Ecoregion Description

131: Gila

Major Habitat Type:

xeric freshwaters and endorheic (closed) basins

Author:

Text modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC, USA. Information also provided by Salvador Contreras Balderas.

Countries:

Mexico; United States

Boundaries:

This ecoregion covers most of southern Arizona, part of southwestern New Mexico, and extends into northern Sonora in Mexico.

Drainages flowing into:

The Gila drains into the Colorado River.

Main rivers or other water bodies:

The major watershed in this ecoregion is that of the Gila River, a tributary to the lower Colorado River. Other important rivers include the San Pedro, the Santa Cruz, and the Salt Rivers, all tributaries to the Gila.

Topography:

The Gila rises in the Black Range in New Mexico and passes through mountain ranges and valleys while following a general downward slope to the Colorado River. Elevations within the ecoregion range from less than 50 m in the southwest to peaks that rise over 3000 m in the northeast.

Climate:

The ecoregion experiences a mesic desert climate with upper altitudes that are cool and moist. Average annual rainfall ranges between 80 to 300 mm in the Sonoran and Chihuahuan deserts. Temperatures range from 10 to 24 ^oC in the lower altitudes (McNab & Avers 1994).  

Freshwater habitats:

The Gila River is one of the largest desert rivers in the world. Once perennial, it is now nearly dry due to irrigation and water diversion efforts.

Terrestrial Habitats:

The Gila drainage covers portions of the Sonoran and Chihuahuan deserts as well as the mountain ranges in central Arizona. Sonoran vegetation includes dense communities of creosote (Larrea divarecata) and white bursage (Ambrosia dermosa) at lower elevations, and palo verde (Cercidium floridum, C. microphyllum), saguaros (Carnegiea gigantia) and other cacti on slopes (Turner et al 1995). Characteristic Chihuahuan desert vegetation includes creosote, tarbush (Florensia cernua), mesquite (Prosopis articulata), and acacia (Acacia spp.), as well as pockets of Sierra Madre Occidental pine-oak forests. Higher elevations in the northern part of the ecoregion are characterized by Pinus species and mixed conifer communities (Ricketts et al. 1999).

Fish Fauna:

As one would expect, the Gila ecoregion shares many of its freshwater species with the Colorado [130], and the Gila and Colorado rivers, in particular, share many distinctive large-river species. However, whereas the other major tributaries to the Colorado, with the exception of the Virgin River, do not have distinct biotas at the species level, the Gila does. Out of roughly 30 native species found in the Gila, an impressive 20% are endemic.

Description of endemic fishes:

Endemic species to the ecoregion include the Santa Cruz pupfish (Cyprinodon arcuatus), Gila chub (Gila intermedia), spikedace (Meda fulgida), Gila trout (Oncorhynchus gilae)  and loach minnow (Rhinichthys cobitis). The woundfin (Plagopterus argentissimus) was historically found in seasonally hot and turbid habitats of the Gila River basin as well as the Virgin [129], but today is limited to the Virgin River system (Minckley et al. 1986; Sigler & Sigler 1994). Related to the woundfin is the spikedace (Meda fulgida), found only in fragmented warmwater reaches of the Gila River. The loach minnow (Rhinichthys  cobitis) is restricted to warmwater reaches of the Gila and San Francisco (a tributary to the Gila) river systems, though it was formerly found more widely in the Gila basin (Page & Burr 1991; Propst & Bestgen 1991). Both the woundfin and loach minnow are significant in that they represent monotypic genera (Robins et al. 1991). The Gila trout (Oncorhynchus gilae) was historically restricted to the headwaters of the Gila and San Francisco Rivers in the Gila National Forest, but has been translocated elsewhere in the ecoregion (Page & Burr 1991; Propst et al. 1992). The longfin dace (Agosia chrysogaster) and Gila topminnow (Poeciliopsis occidentalis) might each be considered near-endemic to the Gila ecoregion, though their ranges extend southward into the Sonoran [160] (Minckley et al. 1986; Page & Burr 1991). The Gila chub (Gila intermedia), whose taxonomy is uncertain, persists in one location in the ecoregion. Colorado squawfish (Ptychocheilus lucius) and razorback sucker (Xyrauchen texanus), endemic to the Colorado complex, may have once occurred in the Gila River, and roundtail chub (Gila robusta) still occurs in three separate reaches of the river.

Evolutionary phenomena:

The Gila ecoregion is noted for speciation in the genus Gila, as well as its higher taxonomic endemism in fish (Abell et al. 2000). The region contains three endemic or near-endemic fish genera (Meda, Plagopterus, and Rhinichthys).

Justification for delineation:

Ecoregion boundaries are taken from Abell et al. (2000) and are based on subregions defined by Maxwell et al. (1995).

Level of taxonomic exploration:

No recent collections known.

References/sources:

Abell, R., Olson, D., et al. (2000). "Freshwater ecoregions of North America" Washington, D.C.: Island Press.

McNab, W. H.,Avers, P. E. (1994) "Ecological subregions of the United States". U.S. Forest Service, ECOMAP Team, WO-WSA-5. Online. http://www.fs.fed.us/land/pubs/ecoregions/index.html..

Minckley, W. L. (2002)"Fishes of the lower most Colorado River, its delta, and estuary: a commentary on biotic change" In Lozano-Vilano, M. (Ed.). Libro Jubilar en Honor al Dr. Salvador Contreras Balderas. Dir. Gral. De Publicaciones. (pp. 63-78) Monterrey, México: Universidad A. de Nuevo León.

Minckley, W. L. (1969). "Enviroments of the Bolson of Cuatro Ciénegas, Coahuila, México, with special refeence to the aquatic biota" University of Texas at El Paso, Sci. Ser. 2 1-65.

Minckley, W. L., Hendrickson, D. A., et al. (1986)"Geography of western North American freshwater fishes: Description and relationships to intracontinental tectonism" In Hocutt, C.H.;Wiley, E.O. (Ed.). The zoogeography of North American freshwater fishes. New York: John Wiley.

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..

Propst, D. L., Stefferud, J. A., et al. (1992). "Conservation and status of Gila trout (Oncorhynchus gilae)" The Southwestern Naturalist 37 117-125.

Propst, D. L.,Bestgen, K. R. (1991). "Habitat and biology of the loach minnow, Tiaroga cobitis, in New Mexico" Copeia 1991 29-38.

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.

Robins, C. R., Bailey, R. M., et al. (1991) "Common and scientific names of fishes from the United States and Canada". Bethesda, Maryland, USA. American Fisheries Society.

Sigler, J. W.,Sigler, W. F. (1994)"Fishes of the Great Basin and Colorado Plateau: Past and present forms" In Harper, K.T.;St. Clair, L.L.;Thornes, K.H.;Hess, W.M. (Ed.). Natural history of the Colorado Plateau and Great Basin. Niwot: University of Colorado Press.

Turner, R.M., J.E. Bowers and T.L. Burgess (1995). "Sonoran desert plants: An ecological atlas" Tuscon, AZ: The University of Arizona Press.