Life history variation among spring-dwelling populations of Gammarus minus Say

Size-selective predators which feed preferentially on larger prey may change the size distribution of prey populations. The hypothesis that the mean and minimum body sizes of prey populations under size-selected predation are smaller than those of populations without predation was tested by using spring-dwelling populations of the amphipod Gammarus minus. Four populations were chosen from each of two areas in West Virginia. In each area, there were two populations exposed to predation by the sculpin Cottus bairdi and two predation-free populations as controls. In one area, both mean and minimum body sizes of ovigerous females, amplexing females and amplexing males were significantly smaller in populations exposed to sculpin predation than in populations free of sculpins as expected. In the other area, this pattern was not observed. The results suggest that the predation effect may be buffered by lower intensity of predation in Jefferson County.