OPTIMAL FORAGING, SPECIALIZATION, AND A SOLUTION TO LIEMS PARADOX

Species that appear highly specialized on the basis of their phenotype (e.g., morphology, behavior, and physiology) also sometimes act as ec ological generalists. This apparent paradox has been used to argue aga inst the importance of competition as a diversifying evolutionary forc e. We provide an alternative explanation based on optimal foraging the ory. Some resources are intrinsically easy to use and are widely prefe rred, while others require specialized phenotypic traits on the part o f the consumer. This asymmetry allows optimally foraging consumers to evolve phenotypic specializations on nonpreferred resources without gr eatly compromising their ability to use preferred resources. The evolu tion of phenotypic specialization on nonpreferred resources can be dri ven 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 d istinguished, based on diet, prey utilization efficiencies, and phenot ypic adaptations. The relationships among these concepts are complex a nd often counterintuitive. Specialists should often reject the very re sources that they have evolved traits to use. The most extreme phenoty pic 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 communit ies than in terrestrial vertebrate communities and provides a mechanis m whereby species can coexist in stable communities despite common pre ferences for some resources.