While it has long been recognized that ecological factors play a significant role in the evolutionary process, the reciprocal influence of evolutionary change on ecological factors has received less attention. Phenotypic changes can alter the overall role of an organism in the community by changing how that organism directly and indirectly interacts with biotic and abiotic factors. In addition, local adaptation can also affect these relationships if population-level characters such as density or size-structure change as a consequence of a change in the phenotype. Here, we experimentally distinguish between the direct and indirect effects of changes in the phenotype and population density of Trinidadian guppies (Poecilia reticulata) on stream ecosystem processes and community structure. We do so in the context of a common garden mesocosm experiment using guppies from two independent origins of the phenotype. Flow-through mesocosms were seeded with benthic invertebrates and organic material from a nearby stream. Guppies were introduced into the mesocosms in a two by two factorial design with phenotype and density of guppies as factors. We also established guppy-free control treatments to evaluate the overall effect of guppies on the ecosystem. We nested electrified fence exclosures within mesocosm treatments to exclude guppies from some parts of the mesocosms so that we could directly quantify the indirect effects. Post guppy introduction, we measured changes in the invertebrate and primary producer community and ecosystem processes over a 28 day period and analyzed results using planned comparisons to separate direct and indirect effects.

Results/Conclusions

We show that direct and indirect effects interact in varied ways to produce the net effects of different phenotypes in the ecosystem. For example, we show that higher consumption of benthic producers by low predation phenotypes directly alters the diversity of the diatom community by decreasing the richness and overall diversity diatom genera. However, the indirect effects of the low predation phenotype result in a smaller, but significant, increase in diatom richness. The net effect is for low predation guppies to decrease the richness of diatoms, but this effect is smaller in magnitude when compared to the direct effect only. Our results show how phenotype and its population-level consequences have a significant impact on the biological community.  Moreover, we show that these two evolutionary effects can occur through complex interactions between direct and indirect ecological pathways.