Plant-plant interactions promote alpine diversification

Plant-plant interactions can promote diversification in harsh environments through (1) natural selection producing divergent adaptations to extreme and varying abiotic conditions in plants that grow in the open, or (2) genetic drift involving little niche differentiation in plants that grow associated with others. We assessed whether alpine plant genera characterized by competitively-excluded or facilitated species are more diverse than genera characterized by habitat-generalist species at both global and local scales. Based on literature data, we characterized plant-plant interactions for 642 alpine species in 254 genera worldwide, using the relative interaction index (RII) that indicates the extent to which a plant species is competitively-excluded or facilitated by others. We tested whether the RII can be considered as a plant trait relatively well-preserved at the generic level and, within a phylogenetic framework, assessed how the number of alpine species per genus varies along the entire gradient of plant-plant interactions at both global and local scales. Species belonging to the same genus tended to be more similar in the RII-values than species from different genera, with ca. 20% of the variation in RII accounted by differences among genera. The relation between the total number of alpine species per genus and mean RII showed two comparable peaks, with genera in the competitive-exclusion or facilitation categories having, on average, more than twice as many species as genera in the neutral category. However, we found that more congeneric species from the competitive-exclusion category coexist locally than congeneric species from the facilitation category. This pattern of plant species richness at the community level was consistent with predictions from the hypotheses that competition promotes adaptive niche divergence at local scales, whereas facilitation promotes divergence with little niche differentiation. We conclude that both negative and positive ecological interactions play an important role as evolutionary drivers of alpine plant diversity.