Optimal foraging, specialization, and a solution to Liem's paradox

Species that appear highly specialized on the basis of their phenotype (e.g., morphology, behavior, and physiology) also sometimes act as ecological generalists. This apparent paradox has been used to argue against the importance of competition as a diversifying evolutionary force. We provide an alternative explanation based on optimal foraging theory. Some resources are intrinsically easy to use and are widely preferred, while others require specialized phenotypic traits on the part of the consumer. This asymmetry allows optimally foraging consumers to evolve phenotypic specializations on nonpreferred resources without greatly compromising their ability to use preferred resources. The evolution of phenotypic specialization on nonpreferred resources can be driven by competition, but the specialists act as ecological generalists whenever their preferred resources are available. Our model identifies at least three different concepts of specialization that need to be distinguished, based on diet, prey utilization efficiencies, and phenotypic adaptations. The relationships among these concepts are complex and often counterintuitive. Specialists should often reject the very resources that they have evolved traits to use. The most extreme phenotypic specializations should occur in the absence of a trade-off between using preferred and nonpreferred resources. Our model may explain why extreme phenotypic-specializations evolve more often in fish communities than in terrestrial vertebrate communities and provides a mechanism whereby species can coexist in stable communities despite common preferences for some resources.