SPATIAL PATTERNS OF DEPLETION IMPOSED BY FORAGING VERTEBRATES - THEORY, REVIEW AND METAANALYSIS

1. A simulation model is used to examine the spatial pattern of resour ce depletion imposed by vertebrates foraging in a heterogeneous enviro nment; this may have important consequences for the dynamics of resour ce populations. It is assumed that consumers distribute themselves to maximize intake rates, that mutual interference occurs between competi ng consumers, and that the coefficient of interference, m, is less tha n one. 2. Where individuals are equal in their susceptibility to inter ference, depletion is density-dependent and variation in resource dens ity is reduced towards zero. However, individuals often differ in thei r susceptibility to interference and incorporating such individual var iation modifies the expected pattern of depletion. Spatial variation i n resource density is no longer reduced to zero. After consumption of a given proportion of the total resource, spatial variation in the den sity of that remaining is greater when there is stronger interference, greater individual variation in susceptibility to interference, or a larger consumer population. With large individual variation and a larg e consumer population, domed or inversely density-dependent patterns o f depletion are predicted, even where the coefficient of interference, m, is less than 1. 3. Field studies that quantify depletion of resour ce patches within the sampling range of a vertebrate population are re viewed and related to patterns predicted by the simulation model. The correlation between depletion and initial resource density is positive in 22 of 24 cases and significantly density-dependent (P < 0.05) in 1 1 of these, while only one correlation is significantly inversely dens ity-dependent. The mean correlation in a meta-analysis of vertebrate p redators of invertebrate prey (22 cases) is significantly positive, sh owing that these groups tend to impose spatially density-dependent dep letion. In contrast, reviews of invertebrate parasitoids find that inv erse and positive density dependence are equally common. This differen ce may result partly from greater mobility of vertebrates, allowing th em to sample the environment and distribute themselves closer to a rat e-maximizing optimum. It is also possible that studies of vertebrates measure depletion at scales appropriate to the aggregative response of the consumers more frequently than studies of invertebrates.