Predator Size Structure Fails to Alter Nonconsumptive Effects in Streams.

Predator population size structure varies over space and time, mediating the top-down, consumptive effects of predators on ecosystems. Yet the role of predator size variation in governing nonconsumptive predator effects has received little targeted research attention. We manipulated stonefly (Acroneuria abnormis) predator size structure and feeding ability and measured effects on the benthic invertebrate prey community in a headwater stream. Field enclosures retained stonefly predators but allowed smaller prey to emigrate as a behavioral avoidance response. Stoneflies caused a ~30% reduction in total prey abundance regardless of whether or not they could feed, indicating a major role for nonconsumptive effects in determining the overall predator effect. This pattern was consistent across two different stonefly predator size structures with equivalent total biomass, as well as most prey responses measured at both the community level and the individual taxon level. Our study demonstrates that stonefly predators cause a community-scale nonconsumptive effect and suggests that predator biomass, rather than predator size structure, might determine the strength of this effect.