Constraints on predator design and behavior

There may also be evolutionary constraints on the anatomy or behavior of predators that limit the lengths of food chains. To feed on prey at a given trophic level, a predator has to be large enough, maneuverable enough and fierce enough to effect a capture. In general, predators are larger than their prey (not true, though, of grazing insects and parasites), and body size tends to increase (and density to decrease) at successive trophic levels (Cohen et al., 2003). There may well be a limit above which design constraints rule out another link in the food chain. It may be impossible to design a predator that is both fast enough to catch an eagle and big and fierce enough to kill it.

Also, consider the arrival in a community of a new carnivore species. Would it do best to feed on the herbivores or the

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Figure 20.16 Sets of model food webs, the dynamics of which were examined to determine the effect of food chain length on stability having accounted for variations in the number of species and the number with self-limitation (•). (a) The original set examined by Pimm and Lawton (1997). (b) Six-species, four-level webs with varying degrees of self-limitation. (c) Six-species webs of self-limited species with varying numbers of trophic levels and species concentrated in the basal level. (d) Eight-species webs of self-limited species with varying numbers of trophic levels and species dispersed among the levels. (e) Eight-species webs of self-limited species with varying numbers of trophic levels and species concentrated in the basal level. (After Sterner et al., 1997a.)

carnivores already there? The herbivores are more abundant and less well protected. The advantage to feeding low down in the food chain can readily be seen. Of course, if all species did this, competition would intensify, and feeding higher in the food chain could reduce competition. But it is difficult to imagine a top predator sticking religiously to a rule that it should prey only on the trophic level immediately below it, especially as the prey there are likely to be larger, fiercer and rarer than species at lower levels. Overall, theoretical explorations (Hastings & Conrad, 1979) suggest that an evolutionarily stable food chain length (one that would be optimal for predator fitness) would be around two (three trophic levels). Such arguments, however, have rather little to offer by way of explanation for the variations in food chain length.

Thus, there are complete answers to neither of our original questions (see p. 595). The constraints on predators are likely to set some general upper limit on the lengths of many food chains. Food chains are likely to be atypically short in especially unproductive environments. Food chain length seems to increase with increases in productive space, but it is unclear whether this is an association with the total energy available in an ecosystem or with ecosystem size alone - and if the latter, it is unclear precisely how size comes to determine food chain length. The two longest established hypotheses - energy per unit area and dynamic fragility - have, if anything, the least support.

Finally, it is important to note that, as with connectance, estimates of food chain length are sensitive to the degree of taxonomic resolution. This may be why many of the more recently documented webs have longer than average chain lengths ranging from five to seven (Hall & Raffaelli, 1993). Moreover, if a well-resolved large web is progressively simplified by lumping taxa together (in a manner analogous to earlier studies), the estimate of food chain length declines (Martinez, 1993). There is clearly a need for rigorous studies of many more food webs before acceptable generalizations can be reached.

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