The relative contributions of competition and abiotic tolerances in determining the geographical distributions of four closely related Yucca species in Texas

Aim: The determinants of species’ geographical ranges are the result of both abiotic and biotic factors. Among co-occurring closely related species, some degree of differentiation in niche characteristics is assumed necessary to allow continued coexistence. As niche requirements are likely to show similarity or overlap, competition among such species may also restrict range limits and help maintain species boundaries. We examined the roles of niche compartmentalization and competition to assess their relative contributions in determining geographical distributions of four closely related species of Yucca co-occurring across a large region of Texas. These species are pollinated by the same moth species, but each yucca is a distinct biological species. Location: Texas, USA. Methods: We used species distribution modelling and niche evaluation methods based on extensive field surveys to evaluate the roles of abiotic factors and competition. We also conducted surveys of flowering activity to determine if species boundaries may be maintained by species-specific shifts in flowering phenology that would limit interspecific gene flow. Results: Niche evaluation methods demonstrate unique niches mostly consistent with the defined ecological regions of Texas. Geographical distributions of yuccas are mostly parapatric with some regions of sympatry. Species distribution models indicate that most species pairings display a region of potential sympatry where one of the species is numerically dominant, suggesting that competitive exclusion is delineating species ranges at parapatric margins. Flowering activity data indicate the potential for cross-pollination among these species. Nevertheless, there is little evidence for hybridization, suggesting that strong selection due to niche requirements is mediating species boundaries. Main conclusions: Results suggest that adaptation to abiotic conditions has been a major force underpinning diversification in these plants. However, competition at the edge of species’ ranges likely limits the spread of any one species, leading to uncharacteristically high species diversity over a relatively small geographical area.