Location and General Description
The Chihuahuan Desert stretches from the southeastern corner of Arizona across southern New Mexico and west Texas to the Edwards Plateau in the United States. It runs deep into central Mexico, including parts of the states of Chihuahua, northwest Coahuila, northeast Durango and several others. This Desert is bounded by the Sierra Madre Occidental to the west and the Sierra Madre Oriental to the east, extending as far south as San Luis Potosi and to disjunct islands of the Chihuahuan vegetation in the states of Queretaro and Hidalgo.
The region contains a series of basins and ranges with a central highland extending from Socorro, New Mexico south into Zacatecas, Mexico. Most sites are located at elevations from 1,100 to 1,500 m (Davis et al. 1997). Due to its generally higher elevation, the Chihuahuan Desert is cooler; the mean annual temperature is 18.6 oC. The hottest temperatures occur in areas of low elevations and in the inter-montane depressions in the region. The climate includes a dry summer and occasional winter rains; mild frosts occur during autumn and winter. This Desert has more rainfall than other warm desert ecoregions, with precipitation ranging from 150 to 400 mm (Davis et al. 1997).
The majority of the region is composed of sedimentary rocks of marine origin, although some portions of the mountains are of igneous origin (Ferrusquía-Villafranca 1993). The region is influenced hydrologically by four river basin systems (Marroquín et al. 1981): the Rio Bravo (Río Grande) that establishes the political boundary between Mexico and the U.S.; the Río Casas Grandes, the Mapimí Bolsón and the Mayrán. Many small and medium-sized rivers cross these basins, but only a few of them supply significant amounts of water.
The dominant plant species throughout the Chihuahuan desert is creosote bush (Larrea tridentata). Depending on diverse factors such as type of soil, altitude, and degree of slope, Larrea tridentata can be found in association with other species. More generally, an association between Larrea, tarbush (Florensia cernua) and viscid acacia (Acacia neovernicosa) dominates the northernmost portion of the desert. The meridional portion is abundant in Yucca and Opuntia, and the southernmost portion is inhabited by Mexican fire-barrel cactus (Ferocactus pringlei) and Arizona rainbow cactus (Echinocereus polyacanthus). Herbaceous elements such as Bouteloua breviseta, Bouteloua gracilis and B. hirsuta, among others, become dominant near the Sierra Madre Occidental. In western Coahuila State, lechuguilla (Agave lechuguilla), Prosopis glandulosa and Opuntia macrocentra and Echinocereus pectinatus are the dominant species. The southeastern portion of the desert scrub is gradually transformed from a strict Larrea-Flourensia scrub to one in which Fouquieria splendens, Yucca filifera, Agave lechuguilla and Cordia boissieri are the most common elements. In areas with shallow soils of good drainage, a scrub dominated by candelilla (Euphorbia antisyphilitica), Agave lechuguilla, Acacia glandulifera and Mimosa zygophylla develops. Near the Sierra Madre Oriental, the scrub becomes a strict combination of Agave victoria-reginae with lechuguilla, guapilla (Hechtia glomerata), barreta (Helietta parviflora), sotol (Dasylirion sp.) and a well-developed herbaceous stratum composed of gramineae, leguminosae and cacti. The grasslands, 20% of this desert, are often mosaics of grass and shrurb. They include side-oats grama (Bouteloua curtipendula), black grama (Bouteloua eriopoda), and purple three-awn (Aristida purpurea). Bottomlands of tobosa (Hilaria mutica) and big alkalai sacaton (Sporobolus wrightii) were probably the species early Spanish explorers encountered when they reported grasses that were "belly high to a horse" (Tweit 1995).
