Predator diet breadth influences the relative importance of bottom-up and top-down control of prey biomass and diversity.

We investigated the effects of predator diet breadth on the relative importance of bottom-up and top-down control of prey assemblages, using microbial food webs containing bacteria, bacterivorous protists and rotifers, and two different top predators. The experiment used a factorial design that independently manipulated productivity and the presence or absence of two top predators with different diet breadths. Predators included a "specialist" predatory ciliate Euplotes aediculatus, which was restricted to feeding on small prey, and a "generalist" predatory ciliate Stentor coeruleus, which could feed on the entire range of prey sizes. Both total prey biomass and prey diversity increased with productivity in the predator-free control and specialist predator treatments, a pattern consistent with bottom-up control, but both remained unchanged by productivity in the generalist predator treatment, a pattern consistent with top-down control. Linear food chain models adequately described responses in the generalist predator treatment, whereas food web models incorporating edible and inedible prey (which can coexist in the absence of predators) adequately described responses in the specialist predator treatment. These results suggest that predator diet breadth can play an important role in modulating the relative strength of bottom-up and top-down forces in ecological communities.