No abstract

Summary 0[ Spatial patterns of freshwater _sh species at regional and local scales were inves! tigated to explore the possible role of interspeci_c interactions in in~uencing dis! tribution and abundance within communities occupying coastal streams of North! Western France[ 1[ Nine sites from nine streams situated in the same biogeographical region were sampled annually over the 5!year period from 0889 to 0884[ 2[ Similar habitats "sites# with richer regional colonization pools exhibited pro! portionally richer local communities in terms of number of species\ total density and total biomass of individuals[ Furthermore\ no negative relationships were found between density and biomass of each of the most common species and local species richness[ 3[ Results of dynamic regression models "applied to the above!mentioned species# suggest an absence of strong competition between all pairs of species[ 4[ The evidence on lack of density compensation for species!poor communities and absence of perceptible interspeci_c competition between species suggest that the communities studied are non!interactive[ 5[ Two main explanations can be advanced[ First\ the local abundance of species in the communities studied could be determined through di}erential responses to unpredictable environmental changes\ rather than through biological interactions[ Second\ as a result of historical events\ the communities studied are reduced in congeneric species which can limit\ in turn\ the in~uence of interspeci_c competition in structuring these communities[ 6[ These results underline the strong in~uence of regional processes in shaping local riverine _sh communities and minimize the possible in~uence of species interactions in governing these communities[ Key!words] density compensation\ _sh assemblages\ interspeci_c interactions\ local species richness\ regional species richness[ Journal of Animal Ecology "0887# 56\ 361Ð373

The behavioral strategies developed by predators to capture and kill their prey are fascinating, notably for predators that forage for prey at, or beyond, the boundaries of their ecosystem. We report here the occurrence of a beaching behavior used by an alien and large-bodied freshwater predatory fish (Silurus glanis) to capture birds on land (i.e. pigeons, Columbia livia). Among a total of 45 beaching behaviors observed and filmed, 28% were successful in bird capture. Stable isotope analyses (δ13C and δ15N) of predators and their putative prey revealed a highly variable dietary contribution of land birds among individuals. Since this extreme behavior has not been reported in the native range of the species, our results suggest that some individuals in introduced predator populations may adapt their behavior to forage on novel prey in new environments, leading to behavioral and trophic specialization to actively cross the water-land interface.