Shorebird predation may cause discrete generations in an amphipod prey

We examine the possible impact of intense, periodic predation by the semipalmated sandpiper Calidris pusilla, on the life history patterns of its amphipod prey, Corophium volutator. We compare populations from two mudflats, one of which is visited by shorebirds on their annual migration south, and one which physically appears very similar but is not visited by the birds. The Corophium population exposed to intense predation had two distinct peaks in density within the season, corresponding to two generations, and the two cohorts had constrained size distributions, and relatively synchronized timing of reproduction. On the mudflat not visited by sandpipers, densities increased in spring and then remained constant through summer. Reproduction was continuous. The mid-summer decline in amphipod density on the mudflat used by sandpipers could not be attributed directly to sandpiper predation as had been previously argued. We interpret the decline as the result of a synchronized natural die-off after reproduction. Selective predation on large amphipods may contribute to the second peak in density by increasing juvenile survivorship due to the removal of competing adults. Size-selective predation by sandpipers causes the overwintering cohort to have a restricted size range in the autumn, and this synchrony persists through to the reproductive period of the following spring. We do not exclude the possibility that these differences in life history have been influenced in an evolutionary sense by the long history of intense periodic predation.