PERMIAN BRACHIOPOD COMMUNITIES OF WEST TEXAS II: APPLICATION OF NEUTRAL ECOLOGICAL THEORY

Hubbell has recently proposed a theory of diversity in ecological communities that assumes that the reproductive fate of any individual in a community of ecologically similar species is independent of interactions with individuals of the same or other species. This neutral model provides a basis for inferring ecological processes that govern species abundance distributions. Application of this theory to fossil brachiopod collections from the Glass Mountains (Texas; Permian) indicates significant changes in the structure of these ancient communities over a ~10 Myr study interval. These collections are particularly suitable for analysis of species abundances for several reasons. First, collections are large, numerous, and diverse (855,047 specimens of 512 species and 142 genera from 191 localities), providing a robust statistical sample. Second, the descriptions and identifications of this material were made by G. Arthur Cooper and Richard Grant, providing a high level of taxonomic consistency. Third, extraction of pervasively silicified fossils by acid dissolution of carbonate matrix produced bulk samples appropriate for abundance analysis. Each of the four third-order, depositional sequences recognized in the study interval has a distinctive brachiopod assemblage. Treating each sequence as a temporally distinct ecological landscape, the neutral model appears to be able to account for observed brachiopod abundance distributions. In addition, significant differences in the abundance distributions of brachiopods from the four sequences suggest that metacommunity dynamics remained consistent within sequences while differing among them. Although the assumptions of the neutral model are overly simple, it appears to provide a useful null for interpreting changes in paleoecological community dynamics through evolutionary time.