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# of Endemic Species
Major Habitat Type:
xeric freshwaters and endorheic (closed) basins
Text modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC, USA.
This ecoregion, part of the Rio Grande complex, extends from east-central and southeastern New Mexico into western Texas.
Drainages flowing into:
The Pecos joins the lower Rio Grande to flow into the Gulf of Mexico (Smith & Miller 1986).
Main rivers or other water bodies:
The major river in this ecoregion is the Pecos, and its drainage area defines the extent of the ecoregion. Major tributaries to the Pecos, virtually all of which arise in the mountains, include the North Seven River, Rio Penasco, Rio Felix, Rio Hondo and its tributary Rio Bonito, Ciénega del macho, and Alamosa Creek. Five major reservoirs have been constructed on the Pecos: Los Esteros Reservoir, Lake Sumner, Lake McMillan, and Avalon Reservoir in New Mexico, and Red Bluff Lake in Texas. Amistad Reservoir, built on the Rio Grande, inundates 10 to 20 km of the Pecos River at its confluence with the Rio Grande (Williams et al. 1985).
The topography of the Pecos ecoregion varies greatly, ranging from 3700 m in the Sangre de Cristo Mountains in north-central Mexico to 366 m at the confluence with the Rio Grande in southwestern Texas (Williams et al. 1985; Minckley et al. 1986). Late Tertiary alluvium blankets the eastern side of the New Mexican Mountains and High Plains on either side of the Pecos River valley (Smith & Miller 1986).
The climate of this ecoregion is predominantly semi-arid and arid (Köppen 1936).
The river contains a variety of habitats, including high-gradient, rocky-bottomed areas; sluggish, soft-bottomed meanders; and rocky riffles.
Most of the southern and central part of the ecoregion is dominated by the dry grasslands of the Chihuahuan Desert, with species such as bush muhly Muhlenbergia porteri), blue grama (Bouteloua gracilis), and big sacaton (Sporobolus wrightii) (McClaran 1995). Along the eastern edge these grasslands transition to the Western Short Grasslands ecoregion, dominated by grama and buffalo grass (Buchloe dactyloides). At higher elevations in the north and west, the grasslands are replaced by Colorado Plateau shrublands, Arizona Mountains forests, and Colorado Rockies forests (Ricketts et al. 1999).
The Pecos ecoregion is relatively species-rich compared to the Upper Rio Grande  and the other Rio Grande tributaries, particularly in terms of fish, with over fifty native species. Of these, none are endemics. However, a handful are near-endemic with the lower Rio Grande . These include the proserpine shiner (Cyprinella proserpina), found only from the lower Pecos River and a few small localities in the lower Rio Grande ; Leon Springs pupfish (Cyprinodon bovinus), known only from Leon Creek and its tributary, Diamond Y Spring, in Pecos County, Texas (Williams et al. 1985; Page & Burr 1991); the Comanche Springs pupfish (C. elegans), which originally inhabited just two isolated spring systems in southwestern Texas; the Pecos River pupfish (C. pecosensis), endemic to the Pecos River and associated springs and gypsum sinkholes; the Pecos gambusia (Gambusia nobilis), whose native habitat consisted of four small springfed systems, two in New Mexico and two in Texas; and the greenthroat darter (Etheostoma lepidum), found in the Pecos system and also on the Edwards Plateau of Texas (in ecoregions  and ). Additionally, the Pecos River is home to the Pecos bluntnose shiner (Notropis simus pecosensis), the only extant form of N. simus.
Description of endemic fishes:
There are no strict endemic species in this ecoregion.
Other noteworthy aquatic biotic elements:
At the species level, there are no endemic unionid mussels, crayfish, amphibians, or aquatic reptiles in this ecoregion, though there are a number of aquatic snails and amphipods found only in the ecoregion’s spring-fed systems. Some of these, notable for their limited distributions, are the Pecos assiminea (Assiminea sp.), native to the Diamond Y Spring complex; the Phantom Spring cochliopa (Cochliopa texana); the Roswell springsnail (Fontelicella sp.); the Blue Spring springsnail (Fontelicella sp.); the Phantom Lake tryonia (Tryonia cheatumi); the Roswell tryonia (Tyronia. sp.); the Diamond Y tryonia (Tyronia. sp.); the Phantom Spring amphipod (Gammarus hyalelloides); the Pecos amphipod (Gammarus pecos); the San Solomon amphipod (Gammarus sp.); and Noel’s amphipod (Gammarus desperatus), endemic to springs along the Pecos River (Williams et al. 1985).
Justification for delineation:
Ecoregion boundaries are taken from Abell et al. (2000) and are based on subregions defined by Maxwell et al. (1995).
Abell, R. A., Olson, D. M., et al. (2000). "Freshwater Ecoregions of North America: A Conservation Assessment" Washington, DC, USA: Island Press.
Minckley, W.L., D. A. Hendrickson, and C. E. Bond (1986)"Geography of western North America fishes: description and relationships to intracontinental tectonism" In Hocutt, C.H.a.E.O.W. (Ed.). The zoogeography of North American freshwater fishes. New York: John Wiley & Sons.
Page, L. M., and B. M. Burr (1991). "A field guide to freshwater fishes: North America north of Mexico" New York, NY: Houghton Mifflin Co..
Ricketts, Taylor H. Dinerstein Eric Olson David M. Loucks Colby J. (1999). "Terrestrial ecoregions of North America: A conservation assessment" Washington, D.C.: World Wildlife Fund.
Smith, M. L.,Miller, R. R. (1986)"The evolution of the Rio Grand Basin as inferred from its fish fauna" In Hocutt, C.H.;Wiley, E.O. (Ed.). The zoogeography of North American freshwater fishes. (pp. 457-485) New York, New York, USA: Wiley.
Williams, J. E., Bowman, D. B., et al. (1985). "Endangered aquatic ecosystems in North American deserts with a list of vanishing fishes of the region" Journal of the Arizona-Nevada Academy of Sciences 20 1-62.