BecallSol' it is single·str.mded. an RNA moI«uIccan hydrogen.bond ...ith itselfand ...ith other RNA molecules. RNA molecules mal;e up the interface bet....een the 1\'1"0 rilosomal subunits, so presumably RNA·RNA bindinghclps hold the ribosome togethtt The bindingsite br mRNA in the ribosomecoukl include rRNA dial can bind the mRNA. (In bet. this !UrnS out to be: me case) Also, complementary bonding ...ithin an RNA molecule ano...-s it tOa55WT1l" a particular ~ shape. and.aIong ...ith the RNA's hmctionaI groups, ~mabIes rRNA tocatalyze peptide bond bmation during rransIation.


Conn'pl Check 17.5 1. In the mRNA, the reading frame downstream from the deletion is shifted, leading to a long string of incorrect amino acids in the polypeptide and, in most cases, a stop codon will arise, leading to premature termination. The polypeptide will most likely be nonfunctional.

2. Norm<\l Dt-JA 5O{.lltf'U. (templlX1e 5trlu"d is ((I"ttJp):

;l'- TA.C.TTGTCCb"T ATC-5"'

5' ~A.TGAAC..AGGCTATAG-3' 5'- AUErA.A.CAGGC.UAUA6-:"

Mut<lfl!'d DNA-se.,.aerce

(-remp"" strand "",top)

3~TACTTGTCCAATATC-~ S/-ATGA ",cAGGTrATAG-3'

mRtJA se.tu~nce: ~i"o a.cid U'tu.ence:

The amino acid sequence is Met·Asn.Arg-l.eu both before and after the mu· tation because the mRNA codons S··eUA·3' and S'·UUA·3' both code for U>u. (The fifth codon is a stop codon.)

Conn'pl Check 17.& 1. No, transcription and translation are separated in space and time in a eukaryotic cell, a result of the eukaryotic cell's nudear compartment. 2. When one ribosome terminates translation and dissociak's, the two subunits would be veryclost'to theap. Thiscould facilitate their rebinding and initiating synthesis of a new polypeptide, thus increasing the elfleiency of translation.

Self.Quiz 1. b 2 d 3 a 4 a 5 d

8.

6 e 7 b Fiq~:kms

""BP<,f R"A M"'~c~' lrriRIJPf -rr~fer R~A

(w')

IObo_IRlJA (c,,"')

~ infv-rno.twn 5fec.iful~

4/lI1(lD a.l.il -Saffetlces

traJ1W"ipt

eillS

P1~ "At"l~ ((iOO~ rdrs 41tI

~trMfu.rDJ rolo

15 <\ prtWf"SQr fh~

rf-pr:

!IN" ~ r~ts. Su~u l\S .cll.pttr f\1DltcJ,the in lY",ein ~rT+tlie5i5j tnvJs.1<\ta mRJ.lA cMDrIS 'rnUl amino iWds.

.fnm

in n

mes.

'*' rnR1-lA, ~/<IA,cr tR~AJ

~ bei;t pr~, Soll'le lntnrtl RN/l M1!. 'l rjbl~ tAt... l~zI~ own ~lidn1'

in. SmAil t'\(l<'.lur' RIJA (SIlRIJ~)

shdur..., tIJId ('4t... lyl1" rolo ,n .$pIietl'!o:lme$, *,,(~le)(e; tf prete.·n ll1Id I)JA ttlat 'p1oll. p«-' ml2kIA .

'Ad'j!>

(HAPTER 18

Figure Questions Figure 18.3 As the concentration of tryptophan in the cell falls, eventually there will be none bound to repressor moll'Cules. These will then take on their inactive shapes and dissociate from the operator, allowing transcription of the operon to resume. The enzymes for tryptophan synthesis will be made, and they will begin to synthesize tryptophan again in the cell. Figure 18.10 The albumin gene enhancer has the threeoontrol elements colored yellow, gray, and red. The sequences in the two cells would be identical, since the cells are in the same organism. Figure 18.16 Even if the mutant MyoD protein couldn't activate the myoD gene, it could still tum on genes for the other proteins in the pathway (other transcription factors, which would tum on the genl'S for muscle-specific proteins, for example). Therefore, some differentiation would occur. But unless

there were other activators that could compensate for the loss of the MyoD pro· tein's activation of the I1l)'oD gene, the cell would not be able to maintain its dif· ferentiated state. Figure 18.19 Nomlal Bicoid protein would be made in the anterior end and compensate for the presence of mutant bicoid mRNA put into the egg by the mother. Development should be normal, with a head present. Figure 18.21 The mutation is likely to be recessive because it is more likely to have an effect ifboth copies of the gene are mutated and code for nonfunctional proteins. If one normal copy of the gene is present, its product could inhibit the cell cycle. (However, there arc also known cases of dominant p53 mutations.)

Concept Check 18.1 1. Binding by the trp corepressor (tryptophan) activates the trp rt'pfl'Ssor, shutting off transcription of the trp operon: binding by the lac inducer (allolactose) inactivates the lac repressor, leading to transcription of the lac operon. 2. The cell would continuously produce ~'galactosidase and the two other en· zymes for lactose utilization, even in the absence of lactose, thus wasting cell resources. 3. With glucose scarce, cAMP would be bound to CAP and CAP would be bound to the promoter, favoring the binding of RNA polymerase. However, in the absence of lactose, the repressor would be bound to the oper· ator, blocking RNA polymerase binding to the promoter. The operon genes would therefore not be transcribed. If another sugar were present and the genes encoding enzymes for its breakdown were in an operon regulated like the lac operon, we might expect to find active transcription of those genes.

Concept Check T8.2

1. Histone acetylation is generally associated with gene expression, while DNA methylation is generally associated with lack of expression. 2. General tran· scription factors function in assembling the transcription initiation compb at the promoters for all genes. Specific transcription factors bind to control elements associated with a particular gene and, once bound, either increase (activators} or decrease (repressors) transcription of that gene. 3. The three genes should have some similar or identical sequences in the control elements of their enhancers. Bl'CaUSC of this similarity, the same sJX-cific transcription factors could bind to th<' enhancers of all thrt'e genes and stimulate their expression coordinately. 4. Degradation of the mRNA, regulation of transla· tion, activation of the protein (by chemical modification, for example), and protein degradation 5. Expression of the gene encoding the yellow activator (YA) must be regulated at one of the steps shown in Figure 18.6. The YA gene might be transcribed only in liver cells oc.:ause tht, necessary activators for tht, enhancer of the YA gene are found only in liver cells.

Concepl Check 18.3

1. Both miRNAs and siRNAs arc small, single-stranded RNAs that associate with a complex of proteins and then can base-pair with mRNAs that have a complementary sequence, This base pairing leads to either degradation of the mRNA or blockage of its translation. Some siRNAs, in association with other proteins, can bind back to the chromatin in a certain region, causing chromatin changes that affect transcription. Both miRNAs and siRNAs are processed from double-stranded RNA precursors by the enzyme Dicer. However, miRNAs art' encoded bygenes in thecell'sgenome, and the singletranscript folds back on it· selfto form one or more double-stranded hairpins, each of which is processed into an miRNA. In contrast, siRNAs arise from a longer stretch of double· stranded RNA, which may be introduced into the cell by a virus or an experi· menter. In some cases, a cellular gene codes for one RNA strand of the prt-cursor molecule, and an enzyme then synthesizes the complementary strand. 2. The mRNA "''QuId persist and be translated into the cell divi· sion-promoting protein, and the cell would probably divide. If the intact miRNA is nl'Cessaryfor inhibition ofcell di\~sion, then division ofthis cell might be inappropriate. Uncontrolled cdl division could lead to formation of a mass of cells (tumor} that prevents proper functioning of the organism. Concepl Check T8.4 1. Cells undergo differentiation during embryonic development. bl'Com· ing different from each other; in the adult organism, there are many highly specialized cell types. 2. By binding to a receptor on the receiving cell's surface and triggering a signal transduction pathway that affects gene ex· pression 3. Because their products, made and deposited into the egg by the mother, determine the head and tail ends, as well as the back and belly, of the embryo (and eventually the adult fly} 4. The lower cell is synthe. sizing signaling molecules because the g<'ne encoding them is activated, meaning that the appropriate specific transcription factors are binding to

Answers

A·14

~

•~

.. c


the gene's enhancer. The genes encoding these specific transcription {actoN; are also being expressed in thisce]] because the transcriptional activatoNi that can turn them on were expressed in the precursor to this celL A similar explana-

tion also applks to the cells expressing the receptor proteins. This scenario lx,gan with specific cytoplasmic determinants localized in specific regions oftbe egg. These cytoplasmic determinants were distributed unevenly to daughter cells, resulting in cells going down different developmental pathways.

Concept (hed, 18.5

~

•~

.. c

1. The protein product of a proto-oncogene is usually involved in a pathway that stimulates cell division. The protein product of a tumor-suppressor gene is usually involved in a p;lthway that inhibits cell division. 2. When an individual has inherited an oncogene or a mutant allele of a tumor-suppressor gene 3. A cancer-causing mutation in a proto-oncogene usually makes the gene product overactive, whereas a cancer-causing mutation in a tumor· suppressor gene usually makes the gene product nonfunctional.

ConeepI Ched:: 19.2 1. Lytic phages can only carry out Iysisofthe host cell, whereas lysogenic phages may either lyse the host cell or integrate into the host chromosotTll'. In the latter case, the viral DNA (prophage) is simply replicated along with the host chromosome. Under certain conditions, a prophage may exit the host chromosomc'and initiate a lytic C)"(le. 2. The genetic material of these viruses is RNA, which is replicated inside the infected cell by enzymes encoded by the virus, The viral genome (or acompletTll'ntarycopyofit} serves as mRNA for the synthesisofviral proteins. 3. Because it synthesizes DNA from its RNA genome. This is the reverse rretro"} of the usual DNA • RNA information flow. 4. There arc many steps that could be interferc-d with: binding of the virus to the cd!. reverse transcriptase function, integration into the host cell chromosome, genome synthesis (in this case. transcription of RNA from the integrated provirus), assembly of the virus inside the cell, and budding ofthe virus. (Many. if not all, of these arc targets of actual medical strategies to block progress of the infection in HIVinfectc-d (K'Ople.)

Self.Quiz

ConeepI Ched:: 19.3

1. d 2. a 3. d 4. a 5. c 6. I' 7. a 8. c 9. b 10. b 11. a, Promoter EnhdrKCr

1. Mutations can lead to a new strain of a virus that can no longer be effectivcly

Gene 1 Gene2

Gene4

::Ui

#

1/

H

-

Gene 5

The purple, blue, and red activator proteins would be present.

b

Enhancer

Promoter

Gene 2 Gene 3

fought by the immune system, even if an animal had been exposed to the orig· inal strain; a \~rus can jump from one species to a new host; and a rare virus can spread if a host population becomes less isolated. 2. In horizontal transmission, a plant is infected from an external source of \~rus, which could enter through a break in the plant's epidermis due to damage by herbivores. In vertical transmission, a plant inherits viruses from its parent either via infc'Cted seeds (sexual reproduction} or via an inf~ted cutting (asexual reproduction). 3. Humans are not within the host range ofTM\'. so they can't be infected by the virus. 4. It is unlikely that human air travel could have spread the virus, since cxistingstrains of the \~rus do not seem to be transmissible from human to hu· man. It is conceivable but unlikely that an infected human traveling from Asia passed the virus to birds in Africa and Europe. It is possible that domc'Stic birds carried the virus, perhaps in shipments of poultry. The likeliest scenario of all may be that migratory wild birds carried the virus during their migrations and passed it to domestic and wild birds in the new locations. To test these latter hypotheses. the timing of the outbreaks should be analyzed to sec if they correlate with recent poultry shiptTll'nts or known wild bird migrations. Any such migratory birds should be tested for the prc'SCnce of the African or European strain of the virus, based on the nucleotide sequences of their genomes.

Self.Quiz Gene 5

Co

Only gene 4 would be transcribed. In nerve cells, the orange, blue, green, and black activators would have to be present. thus activating transcription of genes I, 2, and 4. In skin cells, the red, black, purpk, and blue activators would have to be present, thus activating genes 3 and 5.

CHAPTER 19 Figure Queslions Figure 19.2 Beijerinck might have concluded that the agent was a toxin produced by the plant that was able to pass through a f1Iter but that be· came more and more dilute. In this case, he would have concluded that the infectious agent could nol reproduce. Figure 19.4 Top vertical arrow: Infection. Left upper arrow: Replication. Right upper arrow: Transcription. Right middk arrow: Translation. Lower left and right arrows: Sdfassembly. Bottom middle arrow: Exit. Figure 19.7 Any class V virus, including the viruses that cause influenza (flu), measles, and mumps.

