A central goal of ecology is to explain and predict the distribution of organisms. Ideal Free Distribution theory (IFD) predicts that consumers should be distributed in patches proportionally to the input of resources within those patches. IFD can be achieved through density dependent habitat selection (DDHS) or population dynamics. When individuals actively migrate and select habitats based on resource richness, DDHS is the primary mechanism. Population dynamics, whereby habitats differ in birth and death rates, are often overlooked by theoretical ecologists leading to the perception that IFD is only an appropriate tool at or below the scale of an individual’s maximum movement capabilities. Here, we test conformity to IFD predictions in an individual-based model of a population undergoing both DDHS and population dynamics in a large environment. Feeding success is based on patch quality and number of foragers in the patch, while a marginal value theorem patch departure rule drives “leave or stay” decisions. Population dynamics are modeled by allowing foragers to reproduce at a specific energy threshold and die if they fall below a percentage of their maximum energy level. Simulations were run as a factorial design with multiple levels of spatial heterogeneity, forager reproduction rates, and movement capabilities.
Results/Conclusions
Population distributions closely matched IFD predictions at all levels of forager movement capability and spatial heterogeneity when DDHS and population dynamics were both considered. Exceptions occurred when reproduction rates were low, in which case IFD predictions were only met when movement capabilities were high or when resources were homogenously distributed (no spatial autocorrelation). When movement capabilities are small relative to the environment, individuals optimize their foraging rate at a local scale via DDHS alone. When patch quality is spatial heterogeneous as in most real ecosystems local optimality deviates from global IFD predictions. Even when spatial scales are larger than the movement capabilities of individuals, population dynamics can serve to drive populations towards the IFD. Contrary to previous modeling work which only incorporated DDHS, the results from this study indicate that IFD is a robust tool which can be used to explain the distribution of individuals at landscape scales.
