Predicting the distribution of organisms among a few patches: Problems with detecting departures from the ideal free distribution

Much empirical work has examined whether foraging animals distribute themselves across patches of resources according to an ideal free distribution (IFD). Most studies consider situations with 2 patches, where choice of one patch is synonymous,vith rejection of the alternative. In this paper, we use computer simulations to determine the effect of the number of patches on achievement of an IFD, Identical foragers were allowed to move between patches so as to maximize their intake rate. To generate departures from an IFD, we assigned all foragers a perception Limit of prey numbers such that they could not perceive differences in patch quality that exceed this limit. Foragers would choose randomly from the patches among which they could perceive no differences. We found that, for the same perception limit, departures from an IFD were greatest in the 2-patch situation. In addition, we are less likely to detect perception limit-generated deviations from an IFD (or even a random distribution) in greater than 2-patch situations unless He examine cases in which there are large among-patch differences in quality. Hence, lab and field studies that incorporate the greater realism achieved by using more than 2 patches must be cautious about falsely accepting the hypothesis of an IFD, particularly if the causes of deviations from an IFD are independent of the number of patches. We review published studies and find that conformity to an IFD seems to decrease with an increase in the number of patches. Hence, if deviations from an IFD are perception-induced, discrimination may decrease with the number of patches, presumably because of the increasing physical and computational difficulties of keeping track of each patch.