Role of Evolution in Maintaining Coexistence of Competitors

Species interactions can regulate a population’s density and therefore can act as a selective force on that population. Such evolutionary responses have the potential to feedback and change ecological interactions between species. For species that compete for resources, the interaction between ecological and evolutionary dynamics will regulate the stability of the species interactions, determining whether competing species can coexist. The outcome of competition between species is determined by two factors: (1) niche overlap, or the similarity in how species use resources and are affected by their environment, and (2) fitness differences, or differences in how efficiently each species uses resources in their environment. Decreasing niche overlap will decrease competitive interactions, thereby stabilizing coexistence. Decreasing fitness differences makes species more equal in their competitive abilities, facilitating coexistence. In the absence of evolutionary constraints, both niche overlap and fitness differences among species are subject to change as a consequence of evolution among competitors, and thus ecological dynamics between two species will also change. In this dissertation, I develop a broader understanding of (1) how niche overlap and fitness differences between species change after evolution in response to competition, (2) how changes in niche overlap and fitness differences are mediated through changes in resource use of protists, and (3) what role evolutionary history plays in shaping ecological and evolutionary dynamics. I address these goals with a suite of approaches including theoretical models, an experimental lab system, and comparative methods. I constructed a quantitative genetic model of trait evolution, where the trait of a species determined its resource use, and found that species are prone to change in their niche overlap as well as their fitness differences as a result of trait evolution. However, the magnitude of changes in niche overlap and fitness differences were determined by the resource availability within the environments. When resources were broadly available, species changed more in their niche overlap, whereas when resources were narrowly available, species changed more in their fitness difference. To test these predictions, I developed a system in the laboratory where protists competed for a bacterial resource. Species were allowed to evolve in either monoculture or a two-species mixture; the effects of evolution on competition, niche overlap and fitness differences were quantified using parameterized models. In general I found that species tended to converge in their niche as a result of evolution, however, changes in fitness differences between species were larger and more influential on coexistence than changes in niche differences. Both increases in niche overlap, and increases in fitness differences decreased coexistence among species pairs. By describing the bacterial communities associated with these protists before and after selection I determined that protists tended to converge or not change in which bacteria they were consuming as a result of selection. Additionally, for eleven protist species, I determined whether traits or relatedness predicted competitive ability by placing species on a molecular phylogeny and conducting pairwise competition experiments for all pairs. I found no correlations, suggesting neither traits, nor evolutionary history was informative for explaining current ecological and evolutionary interactions in this deeply divergent clade. There are two major conclusions from this dissertation: (1) when species evolve in response to competition, changes in fitness differences may often be more important than changes in niche overlap, (2) evolution can, and may be likely to, decrease the ability of species to coexist through increases in niche overlap and increases in fitness differences. This work suggests that one must simultaneously consider the role of evolutionary and ecological processes to understand community processes. Specifically, when researchers are attempting to explain mechanisms of coexistence between species, they must consider how evolutionary dynamics may change the ecological interactions within communities of competitors.