Interactions between the direct and indirect effects of predators determine life history evolution in a killifish.

The ecological impacts of the indirect effects of predators are well established, but the evolutionary consequences are unknown. Predators often decrease prey density, which indirectly increases the resources available to surviving prey. This ecological effect could provide a link to evolution because it is generally assumed that resource availability influences life history evolution. Yet, predictions from theory that consider food availability are inconsistent, and evidence for an important role of resources in shaping life history evolution is absent. We compared life history traits in a Trinidadian killifish, Rivulus hartii, from fish communities that differ in predation intensity; predators are associated with lower population density and faster growth rates. To determine whether the indirect effects of predators influence evolutionary change, we reared second-generation-born fish under two food levels that approximated natural differences in resources between communities. Rivulus from sites with predators are younger and smaller at maturity. They have increased reproductive investment and produce many small eggs and smaller hatchlings. Such divergence is predicted as a direct effect of predation. We also found significant interactions between predator community and food level for age and size at maturity, fecundity, and egg size. These interactions, whereby the differences between communities were more pronounced at high-food levels, argue that evolution in Rivulus has been influenced by the indirect effects of predators mediated through resource availability. Rivulus from sites with predators better exploit the higher resources in those habitats. Therefore, both direct and indirect effects of predators have evolutionary consequences.