A REVIEW OF THE DRIFT AND ACTIVITY RESPONSES OF STREAM PREY TO PREDATOR PRESENCE

In streams the impact that predators have on prey density is unclear. A recent review suggests that predators have on average a small to moderate negative effect on prey density and that predatory invertebrates have a stronger impact than predatory vertebrates. We suggest that a reason for this difference between predatory invertebrates and vertebrates is differences in prey emigration responses to the two types of predators. If prey increase their emigration rates in the presence of predators, predator impacts on local prey density will be strong; conversely, if prey decrease their emigration rates in the presence of predators, prey density might actually build up in patches containing predators and predator impact on prey density will be weak or even positive. We reviewed 22 studies that examined the impact that predatory vertebrates and invertebrates have on prey drift and activity (crawling and rates of emergence from refuge). Our review revealed that predatory invertebrates cause prey to increase drift rates more often than expected by chance. Predatory vertebrates had variable effects on prey drift rates (sometimes increasing and sometimes decreasing prey drift rates), but they caused prey to decrease activity more often than expected by chance. These results provide a potential mechanism for the relatively large impact that predatory invertebrates have on prey density as compared to the impact that predatory vertebrates have. We suggest directions for future research that include examining the impact that both types of predators have on prey emigration behavior and benthic density in the same system.