Two species with similar niches are not expected to be able to coexist because they will compete for the same resources. Closely related species often occupy similar niches because of a recent common ancestor. Because of this, closely related species are not expected to coexist, and if they do, they may partition their niches in order to coexist. There are abundant examples of communities in natural systems that violate the competitive exclusion principle, but the reasons species with similar niches are able to coexist in these communities are often not understood. Leavenworthia, a genus of rare plants in Brassicaceae that is endemic to rock outcrops in the eastern United States, is an excellent system for studying the factors facilitating the co-occurrence of closely related species. At many sites, several Leavenworthia species can be found growing in close proximity to each other, and at some sites in Tennessee as many as four species (half the genus) can be found co-occurring. To elucidate the factors facilitating the co-occurrence of Leavenworthia species, we characterized the niche of each species using bioclimatic, geologic, and hydrologic data. Niche divergence was assessed for each species comparison considering i) the total niche space available to each species and ii) a subset of the niche space that is available to both species. The first test determined the degree to which the species overlapped in their niches. If low overlap was detected, the second test determined whether this low overlap was due to true niche divergence or if it was due to substantial differences in the niche space that is accessible to each species.             We found that the niche overlap between species depends on the niche axes being compared. Specifically, niche overlap was generally lower when geology was used to characterize the niche than when bioclimatic variables were used. Niche overlap was low even for species that are sympatric across much of their range, and several niche comparisons revealed that species had access to substantially different areas of niche space. These results demonstrate that despite frequent co-occurrence, Leavenworthia species occupy distinct regions of niche space. Two competing hypotheses could explain this paradoxical finding. One possibility is that Leavenworthia was more widespread historically, species were isolated from one another, and occupied distinct areas of niche space, and these differences are retained in the contemporary distribution of the species. Another possibility is that species co-occurred for most of their evolutionary history and have partitioned their niches in order to persist in an area where their endemic habitat is abundant.