Predator-Prey Relationships in Natural Community Food Webs

Fifty non-aggregated community food webs, containing from 10 to 74 species, were analyzed in order to determine the effects of web size and species' trophic position on predator-prey interactions. As the number of species (S) in the webs increased, so did generalization, the average number of prey exploited by a predator. Vulnerability, the average number of predators exploiting a prey, also increased with S. Those trends reflected a dominance of particle feeders in the communities. They exploit a potentially limitless number of prey species, because feeding is size-dependent rather than species-dependent. Linkage density, the ratio of links per species, increased rapidly with S at the fish-zooplankton and zooplankton-algal interfaces, where particle feeding was the rule. Linkage density increased only slightly with S at the piscivore-planktivore interface, where predators pursue and capture prey one at a time. Contrary to previously published results for aggregated food webs, there were no clear relationships of generalization and vulnerability to the trophic position of predators and prey. This finding reflects the fact that the natural webs studied herein violate the assumption that omnivory on distant trophic levels is common. Actual feeding was more restricted, and occurred primarily at adjacent trophic levels.