Quantitative tests of multitrophic ideal free distribution theory

Ideal free distribution (IFD) theory is often used to explain small-scale spatial distributions of organisms. However, few studies have rigorously tested predictions of IFD models in situations involving multiple species and trophic levels. We fully parameterized and tested predictions of a general interference IFD model for both predators and prey in a tritrophic system: seven-spotted lady beetles (Coccinella septempunctata L., Coleoptera: Coccinellidae), pea aphids (Acyrthosiphon pisum (Harris), Hemiptera: Aphididae), and tic bean plants (Vicia faba L., Fabaceae). We used habitat selection treatments having both predators and prey in the presence and absence of one another. We also performed experiments to quantify the strength of interspecific competition, the functional response of predators and several measures of fitness for the prey. Our results show that whether prey were present or not, predators followed IFD predictions and aggregated most strongly in the patch with the highest-quality resource. Prey in the presence of predators followed the predicted IFD, which was similar to a uniform distribution. However, prey in the absence of predators moved infrequently and were far from ideal free, suggesting that predators instigated habitat selection behaviour. Our results underscore the importance of considering trophic interactions and multiple measures of patch quality in studies of habitat selection. The observed departures from theoretical predictions also usefully suggest promising extensions for future theory and experiments.