Patterns of Endemism and Species Richness in Fishes of the Western Gulf Slope

Taxa richness and endemism among animals are influenced by current environmental conditions as well as ancestral biotic conditions.  We tested patterns in taxa richness and endemism among Western Gulf Slope (WGS) fishes against water availability gradients and their ancestral origins to assess conformity of species diversity among Texas drainages.  A total of 155 species of native fishes was plotted among 196 sub-basins of the WGS.  Taxa richness was highest in eastern WGS and lowest in the western WGS of Texas, closely conforming to a rainfall/productivity gradient.  Numbers of endemic fishes, however, were highest in the western WGS and lowest in the eastern WGS.  Three regions of note are the Brazos Basin, which exhibited a pronounced increase in endemism compared to adjacent Eastern drainages; the Guadalupe Basin, which contained the greatest number of endemic taxa in the Edwards Plateau; and the Rio Grande and Pecos Basins, which contained the greatest numbers of endemic taxa in the WGS.  Widespread fishes exhibiting ranges within as well as beyond the WGS were primarily of eastern ancestral origin, while endemic species largely evolved from ancestors north or southwest of the WGS.  This study found current and historical environmental conditions, ancestral biological conditions, time of residence, isolation, dispersal, and extirpation, all played significant roles in determining current native distributions of WGS fishes.