Biologists have held the tenet that closely related species compete more strongly with each other than with distant relatives since 1859, when Darwin observed that close relatives seldom co-occur in nature and suggested it was because they competitively exclude one another. The expectation that close relatives experience greater competition than distant relatives has become known as the ‘‘competition-relatedness hypothesis (CRH).’’ The CRH is predicated on the assumption that closely related species are more likely to have similar resource requirements than distant relatives, and thus, compete more strongly for limited resources. While this assumption has been popular because it is intuitive, it has also been subject to relatively little experimentation. Over the past decade, a growing number of CRH studies have arrived at divergent conclusions showing that the strength of competitive interactions can increase, decrease, or be independent of evolutionary relatedness. Most of these studies have focused on measuring competition among species pairs as opposed to competition experienced by species when part of whole communities. We tested whether the CRH holds in communities where individual species experience interactions with a variety of other taxa, which we call the ‘resident community’. We performed a laboratory mesocosm study using communities of eight species of freshwater green algae whose evolutionary relationships were quantified using a recently developed multi-gene molecular phylogeny of 59 North American green algae. We grew species alone and in various combinations in polyculture so that we could measure each species’ sensitivity to competition (reduction in intrinsic growth rate when grown alone vs. with a resident community), relative yield, and competitive release (proportional change in biomass of a species when grown in a resident community missing one competitor vs. in a community with all possible competitors). While each of these metrics consistently revealed a prevalence of competitive interactions among the algal species, none were predicted by the relatedness of a species to a resident community. This suggests that the results of prior pairwise studies refuting the competition-relatedness hypothesis for green algae can be extended to larger resident communities in which more complex ecological interactions possibly occur.
All Science Journal Classification (ASJC) codes
- Ecology, Evolution, Behavior and Systematics