The Chihuahuan desert is one of the three most biologically rich and diverse desert ecoregions in the world, rivaled only by the Great Sandy Tanmi Desert of Australia and the Namib-Karoo of southern Africa (Olson and Dinerstein 1998). Approximately 3,500 plant species live in this desert, and estimates of endemism state that there could be up to 1000 endemic species (29%), and at least 16 endemic plant genera (Toledo 1988). The high degree of local endemism is the result of the isolating effects of complex basin and range physiography, and dynamic changes in climate over the last 10,000 years. Another contributing factor to endemism is the colonization of these inhospitable habitats by specialists species with restricted ranges. Taxa with high level of local endemism include cacti, butterflies, spiders, scorpions, ants, lizards and snakes.
In central Coahuila, near the town of Cuatro Ciénegas, five of the 19 species of Heliantheae can be found (Villaseñor 1991). This small area contains a high number of endemic plant species found in desert scrub and gypsum dune lowlands, and is also considered one of the world’s richest foci for locally endemic cacti. According to Rzedowski (1991), the easternmost region of the desert (Coahuila, Nuevo León and San Luis Potosí states) once constituted a refuge from severe climatic changes that occurred in the past, thus facilitating speciation processes in many groups of animals and plants. The desert is considered a center for the radiation and speciation of the members of the family Lamiaceae (Ramamoorthy & Elliot 1993).
The ecoregion is widely recognized for its diversity and high levels of endemism in Cactaceae. Unfortunately, it also contains the largest assemblage of endangered cacti in America (Oldfield 1997). Perhaps one-fifth of all the world’s cacti - as many as 350 of the 1,500 known species - occur in this desert. Two cacti genera, Coryphanta and Opuntia, are among the five most speciose in the entire flora.
Because of its recent origin, few warm-blooded vertebrates are restricted to the
Chihuahuan Desert scrub. However, the Chihuahuan desert supports a large number of wide-ranging mammals, such as the pronghorn antelope (Antilocapra americana), mule deer (Odocoileus hemionus), grey fox (Unocyon cineroargentinus), jaguar (Panthera onca), collared peccary or javelina (Pecari tajacu), desert cottontail (Sylvilagus auduboni), black tailed jack rabbit (Lepus californicus), kangaroo rat (Dipodomys sp.), pocket mice (Perognathus spp.), woodrats (Neotoma spp.) and deer mice (Peromyscus spp.). With only 24 individuals recorded in the state of Chihuahua Antilocapra americana is one of the most endangered species that inhabits this desert (González-Romero & Lafón-Terrazas 1993). The ecoregion also contains a small wild population of the highly endangered American bison (Bison bison) and scattered populations of the highly endangered Mexican prairie dog (Cynomys mexicanus), as well as the common prairie dog (Cynomys ludovicianus).
Common bird species include the greater roadrunner (Geococcyx californianus), curve-billed thrasher (Toxostoma curvirostra), scaled quail (Callipepla squamata), Scott’s oriole (Icterus parisorum), black-throated sparrow (Amphispeza bilineata), phainopepla (Phainopepla nitens), Worthen’s sparrow (Spizella wortheni), and cactus wren (Campylorhynchus brunneicapillus). In addition, numerous raptors inhabit the desert and include the great horned owl (Bubo virginianus), elf owl (Micrathene whitneyi), burrowing owl (Athene cunicularis), Aplomado falcon (Falco columbarius), red-tailed hawk (Buteo jamaicensis), and the rare zone-tailed hawk (Buteo albonotatus).
The Chihuahuan Desert herpetofauna is more strongly associated with the region. Several lizards are centered in the Chihuahuan Desert, and include the Texas horned lizard (Phrynosoma cornutum), Texas banded gecko (Coleonyx brevis), reticulated gecko (C. reticulatus), greater earless lizard (Cophosaurus texanus), several species of spiny lizards (Scelpoprus spp.), and marbled whiptails (Cnemidophorus tigris marmoratus). Two other whiptails (C. neomexicanus and C. tesselatus) occur as all-female parthenogenic clones in select disturbed habitats (Wright and Lowe 1968). Representative snakes include the Trans-Pecos ratsnake (Elaphe subocularis), Texas blackheaded snake (Tantilla atriceps), and whipsnakes (Masticophis taeniatus and M. flagellum lineatus) (Brown 1994). Endemic turtles include the Bolsón desert turtle (Gopherus flavomarginatus), Coahuilan box turtle (Terrapene coahuila) and several softshells turtles. Some reptiles and amphibians restricted to sky island habitats include the ridge-nosed rattlesnake (Crotalus willardi), twin-spot rattlesnake (C. pricei), cat-eyed snake (Leptodeira septentrioalis), Yarrow’s spiny lizard (Sceloporus jarrovii), Chricahua elopard frog (Rana chircahuaensis), and canyon spotted whiptail (Cneidophorus burti) (Dinerstein et al. in prep.).