Concept Check 19.1 1. TMV consists ofone molecule of RNA surrounded by a helical arrayofproleins. The influenza virus has eight moI~ules of RNA, each surrounded by a helical array of proteins. similar to the arrangement of the single RNA molecule in TMV. Another difference is that the influenza virus has an outer envelope. 2. One of the argumenls for regarding virusc'Sas nonliving is that they cannot perfonn any activity characteristic of living organisms unless they arc inside a host cell. This virus challenges that generalization because the virus can change its shape without haVing access to host cell proteins. (Further anal~is suggested that the proj~tions contain proteins related to intermediate filaments that may polymerize spontaneously under certain conditions.) A-IS

Appendix A

1. d 2. b 3. c 4. d S. c 6. As shown below, the viral genome would be translated into capsid proteins and envelope glycoproteins directly, rather than after a complementary RNA copy was made. A complementary RNA strand would still be made. however, that could be used as a template for many new copies of the viral genome.

@""


CHAPTER 20

atic cell (probably by inducing expression of pancreas-specific regulatory genes in the cell).

Figure Questions

Concept Check 20.4

Figure 20.3

5'JAA Gcnll'

,'lAG cnll'

3'lrrcGAAls'

3'~5'

Figure 20.4 Cells containing no plasmid at all would be able to grow; these colonies would be white because they would lack functional lacZ genes. Figure 20,10 Grow each clone of cells in culture. Isolate the plasmids from each and cut them with the restriction enzyme originally used to make the clone (see Figure 20.4). Run each sample on an elfftrophoretic gel, and recover the DNA of the insert from the gel band. Figure 20,16 The reo searchers might have concluded that differentiated cells are irreversibly changed so that they can make only one type oftissu.' in the plant. (This result would support the idea that cloning isn·t possible.) Figure 20.17 None of the eggs with the transplanted nuclei would have developed into a tadpole. Also. the result might include only some of the tissues of a tadpole which might differ depending on which nucleus was transplanted. (This as· sumes that there ...i as some way to tell the four cells apart, as one can in some frog species.)

1. Stern cells continue to reproduce themselves. 2. Herbieide resistance, pest resistance, disease resistance, salinity resistance, delayed ripening, and improved nutritional value 3. Because hepatitis A is an RNA virus, you could isolate RNA from the blood and try to detectcopiesofhepatitis A RNA by one of three methods. first, you could run the RNA on a gel and then do a Northern blot using probes complementary to hepatitis A genome sequences. A second approach would be to use reverse transcriptase to make cDNA from th., RNA in the blood, run th., cDNA on a gcl, and do a South· ern blot using the same probe. However, neither of these methods would be as sensitive as RT·PCR, in which you would reverse transcribe the blood RNA into cDNA and then use PCR to amplify the cDNA, using primers specific to hepatitis A sequences. If you then ran the products on an electrophoretic gel, the presence of a band would support your hypothesis.

Self-Quiz 1. b 2. b 3. 9.

1. The covalent sugar-phosphate bonds of the DNA strands will cut the molffule

2. Yl"S, Pvul

c

4, b 5, a 6. c 7.

+

"

8, d

I

::::::rTTTeG W"''''*'

~­ o"!J'"

V..-U.I

I'

",..",eTTA t j

5' T~ ~AT6i""'TT~T AAA' <:.'i ~TT"'T6AA.rrG'" C6 (;G!I

~

5'lc C T fGA C GAT CGTTA C C G[ 3' .3'IG ~ AAC TG C TAG CAATG G C[ s'

5'(; CTTG,\ CG ATI3' 3'@6 A ACTGgS'

C

I

Concept Check 20.1

4T(o'l TAeTTIl....

e6

AATTCT"'MGCO$CTTIl.T.. ,'

GTGCCEi5'

S'M1TGA

3' GATn·c."eGAAT... CTTU,

" "T

S'ICGTTACCGI3'

3' trAI; CAAII; I;

ij

5'

3. Some human genes are too large to be incorporated into bacterial plasmids. Bacterial cells lack the means to process RNA transcripts into mRNA, and even if the need for RNA processing is avoid,'d by using cDNA. bacteria lack enzymes to catalyze the post-translational processing that many human proteins require to function properly. 4, S'·CGGT-3' and S'-CCTT·3'

Concept Check 20.2 1. Any restriction enzyme will cut genomic DNA in many places, generating such a large number of fragments that they would appear as a smear rather than distinct bands when the gel is stained after electrophoresis. 2. In Southern blotting, Northern blotting, and microarray analysis, the labeled probe binds only to the s(K'Cifk targd Sl.'quence owing to complemenlary nucleic acid hybridization (DNA-DNA hybridization in Southern blotting and microarray analysis, DNA·RNA hybridization in Northern blotting}. In DNA sequencing, primers base-pair to the template, allOWing DNA synthesis to slart. In RT-PCR, the primers must base-pair withtheirtargetsequencesin the DNA mixture. 3. If a spot is green, the gene represented on that spot is expressed only in normal tissue. If red, the gen.' is expressed only in cancerous tissue. If yellow, the gene is exprCSSl.-cl in both. And ifblack, the gen.' isexpreSSl.-cl in neither type of tissue. AI, a researcher interested in cancer development, you would want to study genes represented by spots that are green or red because these are genes for which the expression level differs between the two types of tissues. Some of these genes may be expressed differendy as a result of cancer, but others might playa role in causing cancer.

Concept Check 20.3 1. No, primarily bffause of subtle (and perhaps not so subtle) differences in their environments 2. The state of chromatin modification in the nucleus from the intestinal ccll .....a s undoubtedly less similar to that of a nucleus from a fertilized egg, explaining why many fewer of these nuclei were able to be reprogrammed. In contrast, the chromatin in a nucleus from a cell at the four-cell stage would have been much more like that of a nucleus in a fertilized egg and therefore much more easily programmed to direct development. 3. A tech· nique would have to be work.'d out for turning a human iI'S cell into a pancre-

~

•~

10. A cDNA library, made using mRNA from human lens cells, which would be expected to contain many copies of crystallin mRNAs

CHAPTER 21 Figure Questions Figure 21.3 The fragments in stage 2 of this figure are like those in stage 2 of figure 21.2, but in this figure theirorrler relative to each other is not known and wm be dt1.cnnincd later by computer. The orckr of thcfragments in Figure 21.2 is completely kno.....n before sequencing begins. (Determining the order takes longer but makes the eventual sequence assembly much easier.) Figure 21,9 The transposon would be cutout of the DNA at the original sill' rather than copied, so part (a}would showthe original stretch of DNA without thetransposon after the mobile transposon had been cut out. Figure 21.10 The RNA transcripts exl<"nding from the DNA in each transcription unit are shorter on the left and longer on the right. This means that RNA polymerase must be starting on the left end of the unit and moving toward the right. Figure 21.13 Pseudogenes are nonfunctional They could have arisen by any mutations in the second copy that made the gene product unable to function. Examples would be base changes that introduce stop codons in the sequence, alter amino acids, or chang., a region of the g.'ne promo,,"r so that the gen.'can no longer be expressed. Figure 21.141.et's say a transposable element (TEl existed in the intron to the left of the indicated EGf exon in the EGF gene, and the same TE was present in the intron to the left of the indicated F exon Answers

A-16


in the fibronfftin gene. During meiotic recombination, these TEs could cause nonsister chromatids on the same chromosome to pair up incorrectly, as seen in Figure 21.12. One gene might end up with an Fexon nexttoan EGF exon. Further mistakes in pairing over many generations might result in these twoexons being separated from the rest of the gene and placed next to a single or duplicated Kexon. In general, the presence of repeated sequences in introns and between genes facilitates these processes because it allows incorrect pairing of nonsister chromatids, leading to novel eJion combinations. Figure 21.16 Since you know that chimpanzees do not speak but humans do, you'd probably want to know how many amino acid differences there arc between the human wild-type FOXP2 protein and that of the chimpanzee and whether these changes affect the function of the protein. (As we explain later in the teJit, there are two amino acid differences.) You know that humans with mutations in this gene have severe language impairment. You would want to learn more about the human mutations by checking whether they affIXt the same amino acids in the gene product that the chimpanzl'l' sequence differences affect. If so, those amino acids might play an important role in the function of the protein in language. Going further, you could analyze the differences between the chimpanzee and mouse FOXP2 proteins. You might ask: Are they more similar than the chimpanzee and human proteins? (It turns out that the chimpanzee and mouse proteins have only one amino acid difference and thus are more similar than the chimpanzee and human proteins, which have two differences, and than the human and mouse proteins, which have three differences.)

Concept Ched, 21.1 1. In a linkage map. genes and other markers are ordered with respect to each other, but only the relative distances between them are known. In a physical map, the actual distances between markers, expressed in base pairs, are known. 2. The three-stage approach employed in the Human Genome Project involves linkage mapping, physical mapping, and then sequencing of short, overlapping fragments that preViously have been ordered relative to each other (see Figure 21.2). The whole-genome shotgun approach eliminates the linkage mapping and physical mapping stages; instead, shon fragments generated by multiple restriction enzymes are sequenced and then ordered by computer programs that identify overlapping regions (set· Figure 21.3). 3. Because the two mouse species are very closely related, their genome sequences are expected to be very similar. This means that the field mouse genome fragments could be compared with the assembled lab moose genome, providing valuable information to use in placing the field mouse genome fragments in the corrfft order. In a sense, the lab mouse genome could be used as a rough map for the field mouse genome, removing tht· nt-.:essity to carry out complete genetic and physical mapping for the field mouse.

Concept Check 21.2 1. The Internet allows centralization of databases such as GenBank and softwart· resources such as BLAST, making them freelyaccessib1e. Havingall the data in a central database, easily aceessibleon the Internet, minimizes the possibility oferrors and of researchers working with different data. It streamlines the process ofscience, since all researchers are able to use the same software programs, rather than each having to obtain their own sofrn'are. It speeds up dissemination of data and ensures as much as possible that errors arecorreded in a timely fashion. These are just a few answers; you can probably think of more. 2. Cancer is a disease caused by multiple factors. To focus on a single gene or a single deffft would ignore other factors that may influence the cancer and even the behavior of the single gene being studied. The systems approach, because it takes into account many factors at the same time, is more likely to lead to an understanding of the causes and most useful treatments for cancer. 3. The DNA would first be sequenced and analyzed for whether the mutation is in the coding region for a gene or in a promoterorenhancer, affecting the eJipression of a gene. Ineithercase, the nature of the gene product could be explored by searching the protein database for similar proteins. If similar proteins have known functions, that would provide a clue about the function of your protein. Otherwise, biocbt'mical and othcr methods could provide some ideas about possible function, Software could be used to compare what is known about your protein and similar proteins.

Concept Check 21.3 1. Alternative splicing of RNA transcripts from a gene and post-translational processing of polypeptides 2. The total number of completed genomes is

A-17

Appendix A

found by clicking on "Published Complete Genomes." Add the figures for bacterial, archaeal, and eukaryotic "ongoing genomes" to get the number "in progress." Finally, look at the topofthe Published Completc Genomes page to get numb,'fS of completed genomes for each domain. (Nole: You can dck on the "Size" column and the table will be re-sorted by genome size, Scroll down to get an idea of relative sizes of genomes in the three domains. Remember, though, that most of the se<[uenced genomes are bacterial.) 3. Prokaryotes are generally smaller cells than eukaryotic cells, and they reproduce by binary fission. The evolutionary process involved is natural selfftion for more quickly reproducing cells: The (aster they can replicate their DNA and dividc, the more likely they will be able to dominate a population ofprokaryotes. The less DNA they have to replicate, then, the faster they will reproduce.

Concept Check 21.4 1. The number of genes is higher in mammals, and the amount of noncoding DNA is greater. Also, the presence ofintrons in mammalian genes makes them longer, on average, than prokaryotic genes. 2. Introns are interspersed within the coding sequences of genes. Many copies of each transposable element arc seattered throughout the genome. Simple Se<[uence DNA is concentrated at the centromeres and telomeres and is dustered in other locations. 3. In the rRNA gene family, identical transcription units for the three different RNA products are present in long, tandemly repeated arrays. The large number of copies of the rRNA genes enable organisms to produce the rRNA for enough ribosomes to carry out actiV<o protcin synthesis, and the single transcription unit ensures that the relative amounts of the different rRNA molecules produced are correct. Each globin gene family consists of a relatively small number of nonidentical genes. The differences in the globin proteins encoded by these genes result in production ofhemoglobin molecules adapted to particular developmental stages of the organ· ism. 4. First, you could chIXk the St'quence by translating it into a predicted amino add sequence and see if there are multiple stop codons. If there aren't, the next step would be to see whether the gene is expressed, probably by carrying out a Northern blot or in situ hybridization to look for the mRNA in the cells that express the gene.

Concept Check 21.5 1. If meiosis is faulty. two copies of the entire genome can end up in a single cell. Errors in crossing over during meiosis can lead to one segment being duplicated while another is deleted. During DNA replication, slippage backward along the template strand can result in a duplication. 2. For either gene, a mistake in crossing over during meiosis could have occurred between the two copies of that gene, such that one ended up with a duplicated non. This could have happened several times, resulting in the multiple copies of a particular exon in each gene. 3. Homologous transposable elements scalierI'd throughout the genome provide sites wh,'re fl.·combination can occur between different chromosomes. Movement of these elements into coding or regulatory sequences may change expression of genes, Transposable elements also can carry genes with them, leading to dispersion of genes and in some cases different patterns of expression. Transport of an exon during transposition and its insertion into a gene may add a new functional domain to the originally encoded protcin, a type of non shuffling. 4. Because more offspring arc born to women who have this inversion, it must provide some advantage. It would be expected to persist and spread in the population. (In fact, evidence in the study allowed the researchers to conclude that it has been increasing in proportion in the population. You'll learn more about population genetics in the next unit.)

