Fish predation alters benthic, but not emerging, insects across whole pools of an intermittent stream

Abstract

Predation effects in streams can cascade to terrestrial food webs through the flux of organisms that develop in the stream and emerge as adults to the terrestrial system. This emergence subsidizes some terrestrial predators, an effect that generally varies based on the magnitude of the subsidy. Factors regulating this magnitude are relatively well known, but factors regulating the trophic structure of the subsidy are not. I tested the hypothesis that predatory fish in natural stream pools alter the biomass and trophic structure (proportion of predatory adults) of emerging aquatic insects. I created a 13× gradient of predatory fish biomass (4 species of Lepomis sunfish and the minnow Notropis boops; within the range of natural variation) across 10 pools in Brier Creek, Oklahoma (USA). Pool area and substrate composition varied naturally, so I also measured their effect on insects. At the end of the experiment after the stream became intermittent, fish reduced benthic insect biomass but not emergence to the terrestrial habitat. The proportion of predatory insects emerging from pools was positively associated with pool area, but was unaffected by fish density. The best predictor of emergence biomass among pools on any date was the standing crop of benthic insects before fish manipulation, a result suggesting a time lag between measured benthic standing crop in the stream and subsequent emergence. Fish manipulations occurred during the end of peak summer insect emergence, which may have limited my ability to detect fish effects on emergence. My study demonstrates that variation in the timing of predation may constrain the spatial scale of fish effects in aquatic and terrestrial food webs and suggests that pool size can influence the trophic structure of emerging aquatic insects.