One of the most important floristic sorting periods to affect modern plant communities occurred during the shift from the wet Tertiary period to the unusually dry Quaternary, when most global deserts developed. During this time, a wave of new plant species emerged, presumably in response to the new climate. Interestingly, most Tertiary species that have been tracked through the fossil record did not disappear but remained relatively abundant despite the development of a much more unfavorable climate for species adapted to moist conditions. Here we find, by integrating paleobotanical, ecological, and phylogenetic analyses, that a large number of ancient Tertiary species in Mediterranean-climate ecosystems appear to have been preserved by the facilitative or ''nurse'' effects of modern Quaternary species. Our results indicate that these interdependent relationships among plants have played a central role in the preservation of the global biodiversity and provided a mechanism for stabilizing selection and the conservation of ecological traits over evolutionary time scales.Mediterranean-type ecosystems ͉ Mexical shrubland ͉ phylogenetic niche conservatism ͉ plant facilitation ͉ stabilizing selection T he evolutionary processes that drive speciation and create biological diversity are central to the natural sciences, but we know little regarding the historical ecological processes that have eliminated species from communities or allowed them to persist (1). Current plant communities are the product of historical sorting processes and, therefore, include mixtures of floristic elements originating in different geological times and, presumably, under different ecological scenarios (2). Many extant plant species are millions of years old (3) and, therefore, have persisted through dramatic changes in global climate. Biogeographic and paleontological evidence indicates that communities are derived from species that have originated in different geological times and places, and that these taxa have been mixed and culled independently of each other through various sorting processes (2, 3). The perception that interdependent processes among plant species are insignificant over evolutionary time frames has been central to the idea that communities are ''merely a coincidence'' (4) and the development of neutral model theory on biodiversity (5). Therefore, with few exceptions (6, 7), plant communities are not thought to possess stable properties determined by plant-plant interactions.In contrast to existing theory, our observations of ecological relationships among species in semiarid communities around the world suggest that powerful facilitative ecological interactions among plant species have been a crucial component of the historical and sorting processes responsible for the composition and diversity of contemporary plant communities. Facilitative interactions appear to have been particularly important during the shift from mesic Tertiary period to the unusually dry Quaternary when global deserts developed. Desertification pr...