The role of genotypic diversity in determining grassland community structure under constant environmental conditions

1. A recent experiment varied the genetic diversity of model grassland communities under standardized soil and management conditions and at constant initial species diversity. After 5 years' growth, genetically diverse communities retained more species diversity and became more similar in species composition than genetically impoverished communities. 2. Here we present the results of further investigation within this experimental system. We proposed that two mechanisms - the first invoking genetically determined and constant differences in plant phenotypes and the second invoking genotype-environment interactions - could each underpin these results. This mechanistic framework was used as a tool to interpret our findings. 3. We used inter-simple sequence repeat (ISSR) DNA markers to confirm which of the individuals of six study species initially included in the model communities were unique genotypes. We then used the molecular markers to assess the survival and abundance of each genotype at the end of the 5-year experimental period. 4. The DNA marker data were used to create, for the first time, a genotype abundance hierarchy describing the structure of a community at the level of genotypes. This abundance hierarchy revealed wide variation in the abundance of genotypes within species, and large overlaps in the performance of the genotypes of different species. 5. Each genotype achieved a consistent level of abundance within genetically diverse communities, which differed from that attained by other genotypes of the same species. The abundance hierarchy of genotypes within species also showed consistency across communities differing in their initial level of genetic diversity, such that species abundance in genetically impoverished communities could be predicted, in part, by genotypic identity. 6. Three species (including two canopy-dominants) experienced shifts in their community-level genotype abundance hierarchies that were consistent with an increased influence of genotype-environment interactions in genetically impoverished communities. 7. Our results indicate that under relatively constant environmental conditions the species abundance structure of plant communities can in part be predicted from the genotypic composition of their component populations. Genotype-environment interactions also appear to shape the structure of communities under such conditions, although further experiments are needed to clarify the magnitude and mechanism of these effects.