Phylogenetic Distance Does Not Predict Competitive Ability in Green Algal Communities

Abstract

An assumption often made by ecologists and phylogeneticists—that closely related species possess similar traits and ecology—can be extended into the hypothesis that closely related species compete more heavily than distant relatives due to shared ecology. The intuition that related species occupy similar niches and thus compete intensely for resources, one outcome of which is competitive exclusion and local reduction of biodiversity, was formally introduced by Darwin in 1859. The past decade has seen a steady rise in tests of Darwin’s “competition-relatedness hypothesis” that experimentally manipulate relatedness, or evolutionary history represented by species in a community, and then measure interaction strengths. Despite the initial enthusiasm for using evolutionary history to predict ecology, different competition studies have arrived at different conclusions regarding whether there is a positive, negative, or no relationship between species’ evolutionary relatedness and the strength of competitive interactions between them. Furthermore, these studies have primarily measured competition for pairs of species rather than the overall competition a species experiences within a multi-species community where more complex (e.g., indirect) interactions might be expected to take place. In order to test whether the competition-relatedness hypothesis holds in communities with a species richness more representative of that found in nature, a mesocosm study was performed using communities of eight species of freshwater green algae. Species relatedness was quantified as the phylogenetic distance between species using a comprehensive multi-gene molecular phylogeny of 59 North American green algae. Three metrics of competition strength—sensitivity to competition (reduction in intrinsic growth rate when grown from low density with competitors versus when grown in monoculture), relative yield, and competitive release (proportional change in biomass of a focal species grown with one competitor missing versus when grown with all competitors)—were not predicted by the relatedness of a species to its community. The finding that species’ relatedness to their resident community was unrelated to the strength of competition they experienced concurs with previous findings from studies of interaction strength as a function of relatedness between pairs of species. This finding suggests that the results of prior studies refuting the competition-relatedness hypothesis can be extended to larger communities in which more complex ecological interactions occur.