Testing the link between relatedness and competition in a model microbial community
Ecological communities are comprised of numerous interacting species, many of which experience competition for the same limiting resources. The competition-relatedness hypothesis makes the intuitively sensible prediction that the strength of competition is positively correlated with phylogenetic similarity because genetically similar species are expected to occupy similar ecological niches. However, empirical studies of relatedness and competition in natural communities have yielded mixed results. In order to test this hypothesis in a well-controlled environment, we employed a laboratory-based model microbial community featuring phylogenetically diverse heterotrophic bacteria competing in a carbon-limited environment. We made simple measurements of the growth dynamics and steady-state behavior of each member species in isolation and every possible pair of species. Then, we classified the types of interactions we observed and mapped them onto the phylogenetic distance between each pair.
Across our pairwise competition experiments, we observed each of the three possible steady-state outcomes of competition between two species: coexistence, dominance, and bistability. Despite the relative simplicity of the experimental environment, stable coexistence between species was surprisingly common. Phylogenetic relatedness was estimated using 16S rDNA sequence similarity. Dominance of one species over the other was more common among more closely related pairs, suggesting that competitive exclusion driven by genetic similarity may indeed play a significant role in structuring this model community.