The ideal free distribution: function meets mechanism

Abstract

In this thesis I attempt to synthesize psychological and biological approaches to the study of foraging. The specific optimal foraging model I examine is the Ideal Free Distribution. The Ideal Free Distribution predicts that the equilibrium distribution of organisms between resource sites will equal the distribution of resources between those sites. In the first part of this study I reanalysed previously published tests of the Ideal Free Distribution, drawing on an analogy between the Ideal Free Distribution and a psychological principle termed the matching law. By analysing experiments on the Ideal Free Distribution in the same way as psychological experiments on the matching law, I was able to document that the distribution of organisms is systematically less extreme than the distribution of resources. Violations of the Ideal Free Distribution's assumptions of (1) perfect knowledge (no perceptual constraint), (2) no effect of interference, and (3) no effect of travel, may cause these deviations. In a series of experiments with a free-living population of mallard ducks I examined whether the Ideal Free Distribution is robust to violations of these assumptions. In the first experiment I tested the hypothesis that perceptual constraints account for the observed deviations from the Ideal Free Distribution by manipulating the overall rate of food availability. This experiment supported the predictions of the perceptual constraint hypothesis and suggested that social interactions may also be affecting the distribution of organisms. I simulated the effects of perceptual constraints and social interactions to investigate their potential effects on the distribution of organisms. The simulation showed that quite small perceptual constraints can cause large departures from the Ideal Free Distribution, and that a change in the distribution of interactions may also have a major effect on the distribution of organisms. In the second experiment I manipulated the distribution of agonistic interactions to quantify their effects on the distribution of organisms. The results of this experiment supported the hypothesis that organisms use interactions as cues to assess resource profitability and thus counterintuitively, that interactions act as a benefit rather than as a cost The final experiment examined the effects of travel between food sites on the Ideal Free Distribution. This experiment showed that a relatively small increase in travel time Will increase the departures from the Ideal Free Distribution. This result suggests that increased travel distance decreased the discriminability of the relative food availabilities rather than acting as a cost to the organisms. It is concluded that the incorporation of detailed mechanistic considerations about the perceptual abilities of animals, the cues they use to assess resource availability, and the effects of social interactions would greatly strengthen the predictive power of functional models of foraging.