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.