Responses of a Key Intertidal Predator to Varying Recruitment of Its Prey

How a predator responds to fluctuating prey abundances determines its capacity to regulate the prey's population levels. Unlike many terrestrial and a few marine arthropods, key intertidal invertebrate predators, e.g., sea stars and whelks, have been portrayed as lacking rapid behavioral responses that would permit them to control episodes of massive prey recruitment. Our field experiments demonstrated that intertidal sea stars, Pisaster ochraceus, rapidly change densities and alter diet in response to varying recruitment of their prey, the mussels Mytilus spp. Densities of Pisaster foraging at high tide were positively correlated with an index of recruitment of Mytilus californianus. The sea stars aggregated in episodes of massive mussels recruitment and dispersed as local abundances of juveniles mussels declined. Aggregation and dispersal were reproduced in field experiments by adding or removing masses of 1st—yr Mytilus spp. to naturally occurring mussel beds. Over the 7 yr of the study, the lower margins of the mussel beds, a distributional limit set at least in some circumstances by the sea stars, remained relatively constant, despite significant year—to—year variation in mussel recruitment rates. We speculate that aggregation acts in concert with previously hypothesized long—term responses to stabilize the lower distributional limits of the mussels.