Connecting genetic diversity to seagrass productivity: Effects of genotypic richness, genetic relatedness and trait variation

Background/Question/Methods There is growing evidence that genetic variation within and among populations of key species plays an important role in ecosystem processes. Several experiments provide compelling evidence that the number of genotypes in an assemblage (genotypic richness) can influence critical ecosystem functions including productivity, resistance to disturbance and invasion or colonization success. However, these studies use only the number of genotypes as a measure of genetic diversity. Recent analyses of species diversity experiments show that phylogenetic or trait diversity may be a more reliable predictor of ecosystem functioning than simply the number of species. However, such approaches have not yet been applied to understanding the effects of genetic diversity on ecosystem functioning. Here we use new data on functional and genetic relatedness of eelgrass (Zostera marina) genotypes to re-analyze previous manipulations of the number of genotypes in experimental seagrass plots. Specifically, we ask whether the number, genetic relatedness, or trait diversity of genotypes best explains the productivity of eelgrass assemblages. Because of both its ecological importance and its declining status in many locations, understanding what aspects of intraspecific diversity most closely predict population success may thus offer practical guidance for management and restoration activities.

Results/Conclusions We show that genetic relatedness is a better predictor of eelgrass productivity than simply the number of genotypes in an assemblage. In analyses for which trait data were available, functional diversity was an even better predictor than relatedness for productivity of plots with equivalent richness. Trait diversity and relatedness were strongly positively correlated, suggesting that a higher diversity of traits in more related assemblages may contribute to effects of relatedness on shoot density. Although this initially seems counterintuitive, our experimental mixtures lacked assemblages of very closely related genotypes. The positive relationship between relatedness and productivity may change when assemblages of highly related genotypes are considered. Experimental monocultures (highest possible relatedness) performed poorly relative to all mixtures, supporting this hypothesis. In addition, a field survey revealed a unimodal relationship between relatedness and shoot density suggesting that assemblages of both highly related and highly unrelated individuals perform poorly relative to intermediately related assemblages. Our evidence is consistent with both trait complementarity and cooperation among relatives as drivers of this relationship. Our work extends the relationship between phylogenetic distance and productivity from species diversity experiments to the within-species level.