Density-dependent prey mortality is determined by the spatial scale of predator foraging
Foraging theory predicts which prey patches predators should target. However, in most habitats, what constitutes a “patch” and how prey density is calculated are subjective and depend on the spatial scale at which the predator (or scientist) is observing. Moreover, the predator’s ‘foraging scale’ affects prey population dynamics: predators should produce directly density-dependent (DDD) prey mortality at the foraging scale, but inversely density-dependent (IDD) mortality (safety-in-numbers) at smaller scales. This hypothesis has been supported by indirect evidence in the past, but never tested directly. We performed the first experimental determination of the spatial scale of predator foraging and its effect on the form of prey density-dependent mortality using behavioral assays in a model system: guppies (Poecilia reticulata) feeding on bloodworm ‘prey’ patches. By manipulating prey configuation and density at different foraging scales, we were able to estimate the spatial scale at which predators targeted groups of prey, and then determined whether mortality was directly density-dependent or not.
We first determined that guppies prefer prey patches that have high densities at a spatial scale of 10-12 cm, but are indifferent to high-density clusters that occur at smaller spatial scales. Thus the guppies have a ‘foraging scale’ of 10-12 cm. Based on that result, our test of prey mortality patterns confirmed theoretical predictions: predation was IDD when prey were aggregated at a scale smaller than the foraging scale, but not when prey were aggregated at larger scales. These results could be used to predict outcomes of predator-prey interactions in continuous, non-discrete habitats in the field. Furthermore, this result reveals possible selective pressures on prey aggregation behavior; it would be beneficial to aggregate only at spatial scales smaller than the perceptual foraging scale of the predator.