Concepl Check 21.6 1. Because both humans and macaques are primates, their genomes are expffted to be more similar than the macaque and mouse genomes are. The mouse lineage diverged from the primate lin"age before the human and macaque lineages diverged. 2. Homeotic genes differ in their nonhomeobox sequences, which determine the interactions of homeotic gene products with other transcription factors and hence which genes are regulated by the homeotic genes. These nonhomeobox sequences differ in the two organisms, as do the expression patterns of the homeobox genes. 3. Alu clements must have undcrgone transposition moreactivcly in th.· human genome for some reason. Their increased numbers may have then allowed more rffombination errors in the human genome, resulting in more


or different duplications. The divergence of the organization and content of the m'o genomes presumably accelerated divergence of the two species by making matings less and less likely to result in fertile offspring. Sl'lf-Quiz

1. c 2. 7.

I' 3. a 4. e 5. I' 6. a ATET!.. PK55D T55TT NARRD

2, ATETI .. PK5S£i .T5m .. RlARRD 3, ATETI .. PK5SD .. T55TT .. NARRD 4, ATETI .. PK55D, .T55liT

:iARRD

5 ATET!.. PK55D .T55TT

NARRD

6, VTETI .. PKSSD, .T55TT .. NARRD

population exhibit a range of heritable variations. some of which make it likely that their bearers will leave more offspring than other individuals (for example, the bearer may escape predators more effcrtively or be more tolerant of the physical conditions of the environment). Owr time, natural selection imposed by factors such as predators, lack of food, or the physical conditions of the environment can increase the proportion of individuals with favorable traits in a population (evolutionary adaptation}. 3. The fossil mammal species (or its ancestors) would most likely have colonized the Andes from within South America, whereas ancestors of mammals currently found in African mountains would most likely have coloniJ:<'d those mountains from other parts of Africa. As a result, the Andes fossil species would share a more rcrcnt common ancestor with South American mammals than with mammals in Africa. Thus, for many of its traits, the fossil mammal species would probably more closely resemble mammals that live in South American jungles than mammals that live on African mountains.

a. Lines I, 3, and 5 are the C. G. R species. b. Line 4 is the human sequence.

Concept Check 22.3 1. An environmental factor such asa drug does not create new traits such as

C. Line 6 is the orangutan scquence. d. There isone amino acid diffcrence between thc mouse (line 2) and the C. G, R species; there are three amino acid differences between the mouse and the human. e. Bcrause only one amino acid difference arose during the 60-100 mil· lion years since the mouse and C. G, R species diverged, it is somewhat surprising that two additional amino acid diffen:nces resulted during th., 6 million years since chimpanzees and humans diverged. This indicates that the FOXP2gene has been evolVing faster in the human lineage than in the lineages of other primates.

drug resistance, but rather sclcrts for traits among those that arc already prcsent in the population. 2. (a) Despite their different functions, the forelimbs of different mammals are structurally similar bcrause they all represent modifications of a structure found in the common ancestor. (b) Convergent evolution: The similarities between the sugar glider and flying squirrel indicate that similar environments sckcted for similar adaptations despite different ancestry. 3. At the time that dinosaurs originated, Earth's landmasses formed a single large continent, Pangaea. Because many dinosaurs were large and mobile, it is Ukelythat early members of these groups lived on many different parts of Pangaea. When Pangaea broke apart, fossils of these organisms would have moved with the rocks in which theywcredeposited. As a result, we would predict that fossils of early dinosaurs would have a broad geographic distribution (this prediction has been upheld).

(HAPTER 22 Figure Questions Figure 22,8 More than 5.s million years ago. Figure 22.13 The original pool of the transplanted guppy population contains pike·cichlids, a potent predator of adult guppies. Brightly colored adult males would be at a disadvantage in this pooL Thus, it is likely that color patterns in the guppy popu· lation would become more drab if they were returned to their original pooL Figure 22.19 Based on this evolutionary tree, crocodiles arc more closely related to birds than to lizards because they share a more recent common ancestor wilh birds (ancestor") than with lizards (ancestor

Self.Quiz

1. b 2. c 3. d 4. d S. a 6. d

8. (a)

0).

v

Concept Ched 22.1 1. Hutton and Lyell proposed that events in the past were caused by the same processes operating today. This principle suggested that Earth must be much older than a few thousand years, the age that was widely accepted at that time. Hulton and Lyell also thought that geologic change occurs gradually, stimulating Darwin to reason that the slow accumulation of small changes could ultimately produce the profound changes documented in the fossil rcrord. In this context, the age of Earth was important to Darwin, bcrause unless Earth was very old, he could not envision how there would have been enough time for evolution to occur. 2. By these criteria, Cuvier's explanation of the fos· sil rl'COrd and Lamarck's hypothesis of .'volution arc both scientific. Cuvier suggested that catastrophes and the resulting extinctions were usually confined to local regions, and that such regions were later repopulated by a dif· ferent set of species that immigrated from other areas. These assertions can be tested against the fossil record (they have been found to be false). With reo sped to Lamarck, his principle of use and disuse can be used to make testable prcdictions for fossils of groups such as whale anccstors as thl'y adapt to a new habitat.l.amarck·s principle of the inheritance of acquired characteristics can be tested dircrtly in liVing organisms (it has been found to be false).

Concept Check 22.2 1. Organ isms share characteristics (the unity oflife) because they share com· mon ancestors; the great diversity of life occurs because new species have repeatedly formed when descendant organisms gradually adapted to different environments, becoming different from their ancestors. 2. All species have the potential to produce more offspring (ovcrreproduce) than can be supported by the environment. This ensures th.'re will be what Darwin called a "struggle for existence· in which manyofthe offspring are eaten, starved, diseased, or unable to reproduce for a variety of other reasons. Members of a

,/

'0 1 4

,

10 ll.

(b) The rapid risc in the pcrcl'lltageof mosquitoes resistant to DDT was most likely caused by natural sckction in which mosquitoes resistant to DDT could survive and reproduce while other mosquitoes could not. (c} In Indiawhere DDT resistance first appeared-natural selection would have caused the frequency of resistant mosquitoes to increase over time. If resistant mosquitoes then migrated from India (for example, transported by wind or in planes, trains, or ships} to other parts of the world, the frequency of DDT resistance would increase there as well.

CHAPTER 23 Figure Questions

Figure 23.7 The predicted frequencies are 36% c!c!, 48% c!C w, and 16% CwC w. Figure 23.12 Bl'Causc such a shift in prevailing winds would increase the transfcr of alleles (gene now) from plants living on mine soils to plants living at the location marked by the arrow, the change would probably lead to an increase in the index of copper tolerance of plants at thaI location. Figure 23.16 Crossing a single female's eggs with both an SC and an LC male's sperm allowed the researchers to directly compare the effects of the maks' contribution to the next gencration, since both batches of offspring had the same maternal contribution. This isolation of the male's impact enabled researchers to draw conclusions about differences in genetic "quality· between the SC and LC males. Figure 23.18 The researchers measured

Answers

A-IS

~

•~

.. C


the percentages of successfully reproducing adults out of the breeding adult population that had each phenotype. This approach of determining which phenotype was favored by selection assumes that reproduction was a sufficient indieator of relative fitness (as opposed to counting thl' numbl'r of eggs laid or offspring hatched, for example) and that mouth phenotype was the driving factor determining the fishes' ability to reproduce.

Concept Check 23.1 1. (a) Within a population, genetic differenct'S among individuals provide the raw material on ",-hich natural sekction and other mechanisms can act. Without such differences, allele frequencies could not change over time-and hence the population could not evolve. (b} Genetic variation among populations can arise by natural selection if selection favors different alleles in different populations; this might occur, forexa.mple, if the different populations experienced different environmental conditions. Genetic variation among populations can also arise by genetic drift when the genetic differences between populations are selectively neutral. 2. Many mutations occur in somatic cells that do not produce gametes and so are lost when the organism dies. Of mutations that do occur in cell lines that produce gametes, many do not have a phenotypic effect on which natural selection can act. Others have a harmful effect and are thus unlikely to inerease in frequency bt'C3use they dt'Crease the reproductive success of their bearers. 3. Its genetic variation (whether measured at the level of the gene or at the level of nucleotide sequences} would probably drop over time. During meiosis, crossing over and the independent assortment of chromosomes produce many new combinations of alleles. In addition, a population contains a vast numbcr of possible mating combinations, and fertilization brings together the gametes of individuals with different genetic backgrounds. Thus. via crossing over. independent assortment of chromosomes, and fertilization. sexual reproduction reshuffles alleles into fresh combinations each generation. Without sexual reproduction, new sources of genetic variation would be reduced, causing the overall amount of genetic variation to drop.

Concept Check 23.2 1. 750. Halfthe loci (250) are fixed, meaning only one allele exists for each locus: 250 x I '" 250. There aretwo alleles each for the other loci: 250 x 2 '" 500. 250 + 500 = 750. 2. p2 + 2pq; represents homozygotes with two A alleles, and 2pq represents heterozygotes with one A allele. 3. There are 120 individuals in the population. so there are 240allcles. Oftht'SC. there are 124 A alleles-32 from the l6AA individuals and 92 from the92Aa individuals. Thus. the frequency of the A allele is p '" 124/240 '" 052; hence. the frequency of the a allele is q '" 0.4$. Based on the Hardy-Weinberg equation. if the population were not evolving, the frequency of genotype AA should be = 052 x 052 = 0.27; the frequency of genotype Aa should be 2pq = 2 x 0.52 x 0.48 = 05; and the frequencyofgenorypeaa should be = 0.4$ xO.48 = 0.23. In a population of 120 individuals, these expected genotype frequencies lead us to predict that there would be 32AA individuals (0.27 x 120). 60Aa individuals (05x 120). and 28 atJ individuals (0.23 x 120). The actual numbers for the population (16 AA, 92 Aa. 12 aa) dt'Viate from these expectations (fl'"..cr homozygotes and more heterozygoles than expcckd). This suggests that thl' population is not in HardyWeinberg equilibrium and hence is evolVing.

i

l

i

Concepl Check 23.4 1. Zero. because fitness includes reproductive contribution 10 the next generation, and a sterile mule cannot produce offspring. 2. Although both gcnl' 110w and genetic drift can increase thl' frequency of advantageous alleles in a population, they can also decrease the frequency of advantagt'Ous alldes or increase the frequency of harmful alleles. Only natural selection coflSistc/lliy results in an increase in the frequency of alleles that enhance survival or reproduction. Thus, natural selection is the only mechanism that consistently causes adaptive evolution. 3. The three modes of natural selection (directional, stabilizing, and disruptive} are defined in terms of the selective ad· vantage of diffcrent phenotypes. not different genotypes. Thus. the typl' of selection represented by heterozygote advantage depends on the phenotype of the heterozygotes. In this question. because heterozygous individuals have a more extreme phenotype than either homozygote. heterozygote advantage represents directional selection. Self-Quiz

1. d 2. a 3. 4. b 5. b

I'

7. The frequency of the I4 p 'J' allele forms a cline. decreasing as onc moves from southwest to northeast across Long Island Sound.

1 Site 13 I'p~~.

Site

I'f"'~

5

3. 37 II 5'1

7o+-!--+H+H+t-

t.o

f SD i?

++---+-+++++bl-

'/d++~---+-t-I-Hb¥1H­

J '" ++--++-Jft-+t+t10 ~ Jo+j~::$~ttttt /o-t o-'--t--\-+++~f-++­ I

5W

,sih!.:;

,.l.3~~" 78

A hypothesis that explains the clinl' and accounts for the observations stated in the question is that the cline is maintained by an interaction between se· lection and gene 110w. Under this hypothesis. in the southwest portion of the Sound. salinity is relatively low, and selection against the lap 94 allele is strong. Moving toward thc northeast and into the open ocean, whl're salinity is relatively high, selection favors a high frequency of the lap 94 allele. However. because mussel larvae disperse long distances, gene flow prevents the lap'JI allele from becoming fixed in the open ocean or from declining to zero in the southwestern portion of Long Island Sound.

Concept Check 23.3 1. Natural selection is more ·predictable" in that it altcrs allele f{<'{jul'neies in a nonrandom way: It tends to increase the frequency of alleles that increase the organism's reproductive success in its environment and decrease the frequency of alleles that decrease the organism's reproductive success. Alleles subject to genetic drift increase or decrease in frequency by chance alone. whether or not they are advantageous. 2. Genetic drift results from chance events that cause allek frequencies to nuctuate at random from gcneration to generation; within a population, this process tends to decrease genetic variation over time. Gene now is the exchange of alleles between populations: a process that can introduce new alleles to a population and hence may increase its genetic variation (albeit slightly, since rates of gene 110w are often low}. 3. Selection is not important at this locus; furthermore. the populations arc reasonably largc. and hene<' thl' effects of genetic drift should not be pronounced. Gene now is occurring via the movement of pollen and seeds. Thus. anele and genotype frequencies in these populations should become more similar over time as a result of gene 110w.

