Diet composition in postmetamorphic bullfrogs and green frogs: Implications for interspecific predation and competition

We examined diet composition of postmetamorphic bullfrogs (Rana catesbeiana) and green frogs (R. clamitans) co-occurring at two study sites in southwest Michigan to gain insight into the nature of potential interactions between the species. Observations during sample collection indicated that bullfrogs tended to be found in the water and green frogs tended to be on land within a few meters of the water's edge. This habitat difference was reflected in diet composition. The percentage of the diet composed of aquatic prey items was significantly higher for bullfrogs on three of four collection dates. Comparisons of adult and juvenile classes of the two species indicated interspecific diet similarity was negatively related to the body size difference between classes. Juvenile frogs were common in the diet of adult bullfrogs, but were almost never consumed by green frogs. The small size of frogs consumed by adult bullfrogs indicated that juvenile green frogs constituted the great majority of frogs eaten. Our results suggest that, because of differences in habitat and body size, the opportunity for substantial competition between these species is probably small, and is restricted to individuals of similar body size. The potential for predatory interactions, however, may be substantial, and is highly asymmetrical, with the interaction largely restricted to adult bullfrogs preying on juvenile green frogs.