Understanding spatial variation in organismal density and species interactions is a central issue in ecology. Many studies approach this problem by examining sites of different quality as isolated systems. Yet, animal mobility can connect sites of varying quality and thus modify local and regional patterns. We built a simple two-patch model to study the effect of animal mobility on: a) the (local and regional) density of mobile consumers; b) the density of a sessile resource; and c) the spatial variation in fitness of the consumer. We varied the relative size of both the high and low-quality patches, and incorporated two temporal scales by excluding or including consumer demography. We incorporated mobility via a term describing the rate of fitness-dependent migration.
Results/Conclusions
When consumers were mobile over short timescales (i.e., without demography), their density was always greater in the higher quality patch, but this disparity increased as consumer mobility increased. Consumer fitness (determined by resource density) was most different at low mobility but approached equality as mobility increased. On the other hand, in the presence of demography (births and deaths), numerical responses partially compensated for the inability of low-mobility consumers to equalize fitness across patches (i.e., consumer density was lower but consumer fitness was higher in the high-quality patch compared to the situation without demography). However, this trend reversed as mobility increased further; as mobility approached infinity, consumers achieved equal fitnesses in the two patches, which reflects the classic equilibrium that arises when mobility equals 0: i.e., they achieved an ideal free distribution. Our study shows that intermediate mobility (even when movement responds to fitness gradients) can lead to fitness heterogeneity in a landscape by preventing consumer density from reflecting spatial variation in habitat quality.