A useful phenomenological difference between exploitation and interference in the distribution of ideal free predators

Abstract

It seems an obvious truism that purely phenomenological models of animal interactions will always compare unfavourably to those based on the actual mechanisms by which individuals interact with each other and their environment. This is a conclusion drawn by van der Meer & Ens (1997) in their comprehensive comparison of model predictions for ideal free predators, and echoed by Weber (1998). We cannot do without phenomenological models, however, since they provide the conceptual framework of key processes, such as density-dependent food intake for 'ideal' and 'free' predators, upon which to model the particular mechanisms, such as food-searching behaviour and time-wasting interactions. Phenomenological models often get a rough ride in the ecological literature because they are matched against mechanistic models as if the two types were alternatives. They accomplish different tasks, with the former type predicting the outcome of underlying processes while the latter distinguishes between alternative mechanisms. It is an incomplete appreciation of this difference that leads van der Meer & Ens (1997) to unfairly criticise the searching rate equation of Hassell & Varley (1969), which is one of the more popular phenomenological models of interference. My intention here is to show how this model can make useful predictions about the exploitation of renewable standing stock, when it is used in an appropriate setting. In common with other authors (e.g. Sutherland 1996), van der Meer & Ens draw on a simple conceptual framework to infer that without interference all ideal free predators will aggregate in the patch or patches with the highest standing stock of prey. This result arises because each predator's intake rate of prey is a function of prey density only, and not predator density if there is no interference between predators. The patches with the highest prey density therefore yield the highest intake for all. Van der Meer & Ens note that 'such strong aggregation is, of course, rather unlikely.' Their conclusion is that ideal free exploitation is insufficient in many cases to explain