DIET COMPOSITION IN POSTMETAMORPHIC BULLFROGS AND GREEN FROGS - IMPLICATIONS FOR INTERSPECIFIC PREDATION AND COMPETITION

We examined diet composition of postmetamorphic bullfrogs (Rana catesb eiana) and green frogs (R. clamitans) co-occurring at two study sites in southwest Michigan to gain insight into the nature of potential int eractions between the species. Observations during sample collection i ndicated that bullfrogs tended to be found in the water and green frog s tended to be on land within a few meters of the water's edge. This h abitat 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 simil arity was negatively related to the body size difference between class es. Juvenile frogs were common in the diet of adult bullfrogs, but wer e almost never consumed by green frogs. The small size of frogs consum ed by adult bullfrogs indicated that juvenile green frogs constituted the great majority of frogs eaten. Our results suggest that, because o f differences in habitat and body size, the opportunity for substantia l competition between these species is probably small, and is restrict ed to individuals of similar body size. The potential for predatory in teractions, however, may be substantial, and is highly asymmetrical, w ith the interaction largely restricted to adult bullfrogs preying on j uvenile green frogs.