Food Web Earth Systems

A Food WebFood chains are rarely simple linear progressions from one trophic level to the next. Instead, a single species, such as a hawk, might be a second-order consumer of field mice, a third-order consumer of frogs, and a fourth-order consumer of owls. This is a survival strategy for the hawk, so it doesnŐt become too dependent on one species. It also creates complex interrelationships between species, and a food web is the result.

In most food webs, the relationships are so intricate that ecologists are still discovering the ways that these dynamic networks work. A change in one species will affect the species that prey on it, but it can also trigger unexpected consequences in species with no direct link in the web.

In a pond ecology, for example:

  1. Aquatic invertebrates and minnow eat algae
  2. Minnows and perch eat aquatic invertebrates
  3. Perch and bass eat minnows
  4. Great Blue Herons and bass eat perch
  5. Great Blue herons eat perch.

In this food chain, there is a direct positive effect of the predator on their prey. If we increase the population size of the predator, we expect to see a decline in the population size of they prey (because more predators are eating the prey). Similarly, if we see an decrease in the population size of the predator, we would expect to see an increase in the population size of the prey (because there are fewer predators eating the prey).

Perch, minnows, and algaeHowever, one species can also affect the population size of another species indirectly through its effect on a third species. For example, perch eat minnows, and minnows eat algae. If the population size of perch increases, what happens to algae? More perch means fewer minnows, and fewer minnows means more algae. Thus, perch have an indirect positive effect on algae. So each species is simultaneously being affected by a number of direct and indirect interactions.

What other indirect relationships are there in a pond food web? Let's find out!Next


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