Two bodies of theory have evolved to elucidate what limits the number of species in a local community, and why that number varies among communities?. First, niche theory posits that differences among species in their environmental tolerances, niche requirements, and competitive abilities determine coexistence. Second, and in stark contrast to niche theory, the Neutral Theory of Biodiversity (NTB) suggests that all species are functionally equivalent and that community changes arises as a product of drift, dispersal, and random speciation. We tested the predictions of both niche theory and the NTB using data from elevational diversity gradient in ants. Specifically, we asked whether differences among species in their responses to temperature or competitive interactions shape the elevational diversity gradient, as niche theory would predict, or whether purely neutral processes lead to the elevational diversity gradient, as the NTB predicts.

Results/Conclusions  

Using null models and detailed behavioral observations of species competing for resources, we found that competition does not shape the broad-scale distribution of diversity in this system. Instead, the most likely explanation is that differences among species in their thermal niches shape this elevational diversity gradient. We also found that beta diversity between communities was not related to distance in the way that the NTB predicts. This finding, coupled with the evidence for thermal niches, indicates that there is no support for the hypothesis that drift, dispersal, and speciation shape this gradient. Taken together, the results of this study suggest that understanding how individual taxa respond to climate may shed light on the mechanisms that limit the number of species in local communities and lead to variation in the number of species among communities.