Format: ORAL

Juan Pablo Ortiz-Brunel1, Hilda Flores2, Helga Ochoterena2, and Michael J. Moore3

1 Departamento de Botánica, Universidad de Guadalajara, Jalisco, Mexico. 2 Instituto de Biología, Universidad Nacional Autónoma de México, Ciudad de México, Mexico. 3 Department of Biology, Oberlin College, Ohio, USA.

Isolated gypsum outcrops along Mexico and southwestern USA are diverse in terms of size and flora. Over 250 plant species restricted to gypsum (gypsophytes) are known there, mostly within the Chihuahuan Desert. The dominant North American gypsum endemics are often physiologically and morphologically highly distinctive from their nearest relatives, and many belong to clades that may have originated several million years ago. Nevertheless, biogeographic patterns of endemism and the ultimate origins of the gypsum flora of North America (including endemic and non-endemic taxa) are poorly known. This is particularly true for the gypsum outcrops in tropical Mexico, which are poorly investigated. Recent efforts showed that the gypsicolous flora of tropical Mexico is highly diverse despite inhabiting much smaller outcrops than in the Chihuahuan Desert. However, no comprehensive studies of diversity patterns, including phylogenetic diversity, have been conducted in North America. Here we use different diversity metrics to analyze the composition of the gypsum floras of North America. We used 6,211 herbarium-based records to estimate the spatial distribution of species richness, endemism, phylogenetic diversity, and phylogenetic endemism. We found that tropical gypsum patches possess a more diverse flora but with significantly reduced endemism as compared to those in the arid regions further north. We detected a mix of Nearctic and Neotropical elements, with the latter especially prominent in western and southern Mexico. Total species richness followed global latitudinal diversity gradients, but remarkable cases of high diversity were identified in the Cuatrocinegas Basin, in the Sierra Madre Oriental at Nuevo Len, and in Oaxaca. Categorical analysis revealed examples of neo- and paleo-endemism. Species endemism and phylogenetic endemism are heterogenous throughout the study area, but are higher in the Chihuahuan Desert Region, which may reflect the much greater geographic extent of gypsum outcrops in these areas compared with the tropical patches.