Testing the effects of resource distribution and inherent habitat riskiness on simultaneous habitat selection by predators and prey

Theoretical models have extended the Ideal Free Distribution model to examine predator-prey systems having three trophic levels, when both predator and prey are allowed to move freely. One consistent prediction made by such models is that the spatial distribution of prey should be mainly determined by the inherent habitat riskiness ( e. g. cover level), with prey avoiding the inherently riskier habitats regardless ( or nearly so) of resource distribution. To test this prediction, we conducted laboratory experiments in which both predators (creek chub, Semotilus atromaculatus) and prey ( northern redbelly dace, Phoxinus eos) were free to move between feeding patches differing in resource quantity and habitat riskiness. When alone, creek chub and northern redbelly dace both preferred the more food-rich patch. The spatial distribution of dace was also inversely related to that of creek chub in the presence of chub fenced into feeding patches. When the patches differed in both riskiness and resource quantity, the spatial distribution of dace was mainly influenced by resource distribution and, to a lesser extent, by the inherent habitat riskiness in the absence of predators. In the presence of creek chub, the dace significantly decreased their use of the inherently riskier patch compared to when predators were absent. However, contrary to the models' prediction, food distribution still significantly influenced dace distribution when predators were present. Finally, dace aggregated increasingly in a third, totally safe area ( but one offering no food) as the number of chub present in both patches increased. Thus, the influence of resource distribution and inherent habitat riskiness on prey distribution seems to vary with the level of predation risk. (C) 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.