Species diversity of grasses promotes genotypic diversity of clover populations in simulated communities

Recent studies in community genetics have demonstrated strong effects of intraspecific genetic variation on the diversity of interacting species but largely ignored the potential for effects of species diversity on genetic diversity, which could also create a positive correlation between these two levels of biodiversity. Here we investigated the role that species diversity of competitors could play in shaping the genotypic diversity of a focal species, using a modified version of an existing model of grass-clover dynamics. We simulated communities in which clover genotypes varied in their relative competitive ability when growing adjacent to different grass species. Under many parameter combinations, communities with greater species richness of grasses retained greater genotypic richness within the clover population. Increasing grass species richness effectively increased biotic environmental heterogeneity with respect to clover growth, thereby promoting the maintenance of genotypic diversity. This result depended on three conditions being met: (1) a sufficiently strong tradeoff among genotypes in their fitness when growing with different grass species, (2) partial de-coupling of competition and facilitation, with grass and clover capable of sharing microsites rather than strictly excluding one another, and (3) sufficiently high rates of clover growth and clonal spread, which allowed clover genotypes to 'track' the shifting mosaic of grass species. Our results demonstrate that species diversity can act as an important promoter of genotypic diversity, and they provide testable predictions concerning the conditions that promote this outcome in nature.