Predation risk, unequal competitors and the ideal free distribution

Ideal free distribution (IFD) theory has frequently been used to investigat e habitat selection when fitness payoffs are frequency-dependent. To date, however, researchers have not considered the possibility that individuals m ay simultaneously differ in their ability to compete for resources and thei r 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 competiti ve ability and mortality risk on the equilibrium distribution of competitor s across habitats. For simplicity, we consider the case of two competitor t ypes competing for resources in an environment containing two habitats: a p roductive, but risky habitat and a less productive, safer habitat. In gener al, the model predicts that when individual mortality risk is independent o f the density of competitors within a habitat, competitor types will tend t o be assorted by competitive ability, with the competitor type experiencing the higher ratio of mortality risk across the habitats occurring predomina ntly in the safer, but less productive habitat. In contrast, when individua l mortality risk within a habitat is diluted by competitor number, the mode l predicts that both competitor types will tend to aggregate in the same ha bitat, with the chosen habitat depending on which competitor type experienc es the higher ratio of mortality risk across the habitats. When good compet itors experience a higher ratio of mortality risk than poor competitors, bo th competitor types will tend to aggregate in the risky, but productive hab itat. However, when poor competitors experience the higher ratio of mortali ty risk, both competitor types will tend to aggregate in the safer, but les s productive habitat. Because our model can be applied to both intra- and i nterspecific resource competition, its results suggest a new potential mech anism for the co-existence of competitor types within a habitat.