PREY EXCHANGE-RATES AND THE IMPACT OF PREDATORS ON PREY POPULATIONS IN STREAMS

Presents 4 lines of evidence that the magnitude of prey exchange (immigration/emigration) among substrate patches has on overwhelming influence on the perceived effects of predators on prey populations. 1) Extensive review of the literature on predation effects in benthic and littoral freshwater habitats revealed a significant relationship between prey exchange rate and observed predator impact. In streams, studies showing significant predator effects used cages with smaller mesh sizes than studies showing nonsignificant effects; there was a highly significant correlation between cage mesh size and the magnitude of predator impact on common prey. Prey drift and colonizaiton rate were inversely related to predator impact on benthic prey. 2) These patterns were confirmed by field experiments, and observations. In Colorado streams there was greater predator impacts on Baetis prey when immigration/emigration was restricted vs. when the mesh size of the cage was relatigvely large. The effects of trout in California stream pools were greater when prey turnover rates were low. 3) Re-analysis of Peckarsky's (1985) data shows an inverse relationship between predator impact and prey mobility within a field experiment. 4) A model that incorporates both predation and exchange of prey indicates that we ought to expect a lower magnitude of predator effects when exchange rates are high. -from Authors