CHANGES IN ECOLOGICAL PROCESSES OF LOCAL BRACHIOPOD PALEOCOMMUNITIES IN THE DELAWARE BASIN (WEST TEXAS)

The partitioning of diversity into within-community and among-community components provides a meaningful way to assess the total diversity within a region. Knowing the contribution of each component aids in understanding the underlying ecological mechanisms influencing the distribution of species and communities. Ecological factors such as dispersal and competition operating at different scales can influence diversity, but diversity also can be influenced by environmental conditions. This study evaluates changes in local brachiopod paleocommunities within a larger region (Guadalupe Mountains National Park, Texas) to understand the ecological mechanisms influencing diversity in the Pinery, Rader, Lamar, and Reef Trail Members of the Middle Permian Bell Canyon Formation. For each member, brachiopod abundance data are multiplicatively partitioned into within-community and among-community components using Shannon Entropy values converted into the number of effective genera (Jost, 2007). Results from partitioning show that the percentage of effective genera in the metacommunity absent from the average local community decreases from 66% in the Pinery Member to 59% in the Rader Member to 38% in the Lamar Member, but increases to 161% in the overlying Reef Trail Member. Changes in the relative proportion of the diversity components among members suggest an increase in the effectiveness of dispersal of brachiopods in the Lamar Member (which have a larger proportion of taxa in common) and a decrease in effectiveness of dispersal in the Reef Trail Member. These changes appear to be associated with major disruptions in the basin related to sea-level change. Specifically, changes in the taxonomic composition, as well as the partitioning of diversity, in Lamar Member paleocommunities documents the displacement of previously incumbent taxa with new dominant taxa (including two invaders from outside the basin): Taxa are able to disperse into communities because incumbent taxa are not barriers to immigration. This study reveals that a fundamental change occurred in the relative importance of processes (e.g., dispersal and competition) controlling the diversity of paleocommunities across the region.