Areas of Endemism of Plants in the South-Central and Southwestern Nearctic Region

The history and connections of the flora of south-central/southwestern (SC/SW) North America have been studied and summarized in biogeographic regionalization. However, some studies show contrasting delineations of the SC/SW North American provinces that could be better determined using quantitative methods. We aimed to find spatial patterns of a set of plants in the SC/SW United States by conducting endemicity analysis (EA) on different scales. We first built a dataset with 81,965 specimen point records of 400 species from 174 genera and 61 families of angiosperms and two genera of gymnosperms using digitized specimen data from iDigBio. We then performed EA at four different scales to identify the areas of endemism (AoEs). We obtained 28 AoEs with different cell sizes by selecting each AoE under the grid size that yielded the highest number of high-scoring species. The study region split into two significant centers of accumulation of nested or partially overlapping AoEs: the SW and SC consensus areas. In these parts of the Nearctic region, many genera/clades, among those in the dataset, showed a geographic split into western and eastern clades. The split corresponded to an environmental and physical barrier known as Cochise Filter Barrier. The Sonora–Mojave arid center, parts of South Texas, and the Chihuahuan Desert harbor basally branching taxa of several genera and even families, based on the observations of some species, which allowed identification of the AoEs.