Interdependence Among Organisms in an Ecosystem
Senior 3 · Biology
Interdependence Among Organisms in an Ecosystem
Lesson 1 of 5
25 min
In 2015, lions were brought back to Akagera National Park, where the last wild lions had disappeared years before. African Parks, which manages Akagera with the Rwanda Development Board, moved seven lions from South Africa to start a new population. Park managers did not only ask "Will the lions survive?" They also asked what the lions would do to the impalas and zebras, and what fewer grazers would then mean for the grass. No organism lives alone: a change to one population travels along the feeding links to many others.
In Senior 2 you drew food chains and food webs. In this unit you use them to explain interdependence: how every organism in an ecosystem depends on others, and what happens when one population changes.
Definition
Interdependence: the way the organisms in an ecosystem depend on one another for food, shelter, pollination, seed dispersal and other needs, so that a change in one population affects others. Example: impalas depend on grass for food; lions depend on impalas.
Food web: a diagram of all the linked food chains in an ecosystem. Each arrow points from the food to the organism that eats it, in the direction energy flows.
Omnivore: an animal that eats both plants and animals, so it feeds at more than one trophic level. Example: a small cichlid fish in Lake Kivu that eats algae (as a primary consumer) and insect larvae (as a secondary consumer).
Apex (top) predator: a predator at the top of a food web that no other animal normally hunts. Example: the lion in Akagera.
Akagera, in eastern Rwanda, is a savanna of grassland, scattered acacia trees, lakes and swamps. Figure 1 shows some of its feeding links. It is simplified: a real web has hundreds of species, including the decomposers (bacteria and fungi) that feed on every dead organism and every dropping.
Figure 1: A simplified Akagera food web. Green: producers; yellow: herbivores (primary consumers); red: carnivores (secondary consumers). Each arrow points from the food to the eater.
Worked example 1: predicting what happens when lions return
Use Figure 1. Lions are added to Akagera. Predict the effect on (a) impalas, (b) grasses and acacia trees, (c) leopards and spotted hyenas.
Answer: (a) impalas decrease; (b) grasses increase, and acacias may increase slightly; (c) leopards and hyenas may decrease, because they compete with lions for the same prey.
Nyungwe is a mountain rainforest in south-western Rwanda. The park reports 13 primate species, among them chimpanzees, L'Hoest's monkeys and a very large troop of black-and-white colobus monkeys. In a forest most food is up in the trees, so the web is built on leaves and fruit.
Figure 2: A simplified Nyungwe food web. The crowned eagle, a forest eagle that hunts monkeys in the canopy, is the top predator in this web.
Notice two things. First, the colobus monkey eats mostly leaves, while the L'Hoest's monkey and the chimpanzee eat both fruit and leaves: they share foods, so they are linked even though they never eat each other. Second, if the fruiting trees failed in a dry year, the effect would reach the chimpanzees, the L'Hoest's monkeys and the fruit-eating birds, and through the monkeys the eagle as well.
In a lake, the producers are phytoplankton: microscopic algae floating in the sunlit surface water. They are eaten by zooplankton, tiny animals such as copepods, which are in turn eaten by fish.
Figure 3: A simplified food web of Lake Kivu. Purple: tertiary consumers. The small cichlid fish is an omnivore: it eats phytoplankton and animals.
Isambaza (the Tanganyika sardine, Limnothrissa miodon) is not native to Lake Kivu. It was brought from Lake Tanganyika in 1959 to live in the open water far from the shore, which had no fish adapted to feed on the zooplankton there; the lake's small native fish, mostly cichlids, live near the shores. Isambaza now support a large fishery. This introduction is a real test of interdependence.
Worked example 2: an introduced fish in Lake Kivu
Use Figure 3. Isambaza were added to a lake that had no fish feeding in its open water. (a) Predict the effect on the zooplankton. (b) Predict the effect on the phytoplankton. (c) Explain why the effect could reach the small cichlid fish.
What was observed. Studies after the introduction found that the zooplankton had a lower total mass and smaller copepods than before, as predicted in (a). Yet the isambaza fishery did not collapse: surveys in 2008 found a stable stock. The web adjusted to a new balance instead of breaking down.
Warning
Common mistakes. (1) Drawing the arrow from the eater to the food: the arrow shows where the energy goes, so it points to the eater. (2) Saying that removing a predator makes its prey increase "forever": with more prey, food runs short and disease spreads, so the prey population rises and then falls or levels off. (3) Giving an omnivore only one trophic level: name every level it feeds at.
Moving seven lions to Akagera National Park to establish a founder population (African Parks). Watch how the lions were moved from South Africa and kept in a holding area before release, and think about which populations in Figure 1 their return would affect.
Tip
Exam technique: "follow the arrows". To predict the effect of a change, go one arrow at a time: (1) what the changed organism eats, (2) what eats it, (3) what else eats the same food (its competitors). For each, say increase or decrease and give the reason.
Practice: food webs and interdependence
Key points
Sources and credits
Written by Ganzaa: Figures 1 to 3 (simplified webs), the worked examples and the practice questions.
