Phylogeny, niche differences, and the outcome of competition

Background/Question/Methods

Explanations for the maintenance of the wide diversity of life on earth despite competitive exclusion have centered on species differences. Ecology was virtually founded on the principle that coexistence is determined by niche differences, and as a consequence, competition would tend to exclude closely related species presumed to be too similar in their traits. However, recent advances in coexistence theory predict that competition can also preferentially exclude distantly related taxa if differences in species’ competitive ability are phylogeneticaly conserved. The balance between these opposing effects of competitive exclusion on the phylogenetic structure of communities is determined by the relative degree to which phylogenetic distance predicts niche versus competitive ability differences.

To test how competition influences patterns of relatedness in communities, we quantified stabilizing niche differences and relative fitness differences between 18 species of annual plant found in California’s serpentine grasslands. We evaluated the degree to which the exclusion of closely or distantly related species depended on the relative scaling of niche differences and competitive ability differences with phylogenetic distance. 

Results/Conclusions

More distantly related species showed on average greater niche and competitive ability differences than closely related species, though the relationships were best described by an increasing upper bound. However, the fitness ratios increased exponentially with phylogenetic distance while the niche differences increased only linearly, leading to a general competitive hierarchy, with little coexistence between species pairs. Competitive dominance proved phylogenetically conserved because species’ demography and their response to competition were conserved. Pairwise comparisons revealed that the few species pairs predicted to coexist (12 of 153) were not a random sample of the species’ pool. Coexisting pairs were species more distantly related (average phylogenetic distance, 270.5 Mya) with an average niche differences above the 97.5^th percentile and an average fitness differences below the 2.5^th percentile. These results suggest that the influence of species relatedness on community assembly may depend on evolutionary history constraining upper bounds to both niche differences and differences in competitive ability.