Habitat assessment by parasitoids: consequences for population distribution

The ideal free distribution (IFD) is a stable distribution of competitors among resource patches. For equally efficient competitors, equilibrium is reached when the per capita rate of intake equalizes across patches. The seminal version of the IFD assumes omniscience, but populations may still converge toward the equilibrium provided that competitors 1) accurately assess their environment by learning and 2) remain for an optimal (rate-maximizing) time on each encountered patch. In the companion article (Tentelier C, Desouhant E, Fauvergue X. 2006. Habitat assessment by parasitoids: mechanisms for patch time allocation. Behav Ecol. Forthcoming), it is shown that the parasitoid wasp Lysiphlebus testaceipes adapts its exploitation of aphid host colonies based on previous experience, in a manner consistent with these two conditions. We therefore predicted that a randomly distributed population of initially naive wasps should converge toward the IFD. We tested this prediction by introducing 1300 L. testaceipes females into a 110-m-super-2 greenhouse containing 40 host patches. Just after introduction, the parasitoid rate of gain was positively affected by host number and negatively affected by parasitoid number but, as predicted, these effects vanished in the course of the experiment. Six hours after introduction, the expected rate of gain reached a constant. Surprisingly, this passage through equilibrium was not accompanied by a decrease in the coefficient of variation among gain rates or by a shift from a random to an aggregated distribution of parasitoids. These results challenge our understanding of the link between individual behavior and population distribution. Copyright 2006.