A simulation model of foraging behaviour and the effect of predation risk

Summary 1. The effect of predation risk on the distribution of animals foraging in a patchy environment was explored using a simulation model based on the work of Bernstein, Kacelnik & Krebs (1988, 1991), extended to incorporate predation risk. 2. Modelled foragers consume prey within patches, having different prey densities, at a rate that depends only on prey density and interference from other foragers in the patch. Prey density remains constant (no depletion). A forager leaves a patch if its intake rate drops below its estimate of the average intake rate available in the environment. This estimate is continually updated by taking a weighted average of current intake and the previous estimate, giving a simple learning process. Decisions on whether to leave are made at regular intervals and a forager leaving a patch may arrive at random at any other patch. 3. Simulation results were sensitive to the computational scheme used to represent estimates of average intake rate in the environment and to the way in which foragers’ decisions were distributed over time, illustrating the need for careful formulation of such models. 4. Predation risk was modelled as a cost that reduced effective intake, and could be distributed uniformly or skewed to patches of high prey density. The effects of different levels and distributions of predation risk on distribution of foragers were examined using model parameters broadly appropriate to hedgehogs (Erinaceous europaeus L.) foraging for earthworms (Lumbricus spp.) and risking opportunistic predation by badgers (Meles meles L.). The distribution of foragers with respect to prey density took an ‘ideal free’ form for zero or uniform risk, and was domed when cost of predation risk was relatively high and concentrated in the richer patches, as might be the case when predators and foragers share a common food resource.