A-19

Appendix A

(HAPTER 24

Figure Questions Figure 24.3 Allele I (found in some birds in I'opulation B) is more closely related to alleles found in Population A than to other alleles found in Population B. This implies that the ancestral allele from which allele I descended existed in Population A. Hence, the direction of gene now was from Population A to Population B. Figure 24.9 This change would have thc effect of incrl'asing gcne now between thl' populations. which would make the evolution of reproductive isolation more difficult. Figure 24.12 Such results would suggest that mate choice based on coloration does not provide a reproductive barrier between these two cichlid species. Figure 24.14 Because the populations had only just begun to diverge from onl' anothcr at this point in the process, it is likely that any existing barriers to reproduction would weaken over time. Figure 24.20 The presence of M. cardinails plants that carry the M. lewisii yup allele


would make it more likely that bumblebees would transfer pollen hetween the two monkey Oower spl'(:ies. As a result, we would eXpl'(:t the number of hybrid offspring to increase.

might enhance the effect of the )'UP lows (by modifying !lower color) or cause entirely different barriers to reproduction (for example, gametic isolation or a postlygotic barrier).

Concept Check 24.1 1. (a) All except the biological species concept Gin be applied to both asex· ual and sexual species because they define spedes on the basis of characteristics other than ability to reproduce. In contrast, the biological species concept can he applied only to sexual spl'(:ies. (b} Theeasiest spl'(:ies concept to apply in the field would he the morphological species concept because it is based only on the appearance of the organism. Additional information about its ecological habits, evolutionary history, and reproduction are not reo quired. 2. Because these birds live in fairly similar environments and can breed successfully in captivity, the reproductive barrier in nature is probably prezygotic; given the species differences in habitat preference, this barrier could result from habitat isolation.

Self-Quiz

Concept Check 24.2 1. In allopatric spl'(:iation, a new species forms while in geographic isolation from its parent species; in sympatric speciation, a new spl'(:ies forms in the absence of geographic isolation. Geographic isolation greatly reduces gene flow between populations, whereas ongoing gene flow is more likely in sym· patry. Asa result, sympatric speciation is less common than allopatric speciation. 2. Genc flow betwcen subsets of a population that live in the samc area can he reduced in a variety of ways. In some species-especially plantschanges in chromosome numher can block gene Oow and establish reproductive isolation in a single generation. Gene Oow can also he reduced in sympatric populations by habitat differentiation (as seen in the apple maggot fly, Rhagolelis} and sexual selcction (as seen in Lake Victoria cichlids). 3. AIlopatric speciation would be less likely to occur on a nearby island than on an isolated island of the same size. The reason we expect this result is that continued gene Oow hetween mainland populations and those on a nearby island reduces the chance that enough genetic divergence will take place for allopatric speciation to occur. Concept Check 24.3 1. Hybrid wnes are regions in which memhersof different species meet and mate, producing some offspring of mixed ancestry. Such regions are "natural laboratories' in which to study speciation because scientists can directly ob· serve factors that cause (or fail to cause) reproductive isolation. 2. (a) If hybrids consistently survive and reproduce poorly compared to the off· spring of intraspecific matings, it is possible that reinforcement would occur. If it did, natural selection would cause prezygotic barriers to reproduction between the parent species to strengthen over time, decreasing the production of unfit hybrids and leading to a completion of the spe· ciation process. (b) lfhybrid offspring survive and reproduce as well as the offspring of intraspl'(:ific matings, indiscriminate mating hetween the parent spl'(:ies would lead to the production of large numbers of hybrid offspring. As these hybrids mated with each other and with members of both parent species, the gene pools of the parent species could fuse over time, reversing the speciation process. Concept Check 24.4 1. The time hem'een speciation events includes (I} the length of time that it takes for populations of a newly formed species to begin diverging reproductively from one another and (2) the time it takes for speciation to be com· plete once this divcrg,'nce bl'gins. Although speciation can occur rapidly once populations have begun to diverge from one another, it may take millions of years for that divergence to begin. 2. Investigators transferred alleles at the )'up locus (which influences flower color} from each parent species to the other. M. lewisii plants with an M. cardinalis YIIP allele reo ceived many more visits from hummingbirds than usual; hummingbirds usually pollinate M. eardinalis but avoid M. lewisii. Similarly, M. cardinalis plants with an M. lewisii YIIP allele received many more visits from bumblebees than usual; bumblebees usually pollinate M. lewisii and avoid M. cardinalis. Thus. alleles at the yup locus can inOuence pollinator choice. which in these species provides the primary barrier to interspecific mating. Nevertheless, the experiment does not prove that the YIIP locus alone con· trois barriers to reproduction between M. lewisii and M. cardinalis; other genes

1. b 2. a 3. c 4. e 5. d 6. c 8. One possible process is

I I

,"",14) AA X

@J Cl>FI'/)

~

~ (,""Ie)

~

Jmeiotic error

o.n'Jl![A~B81

•~

.. c

x ~"JFI4) ~

~ (srer;Je)

t

rntid-ic et"(br

IMBB !llli ()"dJZ) CHAPTER 25 Figure Questions Figure 25.5 Becauseuranium-2J8hasa half-life of4.5 billion years,thex-axis would be relabeled (in billions of rears) as:4.5, 9, 13.5, and 18. Figure 25.23 The coding St'{juence of the Pitxl gene would differ bdween the marine and lake populations, but patterns of gene expression \\"oold not. Concept Check 25.1 1. The hypothesis that conditions on early Earth could have permitted the synthesis of organic molecules from inorganic ingredients 2. In contrast to random mingling of molecules in an open solution, segregation of molecular systems by membranes could concentrate organic molecules, assisting biochemical reactions. 3. No. Such a result would only show that life could have begun as in the experiment. Concepl Check 25.2 1. 22,920 years (four half-lives: 5,730 x 4) 2. The fossil record shows that different groups of organisms dominated life on Earth at different points in time and that many organisms once alive are now extinct; specific examples of these points can be found in Figure 25.4. The fossil record also indicates that new groups of organisms can arise via the gradual modification of previously existing organisms, as illustrated by fossils that document the origin of mammals from cynodont ancestors. 3, The discowry of such a (hypothetical} fossil organism would indicate that aspects of our current understanding of the origin of mammals are not correct because mammals are thought to have originated much more recently (see Figure 25.6). For example, such a discovery could suggest that the dates of previous fossil discoveries are not correct or that the lineages shown in Figure 25.6 shared features with mammals but were not their direct ancestors. Such a discovery would also suggest that radical changes in multiple aspects of the skeletal structure of organisms could arise suddenly-an idea that is not supported by the known fossil record. Concept Check 25.3 1. Free oxygen attacks chemical bonds and can inhibit enzymes and damage cells. 2. All eukaryotes have mitochondria or remnants of these organelles, but not all eukaryotes have plastids. 3. A fossil record of life today would include many organisms with hard body parts (such as H'rtebrates

Answers

A-20


and many marine invertebrates), but might not indude some species we are very familiar with, such as those that have small geographic ranges and/or small population sizes (for example, all five rhinoceros spedes). Concept Check 25.4 1. Continental drift alters the physical geography and climate of Earth, as well as the extent to which organisms are geographically isolated. Because these factors affect extinction and spedation rail'S. continental drift has a major impact on tifeon Earth. 2. Mass extinctions; major evolutionary innovations; the diversification of another group of organisms (which can provide new !iOurces of food); migration to new locations where few competitor species exist 3. Their fossils should be present right up to the time of the catastrophic evem, then disappear. Reality is a bit more complicated because the fossil record is not perft'Ct. So the most fl'CCnt fossil for a species might be a million years before the mass extinction. even if the species did not become extinct until the mass extinction. Concept Check 25.5 1. Heterochrony can cause a variety of morphological changes, For example, iftheonset of sexual maturity changes, a retention of juvenile characteristics (paedomorphosis) may result. Paedomorphosis can be caused by small genetic changes that result in large changes in morphology, as seen in the axolotl salamander. 2. In animal embryos. Hm genes inf1uence the development of structurt'S such as limbs or feeding appendages. As a result, changes in these genes-or in the regulation of these genes-are likely to have major effects on morphology. 3. From genetics, we know that gene regulation is altered by how well transcription factors bind to noncoding DNA sequences called control clements. Thus, if changes in morphology arc often caused by changes in gene regulation, portions of noncoding DNA that contain control elements arc likely to be strongly afft'Cted by natural selection. Concept Chc拢k 25.6 1. Complex structures do not evolve all at once. but in increments. with natural seledion selecting for adaptive variants of the earlier versions. 2. Although the myxoma virus is highly lethal, initially some of the rabbits are resistant (0.2% of inft'Cted rabbits are not killed). Thus, assuming resistance is an inherited trait. we would expect the rabbit population to show a trend for increased resistance to the virus. We would also expect the virus to show an evolutionarytrend toward reduced lethality. We would expect this trend because a rabbit infected with a less lethal virus would be more likely to live long enough for a mosquito to bite it and hence potentially transmit the virus to another rabbit. (A virus that kills its rabbit host before a mosquito transmits it to another rabbit dies with its host.)

CHAPTER 26 Figure Questions Figure 26.5 This new version docs not alter any of the evolutionary relationships sho.....n in Figure 26.5. For example. Band C remain sister taxa, taxon A is still as dosely related to taxon B as it is to taxon C. and so on.

,

TAl'a~ II

IIll/t'tlE T/ll(i;~F

T6.vCOl Pc To<f'lC r""o~ IS

Figure 26.6 Mistakes can occur while performing the experiment (such as errors in DNA sequencing) and analyzing the results (such as misaligning the DNA sequences of different species). But an erroneous condusion-such as concluding that a sample was from a humphack whale when in fact it came from a gray whale-could be reached even if no such errors were made. A particular humpback whale might. for example, happen to have a DNA sequence that was rare for its species. yet common for another spt'Cies. To rt'(\uce the chance that such events could lead to an erroneous conclusion. gene trees could be con路 structed for multiple genes; if similar results emerged from aU of these gene trees. there would be little rea!iOn to doubt the conclusions. Figure 26.9 There arc four possible bases (A, C. G, I) at each nucleotide position. If the base at each position depends on chance, not common descent. we would cxpt'Ct roughly one out offour (25%) of them to be the same. Figure 26.12 The lebrafish lineage; of the five vertebrate lineages shown. its branch length is the longest. Figure 26.19 The molecular clock indicates that the divergence time is roughIy45-50 million years. Figure 26.21 Bacteria was the first to emerge. Archaea is the sister domain to Eukarya. Concept Check 26.1 1. We are classified the same down to the class level; both the leopard and human arc mammals. Leopards belong to order Carnivora. whereas humans do not. 2. The branching pattern of the tree indicates that the badger and the wolf share a common ancestor that is more recent than the ancestor that these two animats share with the leopard. 3. The tree in (c) shows a different pattern of evolutionary relationships. In (c), C and B arc sister taxa. whereas C and D arc sister taxa in (a) and (b).

4.

r - - - - - - T"l'tnA

Self-Quiz

1. c 2. a 3. e 4. b 5. c 6. d 7. b 8.

>"""", lKljirt.,f mAmmals (..,

",.'

Cr~fDuw,s /"I'IIW:

tlLHneh'on

(~s!S ~I:I.)

'PUOl;4/l

~-_ _

"",0,",,,

{", m11 CunbnifJlaposiC1I

(~3r; -5'2.S' 11IJ')

~",""iluJ<lY/

"""'j""

Sinjle-ooltd

wW.,n" A-21

Appendix A

p,'~j,*" ~phUico~en

Concept Check 26.2 1. (a) Analogy, since porcupines and cacti are not closely related and since most other animals and plants do not have similar structures; (b) homology. since cats and humans are both mammals and have homologous forelimbs. of which the hand and paw are the lower part; (c) analogy. since owls and hor路 nets are not closely related and since the structure of their wings is very different. 2. Species 2 and 3 are more likely to be closely related. Small genetic changes (as between species 2 and 3) can produce divergent physical appearances. whereas if genes have diverged greatly (as in spt'Cics 1 and 2). that suggests that the lineages have been separate for a long time. Concept Cheel< 26.3 1. No; hair isa shared ancestral character common toall mammals and thus is not helpful in distinguishing different mammalian subgroups. 2. The princi路 pleof maximum parsimony states that the hypothesis about nature we investigate first should be the simplest explanation found to be consistent with the facts. Actual evolutionary relationships may differ from those inferred by


parsimony owing to complicating factors such as convergent evolution.

9. l.tl

i.MI:tld·

3. The traditiooal classification providesa poor match toevolutionary history, thus violating the basic principle of cladistics-that classification should be based on

(c."rt'lf) l..4.Mprtlj

common descent. Both birds and mammals originated from groups traditionally designated as reptiles, making reptiles as traditionally delineated a paraphyletic group. These problems can be addressed by removing Dimetro@llandcynodonts from the reptiles, and by considering birds as a group of reptiles (specifically. as a group of dinosaurs).

