Resource subsidies, top predators, and community regulation in stream ecosystems

Resource subsidies in the form of energy, materials, and organisms support the productivity of recipient ecosystems. When subsidies increase the abundance of top predators, theory predicts that top-down interactions will be strengthened. However, empirical evidence demonstrating stronger trophic cascades in the presence of resource subsidies is limited, and the degree to which subsidies intensify predation should be constrained by the strength of interactions between predators and their prey. Using both experimental and modeling approaches we tested the degree to which short term patterns of predation are mediated by the availability of terrestrial subsidies, and whether resource subsidies supported two stream predators, steelhead (Oncorhynchus mykiss) and Pacific giant salamander (Dicamptodon tenebrosus), potentially resulting in stronger top-down control. Results were consistent between both experimental and model food webs where despite high predator biomass, there was little support for a trophic cascade or increased predation of most herbivores in the absence of terrestrial subsidies. Most herbivores were relatively invulnerable to predation, emphasizing that behavioral and morphological adaptations can temper predator prey interactions in highly subsidized ecosystems.