BULLFROG (RANA-CATESBEIANA) INVASION OF A CALIFORNIA RIVER - THE ROLEOF LARVAL COMPETITION

I studied the invasion of Rana catesbeiana (the bullfrog) into a north ern California river system where bullfrogs are not native. Native yel low-legged frogs, Rana boylii, a species of special concern, were almo st an order of magnitude less abundant in reaches where bullfrogs were well established. I assessed the potential role of larval competition in contributing to this displacement in a series of field manipulatio ns of tadpole density and species composition. The impact of R. catesb eiana on native tadpoles in the natural community agreed with the outc ome of more artificial experiments testing pairwise and three-way inte ractions. In 2-m(2) enclosures with ambient densities of tadpoles and natural river biota, bullfrog tadpoles caused a 48% reduction in survi vorship of R. boylii, and a 24% decline in mass at metamorphosis. Bull frog larvae had smaller impacts on Pacific treefrogs, Hyla regilla, ca using 16% reduction in metamorph size, and no significant effect on su rvivorship. Bullfrog tadpoles significantly affected benthic algae, al though effects varied across sites. Responses to bullfrogs in field se ttings were similar qualitatively to results seen in smaller-scale exp eriments designed to study size-structured competition among disparate age/size classes of species pairs and trios. Competition from large o verwintering bullfrog larvae significantly decreased survivorship and growth of native tadpoles. Competition from recently hatched bullfrog larvae also decreased survivorship of R. boylii and H. regilla. Native species competed weakly, both interspecifically and intraspecifically . The only suggestion of a negative impact of a native species on bull frogs was a weak effect of H. regilla on recent hatchlings. Competitio n appeared to be mediated by algal resources, and there was no evidenc e for behavioral or chemical interference. These results indicate that , through larval interactions, bullfrogs can exert differential effect s on native frogs and perturb aquatic community structure.