,

S.Iun.o.ndtr

, - - - - - - - LiwdswSllAUs

Tw+"

-

"'f""

,

~

~Lrtdll4J1 ,i,'fW4!Ul

L..--

-

--l,--_....

,-------/)illtdraU"

'---L_--{====o,,,,,,,

~

..

110

C

-

;-

1. A molecular clock is a method of estimating the actual time of evolutionary events based on numbers of base changes in orthologous genes. It is based on the assumption that the regions of genomes being compared evolve at constant rates. 2. There are many portions of the genome that do not code for genes; many base changes in these n-gions could accumulate through drift without affecting an organism's fitness. Even in coding regions of the genome, some mutations may not have a critical effect on genes or proteins, 3. The gene (or genes) used for the molecular clock may have evolved more slowly in these two taxa than in the species used to calibrate the clock: as a result, the clock would underestimate the time at which the taxa diverged from one another.

Conn'pt Check 26.6 1. The kingdom Monera included bacteria and archaea, but we now know that these organisms are in separate domains. Kingdoms are subsets of domains, so a single kingdom (like Monera) that includes taxa from different domains is not valid (it is polyphyletic). 2. Because of horizontal gene transfer, some genes in eukaryotes are more closely related to bacteria, while others are more closely related to archaea; thus, depending on which genes are used, phylogenetic trees constructed from DNA data can yield conflict· ing results.

3.

-cc

e.",,,. Ard'IAttl

E:.l.'W.1'

The third tree, in which the eukaryotic lineage diverged first, is not likely to receive support from genetic data because the fossil record shows that prokaryotes originated long before eukaryotes.

Self.Quiz 1. b 2. d 3. a 4. d S. c 6. d 7. d

,

, ,.

,

Concept Check 26.4

Concept Check 26.5

WpI1in

•~

~&Il,,"l1s

1. Proteins are gene products. Their amino acid sequences are determined by the nucleotide sequences of the DNA that codes for them. Thus, differences between comparable proteins in tv.'o species reflect underlying genetic differences. 2. These observations suggest that the evolutionary lineages leading to species 1 and species 2 diverged from one another before a gene duplication event in species 1 produced gene B from gene A.

T_

,

TwHt

, 5

(c) The tree in (a) requires seven evolutionary changes, while the tree in (b) re<juires nine evolutionary changes. Thus, the tree in (a) is the most parsi· monious, since it requires fewer evolutionary changes.

(HAPTER 27 Figure Questions Figure 27.10 It is likely that the expression or sequence of genes that affect glucose metabolism may have changed: genes for metabolic processes no longer needed by the cell also may have changed. Figure 27.12 The pop. ulation that includl'd individuals capable of conjugation would probably be more successful, since some of its members could form recombinant cells whose new gene combinations might be advantagrous in a novel environment. Figure 27.17 Thermophiles live in very hot environments, so it is likely that their enzymes can continue to function normally at much highrr temperatures than do the enzymes of other organisms. At low temperatures, however, thl' enzymes of thermophiles may not function as well as the en· zymes of other organisms. Figure 27.19 From the graph, plant uptake can be estimated as 0.7, 0,6, and 0,95 (mg K) for strains I, 2, and 3, respectively. These values average to 0.75 mg K. If bacteria had no effect. the average plant uptake of potassium forstrains 1,2, and 3 should be close to 0.5 mg K, thl' value observed for plants grown in bacteria-free soil.

Concept Check 27.1 1. Adaptations include the capsule (shields prokaryotes from host's immune system) and mdospores (enable cells to survive harsh conditions and to revive when the environment becomes favorable). 2. i>rokaryotic cells generally lack the internal compartmentalization of eukaryotic cells. Prokaryotic genomes have much less DNA than eukaryotic genomes, and most of this DNA is contained in a single ring-shaped chromosome located in the nucleoid rather than within a true membrane-bounded nucleus. In addition, many prokaryotes also have plasm ids, small ring-shaped DNA molecules containing a few genes. 3. Bl'Causc prokaryotic populations evolve rapidly in response to their environment, it is likely that bacteria from endospores that formed 4tl years ago would already be adapted to the polluted conditions. Hence, at least initially, these bacteria would probably grow better than bacteria from endospores that formed ISO years ago, when the lake was not polluted.

Answers

A-22


Concept Che<:k 27.2 1. Prokaryotes have extremely large population sizes, in part because they have short generation times. The large number of individuals in prokaryotic populations makes it likely that in each generation there will be thousands of individuals that have new mutations at any particular gene, thereby adding considerable genetic diversity to the population. 2. In transformation, naked, foreign DNA from the environment is taken up by a bacterial celL In transduction, phages carry bacterial genes from one bacterial ceHto another. In conjugation, a bacterial cell directly transfers plasmid or chromosomal DNA to another cell via a mating bridge that temporarily connects the tv.'o cells. 3. Yes. Genes for antibiotic resistance could be transferred (by trans· formation, transduction, or conjugation) from the nonpathogenic bacterium to a pathogenic bacterium; this could make the pathogen an even greater threat to human health. In general, transformation, transduction, and conjugation tend to increase the spread of resistance genes. Concept Che<:k 27.3 1. A phoIntroph derr,'l5 its energy fron11ight. v.ttile a dlemorroph gets itsenergy &001 chemical sources. All autotroph derives itscarbon fiun in~ sources (dtcn C00, while a heterotroph gets itselrlxJn fiun organic sources. Thus. there are k>ur nubitional modes: phcroautdrophic, phoInhcU'rotrophic (Wlique 10 prokaryotes), chemoautotrophic (unique to prokaryotes), and chemohcterotrophic. 2. Otemoheteroouphy; the bacterium must rely on chemiell SOUlU'S of energy, since it is not exposed to light, and it must be a heteroouph if it requires an organic SOlllU' of carbon rather than COz (or anothcr inotpnic SOlllU', like bicarbonate). 3. If hu· mans could fix nitrogen, we could build proteins using atmospheric N2 and lK'Ilce would not need toeat high-protein foods such as meat or fish. Ourdiet would, however, need to include a sourceofcarbon, along with minerals and water. Thus, a typical meal might consist ofcarbohydratl'S as a carbon source, along with fruits and vegetables to provide essential minerals (and additional carbon). Concept Che<:k 27.4 1. Before molecular systematics, taxonomists classified prokaryotes according 10 phenotypic characters that did not clarify t'volutionary relationships. Moh:ular oomparisons-ofDNA in particular-indielte key divergences in prokaryotic lineages. 2. By not requiring that organisms becultured in the laboratory, genetic prospecting has revealed an immense diversity of previously unknown prokaryotic species. Over time, the ongoing discovery of new species by genetic prospecting is likely to alter our understanding of prokaryotic phylogeny greatly. 3. At present, all knov.'Il methanogens are archaca in the clade Euryarchat'Ota; this suggests that this unique metabolic pathway arose in anQ'Stral species within Eur· yarchaeota. Since Bacteria and Archaea have been separate evolutionary lineages for billions of years, the discovery of a mcthanogen from the domain Bacteria \'«)IIld suggest that adaptations that enabled the use of CO 2to oxidize H 2 evolved at least twice-once in Archaea (within Euryarchaeota) and once in Bacteria. Concept Che<:k 27.5 1. Although prokaryotes are small, their large numbers and metabolic abilities enable them to play key roles in ecosystems by decomposing wastes, recycling chemicals, and affecting the concentrations of nutrients available to other organisms. 2. Bacteroifks thctawuwmicrol1, which lives inside the human intestine, benefits by obtaining nutrients from the digestive system and by receiving protection from competing bacteria from host-produced antimicrobialcompounds to which it is not sensitive.1be human host benefits because the bacterium manufacturcscarbohydratcs, vitamins, and other nutrients. 3. Some of the many differt'llt species of prokaryott'S that he in the human gut compete with one another for resources (in the food )'Ou eat). Because different prokaryotic species have different adaptations, a change in diet may alter which species can grow most rapidly, thus altering species abundance. Concept Che<:k 27.6 1. Sample answers: eating fermented foods such as yogun, sourdough bread, or cheese; receiving clean water from sewage trl'atmenl; taking medicines produced by bacteria. 2. No. If the poison is secreted as an aotoxin,livc bactt'l'ia could be transmitit'd to another person. But the same is true ifthe poison is an endotoxinonly in this case, the live bacteria that are transmitted may be descendants of the (n()\',··dead) bacteria that produced the poison. 3. Strain 1(-12 may have lost genes by deletion mutations. A phylogenetic analysis would help distinguish betv.-een these hypotheses-if some of the genes found in 0157:H7 but not in 1(·12 are present in the common ancestor of the tv.'O strains, that would suggest that strain K-1210st tht'SC gent'S O\'t'l" the course of its evolution.

A-23

Appendix A

Self-Quiz

1.

I'

8. (a)

2. a 3. d 4. d 5. b 6. a 1-

-

r-

r-

~

o

n

,

(b) Some Rhizobium slrains are much more effective at promoting plant growth than are other Rhizobium strains; the most ineffc<:tive strains have little positive effect (plant growth with these strains differs little from plant growth in the absence of Rhizobium). The ineffective strains may transfer relatively little nitrogen to their plant host, hence limiting plant growth.

(HAPTER 28 Figure Questions Figure 28.10 Merozoites are produced by the asexual (mitotic) cen division of haploid sporowites; similarly, gametocytes are produced by the asexual cell division of merozoites. Hence, it is likely that individuals in tht'SC three stages have the same complement of gent'S and that morphological differenct'S between them result from changes in gene expression. Figure 28.22 The fol· lOWing stage should be circled: step 6, where a maturc cell undergoes mitosis and forms four or more daughter cens. In step 7, the zoospores eventually grow into mature haploid cells, but they do not produce new daughter cells. likewise, in step 2, a mature cell develops into a gamete, but it does not produce new daughter cells. Figure 28.23 If the assumption is correct, then thcir results indicate that the DHFR-TS gene fusion maybe a derived trait shared by members of four supergroups of eukaryotes (Excavata, Chromalveolata, Rhizaria, and Archaeplastida). However, if the assumption is not correct, the presence or absence of the gene fusion may tell little about phylogenetic his· tory. For example, if the genes fused multiple times, groups could share the trait hccause of convergent evolution rather than common descent. If the genes were ~ondarily split, a group with such a split could be placed (incor· rectly) in Unikonta rather than its correct placement in one of the other four supergroups. Concept Cheel< 28.1 1. Sample response: Protists inelude unicellular, colonial, and multicellular organisms; photoautotrophs, heterotrophs, and mixotrophs; species that reproduce asexually, sexually, or both ways; and organisms with diverse physi. cal forms and adaptations. 2. Strong evidence shows that eukaryotes acquired mitochondria after an early eukaryote first engulfed and then formed an endosymbiotic association with an alpha proteobacterium. Similarly, chloroplasts in red and green algae appear to have descended from a photosynthetic cyanobacterium that was engulfed by an ancient heterotrophic eukaryote. Secondary endosymbiosis also played an important role: Various protist lineages acquired plastids by engulfing unicellular red or green algae. 3. The modified tree would look as follo","-s:

""....

Co"" C-hro.wtto!cb. Rht~Ul

0<

""""'

CN'."'<lI~dl

ArIhltrlo.l!it:la.

Rliiwt.\

""""'.

AaliMp...sHdo..


Conn'pt Check 28.2 1. Their mitochondria do not have an electron transport chain and so cannot function in aerobic respiration. 2. Since the unknown protist is more closely related to diplomonads than to euglenids, it must have evolved after the diplomonads and parabasalids divergl-d from the euglcnozoans. In addition, since the unknown species has fully functional mitochondria-yet both diplomonads and parabasalids do not-it is likely that the unknown spedes evolved before the last common ancestor of the diplomonads and parabasalids.

Concept Check 28.3 1. Some DNA data indicate that Chromalveolata is a monophyletic group, but other DNA data fail to support this result. In support of monophyly, for many species in the group, the structure of their plastids and the st:quence oftheir plastid DNA suggest that the grouporiginated by a Sl'COndary endosymbiosis event (in which a red alga was engulfed). However, other species in the group lack plastids entirely, making the secondary endosymbiosis hypothesis difficult to test. 2. Figure 13.6b. Algae and plants with alternation of generations have a multicellular haploid stage and a multicellular diploid stage. In the other hm life cycles, either the haploid stage or the diploid stage is unicellular. 3. The plastid DNA would likely be more similar to the chromosomal DNA of cyanobacteria based on the well-supported hypothesis that eUkaryotic plastids (such as those found in the eukaryotic groups listed) originated by an endosymbiosis event in which a eukaryote engulfed a cyanobacterium. If the plastid is derived from the cyanobacterium, its DNA would be derived from the bacterial DNA.

Concept Check 28.4 1. Because foram tests are hardened with calcium carbonate, they form longlasting fossils in marine sediments and sedimentary rocks. 2. Convergent evolution. The different organisms have corne to display similar morphological adaptations over time owing to their similar lifestyles.

