Growth trades off with habitat specialization.

How differences in niche breadth evolve and are maintained remains largely unknown. The 'jack of all trades is master of none' model of resource specialization has been widely considered, but, to our knowledge, never before supported empirically. It invokes performance trade-offs associated with specialization. Specialists should outperform generalists on a subset of resources, but be unable to maintain high performance over a broader range of resources. By contrast, generalists should perform less well, on average, using a greater diversity of resources. We report such trade-offs among four coral goby species in the wild. Habitat specialists grew faster than generalists in one of two habitats. Average growth rates of generalists were less than that of specialists, but more consistent between habitats. Performance trade-offs associated with resource specialization could influence the evolution and maintenance of narrow niche breadth.