Predation risk, unequal competitors and the ideal free distribution

Ideal free distribution (IFD) theory has frequently been used to investigate habitat selection when fitness payoffs are frequency-dependent. To date, however, researchers have not considered the possibility that individuals may simultaneously differ in their ability to compete for resources and their susceptibility to predation. Such differences might be expected to occur as a consequence of differences in body size, morphology or behaviour. Here, we develop a model to investigate the effects of differences in competitive ability and mortality risk on the equilibrium distribution of competitors across habitats. For simplicity, we consider the case of two competitor types competing for resources in an environment containing two habitats: a productive, but risky habitat and a less productive, safer habitat. In general, the model predicts that when individual mortality risk is independent of the density of competitors within a habitat, competitor types will tend to be assorted by competitive ability, with the competitor type experiencing the higher ratio of mortality risk across the habitats occurring predominantly in the safer, but less productive habitat. In contrast, when individual mortality risk within a habitat is diluted by competitor number, the model predicts that both competitor types will tend to aggregate in the same habitat, with the chosen habitat depending on which competitor type experiences the higher ratio of mortality risk across the habitats. When good competitors experience a higher ratio of mortality risk than poor competitors, both competitor types will tend to aggregate in the risky, but productive habitat. However, when poor competitors experience the higher ratio of mortality risk, both competitor types will tend to aggregate in the safer, but less productive habitat. Because our model can be applied to both intraand interspecific resource competition, its results suggest a new potential mechanism for the co-existence of competitor types within a habitat.