High extinction rates have prompted interest into the importance of biodiversity. Several decades of research have shown that increasing species diversity can differentially affect individual species and can have important consequences for community dynamics and ecosystem function.  More recent attention has been paid to variation within a single species and it was found that increasing genotypic diversity can affect these same processes. We provide the first direct comparison of the relative importance of genotypic and species diversity for plant communities and individuals within those communities.  We established four replicated diversity treatments in the field using a substitutive design and a pool of 24 Oenothera biennis genotypes and 24 common old field species.  Genotypic diversity was either low (one O. biennis genotype) or high (eight genotypes) and similarly species diversity was low (one old field species) or high (eight species).  We assessed and compared the impact of these different diversity types on plant communities by measuring multiple traits related to the phenology and productivity of these plots. In addition, we placed a sentinel O. biennis individual in the middle of each plot to determine how community context influences O. biennis traits and whether natural section on those traits varies among communities.

Results/Conclusions

For the plant communities, we found that O. biennis plots were more productive and had more extended flowering phenologies than the species plots, and that high diversity plots were significantly more productive and than low diversity plots for both genotypes and species mixtures. Interestingly, we found that increasing genotypic diversity had equivalent effects on community responses to that of increasing species diversity. For the sentinel plants, we found that selection was only significant for size related traits, suggesting that competitive ability was important in this experiment. Although the direction of selection was positive in all communities, the strength varied significantly. We found that selection for increased biomass became stronger with increased similarity of the community to the sentinel plant.  Weakest selection was found in the species plots and strongest selection in the genotypic monocultures. This experiment is unique in its scope – we not only manipulated genotypic and species diversity simultaneously, we also revealed the consequences of that diversity on individuals and the community. We conclude that the relative importance of species diversity and genotypic diversity is equivalent for plant communities, and that diversity also affects natural selection for at least one member of that community.