Patterns, causes and consequences of regional variation in the ecology and life history of a reef fish

Many species vary in their ecology across their geographic ranges in response to gradients in environmental conditions. Such variation, which can influence life history traits and subsequent demography of populations, usually occurs over large spatial scales. However, describing and understanding the causes of such variation is difficult precisely because it occurs over such large spatial scales. In this study, we document spatial variation in the ecology of a common reef fish, Stegastes beebei, in the Galápagos Islands and test a number of potential causal mechanisms. The pattern resembles that seen in latitudinal variation: individuals are larger, occur in higher densities, and live longer in the coldest region of the islands than those in the warmest region. However, in this system, demography varies among regional populations separated by <150 km. Preferred nutritious algae are more available in the cold region and comprise a greater proportion of the diet of fish in this region. Per gram reproductive effort appears to be strongly related to temperature, despite differences in the gross magnitude and timing of reproduction in different regions. A model of reproductive output suggests that fish in the warmest region are allocating a greater proportion of available energy to reproduction, resulting in apparent regional life history tradeoffs. Our data suggest that regional demographic differences in S. beebei may be driven by a combination of variation in food availability and an environmentally mediated life history tradeoff.