6.4 Population size depends on abiotic and biotic factors

Learning intentions

By the end of this topic, you will be able to: • distinguish between abiotic and biotic factors • explain how different factors can cause a population to increase or decrease.

abiotic factors non-living factors that in uence an ecosystem such as wind, water, salinity and temperature

biotic factors living factors that in uence an ecosystem such as animals, plants and bacteria

immigrate when an animal enters a new ecosystem

emigrate when an animal leaves an ecosystem

Key ideas

• All populations are in a constantly changing equilibrium. • Deaths and emigration can decrease a population. • Births and immigration can increase a population.

Populations A dynamic balance

Like a population of humans in a city, the population of living organisms in an ecosystem is constantly changing. In a drought there are high temperatures and little rain. This can make it hard for plant and animal populations to survive. The amount of water or temperature are non-living factors (called abiotic factors ) that can affect the size of a population. Other abiotic factors that can affect the number of organisms in a population include the amount of sunlight, salt in the water, soil, available nutrients or places for the organism to hide. Sometimes it is the number of other organisms competing for food or hunting them, that can affect the number of organisms in a population. These ‘living’ factors that affect the survival of organisms are called biotic factors .

All organisms live in a complex web of interdependent relationships – with each other and with their environment. In an ecosystem, there needs to be a balance so that all species can survive. The more types of organisms in an ecosystem, the healthier the community of organisms in the ecosystem. Many things can cause a population of organisms to increase in number. A population of birds will increase when new chicks are born, or when new birds y into the ecosystem (immigrate). In contrast, the number of birds in the population will decrease if some birds die because of hunger or because they are hunted. Sometimes the birds will leave to y to another ecosystem (emigrate) (Figure 2). Biotic factors (living) Abiotic factors (non-living)DRAFT

Bacteria Fungi Air Salinity Soil

Plants Archaea Temperature Light Water

Figure 1 A comparison of abiotic and biotic factors Animals Protists Minerals pH Humidity

Births

Population size Immigration

Ecosystem balance constantly changes before returning to a new stable balance. This is called a dynamic equilibrium. Changes may upset the equilibrium, but another new equilibrium arises. Often, the change is only a small one. Changes in ecosystems occur naturally but they may be intensi ed by external abiotic factors such as oods and bush res. Reproduction, death, migration, natural events (such as seasonal changes), disasters ( oods, droughts, earthquakes) and human intervention occur regularly. Biotic factors such as the loss of a species or the introduction of a new species can also change a population.

Emigration when food source declines

Deaths Figure 2 The size of a galah population in a particular area depends on the food available and the number of births and deaths. Whenever the size of a population changes, it can affect other organisms in the ecosystem’s food web. Consider the food web for an ecosystem shown in Figure 3. If the number of frogs decreased in this ecosystem, the effects could include: > an increase in grasshopper numbers, causing the amount of grass to decrease > an initial increase in praying mantis numbers because of more grasshoppers > a decrease in lizard numbers > more birds eating praying mantises rather than frogs and lizards > a decrease in praying mantis numbers because they are being eaten by birds > further increase in grasshopper numbers who then start eating the grass. If this was severe enough, the ecosystem would be at risk, as all food webs depend on a good supply of producers. The most likely outcome is that the bird population would decrease, so all species would return to balance with smaller population sizes. A positive effect is that decreased bird numbers might enable the frog population to recover.

Figure 3 A food web for an ecosystem

Grass Grasshopper Praying mantisFrog DRAFT Lizard Bird Population dynamics Population dynamics is the study of changes in population numbers within ecosystems. If scientists can measure how many of each species are in a certain location, they can make predictions and try to prevent a species becoming extinct. Regular sampling provides information about increases and decreases in population numbers, and causes can be identi ed.

population dynamics the study of changes in species population numbers and the factors that may contribute to these changes

quadrat a randomly selected square plot used to estimate the number of organisms

capture–recapture a method of estimating the number of organisms by capturing, marking and releasing a sample of the organisms

