UC San Diego

Food availability and predation risk are the primary factors that influence an individual's diet composition. Reduced food availability will result in populations with broader diet breadths due to individual diet specialization. This effect can be enhanced when predators limit foraging movement of prey. Predators can also force prey to forage in lower-risk habitats. An opportunity for us to study these influences on fish diets in a natural system exists in coral reefs. The reefs of the Northern Line Islands vary in their predator biomass and the amount of food available to each trophic guild. To determine the effects of changing food availability and predator densities on the diets of fishes we quantified the diets of species from three trophic guilds from the Northern Line Islands. Populations experiencing higher predation pressure and/or lower food availability are expected to have wider diet breadths. Trends in diet breadth varied across species. The results do not offer a clear indication that fish are shifting their foraging habitats to be closer to reef refuges in response to increased predation pressure. So, changes in predator density and the amount of food available seem to influence shifts in diet breadth, but the influences vary across species. There are two factors that may also influence diets: habitat complexity and resource patchiness