Genotypic diversity within a species can have extended consequences for community structure but our understanding of the ecological and evolutionary implications of within-species genotype interactions is limited. If an individual’s phenotype is in part determined by the composition of neighboring individuals, this can greatly alter the expression of phenotypic traits and evolutionary trajectories. Here we determine how genotypes of an invasive grass perform when grown under different genotypic diversity neighborhoods and environmental contexts. Phalaris arundinacea is a cool season perennial grass that is currently invading large areas of the North Eastern United States. We established 120 plots of increasing genotypic diversity (1, 2, 4 or 8 eight genotypes) under two disturbance conditions (disturbed and undisturbed) to determine how small scale genotypic diversity and environmental conditions influence Phalaris arundinacea performance and evolutionary potential.
We show that different genotypes of Phalaris arundinacea, in both the disturbed and undisturbed environments, had a range of positive and negative responses to increasing genotypic diversity. Moreover, unique sets of genotypes responded to disturbed and undisturbed conditions. Overall productivity and survivorship of genotypes was much greater under disturbed conditions; with genotypes producing 85% greater total biomass (p < 0.001) and having 36% greater survivorship (p < 0.001) than undisturbed plots. Genotypes had significantly greater heritable variation under mixture plots (2, 4 or 8 genotypes) than when grown under monoculture and for all biomass measurements we found that the within population genetic variance (Vg) increased with increasing genotypic diversity. Our work demonstrates that genotypic neighborhood influences a genotype’s potential response to selection and that genotypes within a species do not have the same magnitude and direction of responses to their local genotypic neighborhood and environmental condition, as is often assumed in models. For invasive species, a range of responses to different genotype neighborhoods may facilitate invasion success by enabling genotypes to adapt to a wide range of environmental conditions.