Fish Assemblages and Declining Water Quantity and Quality in the Rio Grande Basin, West Texas: How Low Can You Go?

Desert aquatic ecosystems around the world are declining in water quantity, quality and native species diversity.   Fish assemblages in these ecosystems are increasingly becoming homogenized, and these changes interact with historical patterns of species diversity and environmental change to determine resulting patterns.  We examined the spatial scale of similarity for fish assemblages in the highly modified Rio Grande basin in the Chihuahuan desert of West Texas using a well-documented historical data set and recent collections that provided a paired analytical design across 29 localities spanning approximately 30 years of time.  Results indicated that homogenization and differentiation occurred, but these patterns depended on the spatial resolution of comparison.  The greatest changes were found for between stream reach comparisons within a sub-basin.  The Rio Grande sub-basin showed strong convergence between upstream and downstream reaches that corresponded to declining water quality and quantity from the Rio Conchos in Mexico.  The Pecos River sub-basin showed strong differentiation between upstream and downstream reaches.  Assemblage similarities within specific river reaches of each sub-basin were variable in their degree of historical change, which was largely dependent on anthropogenic modification to the flow regime change and variability in the success of invasive species.