Concept Check 28.5 1. Many red algae contain an accessory pigment calk-d phycoerythrin, which gives them a reddish color and allows them to carry out photosynthesis in relativelydeep coastal water. Also unlike brown algae, red algae have no tlagellated stages in their life cycle and must depend on water currents to bring gametes together for fertilization. 2. llll'a's thallus contains many cells and is differentiated into leaflike blades and a rootlike holdfast. Caulerpa's thallus is composed of multinucleate filaments without cross-walls, so it is essentially ont' large cell 3. Red algae have no flagellated stages in their life cycle and hence must depend on water currents to bring their gametes together. This feature of their biology might increase the diftkulty of reproducing on land. In contrast, the gametes of green algae are flagellated, making it possible for them to swim in thin films of water. [n addition, a varicty of grecn algae contain compounds in their cytoplasm, cell wall, or zygok coat that protect against intense sunlight and other terrestrial conditions. Such compounds may have increased the chance that descendants of green algae could survive on land

be available for fishes and other species that eat cora!. Asa result. populations of these species may decline, and that, in rum, might cause populations of their prcdators to d"dine.

Self-Quiz 1. d 2. b 3. c 4. d 5. e 6. d 7.

&M1ld-1l,

r----- Cir_

.......

"'--

"'""'"

......

.....tlw~orlidll,

Am.:"bou:ll.lU

-

i\rl;1III.!.l

-1

ChcooI1..f~IWu

~

•~

\N.ikDIlhl.

..

,,",,;

-1

c

NIlele.v1·~

Pathogens that share a relatively recent common ancestor with humans should also share metabolic and structural characteristics with humans. Because drugs target the pathogcn's metabolism or structure, developing drugs that harm the pathogen but not the patient should be most difficult for pathogens with whom we share the most recent evolutionary history. Working backward in time, we can use the phylogenetic tree to determine the order in which humans shared a common ancestor with pathogens in different taxa. This process leads to the prediction that it should be hardest to develop drugs to combat animal pathogens, follo.....cd by choanoflagellatc pathogens, fungal and nudeariid pathogens, amocbozoans, other protists, and finally prokaryotes.

(HAPTER 29

Figure Questions Figure 29.7

r - - - - """,, L--

..

~

...

Concept Check 28.6 1. Amochozoans have lobe-shaped pSl'Udopodia, whereas for.lms havl' threadlike pseudopodia. 2. Slime molds are fungus-like in that they produce fruiting bodies that aid in the dispersal of spores, and they are animal-like in that they are motile and ingest food. However. slime molds are more closely related to gymnamoebas and entamoehas than to fungi or animals. 3. Support. Unikonts lack the unique cytoskeletal features shared by many excavates (seeConcept 28.2). Thus, if the unikonts wcre the fIrst group of cukaryok's to diverge from other eukaryotes (as sho.....n in Figure 28.23}, it would be unlikely that the eukaryote common ancestor had the c)'toskeletal features found today in many excavates. Such a result would strengthen the case that many excavates share cytoskeletal features because they are members of a monophyletic group, the Excavata.

Conn'pt Check 28.7 1. Because photosynthetic protists lie at the base of aquatic food webs, many aquatic organisms depend on them for food, either directly or indirectly. (In addition, a substantial percentage of the oxygen produced in photosynthesis on Earth is made by photosynthetic protists.) 2. Protists form mutualistic and parasitic associations with other organisms. Examples include parabasalids that form a mutualistic symbiosis with termites. as well as the oom}'cete Phytophthora ramorum, a parasite of oak trees. 3. Corals de· pend on their dinoflagellate symbionts for nourishment. so coral blcaching would be expected to cause the corals to die. As the corals die, less food will

Figure 29.10 Because the moss reduces nitrogen loss from the ecosystem, species that typically colonize the soils after the moss probably experience higher soil nitrogen levels than they otherwise would-an effect that may bencfit thesc species, since nitrogen is an essential nutrient that often is in short supply. Figure 29.13 A fern that had wind-dispersed sperm would not require water for fertilization, thus removing a difficulty that ferns face when they live in arid environments. The fern would also be under strong selection to produce sperm above ground (as opposed to the current situation, where some fern gametophytes arc located below ground).

Concept Check 29.1 1. Land plants share some key traits only with charophytes: rosette cellulose-synthesizing complexes, presence of peroxisome enzymes, similarity in spenn structure, and the formation of a phragmoplast in cell division. GHnparisons of nuclear and chloroplast gtonl'S also point to a common ana'Stry. 2. Spore walls toughened by sporopollenin (protects against harsh enlironmental conditions}; multicellular, dependent embryos (provides nutrients and protection to the developing embryo); cuticle (reduces waterloss). 3. The multicellular diploid stage of the life cycle ....uu1d not reproduce sexually. Instead both males and females would produce haploid spores by meiosis. These spores would give rise to multicellular male and [(wale haploid stages-a major change from the single-celled haploid stages (spenn

Answers

A-24


and eggs) that we actuaUy have. The multicenular haploid stages woukl produce gametes and reproduce sexually. An individual at the multicellular haploid stage ofthe human life cycle might look Uke us, or it might look completely different.

Concept Check 29.2 1. Bryophytes do not have an extensive vascular transport system, and their life cycle is dominated by gametophytl'S rather than sporophytes.

2. Answers

may include the following: Larg" surface area of protoncma enhances absorption of water and minerals; the vase-shaped archegonia protect eggs during fertilization and transport nutrients 10 the embryos via placental transfer cells; the stalk-like seta conducts nutrients from the gametophyte to the capsule, where spores are produced; the peristome enables gradual spore discharge; stomata

enable C02i'Oz exchange wbile minimizing water loss; lightweight sporl'S are rcadilydispcrscd by wind.

~

•~

..

,.

±!lw¥1

c

r-----I"''''"~wort.s , _ _ H_ l..<--'LyCDfh~tes

P"t1ji~r~

1lye>tt«I> Z r-----f-trnworh(or

SfoM....11t p~ESE"'T

1. Lycophytes have microphylls. whereas seed plants and pterophytes (ferns and their relatives) have megaphylls. Pterophytes and seed plants also share other traits not found in lycophytes. such as overtopping growth and the initiation of new root branches at various points along the length of an existing root. 2. Both seedless vascular plants and bryophytes have flagellated sperm that re<juire moisture for fertilization; this shared similarity poses challenges for these species in arid regions. With respect to key differences. seedless vascular plants have lignified, well-developed vascular tissue, a trait that enables the sporophyte to grow tall and that has transformed life on Earth (via th(' formation of forests). Secdkss vascular plants also have true leaves and roots, which, when compared to bryophytes, provides increased surface area for photosynthesis and improves their ability to extract nutrients from soil. 3. If lycophytes and pterophytes formed a clade, the traits shared by pterophytes and seed plants might have been present in the com· mon ancestor of all vascular plants, but lost in the lycophytes. Alternatively, the common ancestor of all vascular plants may have lacked the traits shared by pterophytes and seed plants; in this case, pterophytes and seed plants would share these traits as a result of convergent evolution.

Self-Quiz

'i~:;pt;";;'''''~~ A)

Concept Check 29.3

~)

,-j---- Li\lerwo"'h,

1. b 2.

I' 3. a 4. d 5. c 6. b 7. c 8. a. diploid; b. haploid: c. haploid; d. diploid: e. haploid 9. Based on our current understanding of the evolution of major plant groups, thl' phylogeny has thl' four branch points shown here:

,

cw""'" ,------

lD5e !.1'Ifl\Ata ,

1 ' - - - "',,,..

_ _ ~/l.l.JeS (rtf' NJ"lIUlo'r1's)

4lCll ph'lte..s f'mP"Jt<l ~mrWpUrY6

Mj"'I"'''' BJ

r---+-.!J·vt:Nr~

[.0><_

fl,e......"

",,'"

~-l'J'p"Jtt,

r--~1"

"'"""'I""'" A-r5iospums

Derived characters unique to the charophyte and land plant dade (indicated by branch point I} include rosette cellulose-synthesizing complexes, perox· isome enzymes, flagellated sperm structure, and a phragmoplast. Derived characters unique to the land plant clade (branch point 2) include apical mcristems, alternation of generations, walled spores produced in sporangia, and multicellular gametangia. Derived characters unique to the vascular plant clade (branch point 3) include life cycles with dominant sporophytes, complex vascular systems (xylem and phloem}, and well·developed roots and leaves. Derived characters unique to the pterophyte and seed plant dade (branch point 4} include megaphylls and overtopping growth.

CHAPTER 30 Figure Questions Figure 30.3 Three: (I) the current sporophyte (cells of ploidy 2n, found in the integum('nt, or sel'd coat); (2} thl' female gametophyte (cens of ploidy n, found in the food supply}; and (3) the sporophyte of the next generation (cells of ploidy 2/1, found in the embryo). Figure 30.12 No. The branching or· der shown could still be correct if Amborella and other early angiosperms had originated prior to ISO million years ago but angiosperm fossils ofthat age had not yet been discovered. In such a situation, the 140.million-year-old date for the origin ofth(' angiosperms shown on the phylogeny would be incorrect. Figure 30.14 This study establishes a correlation between the type of floral symmetry and the rate of plant speciation-but it is possible that floral symmetry is correlated with another factor that was the actual cause of the observed results. Note, however, that floral symmetry was associated with increased speciation rates in a variety of different plant lineages. This suggests-but does not establish-that differences in floral symmetry cause differences in speciation rates. In general, strong evidence for causa· tion can come from controlled, manipulative experiments, but such experi· ments are usually not possible for studies of past evolutionary events.

Concept Check 30.1 1. To have any chance of reaching the eggs, the flagellated sperm of seedless vascular plants must swim through a film of water, usually over a distance of no more than a few centimeters. In contrast, the sperm of seed plants do not A-25

Appendix A


require water because they are produced within pollen grains that can be transported long distances by wind or by animal pollinators. Although flagellated in some spedes, the sperm of seed plants do not require mobility because pollen tubes convey them from the point at which the pollen grain is deposited (near the ovules) dill,'(;tly to the eggs. 2. The reduced gametophytesof seed plants art nurtured by sporophytes and protected from stress, such as drought conditions and UV radiation. Pollen grains have tough protective walls. S«ds ha\1! one or two la}...n of protectiw tissue, the Sl"I.'d coat. that improve survival by prO\;ding more protection from en\;ronmental Sl~ than do the- walls of spores. Seeds also contOlin a stored supply of food, Ito'hich mabltS ~ to U\'e longer thlJn sportS and provides dC\'eloping tmbryos with nourishment for grolto'th. 3. If seed plants \O'ere homosporous. OIlly one typt ofspore would be produced-as opposed to the actual situation in which microspores &"\" rise to sperm crUs '<Iithin pollen grains, and megaSporC5 gk"l' rise to eggs within O\-wes. Thus, if structurl'5like pollen grains and seeds were produced. they would arist in a .·ery different way from how they now are formed..

Conctpt Chedc 30.2 1_ Although gymnosptnns are similar in not having their ~ enclosed in ow.ries and fruits, their seed-bearing structures vary greatly. For installtt,

cycads hlJ<.'C la~ conn, ",tlneas some gylTlflO!ip<'"llS such as Gint,o and Gndum, ha\'C small cones that look SOIIlI'Yoitat like berries. r ..en though they are not fruiu. ~af shlJpe also varies greatly, from the nttdIes of many conifers to the palmlike lea\·n ofcycads to G~lum leaVI'5 that look like those of flowtring planu. 2. 1lle life cycle iUustratn heterospory, as ovulate cones produce mtgasports and pollen conn produce microspores. The reduced gametophytes are tvident in the form ofthe microscopic pollen gnins and the microscopic female gametopbyte '<Iithin the megaspore. The egg is shown de\·eIoping within an O\'Ule, and a pollen tube issoown con'TYing the sperm. The figure also shows the protectk'C and nutriti\"e features of a Sl"l'd. 3. No. Fossil tvidtnce indicates that gymnosperms originated at least 305 million years ago, but this does not mean that angiosperms are that oldonly that the most recent common ancestor of gymnosperms and angiosperms must be that old. Concept Cked 30.3 1. In the oak·s life cycle, the tree (the sporophyte) produces flowers, '<I-hich contain gametophytes in pollen grains and ovules; the eggs in ovules are fertilized: the mature ovaries develop into dry fruits called acorns. We can view the oak's life cycle as starting when the acorn seeds germinate, resulting in embryos giving rise to seedlings and finally to mature trees, which produce flowers-and then more acorns. 2. Pine cones and flowers both have sporophylls, modified leaves that produce spores. Pine trees have separate pollen conl'S (with pollen grains) and ovulate cones (with ovules inside cone scales}. In flowers, pollen grains are produced by the anthers of stamens, and ovules are within the ovaries ofcarpels. Unlike pinecones, many flowers pro· duce both pollen and ovules. 3. Such a discovery would remove support for the idea, based on 12S.million-year-old Archaefructus sinensis fossils, that the earliest angiosperms may haVl' been herbaceous, aquatic plants. Concept Checl.: 30.4 1. Because extinction is irreversible, it dl'Creases the total diversity of plants, many of which may ha\"l' brought important l>el1l'fits to humans. 2. A detailed phylogeny of the seed plants would identify many different monophyletic groups of seed plants. Using this phylogeny. researchers could look for clades that contained species in '<I-hich medicinally useful compounds had already been disco\'et"l'd. IdentifICation of such clades would alJow researchers to concentrate their search for new medicinal compounds among dade members-as opposed toSol'arChing for newoompounds in species that were selected at random from the more than 250.000 existing species ofseed plants. Self-Quiz 1. d 2. a 3. b 4. a S. d 6.

