INTERSPECIFIC COMPETITIVE INTERACTIONS AND THE MAINTENANCE OF GENOTYPIC VARIATION WITHIN TWO PERENNIAL GRASSES.

Theoretical models predict that natural selection acting through competitive interactions should lead to increased divergence in resource use and to more equal competitive abilities among different genotypes. The consequent overcompensatory interactions and similar competitive abilities will favor maintenance of genotypic diversity. In field experiments we found that naturally co-occurring genotypes of two perennial grasses, Danthonia spicata and Anthoxanthum odoratum, differed in interspecific competitive performance. The competitive performance of a given genotype often depended on the genotypic identity of the competing species, especially in Danthonia spicata. Both overcompensatory and undercompensatory interactions were found, but neither was prevalent for both species. These data indicate that interactions between species are complex and have an important genetic component. The results from this experiment are compatible with a competitive explanation for the maintenance of genetic variation, but the persistence of weakly competitive genotypes cannot be explained.