Phenotypic differences among species have long been used predictors of community assembly in ecology. More recently, trait-based ecology has been used to help predict patterns of ecosystem functioning. However the plastic nature of many traits and the challenge of finding meaningful traits to compare across diverse taxa has spurred interest in phylogenetic proxies for traits that might better predict ecosystem functioning than the number of species. Analogously, within species, the number of genotypes present in an assemblage can affect assemblage productivity, but trait diversity or genetic relatedness could provide more precise and mechanistic predictions of ecological outcomes. Eelgrass (Zostera marina) is a marine foundation species that forms monospecific beds throughout the northern hemisphere and we have previously shown that genotypic richness in experimental plots increases plot biomass and resistance to disturbance. Here we combine measures of genetic relatedness and diversity, common garden trait measurements, and field outplants to assess the contribution of trait vs genetic differentiation for the coexistence of genotypes, the productivity of populations, and the structure of the community of animals that use Zostera as habitat.
We find evidence that genetic and trait differentiation both contribute to eelgrass community structure and function. In general, trait differentiation was uncorrelated with genetic relatedness among individuals, considering both individual traits and multivariate metrics of trait differentiation. In pairwise experiments, trait differentiation strongly influenced coexistence—more distinct genotypes were less likely to coexist, probably because fitness differences among clones in the planting environment were too large to be overcome by stabilizing niche differences. In experiments with higher diversity (up to 6 genotypes and a range of relatedness), genetic diversity (richness+evenness) was consistently the strongest predictor of plant biomass accumulation. Increasing genetic diversity and decreasing relatedness both increased total plant biomass and altered the diversity and abundance of associated animals. Genetic diversity (but not relatedness) led to increased trait diversity, suggesting that greater trait range is responsible for at least some of this effect. Overall our results suggest that eelgrass genetic relatedness and genotypic diversity capture fundamentally different components of intraspecific variation and should be treated as complementary rather than competing dimensions of biodiversity that contribute independently to local ecosystem functioning.