Facilitation, interference, and scale: the spatial distribution of prey patches affects predation rates in an estuarine benthic community

The interaction of prey distribution patterns and predator behavior can mediate predator–prey dynamics. Inter-patch distance (lag) may be especially important in the interacting effects of aggregation and interference among predators on their search and prey-handling ability. Interactions of blue crabs Callinectes sapidus preying upon thin-shelled clams Macoma balthica in Chesapeake Bay provide a test of how the opposing forces of aggregation and interference interact with the spatial distribution of prey patches to influence rates of prey consumption. Blue crabs can detect clam patches from up to 15 m away using chemosensory cues, and they aggregate on them, thus facilitating predation, but exhibit agonistic behaviors when closer than 5 m to another crab, thus reducing feeding efficiency. We used these patterns of aggregation and interference to modify a generalized functional response model to describe individual crab foraging efficiency as a function of distance between patches. The model predicted highest predation rates at an intermediate lag of 6.6 m. We tested this a priori hypothesis with a set of field experiments wherein prey patches were established with lags of 1, 7, 10, and 50 m. Predation rates were highest at intermediate lags, as predicted. This work highlights the importance of the interaction between spatial scales and ecological processes, demonstrating that spatial heterogeneity is not noise that obscures processes, but an active component of the predator–prey dynamic.