Increasing isolation reduces predator:prey species richness ratios in aquatic food webs

Abstract

The number of species that live in a habitat typically declines as that habitat becomes more isolated. However, the influence of habitat isolation on patterns of food web structure, in particular the ratio of predator to prey species richness, is less well understood. We placed aquatic mesocosms at varying distances from ponds that acted as sources of potential colonists; then we examined how isolation affected the ratio of predator:prey species richness in the communities that assembled. In the final sampling, a total of 21 species (12 prey and 9 predators) of insects, crustaceans, and amphibians had colonized the mesocosms. We found that total species richness, as well as the richness of predators and prey, declined with increasing isolation. However, predator richness declined more rapidly than prey richness with increasing isolation, which lead to decreasing predator:prey ratios. This result conflicts with prior demonstrations of invariant predator:prey ratios in freshwater communities.