The ecoregion is also an important center of insect endemism, constituting one of only three in Mexico (Challenger 1998), and it contains the highest diversity of bees in Mexico (Ayala-Barajas & Griswold 1988).
The Chihuahuan Desert has been altered by human activities over the last centuries. Vast portions of the Chihuahuan desert have been transformed into secondary and successional vegetation. Agricultural activities constitute the strongest pressure on the native plant communities. The preferred soils are those occupied by Yucca filifera and mesquite (Prosopis juliflora), because they possess the desired cultivable characteristics (e.g. deep soils that retain water) (Marroquín et al. 1981). Changes in grazing and fire regimes, and depletion and diversion of water sources have also affected the natural vegetation. The heavily grazed areas in all the ecoregion are characterized by increasing dominance of creosote bush, mesquite, tarbush, acacia, and drastic alteration of native grasses (Brown 1995).
Due to habitat loss, large vertebrates, particularly in lowland habitats, are now rare and isolated. Brown bears, wolves, bison, pronghorn, and large cats have almost been eliminated from the region. The loss of riparian habitats and water sources has also affected terrestrial vertebrates and invertebrates dependent on water (Dinerstein et al. in prep.).
The Chihuahua ecoregion as a whole suffers from lack of protection. Some protected areas include Big Bend National Park (286,572 ha), Guadalupe Mountains National Park (30,867 ha), Rio Grande Wild and Scenic River Complex (3,885 ha), Bosque del Apache Wildlife Refuge (24,144 ha), White Sands National Monument (58,614 ha), and Carlsbad Caverns (18,921). Relatively intact habitats are rare and are mainly found in montane areas, inaccessible slopes, gypsum dunes and saline playas. CONABIO has identified the following terrestrial priority sites within this ecoregion: El Berrendo, Laguna Jaco, Mapimí, Cuchillas de la Zarca, Sierra La Fragua, Cuatrociénegas, Sierra de La Madera, Sierra del Nido-Pastizal de Flores Magón, Médanos de Samalayuca, Cañón de Santa Elena, Bavispe-El Tigre, Sierra de San Luis-Janos, and Cananea-San Pedro. A number of important areas for bird conservation has been identified in this ecoregion including the Sistema de Islas Sierra Madre Occidental, Janos-Nuevo Casas Grandes, Mesa de Guacamayas, Baserac-Sierra Tabaco-Rio Bavispe, Sierra del Nido, Babícora, Laguna de Mexicanos, Laguna de Bustillos, Laguna de Jaco, and Mapimí, to name a few. However, these sites do not receive formal protection unless their boundaries fall within a designated protected area such as a reserve or park.
Types and Severity of Threats
The major conversion threats are urbanization, agricultural expansion, and resource extraction. Urban and suburban expansion around Cruces, New Mexico; El Paso, Texas; and other cities is threatening surrounding areas. Degradation threats include increasing off-road vehicle use in some areas, invasions of non-native species, and increasing dominance of native shrub species in areas historically characterized by open grasslands. Cattle farming threatens the fragile and diverse scrub associations that are still present in the desert. This is most extensive in the Chihuahuan desert, but also occurs in the central plateau and in the Tamaulipan matorral. Candelilla (Euphorbia antisyphilitica), nopal (Opuntia spp.), lechuguilla (Agave lechuguilla) and palma (Yucca spp.) are the most exploited species.
