Trophic interception: how a boundary-foraging organism influences cross-ecosystem fluxes

The rate at which subsidies move between habitats is a function not only of those factors that facilitate such transfers, but also of factors that limit or restrict the movement of subsidies. The interruption or redirection of subsidies by organisms foraging at the boundary between habitats, or trophic interception, has the potential to substantially restructure the food webs of recipient habitats. In this study we describe how a waterstrider, Aquarius remigis, limits the transfer of a subsidy across the land-water boundary. Prey interception varied with the type of stream habitat; on average, waterstriders in isolated pools intercepted 71.8 % of experimental prey inputs of all sizes compared with 21.5 % in connected pools and 0.8 % in riffles. Across all stream habitat types, waterstriders consumed 43.2 % of the experimental inputs of the smallest prey used, the prey size class most similar to natural inputs in our study area. We estimate that foraging waterstriders may intercept 0.31.2 kg of terrestrial prey subsidy for every 100 m of stream channel during three summer months, resources that could otherwise support 1358 young-of-the-year salmonids. In controlled trials, waterstriders significantly altered the amount of terrestrial prey directly consumed by fish, while fish also altered waterstrider interception of those prey. Interestingly, when waterstriders and fish were present together, more terrestrial prey were lost to the streambed than when either or both of these consumers were absent, making this resource available to benthic detritivores, and facilitating the direct incorporation of terrestrial nutrients into stream detrital webs. Overall, w