Community Context Evolution

Species do not evolve in isolation, but rather exist within a complex network of species interactions that all may influence how traits evolve. Moreover, indirect effects in multispecies communities may also affect trait evolution. We are interested in such multispecies effects to understand how species evolve in natural communities.

In natural biological communities, species interact with many other species. Multiple species interactions lead to indirect ecological effects that have important fitness consequences and can also result in non-additive patterns of natural selection. Ecologists have long recognized the importance of indirect ecological effects on species abundances, coexistence, and diversity. However, the evolutionary consequences of indirect interactions are rarely considered. I conducted selection experiments and examined the evolutionary response of Colpoda sp., a ciliated protozoan, to other members of the inquiline community of purple pitcher plants (Sarracenia purpurea). I measured the evolution of six traits in response to (1) predation by mosquito larvae, (2) competition from other ciliated protozoans, and (3) simultaneous predation and competition. The latter treatment incorporated both direct effects and indirect effects due to interactions between predators and competitors. Population growth rate and cell size evolved in response to direct effects of predators and competitors. However, trait values in the multispecies treatment were similar to those in the monoculture treatment, indicating that direct effects were offset by strong indirect effects on the evolution of traits. For most of the traits measured, indirect effects were opposed to, and often stronger than, direct effects. These indirect effects occurred as a result of behavioral changes of the predator in the presence of competitors and as a result of reduced densities of competitors in the presence of predators. Incorporating indirect effects provides a more realistic description of how species evolve in complex natural communities.

Given that ecological indirect effects are common in nature, non-additive selection may also be quite common, and thus may be critical for predicting evolution in natural communities composed of many interacting species. In collaboration with several others at the Kellogg Biological Station, we developed a clear method for testing for non-additive selection and considered how it might affect adaptation in multispecies communities. We developed a null hypothesis and used a simulation model to validate our approach. Additionally, we described how to quantify the strength of non-additive selection in comparison to the relative strength of pairwise selection within and across studies. We used two case studies to illustrate how our method can be applied to empirical datasets. Our results suggest that non-additive selection may be common in nature, but further studies using the methodology outlined are needed to determine just how common non-additive selection is and under what conditions it is most likely to occur.