Counting organisms

or two days (or nights) later. Some of these recaptured animals will have tags. An estimate There are many ways to measure the size of of the population is then obtained using these a population. The simplest way would be to three numbers: count all the organisms, but in practice this Total number of animals = number of tagged is very dif cult. It is easier to estimate the animals × number of recaptured animals ÷ total population by counting a sample from a number of tagged animals recapturedhelicopter, or by using quadrats or capture–recapture methods. For human populations, a census is the usual method. Capture–recapture is a good way to estimate the population size of small Australian mammals, such as the marsupial Antechinus

For plants and stationary animals, quadrats (the common bush rat). Because most native (randomly selected square plots) are marked Australian mammals are nocturnal, the in an ecosystem (Figure 4). The organisms in traps may be set at night and checked the each plot are counted and used to calculate the next morning.average number of organisms in each square Modern counting methods When tags or quadrats are used to count organisms, it can disrupt their environment and affect the way the animals behave. Modern counting methods can avoid this disturbance by using remote sensors that detect movement and turn on unmanned cameras (Figure 6). These images allow scientists to count the number of animals moving in an area and to study how animals behave when humans are not around. Other ways to identify animals is to record the calls they make to each other. This recording can then be used to identify the species of animal and the number of them making and replying to the calls. plot. The total number of organisms in the whole ecosystem can then be calculated by multiplying the average number of organisms by the area of the ecosystem. This method works well if a large number of quadrats are used and the organisms are evenly spread throughout the ecosystem. For animals that are moving, capture–recapture is a popular method. Animals are captured in traps and marked with tags, correction uid or permanent marker on their tails. The number counted on the rst capture are called tagged animals (Figure 5). The animals are then released and it is assumed that they move evenly throughout the population. Another capture (recapture) is made one DRAFT

Figure 5 A scientist tagging a bird

Figure 4 Using a quadrat

Worked example 6.4: Calculating population size

Scientists wanted to determine the size of a bilby population in a small reserve. They used the capture–recapture method to estimate the size of the population. They captured and marked 9 bilbies on the rst night and 8 bilbies (4 marked) one week later. Calculate the size of the bilby population.

Solution Number of tagged/marked animals = 9 Number of recaptured animals = 8 Number of tagged animals recaptured = 4 E s t i m a t e d n u m b e r o f b i l b i e s = n u m b e r o f t a g g e d a n i m a l s × n u m b e r o f r e c a p t u r e d a n i m a l s ÷ n u m b e r o f t a g g e d a n i m a l s r e c a p t u r e d

= 9 × 8 ÷ 4

= 18

Retrieve 1 Identify two examples of abiotic factors and two examples of biotic factors. Comprehend 2 Describe two ways a population can: a increase b decrease. 3 Describe suitable methods for estimating the size of populations of: a plants and stationary animals b other animals. 4 Explain how predator–prey relationships achieve a state of balance by describing what happens to the number of predators when: a prey numbers increase b prey numbers decrease c predator numbers increase d predators numbers decrease. Analyse 5 Students on a eld trip with a national park ranger set traps for a small nocturnal marsupial, Antechinus stuartii , in a heathland ecosystem. They captured eight animals on the rst night and marked white dots on their tails. Then they released them. On the second night, they captured 10 animals, of which four were marked.

a Calculate the estimated population size of A. stuartii in this ecosystem. b Describe one way the students could check if their estimated population size was correct. Apply 6 Desalination plants take the salt out from sea water to produce fresh water for us to drink. The remaining sea water with high levels of salt is returned to the ocean through a fast ow pipe. A study of the desalination plant in Sydney found that the population of the mobile sponges decreased near the returning pipe, while populations of 6.4 Check your learning DRAFT barnacles sticking to the rocks increased. Compare the two populations and propose a hypothesis that might explain the difference between the survival of the two populations. 7 Investigate the rules that regulate the type, number and size of sh that can be caught in your local area. Write a letter to a local paper explaining why these rules are needed. Quiz me Complete the Quiz me to check how well you’ve mastered the learning intentions and to be assigned a worksheet at your level. Figure 6 Remote sensors can be used to record and identify animals without disturbing their normal behaviour.