life11e_ch57

The number of trophic levels can control the flow of energy through food webs

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You know from the previous section that changes in the abundance of organisms at one trophic level can influence the energy flow at multiple trophic levels. You might imagine that the number of trophic levels in a food web could be important in the magnitude of that influence. Indeed, all things being equal, a three-level food web, in which the secondary consumer affects the primary consumer’s abundance, should have higher NPP than a two- or four-level food web in which primary consumers have less secondary consumer control (Figure 57.10B). Omnivory, or feeding on more than one trophic level, can also change the way energy is transferred in food webs by essentially “collapsing” trophic levels on one another. For example, a three-level food web with a secondary consumer feeding on both the primary consumer and primary producer should have lower NPP than a three-level food web without omnivory (Figure 57.10C). That’s because even though the secondary consumer indirectly benefits the primary producer by feeding at the primary consumer level, it partially negates this effect by directly feeding on the primary producer as well.

While the number of trophic levels in foods webs can vary, most food webs have three or four trophic levels, with the rare exception of five or more in some systems. What factors might be important in limiting the number of trophic levels in food webs? Several factors have been hypothesized to be important:

The amount of NPP entering the system: This hypothesis suggests that bottom-up factors can influence trophic structure. Given that a large amount of energy is lost from one trophic level to the next (see Figure 57.9), the number of trophic levels will be limited by the amount of energy that can sustain populations at higher trophic levels.
The amount of disturbance: This hypothesis suggests that longer food webs are less likely to recover from disturbances than shorter food webs, which can presumably reassemble more quickly given the fewer trophic levels, and species, involved.
The evolutionary constraints on top predators: This hypothesis focuses on the idea that no organisms on Earth today, with the exception of humans, have evolved to capture apex predators such as birds of prey, sharks, killer whales, and polar bears. With evolutionary constraints maintaining top predators at the pinnacle of food webs, the number of higher trophic levels is limited.