Background/Question/Methods Evolution has been shown to be a critical determinant of ecological processes in some systems, but its importance relative to traditional ecological drivers is not well known. In addition, almost nothing is known about the role of ongoing coevolution in shaping ecosystems. One way to gauge the relative importance of evolution or coevolution is to contrast the magnitude of ecosystem responses associated with manipulation of intraspecific diversity against the magnitude of responses due to varying some general ecological factor. Here, we experimentally evaluated the relative effects of species invasion (a traditional ecological effect), evolution, and coevolution on ecosystem processes in Trinidadian streams. To do so, we manipulated the presence and population-of-origin of two common fish species, the guppy (Poecilia reticulata) and the Hart’s rivulus (Rivulus hartii) in experimental stream channels, and measured resulting effects on epilithic algal biomass and accrual, aquatic invertebrate biomass, and detrital decomposition.

Results/Conclusions Our results show that for some ecosystem responses the effects of evolution and coevolution were actually larger than the effects of species invasion (i.e., alteration of species assemblage). Guppies originating from a source that experiences intense predation increased algal biomass and accrual rates relative to guppies from a site with less risk of predation. These effects on primary producers appear to be associated with divergence in rates of nutrient excretion and algae consumption. Rivulus-guppy coevolution significantly influenced the biomass of aquatic invertebrates. Specifically, locally coevolved populations reduced invertebrate biomass relative to non-coevolved populations. In combination, these findings challenge the general assumption that population diversity is a less critical determinant of ecosystem function than is interspecific diversity. Moreover, given existing evidence for contemporary evolution in these fishes along similar ecological gradients, our findings suggest considerable potential for eco-evolutionary feedbacks to operate as populations adapt to natural or anthropogenic perturbations.