Assembling the Pecos River fish fauna: barrier displacement on the Southern Great Plains, North America

Barrier displacement by river capture is an important mechanism for the assembly of freshwater fish faunas. The production of increasingly comprehensive and rigorously dated phylogentic trees for major clades of fishes, along with improved resolution in historical geomorphology, provide an unprecedented opportunity to develop thorough biogeographical scenarios of faunal assembly that synthesise existing knowledge and provide detailed context for future study. The Pecos River of southwestern North America is a textbook example of drainage formation by river capture and provides a straightforward case of freshwater fish faunal assembly by river capture. Fishes ultimately confined to the middle section of the Pecos River (Capitan area of endemism) have their closest relatives in the Brazos, Colorado (Texas), and Red rivers, which served as ancient dispersal corridors from the Mississippi River drainage. The Capitan area of endemism developed in association with two dissolution basins that, in the Late Miocene, captured headwaters of these rivers. In the Late Pliocene or Early Pleistocene, the endorheic middle Pecos River (Capitan area of endemism) was captured by or overflowed into a tributary to the Río Grande, which became the lower Pecos River. The nascent lower Pecos River also harboured an endemic-fish assemblage as part of a nexus of springfed rivers (ancestral Río Grande, Devils River, lower Pecos River) that comprised the Devils area of endemism. Even after a through-flowing Pecos River linked the Capitan and Devils areas of endemism, many endemic species remained only within their original area of endemism, giving the Pecos River a composite fish fauna. The Río Grande connection later allowed fishes dispersing along the Gulf of Mexico coast, aided by Late Pleistocene sea-level falls and glacial outbursts, not only to populate the Río Grande, but also to disperse up the Pecos River. Incision of the lower Río Grande valley and uplift in the Sangre de Cristo Mountains empowered the Pecos River to capture headwater streams from the adjacent South Canadian River and middle Río Grande, bringing in additional fishes. More recently, humans introduced at least 50 species to the drainage, while human impacts fragmented the native fauna. Non-native versus native status remains uncertain for several species (e.g. Miniellus stramineus) and multiple lineages of diverse origin may exist for some widespread, polytypic species like Cyprinella lutrensis and Pimephales promelas. The composite fish fauna of the Pecos River is a biogeographical anomaly explainable by the complex geomorphological history that produced it. As such, it provides a unique opportunity for studies of fish-assemblage evolutionary ecology. Also, its historical association with neighbouring drainages helps clarify their biogeography (as detailed here). Further broadening of this synthesis could support biogeographical scenarios at large spatial scales, illustrating the potential that now exists for reconstructing regional river-drainage faunas.