Community Ecology- Food Webs

The take home point of my classes on Community Ecology is the "the world is complicated!" It is relatively simple to think about species interactions when we are studying interactions between only two species at at a time. However, species exist in very complex food webs. All species are potentially a source of food for other species and all species (except for plants) must use another species as food. (of course, plants are competing with each other for light, nutrients, and water!).

Direct Effects

Species can directly influence the growth rate or population size of another species directly by (1) acting as a source of food, (2) by using the other species as a source of food, or (3) by interference competition.

Let's try an example. Imagine that we are interested in studying the ecology of the Serengetti National Park in Tanzania. To keep things simple lets say that (1) lions eat wildebeasts, and (2) leopards eat wildebeasts. If we increase the population size of lions, then we would expect the population size of their prey (say wildebeasts) to decrease because there are more lions preying on wildebeasts. Alternatively, if we increase the population size of wildebeasts then we would expect the population size of lions to increase because there is less food for lions. These would both be examples of "direct effects"

Indirect Effects

What happens to the population size of leopards if we increase the population size of lions? If we assume that there is not interference competition going on between lions and leopards then we see that there are no direct effects of lions on leopards. However, it is easy to see that changing the population size of lions will affect the population size of leopards. If we increase the population size of lions that would decrease the population size of wildebeasts and decreasing the population size of wildebeasts would decrease the population size of leopards. Thus, lions have an indirect effect on leopards through their effects on the population size of wildebeasts. Lions and leopards are expoloitative competitors which tells us that exploitative competition is an indirect interaction.

The World is Complicated

Thus, all species are being influenced by a variety of direct and indirect effects. Thus, altering the population size of a single species may have effects on many other species and the population size of a single species can be influenced by many other species.

Actually determing how changing the population size of a specific species will affect the rest of the community is difficult to determine without careful long-term manipulative experiments in the field. These studies are costly and have therefore only been conducted in a few ecosystems.

Keystone Species

One of the interesting outcomes of studies of complex interactions is the discovery of "keystone species". Emmet Duffy's article on the EoE discusses some of the classic examples of keystone species

Expected Learning Outcomes (slightly modified from the Ecology Reader)

At the end of this course a fully engaged student should be able to

- determine the position of a species in the food chain (TEKS 112.43 10D, 12B, 12E and TEKS 112.44 6B)

- distinguish between direct and indirect effects and provide examples of indirect ecological effects occurring in specific communities (TEKS 112.43 12B)

- identify examples of keystone species (TEKS 112.43 12B and TEKS 112.44 4D, 4E)