THE EFFECT OF SPATIAL SCALE ON THE FUNCTIONAL RESPONSE OF FOX SQUIRRELS

Abstract

The functional response is an essential component of any predator–prey interaction. We present a model to predict changes in a predator’s functional response at varying spatial scales of resource distribution and abundance. The model was tested using free-ranging fox squirrels, Sciurus niger, foraging for sunflower seeds in experimental food patches. We manipulated initial density of food at three spatial scales: foraging (initial abundance varied from patch to patch at a single station); intermediate (initial abundance varied from station to station at a single site); and landscape (initial abundance varied from site to site). We measured the giving-up density of seeds (GUD: food remaining in patches) following a day of foraging, allowing determination of the proportion and amount of food harvested. We predicted that foragers should harvest more resources from rich patches than from poor patches at the foraging scale and that foragers should harvest more resources from a patch in a poor environment than in a rich environment at the landscape scale. The results tightly corresponded with the predictions. At the foraging scale, there was an increase in the proportion of seeds harvested with increasing initial food abundance. At the landscape scale, the opposite was true. At the intermediate scale, the proportion of food harvested did not vary with initial abundance of seeds. The shapes of the functional response curves differed at each of the three spatial scales. With respect to proportion of food harvested with food abundance, they were similar to a Holling Type III (sigmoid) response at the foraging scale, to a Type I (linear) response at the intermediate scale, and to a Type II (decelerating) response at the landscape scale.