Predator diversity effects cascade across an ecosystem boundary

Food webs in different ecosystems are often connected through spatial resource subsidies. As a result, biodiversity effects in one ecosystem may cascade to adjacent ecosystems. I tested the hypothesis that aquatic predator diversity effects cascade to terrestrial food webs by altering a prey subsidy (biomass and trophic structure of emerging aquatic insects) entering terrestrial food webs, in turn altering the distribution of a terrestrial consumer (spider) that feeds on emerging aquatic insects. Fish presence, but not diversity, altered the trophic structure of emerging aquatic insects by strongly reducing the biomass of emerging predators (dragonflies) relative to non‐feeding taxa (chironomid midges). Fish diversity reduced emerging insect biomass through enhanced effects on the most common prey taxa: predatory dragonflies Pantala flavescens and non‐feeding chironomids. Terrestrial spiders (Tetragnathidae) primarily captured emerging chironomids, which were reduced in the high richness (3 spp.) treatment relative to the 1 and 2 species treatments. As a result, terrestrial spider abundance was lower above pools with high fish richness (3 species) than pools with 1 and 2 species. Synergistic predation effects were mostly limited to the high richness treatment, in which fish occupied each level of vertical microhabitat in the water‐column (benthic, middle, surface). This study demonstrates that predator diversity effects are not limited to the habitat of the predator, but can propagate to adjacent ecosystems, and demonstrates the utility of using simple predator functional traits (foraging domain) to more accurately predict the direction of predator diversity effects.