Piscivore addition causes a trophic cascade within and across ecosystem boundaries

Th e addition of predators can play a key role in structuring ecological communities through both consumptive and nonconsumptive eff ects. Stocking of piscivorous fi sh in lakes and similar experimental introductions have provided fundamental evidence in support of trophic cascade theory. Yet, the impact of piscivore addition on cross ecosystem subsidies and meso-predator resource use has not been well studied. Here, we use a replicated pond experiment to document the trophic impacts of a piscivore, cutthroat trout Onchorhynchus clarkii , on aquatic communities already containing a meso-predatory fi sh (threespine stickleback Gasterosteus aculeatus ) and neighbouring terrestrial ecosystems. We fi nd that piscivore addition led to a trophic cascade that extended across an ecosystem boundary: trout addition increased the biomass and average size of insects emerging into the terrestrial system. Piscivores caused a diet shift in stickleback, a non-consumptive eff ect that was likely mainly responsible for the increase in emerging insect biomass. We additionally show that heterogeneity in the strength of the pelagic trophic cascade was more closely correlated with the magnitude of diet shift (refl ecting a non-consumptive eff ect) than decreases in stickleback abundance (a consumptive eff ect). Taken together, our experiment demonstrates that the addition of a piscivore causes a trophic cascade that can extend beyond the aquatic system and suggests that non-consumptive eff ects may more strongly infl uence the strength of a trophic cascade than has been previously recognized.