Impact on stream benthic prey by benthic vs drift feeding predators: a meta-analysis

The impact that predators have on invertebrate prey density in running waters is the subject of debate. Some studies show strong effects of fish predation on benthic prey density whereas others show little or no effects. Recent reviews discuss different factors that affect interactions between predators and invertebrate prey. These include differences in enclosure size, mesh size, predator densities, small sample sizes and the role of invertebrate predators. We suggest the complementary hypothesis that vertebrate predators used in experiments differ in foraging methods. Some predators feed primarily on benthic prey (e.g. sculpins, blacknose dace, creek chub, darters) whereas other predators feed on both drift and benthic prey (predominantly salmonids). Potentially, if the amount of drifting terrestrial animals is large, the impact of drift-feeding fish on the benthic community should be low. We conducted a meta-analysis on the results of field studies that manipulated these two types of predators and found that benthic feeding predators have significantly larger impacts on benthic prey than drift feeding predators. Four methodological variables, mesh size, size of enclosures, duration of experiments, and predator density were analysed to determine whether these could explain the differences between studies involving benthic and drift feeding predators. No correlations existed between any of these four methodological variables and predator impact, indicating that the difference was not due to methodological differences between experiments.