(h¥ophytegreen~

....

8.

(a)

-

qealb.lIpb;~ • a/If, dip~

7«-Ill, trr,Iad

(b) 1lle phylogeny indicales thaI basal angiosperms differed from other an· giosperms in ttrmS of the number of cells in femak gametophytes and tilt ploidy of tilt endosperm. The ancestral Slate of tilt angiosperms cannot be: determined from these data alone. It is possible that the common ancestor of angiosperms had SC\'Cf'I-cel1ed female gamctoph)11'5 and [riploid endosperm and helltt that tilt eight·celled and four--<elled conditions found in basal angiosperms I'eprCSCnt <!em'ed traits for those lineages. A1temati\"l'!y, either tilt dght-cel~01'" four·ceIled condition may rtpreSellt tht ancestral state.

CHAffiRJl Figure Q~tions Figure 31.2 DNA from each of these mushrooms would be: identicaJ ifeach mushroom is part of a single hyphal nmo·ark. as is likely. Figure 31.16 One or both of the following would apply to each specin: DNA analyses I"oold I"l'\'eal that it is a member ofthe ascomycetes clade, or aspects ofits sex· uallife cycle would indicate that it is an ascomycete (foc enmple, it would produce asci and ascospores). Figure 31.21 T....o possible controls would be: E- p- and E+P-. Rnults from an E-P- control could be:compared with results from tht E- P+ experiment, and results from an E+ P- control could be: compared with results from the E+ P+ experiment; together, these two comparisons would indicate whether the addition of the pathogen causes an Incrt'ase in leaf mortality. Results from an E-P- experiment could also be compart'd with results from the second control (E+P-) to determine whether adding tht endophytes has a negative effect on the plant. Concept Check 31.1 1. Both a fungus and a human arc heterotrophs. Many fungi digest their food nternally by secreting enzymes into the food and then absorbing the small molecules that result from digestion. Other fungi absorb such small molecules dir~tly from their <,nvironment. In contrast, humans (and most other animals) ingest relatively large pi~esoffood and digest the food within their bodies. 2. The ancestors of such a mutualist most likely secreted powerful enzymes to digest the body oftheir ins~t host. Since such enzymes would harm a living host, It Is likely that the mutualist would not produce such enzymes or would restrict their secretion and use. Concepl Check 31.2 1. The majority of the fungal life cycle is spent in the haploid stage, whereas the majority of the human life cycle is spent in the diploid stage. 2. The two mushrooms might be reproductive structures ofthe same mycelium (the same organism). Or they might be parts oft.....o separate organisms that have arisen from a single parent organism through aso:ual reproduction and thus carry the same genetic information. Concept Checlr 31.3 1. DNA evidence indicates that fungi. animals, and their protistan relatiyes fonn a c1adt. tht! opisthokonts. Furthermore, an early-dk'Cl'ging fungal lint· agt. the Ch)1rids. hlJI'e posterior flagelb, as do most other opisthokonts. This suggests that othtr fungallintage:s lost their flagella after di\·erging from ch)'uids. 2. This indicates that fungi hlJd already established muwalistic relationships ""ith plants by the date the fossils of the earliest \'asCU1ar plants had formed. 3. Fungi are heterotrophs. Priorto thecoloniution ofland by plants. ttTTt$lrial fungi could ha\'Cli\"ed only where other organisms (or their remains) were present and provided a wurcr offood Thus, iHungi had colonized land btfol'e plants. they couJd have fed on any prokaryotes or protists that Jr,"ed on land or by the water's edge-but not 011 the plants oc animal.s on .....hich many fungi feed today. Aos'<l'Crs

A-26


Concept Che<:k 31.4 1. Flagellated spores 2. Possible answers include the following: In zygomycetes, the sturdy, thick-walled zygosporangium can withstand harsh conditions and then undergo karyogamy and meiosis when the environment is favorabk for reproduction. In glomeromyeetes, the hyphae have a specialized morphology that enables the fungi to form arbuscular mycorrhizal.' with plant roots. In ascomycetes, the asexual spores (conidia) are often produced in chains or clusters at the tips of conidiophores, where they are easily dispersed by wind. The often cup-shaped ascocarps house the sexual sporeforming asci. In basidiomycetes, the basidiocarp supports and protects a large surface area of basidia, from which spores arc dispersed. 3. Such a change to the life cycle of an ascomycete would reduce the number and ge· netic diversity of ascospores that result from a mating event. Ascospore number would drop because a mating event would lead to the formation of only one ascus. Ascospore genetic diversity would also drop because in ascomycetes, one mating event leads to the formation of asci by many different dikaryotic cells. As a result, genetic rl'Combination and meiosis occurs independently many different times-which could not happen if only a single ascus was formed. It is also likely that if such an ascomycete formed an ascocarp, the shape of the ascocarp would differ considerably from that found in its close relatives. Concept Che<:k 31.5 1. A suitable environment for growth, retention of water and minerals, protl'Ction from intense sunlight. and protection from being eaten 2. A hardy spore stage enables dispersal to host organisms through a variety of mechanisms; their ability to grow rapidly in a favorable new environment enables them to capitalize on the host's resources. 3. Many different outcomes might have occurred. Organisms that currently form mutualisms with fungi might have gained the ability to perform the tasks currently done by their fungal partners, or they might have formed similar mutualisms with other organisms (such as bacteria). Alternatively, organisms that currently form mutualisms with fungi might be less effective at liVing in their present envi· ronments. For example, the colonization of land by plants might have bffn more difficult. And if plants did eventually colonize land without fungal mutualists, natural selection might have favored plants that formed more highly divided and cxtensive root systems (in part replacing mycorrhizae). Self-Quiz

1. b 2. c 3. d 4. e S. b 6. a 8.

o

No eM,r.J'" ("-) IZI W'phjh! l"",nt/lO.)

As indicated by the raw data and bar graph, grass plants with endophytes (E +) produced more new shoots and had greater biomass than did grass plants that lacked endophytes (E -). These differences were especially pronounced at thl' highest soil temperature, wherl' E- grass plants producl'd no new shoots and had a biomass of zero (indicating they wl're dead).

CHAPTER 32 Figure Queslions

0

Figure 32.3 As described in and f), choanotlagellates and a broad range of animals have collar cells. Since collar cells have never been observed in plants. fungi, or non-choanotlagellate protists, this suggests that choanollagellates may be more closely related to animals than to other eukaryotes. If choanotlagellates are morl' closely related to animals than is any

A-27

Appendix A

other group of eukaryotes, choanollagellates and animals should share other traits that are not found in other eukaryotes. The data described in Oare consistent with this prediction. Figure 32.6 The sea anemone embryos could be infused with a protein that can bind to l3-catenin's DNA-binding site, thereby limiting the extent to which l3-catenin activates the transcrip· tion of genes necessary for gastrulation. Such an experiment would provide an independent check of the results shown in step 4. Figure 32.10 Ctenophora is the sister phylum in this figure, while Cnidaria is the sister phylum in Figure 32.11. Concepl Check 32.1 1. In most animals, the zygote undergoes cleavage, which leads to the formation of a blastula. Next, in gastrulation, one end of the embryo folds in· ward, producing layers of embryonic tissue. As the cells of these lay<'rs differentiate, a wide variety of animal forms result. Despite the diversity of animal forms, animal development is controlled by a similar set of Hox genes across a broad range of taxa. 2. The imaginary plant would require tissues composed of cells that were analogous to the muscle and nerve cells found in animals: "muscle" tissue would be necessary for the plant to chase prcy, and "nerve" tissue would be required for the plant to coordinate its movements when chasing prey. To digest captured prey, the plant would need to either secrete enzymes into one or more digestive cavities (which could be modified leaves, as in a Venus'llytrap), or secrete enzymes outside of its body and feed by absorption. To extract nutrients from the soil-yet be able to chase prey-the plant would nced something othcr than fixed roots, perhaps retraetabk "roots" or a way to ingest soil. Toconduct photosynthesis, the plant would require chloroplasts. Overall, such an imaginary plant would be very similar to an animal that had chloroplasts and retractable roots. Concept Ched< 32.2 1. c, b, a, d 2. We cannot infer whether animals originated before or after fungi. If correct, the date provided for the most recent common ancestor of fungi and animals would indicate that animals originated some time within the last billion years. The fossil record indicates that animals originated at least 565 million years ago. Thus, we could conclude only that animals originated some time between 565 million years ago and I billion years ago. Concept Check 32.3 1. Grade·level characteristics are those that multiple lineages share regardless of evolutionary history. Some grade-level characteristics may have evolved multiple times independently. Featufl'S that unite c1adl'S arc derived charac· teristics that originated in a common ancestor and were passed on to the var· ious descendants. 2. A snail has a spiral and determinate cleavage pattern; a human has radial, indeterminate cleavage. In a snail, the coelomic cavity is formed by splitting of mesoderm masSI'S; in a human, the coelom forms from folds of archenteron. In a snail, the mouth forms from the blastopore; in a human, the anus develops from the blastopore. 3. Most codomate triploblasts have two openings to their digestive tract, a mouth and an anus. As such, their bodies have a structure that is analogous to that of a doughnut: The digestive tract (the hole of the doughnut) runs from the mouth to the anus and is surrounded by various tissues (the solid part of the doughnut). The doughnut analogy is most obvious at early stages of development (sec Figure 32.9<:). Concepl Chl'ck 32.4 1. Cnidarians possess true tissues, while sponges do not. Also unlike sponges, cnidarians exhibit body symmetry, though it is radial and not bilateral as in other animal phyla. 2. The morphology-bas<'« tree dividl'S Bilateria into two major clades: Deuterostomia and Protostomia. The molecular·bascd tree recog· nizes three major clades: Deuterostomia, Ecdysozoa, and Lophotrochozoa. 3. Both statements could be correct. Figun:' 32.11 shows that the lineage leading to Deuterostomia was the first to diverge from the other two main bilaterian line· ages (those leading to Lophotrochozoa and Ecdysozoa). By itself, however, this information docs not indicate whether the most fl'<Xnt common anCl'Stor ofthe Deuterostomia lived before or after the first arthropods. For example, the an· cestors of Deuterostomia could have diverged from the ancestors of Lophotrochozoa and Ecdysozoa S70 million years ago; it could have then taken 35 million years for the clade Deuterostomia to originate, but only 10 million years for first Ecdysozoa and thm the arthropod clades tooriginate. Self-Quiz

1. a 2. d 3. b 4. e S. c 6. e


,

8

-

(l..-

-j '-

, , ,, , s

, , ,

s '-

s

,

would suggest that the life cycle of basal cnidarians was probably dominated by the medusa stage. Over time. the polyp stage arne to be increasingly important in some groups. such as Hydrozoa. which alternate bctv.·ccn medusa and polyp stages. and Anthozoa. which lack the medusa stage entirely.

Concept Check 33.3 1. Tapeworms can absorb food from their environment and release ammonia into their environment through their body surface bca.usc their body is w:ry nat. due in part to the lack 0( a coelom. 2. The function of the foot reflects the !(). mmotion required in each cbss. Gastropods usc their foot as a hoIdfast or to mall,' sJov,1y on the substrate. In cephalopods, the foot fwJctions as a siphon and tentades.. 3. The inner tube is the alimentary cmaI, ....hich runs the length of the body. The outer tube is the body ...oill. The t....o rubes are separated by the coelom.. 4. Many \opho(roctlozoans lack skeIrtons 01' other stnJctures that could support their toft. bodies against the ba: ~ pity. making it difficult for them to Ir.'e abct.'e the surface of the soil Sor'ne species, sud! as ectoprocts (bryozoans). ha\'e a sturdyexoskdeton, but thl-yare5tolliorwyand so ....ould fin1 it dif· ficuk to capture food on land (Note that thorie Iopbotrochowan! that do Ir.'e aOO.'e the soil ~, such as slugs. 113\" some bm ofhydmstatic skdeton.)

Concepl Ched: 33.4

rtbtionships IO.ithin I..opbotrochclw all' not resolved, we cannot estimate me precist nurnberoC times mat cleavage patterns hzo,,. changed 0\'eI' lhe course of CYOlution. If. for example. P1aryhelmin~ Mollusca. and Annelida form 11 clade. it would bl' reasonabIl' 10 infer t1m.'t' cleavage pattern changn (one in Acocb., 00l' in the anctStoc of this hypothetical dade. and one in Arthropoda~ Various other possibk- rdationships 1IIOOng Iophotrochozoans !t'ad to ~ ~imales.

