Genetic diversity of a dominant plant alters interspecific interactions between two subordinate species and facilitates their coexistence

Environmental heterogeneity of various kinds can promote species coexistence. We hypothesized that greater genetic diversity of a competitively dominant species in a plant community can promote coexistence of subordinate species. To test this hypothesis, we grew the duckweeds Lemna minor and Spirodela polyrhiza apart and together without or with 1, 2, and 4 genotypes of a much larger, co-occurring species, Hydrocotyle verticillata. The presence of H. verticillata strongly reduced the growth of both duckweed species, consistent with its dominance. The presence of S. polyrhiza significantly increased biomass of L. minor and this effect did not depend on the presence or absence of H. verticillata or its genotypic richness. Without H. verticillata, the presence of L. minor drastically reduced biomass (-94%) of S. polyrhiza. However, the inhibitory effect of L. minor on S. polyrhiza became much weaker in the presence of one and two genotypes of H. verticillata and completely disappeared in the presence of four genotypes of H. verticillata. These results suggest that both the presence of H. verticillata and its genotypic richness can alter interspecific interactions between the two duckweed species. We conclude that genetic diversity of a single, highly dominant species may promote coexistence of subordinate species when they differ strongly in the ability to compete with each other. We suggest testing further for effects of genetic diversity of dominant species on subordinate species with similar competitive abilities.