Phenotypic diversification across an environmental gradient: a role for predators and resource availability on the evolution of life histories.

Changes in age/size-specific mortality, due to such factors as predation, have potent evolutionary consequences. However, interactions with predators commonly impact prey growth rates and food availability and such indirect effects may also influence evolutionary change. We evaluated life-history differences in Trinidadian killifish, Rivulus hartii, across a gradient in predation. Rivulus are located in (1) "high predation" sites with large piscivores, (2) "Rivulus/guppy" sites with guppies, and (3) "Rivulus-only" sites with just Rivulus. Rivulus suffer higher mortality with large predators, and guppies may prey upon small/young Rivulus in Rivulus/guppy environments. In turn, population densities decline while growth rates increase in both localities compared to Rivulus-only sites. To explore how the direct and indirect effects of predators and guppies influence trait diversification in Rivulus, we examined life-history phenotypes across five rivers. High predation phenotypes exhibited a smaller size at reproduction, a greater number of eggs that were smaller, and increased reproductive allotment. Such changes are consistent with a direct response to predation. Rivulus from Rivulus/guppy sites were intermediate; they exhibited a smaller size at reproduction, increased fecundity, smaller eggs, and larger reproductive allotment than Rivulus-only fish. These changes are consistent with models that incorporate the impacts of growth and resources.