Figure 14.1 Several types of trophic pyramids. (a) Pyramid of numbers of individuals per 0.1 ha, not including microorganisms and soil animals. (b) Pyramid of biomass (grams dry weight per square meter). (c) Standing crop (kcal/m2) vs. energy flow (kcal/m2- yr) pyramids for Silver Springs, Florida. (d) Seasonal changes in the biomass pyramid in the water column of an Italian lake (planktonic organisms only) (mg/m3). (Based on Odum, 1987.)

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Of course, some organisms may feed at more than one trophic level. One example is the owl mentioned just above, which eats an herbivore and a carnivore. Humans are an example of an omnivore, which eats both plants and animals. However, this is unusual in natural systems. An exception is certain fishes that eat their way up the food chain as they grow. Other examples are the fox, skunk, and black bear.

The complexity of real systems can be described more precisely as a food web, which shows feeding relationships on a species-by-species basis. The food web shows the ecological dependencies of individual species more graphically than the energy pyramid. A web can be used to help predict the effect of a disturbance. For example, elimination of any one species from an ecosystem may increase the abundances of species that it feeds on and decrease the abundance of those that feed on it. Figure 14.2 shows two

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