Scale and the guild functional response: Density-dependent predation varies with plot size

The predator functional response is an important mechanism determining the persistence of prey species; however, little is known about the effects of spatial scale on the functional response. We used a manipulative field experiment to quantify the effects of plot size on the guild functional response on the clam Mercenaria mercenaria, replicating the experiment in the summer in Chesapeake Bay, Virginia, and in the spring and fall in Indian River Lagoon, Florida, to examine the effects of predator and alternative prey abundance. In Virginia, the predation rate increased with both patch size and predator density, and was described by a modified sigmoid Type III functional response model that incorporated the effects of patch size. In Florida in the spring, the predator functional response was a Type III and did not vary with plot size, but in the fall it was a linear Type I at small plot sizes, and a Type III at a larger plot size. We hypothesize that the difference is primarily driven by changes in predator abundance and species between sites. In showing that the functional response can vary with plot size and season, our results indicate that small-scale experiments do not always scale up spatially or temporally. We suggest that the predictive power of such experiments may be limited by the complexity of the food web.

► We determined the effect of plot size on the guild functional response on clams.
► The experiment was performed in Virginia and Florida.
► In Virginia both prey density and plot size increased predation.
► In Florida effects of prey density and plot size varied with season.
► Food web complexity can reduce the predictive power of functional response models.