6.2 All organisms have a role in an ecosystem

Learning intentions

By the end of this topic, you will be able to: • define the term

‘ecosystem’ • explain the different types of relationships that can exist within ecosystems.

ecosystem a community of living organisms and their nonliving surroundings population a group of individuals of the same species living in the same location at the same time community different populations living in the same location at the same time predator an animal that hunts and feeds on another (prey) for food prey an animal that is hunted and killed by another (predator) for food

Key ideas

• An ecosystem is a community of living organisms and their surroundings. • Predators eat prey. • Symbiotic relationships are long term relationships.

Ecosystems

When studying the environment, we can look at a small part such as the organisms living in or on a log, or we can study a whole forest. When we study a large area with a lot of living organisms and non-living things (such as hot temperatures, little light or water), we say that we are studying an ecosystem.

Ecosystems vary in size. They can be as small as a puddle or as large as the Earth itself. Ecosystems are made up of populations. A population is a group of living organisms that are the same species, living in the same place at the same time. When different populations interact with each other, they are called a community. For example, a population of humans can live in a town together. When all the plants in their gardens and their pets are included, then it becomes a community.

An ecosystem must supply all the needs of the organisms, such as food, water, warm temperatures, oxygen and minerals. These make up the non-living parts of the ecosystem. If these conditions are wrong for a population, then the individuals in that population will move to a better ecosystem or they will die out. Relationships between different species Predator-prey In a predator–prey relationship, one organism (the predator) eats another (the prey). This relationship only lasts for a short time (until the prey is eaten) and it only happens when a predator is hungry. An example of this is the relationship between a fox and a rabbit (Figure 2). The two species only meet when the fox is hungry and starts hunting the rabbit (the prey). When the rabbit is caught, the relationship ends.

Figure 1 Wetlands such as those in Kakadu National Park in the Northern Territory are an example of an ecosystem.

DRAFT

Figure 2 A fox is the predator of a rabbit (the prey). Predator species and their prey species have a balanced relationship. If all the prey are eaten, then the predators will starve. As the number of predators decreases (shown by the graph going down), the number of prey will increase again because they are not being hunted as often. This makes the prey numbers in a graph go up. When the number of prey goes up, the predators can hunt more food. This means a predator–prey graph will always show the predator line following the prey line. Figure 3 shows this pattern.

Numbers of predators and prey

Prey

Figure 3 A predator–prey graph – the scales aren’t shown but the prey numbers are mostly greater than those of the predators. Notice that the increase and decrease in prey numbers usually comes before the increase and decrease in predator numbers. Time Predator Periwinkle Competition Competition may also exist between members of different species that need the same food (Figure 4) or shelter. If one species grows quicker, then any species trying to compete may ‘lose’ and die. Usually, competing species will balance their numbers so that neither one dominates. Sometimes inhibition competition occurs when one organism produces a chemical that directly inhibits or blocks the survival or growth of another organism (Figures 5 and 6). DRAFT Limpet

Figure 4 A black periwinkle (Nerita) competes for food with the limpet (Cellana) on a rock platform – both species feed on algae growing on the rocks. The periwinkle moves faster but does not eat all the algae in its path. This means there is some food left for the limpet. Both species can survive because the periwinkles usually leave some algae behind for the limpets. However, when the periwinkles are removed, the limpet population increases. Figure 5 Penicillium fungus mould (seen here growing on an orange) produces an antibiotic called penicillin that inhibits or blocks the growth of many bacteria.

Figure 6 The Lantana plant was introduced into Australia and has become a weed. It releases a chemical into the soil that inhibits or blocks the growth of native plant species.

competition contest between organisms that require or seek similar resources such as shelter and food

symbiosis a close physical relationship between two organisms of different species

mutualism a type of relationship between two organisms of different species, in which both organisms bene t

commensalism a type of relationship between two organisms of different species, in which one organism bene ts and the other is not affected

parasitism a relationship in which one organism (parasite) lives in or on the body of another organism (host) and bene ts while the host is harmed

Symbiosis

Symbiosis happens when two organisms of different species live physically close to each other for a long time. Mutualism, commensalism and parasitism are all examples of symbiosis. Mutualism is a relationship between two organisms in which both organisms are able to help each other to survive. They both bene t. Commensalism is a relationship in which one organism bene ts and survives while the other organism does not notice and is not affected. Commensalism does not happen very often in an ecosystem because it is unlikely that an organism does not notice another organism is living so closely to it. Parasitism is a relationship in which one organism (the parasite) lives in or on the body of another (the host). The parasite bene ts by taking some of the food and nutrients from the host. This means the host often struggles to survive with less food. Sometimes that parasite can even kill the host. Figure 7 Mutualism A lichen is an alga and a fungus, although you cannot see the two organisms separately (except under a microscope). The alga produces energy from the sun for both organisms, and the fungus provides support and other nutrients. Figure 8 Mutualism The anemone fish hides within the tentacles of the sea anemone, where it is camouflaged from its predators. The fish cleans all the algae from the sea anemone. Figure 9 Commensalism. Some animals such as cattle and water buffalo make insects fly up as they walk through the grass. Birds such as cattle egrets benefit by feeding on the insects. Figure 10 Commensalism Certain plants have seeds with tiny hooks. These seeds stick to animal fur and are carried away from the parent plant. This means the parent plant will be able to keep all the sunlight and nutrients for themselves. DRAFT Figure 11 Parasitism Ticks attach to the skin of animals and slowly drink their blood. Although the amount of blood is small, the tick can infect the animal with bacteria, causing it to die.

Figure 13 Parasitism Hookworms attach to the inside of a human or animal intestine, feeding on passing nutrients. If the host doesn’t eat enough, the worm has been known to burrow out of the intestines and travel to other organs, where they can cause damage to the host. Retrieve 1 De ne symbiosis. Comprehend 2 Describe an example of the following relationships. a predator–prey b mutualism c commensalism d parasite–host 3 Explain why a large plant that produces a lot of shade prevents smaller plants from growing. Analyse 6.2 Check your learning Figure 12 Parasitism Leeches are parasites that survive by sucking the blood from other animals. DRAFT 4 Contrast (note the similarities and differences between) a predator–prey relationship and parasitism. Apply

5 Some eucalyptus trees have mistletoe plants living on them. Mistletoe has very similar leaves to eucalyptus leaves.

Mistletoe can make its own food, but the stems send suckers into the eucalypt to obtain water and minerals. If too

much water and minerals are removed, the eucalypt can die. Identify the type of relationship that exists between the eucalypt and the mistletoe. Justify your answer (by de ning the relationship and matching it to this example). 6 Epiphytes are plants, such as ferns and some orchids, that grow high in the branches of other trees, especially rainforest trees. The epiphytes obtain suf cient light to make their own food, collect water from the moist air and obtain minerals from the decaying leaf litter that they catch at their leaf bases.

The tree is not affected by these plants. Identify the type of relationship that is described. Justify your answer (by describing how each species bene ts from, doesn’t notice or is harmed by the relationship).

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