1. Nemalodes bd: body segments and a true coelom; annelids ha\'e both. 2. Arthropod mouthparU all' modified appendagcs, which are bilaterally paill'd. 3. The arthropod ooskeleton. ...tlich had alll'ady ~'(l/\'ed in the ocean, allo....ed terrestrial 5peOet: to ll'Uin w:roter and support their bodies on Land. Wings allO"'ed them to disperse quickly to 0C\f0' habitats and to fmd food and mates. 1M tracheal system alJo.,.-s for efficient gas exdlange despite the presence of an e:xosl;deton. 4. YC'$. Under the tnditional hypothesis.....e ....ould Olp«t body 5CglT1OltaOOn to be mntrolled by similar Hox genes in annelids and arthropod$. Hew.'cver. ifannelids are in l.ophotnxho:roa and arthropods are in Ealysowa. body segmentation may ha\'e C\vl\'ed independently in these tv.v groups. In soch a case. "'"c might expect thai different Hox genes "''QU1d control the de\"Clopment ofbody segmentation in the 1'.1"0 clades.

CHAPTER 33

Concept Check 33.5

I

From lhc phylogeny. it a~ duol mw cJnv..r.gc is the ancestral condition for tuJl'lel:aZo;,f1S,. H~"tWr, btallSC:

Figure Questions Figure 33.8 Within a reproductive polyp. a cell that gives rise toa medusa ....ould have to divide by meiosis. A resulting haploid cell would then divide repeatedly (by mitosis), forming a haploid medusa. Later. cells in the

mcduSOl's gonads would divide by mitosis, forming the haploid e-ggs and sperm.

Figure 33.11 Adding fertilizer to the ....'ater supply would proba-

bly increase the abundance of algae. This. in tum, might increase the abundance of both snails (which eat algae) and blood flukes (which require snails as an intermediate host). As a result, the occurrence ofschistosomiasis might increase, Figure 33.28 Such a result would be consistent with the Ubx and nbd-A Ho:rgenes having played a major role in the evolution ofincreased body segment diversity in arthropods. However. by itself, such a result would simply show that the presence of the Ubx and afxf-A Hox genes was correlated with an increase in body segment diversity in arthropods; it would not provide direct experimental evidence that the acquisition of the Ubx and ndb-A genes caused an in(rease in arthropod body segment diversity,

1. Each tube foot mnsists of an ampulla and a podiwn. When the ampulla squeez.cs. it IOrces ....ater inlO the podium.....hich causes the podium to expand and contact the substrate. Adl\csr.'t' chemicals are then secreted from the base of the podium. thereby attaching the podium to the substrate. 2. These tv.·o organisms look very different from one another. bUI they share features found in all echinoderms. such as a water vascular system and rube fed. Hence, their shared characteristics probably result from homology. not analogy. 3. Both insects and nematodes are ml'lllbcl'$ ofEcdysowa. one of the thrce major clades oIbilatt'Tians. Therefore. achal'1lClcristicsharcd by Drosophila and Cm!/lorhafxfitis may be informative for other members of their clade-but not necessarily for ml'mbers of Deuterostomia. Instead. Figure 33.2 suggests that a species within Echinodermata or Chordata might be a more appropriate invertebrate modd organism from which to draw inferences about humans and othervertebratcs.

Self-Quiz 1. c 2. a 3. d 4. e 5. b 6. 7. A

Concept Checlc 33.1

-f':..c

1. Thl' flagella of choonocytcs draw water through their collars, which trap food particles. The partic~ arl' engulfl'd by phagocytosis and digested.dtller by choonocytcs 01' by amoebocytes. 2. The collar cells of spongcs (and other animals-Sl'<' Chapter 32) bear a striking resemblance to a choonoflageUate cell This suggests that the last common ancestor of animals and their protist sister group may have resembled a choanollagellate. Ne\'Crthelcs.s, mesomycetozoans could still be the sistcr group of animals. If this is the case. the lack of collar cells in mcsom)'l::etozoans would indicatl' that 0\T'r time their structure C'o'olved in ways that auscd it to 1"10 longer resemble a choanoflagcllate cell.

Concept Ched: 33.2 1. Both the polyp and the medll$l an' compo5l'd ofan outer epidermis and an inner pstrodennis separated by a gelatinous layer. the mesogIea. The polyp is a cyUndrical form that adheres to the substr.r.te by its aboral end; the medllSal is a flattened. mouth-dO"ll form that mc....es frcdy in the w:roter: 2_ Cnidarian stinging cells (cnidocytes) function in defense and pay captun'. They contl.in capsule-like organeIJcs (cniclae)....i\kh in rum ronbin coiled threads. The threads either inject poison or stick to and entangle small prey. 3_ This

_.

I'

~""'"

.."

, ,'......... •~ •~ , EaeprllC-tl.

Mol...

c

• -

, ,

,

(a) Both ph)1a in Dcutero5tOmia afl'coeIomaus, su~ that their most mrnt common aI'IC£StOf had a true codom.l..opbottodJozo mnuins one ph)1um of

Ans....ers

A-28


acoelomates (Platyhelminthes), one phylum ofpseudocoelomates (Rotifera), and four phyla of coelomates (Ectoprocta, Brachiopoda, Mollusca, Annelida); thus, we cannot from this information alone infer the condition of the most recent common ancestor shal\-d by thl'S<' phyla. Similarly, since Ecdysowa contains one phylum of pseudocoelomates (Nematoda) and one phylum of coelomates (Arthropoda). we cannot infer whether their most recent common ancestor had a true coelom or not. (b) Depending on whether or not the last common ancestor of Bilateria had a true coelom, the presence of a true coelom has either been lost or gained multiple times during theevolutionary history ofbilaterians. Thus, the presence of a true coelom appears to change over the course of evolution.

CHAPTER 34 Figure Queslions Figure 34.20 Amphibians must have originated some time between thedate that the most recent common ancestor of Hynerpcum and later tetropods originated (380 mya) and the date of the earliest known fossils of amphibians (shown in the figure as 340 mya). Figure 34.37 The phylogeny shows humans as the sister group to the genus Pan. This relationship is consistent with humans being placcd in Pan along with its two living members. chimpanzees and bonobos. Figure 34A3 It is not likely that these two sources of error significantly influenced the results. We can conclude this in part because the results were reproducible: similar SC(juences were found for mtDNA obtained from two different Neanderthal fossils and SC(juenced by t'.'o'o different research teams. In addition, the close relationship of the two Neanderthal mtDNA sequences to each other would not be expected if the fossil DNA had broken down considerably. Similarly, the fact that Europeans and other liVing humans formed a sister group to the Neanderthals, and that chimpanzees formed a sister group to the human/Neanderthal clade also would not be expected had the DNA broken down greatly-nor would these results be cxpected if the fossil DNA sequences were contaminated (for example, by DNA from microorganisms or from liVing humans).

Concept Check 34.1 1. As water passes through the slits. food particles arc filtered from the water and transported to the digestive system. 2. In humans. these characters are present only in the embryo. The notochord becomes disks between the vertebrae, the tail is almost completely lost, and the pharyngeal clefts develop into various adult structures. 3. Not necessarily. It would be possible that the chordate common ancestor had this gene, which was then lost in the lancelet lineage and retained in other chordaks. However. it would also be possible that the chordate common ancestor lacked this gene-this could occur if the gene originated after lancelets diverged from other chordates yet before tunicates diverged from other chordates.

and paired fins and a tail (adaptations for swimming). Aquatic gnathostomes also typically have streamlined bodies for efficient swimming and swim blad· ders orolhcrmechanisms (such asoilstorage in sharks) for buoyancy. 3. Yes, that could have happened. The paired appmdages of aquatic gnathostomcs other than the lobe-fins could have served as a starting point for theevolution of limbs. The colonization of land by aquatic gnathostomes other than the lobe-fins might have been facilitated in lineages that possessed lungs, as that would have enabled those organisms to breathe air. Concept Check 34.5 1. Tetrapods arc thought to have originated about 360 million years ago when the fins of some lobe-fins evolved into the limbs of tetrapods. tn addi· tion to their four limbs-a key derived trait for which the group is namedother derived traits of tetrapods include a neck (consisting of vertebrae that separate the head from the rest of the body), a pelvic girdle that is fused to the backbone, and a lack of gill slits. 2. Some fully aquatic species are paedomorphic, retaining larval features for life in water as adults. Species that live in dry environments may avoid dehydration by burrowing or living under moist leaves, and they protect their eggs with foam nl'Sts. viviparity, and other adaptations. 3. Many amphibians spend part of their life cycle in aquatic environments and part on land. Thus. they may be exposed to a wide range of environmental problems. including water and air pollution and the loss or degradation of aquatic and/or terrestrial habitats. tn addition, am· phibians have highly permeable skin, providing relatively little protection from external conditions, and their eggs do not have a protective sheiL

Concept Cheel<

34,6 1. The amniotic egg provides protection to the embryo and allows the em· bryo to develop on land, eliminating the necessity of a watery environment for reproduction. Another key adaptation is rib cage ventilation, which improves the efficiency of air intake and may have allowed early amniotes to dispense with breathing through their skin. And not breathing through their skin allowed amniotes to develop relatively impermeable skin, thereby conserving water. 2. Birds have weight· saving modifications, including the absence of teeth. a urinary bladder, and a second ovary in females. The wings and feathers are adaptations that facilitate flight, and so are efficient respiratory and circulatory systems that support a high metabolic rate.

,.

if

r--'1llCrttDdiliN\S l>

Concept Check 34.2 1. Hagfishes have a head and skull made of cartilage, plus a small brain, sensory organs, and tooth-like structures. They have a neural crest, gill slits, and more extensive organ systems. In addition, hagfishes have slime glands that ward off predators and may repel competing scavengers. 2. My{{okunmingia. Fossils of this organism provide evidence of ear capsules and e)'e capsules; these structures are part of the skulL Thus. My{{okunmingia is considered a craniate, as are humans. HaikOu£lIa did not have a skull. 3. Such a finding suggests that early organisms with a head were favored by namral selection in several different evolutionary lineages. However, while a logical argument can be made that having a head was advantageous, fossils alone do not constitute proof. Concept Check 34.3 1. Lampreys have a round, rasping mouth, which they usc to attach to fish. Conodonts had two sets of mineralized dental clements, which may have been used to impale prq'and cut it into smaller pieces. 2. In annorl-djawk'SS vertebrates, bone served as external armor that may have provided protection from predators. Some species also had mineralized mouthparts, which coold be used for either predation or scavenging. Still others had mineralized fin rars. which may have enabled them to swim more rapidly and with greater steering control. Concept Che£k 34.4 1. Both arc gnathostomesand have jaws, four clusters of Hoxgenes, enlarged forebrains, and lateral line systems. Shark skeletons consist mainly of cartilage, whereas tuna have bony skeletons. Sharks also have a spiral valve. Tuna have an operculum and a swim bladder, as well as nexible rays supporting their fins. 2. Aquatic gnathostomes have jaws (an adaptation for feeding)

A-29

Appendix A

"• ~ ~

-

fur""" '--

Ornrth~"

,

"l

d~t</!.

2 '~~"

cr

;;

~.

J c...

-

-

Sau.r~l4n

~""''''' "II,,,,,,,, virk. ~lfd~

C/ltnmon

,..-,iw'ed bJ

"P""

-

r;

"~ ~ l\

~

Under this convention, the reptiles wouldconsistofall groups in Figure 34,24 except para reptiles and mammals.


Conn'pl Check 34.7 1. Monotremes lay eggs. Marsupials give birth to very small live young that attach to a nipple in the mother's pouch, where they complete development. Euth,'rians give birth to more developed live young. 2. Hands and feet adapted for grasping, flat nails, large brain, forward-looking eyes on a flat face, parental care, moveable big toe and thumb. 3. Mammals are endothermic, enabling them to live in a wide range of habitats. Milk provides young with a balanced set of nutrients, and hair and a layer of fat under the skin help mammals retain heat. Mammals have differentiated teeth. enabling them to eat many different kinds offood. Mammals also have relatively large brains, and many species arc capable learners. Following the mass extinction at the end of the Cretaceous period, the absence oflarge terrestrial dinosaurs may have opened many new ecological niches to mammals, promoting their adaptive radiation. Continental drift also isolated many groups o( mammals from one another, promoting the formation of many new species.

Conn'pl Check 34.8 1. Hominins are a clade within the ape clade that includes humans and all s(K-ck-s more closely related to humans than other apes. The derived characters of hominins include bipedal locomotion and relatively larger brains. 2. In hominins, bipedal locomotion evolved long before large brain size. Homo crgaster, for example, was fully upright, bipedal. and as tall as modern humans, but its brain was signifICantly smaller than thai of modern humans. 3. Yes. both can be correct. Homo sapiens may have established populations outside of Africa as early as 115,OCXIyears ago, as indicated by the fossil r('COrd. However, those populations may have left f,....., or no descendants today. Instead, all living humans may have descended from Africans that spread from Africa roughly 5O,0CXI years ago, as indicated by genetic data. Self-Quiz 1. e 2. c 3. a 4. d 5. b 6. c 7. c 9. (a) Because brain size tends to increase consistently in such lineages, we can conclude that natural selection favored the evolution of larger brains and hmce that the benefits outv..cighed the costs. (b) As long as the benefits of brains that are large relative to body size are greater than the eosts, large brains ean evolve. Natural selection might favor the evolution of brains that are large relative to body size because such brains confer an advantage in ob路 taining mates and/or an advantage in survival.

0.'

--- ---.l.

.,

Dtliiorllm

0

---