Secondary contact between closely related species often results in negative interspecific interactions, such as competition or reproductive interference. For this reason, closely related species are generally less likely to coexist in the same habitat. In this study, we focused on two congeneric species, Viola mandshurica and V. yedoensis var. pseudo-japonica which coexist sympatrically in southwestern Japan. We first examined the roles of resource competition and reproductive interference as potential negative interactions between the two species. We then investigated the role of niche partitioning and autonomous selfing as factors that could facilitate the coexistence of these species. An artificial pollination experiment demonstrated that the seed set rate of V. yedoensis significantly decreased when pollinated with a mixture of V. yedoensis and V. mandshurica suggesting the possible existence of reproductive interference.
We found no significant differences in the abiotic niches of the two species; however, V. yedoensis occupied a qualitatively broader abiotic niche than V. mandshurica, suggesting that V. mandshurica is nested within the larger-ranged V. yedoensis. Both species produced a substantial number of cleistogamous flowers, and the seed germination rate of cleistogamous flowers ranged from 70% to 90% for both species, indicating that the degree of inbreeding depression is relatively weak. These results suggest that autonomous selfing by cleistogamous flowers could mitigate the negative effects of reproductive interference and facilitate the coexistence of the two species in urban areas.