Wildlife and exotic plant extraction, and many human activities in general have reduced the populations of some vertebrates. The American black bear (Ursus americanus) is widely hunted for its fur and body parts; however, there is still a chance for these populations to recover. Illegal trade of cacti and other exotic species of desert plants are a threat for the region; accelerated loss of habitat is also reducing cacti populations.
Threats to this ecoregion are also related to water resources. Wetland and riparian areas suffer from water loss and water reduction from irrigation and livestock. Water pollution in the Rio Grande has increase due to the growth of El Paso-Ciudad Juarez metropolitan area. Over-pumping of groundwater for agriculture and use by growing urban areas is affecting the flow of Chihuahuan rivers, including the San Pedro, Pecos, Río Grande, Río Conchos, Río Extorax, and Río Aguanaval.
Justification of Ecoregion Delineation
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Brown, D. E., editor. 1994. Biotic communities: Southwestern United States and northwestern Mexico. University of Utah Press, Salt Lake City.
Brown, J. H. 1995. Macroecology. University of Chicago Press, Chicago.
Challenger, A. 1998. Utilización y conservación de los ecosistemas terrestres de México. Pasado, presente y futuro. Conabio, IBUNAM y Agrupación Sierra Madre, México.
Davis, S. D., Heywood, V. H., Herrera-MacBryde, O., Villa-Lobos, J. and A.C. Hamilton, editors. 1997. Centres of Plant Diversity. Vol. 3 The Americas. WWF, IUCN, Oxford, U.K.
Dinerstein, E., Olson, D., Atchley, J., Loucks, C., Contreras-Balderas, S., Abell, R., Iñigo, E., Enkerlin E., Williams, C. and G. Castilleja. 2000. (In preparation). Ecoregion-Based Conservation in the Chihuahuan Desert: A Biological Assessment.
Ferrusquía-Villafranca, I. 1993. Geología de México: Una Sinopsis. Pages 3-108 in T.P. Ramamoorthy, R. Bye, A.Lot, y J. Fa (editors). Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM, Mexico.
González-Romero, A., y A. Lafón-Terrazas. 1993. Distribución y estado actual del berrendo (Antilocapra americana). R. A. Medellín, y G. Ceballos (editors). Avances en el estudio de los mamíferos de México. Publicaciones Especiales Vol. 1. Asociación Mexicana de Mastozoología, A.C. México, D. F.
Marroquín, J. S., G. Borja, R. Velázquez, y J. A. de la Cruz. 1981. Estudio ecológico y dasonómico de las zonas áridas del norte de México, Publicación Especial 2. Instituto Nacional de Investigaciones Forestales, SARH., México.
Oldfield, S. (Comp). 1997. Cactus and succulent plants: Status survey and conservation action plan. IUCN, Cambridge, U.K.
Olson, D. M. and E. Dinerstein. 1998. The Global 200: A representation approach to conserving the earth's most biologically valuable ecoregions. Conservation Biology 3:502-512.
Ramamoorthy, T. P., y M. Elliott. 1993. Lamiaceae de México: diversidad, distribución, endemismo y evolución. T. P. Ramamoorthy, R. Bye, A. Lot, y J. Fa , editors. Diversidad Orígenes y Distribución. Instituto de Biología, UNAM, Mexico.
Rzedowski, J. 1991. El endemismo en la flora fanerógamica mexicana: una apreciación analítica preliminar. Acta Botánica Mexicana 15: 47-64.
Toledo, V. M. 1988. La diversidad biológica de México. Ciencia y Desarrollo XIV(81): 17-30.
Tweit, S. J. 1995. Barren, wild and worthless: Living in the Chihuahuan Desert. University of New Mexico Press, Albuquerque.
Villaseñor, J. L. 1991. Las Heliantheae endémicas a México: una guía hacia la conservación. Acta Bot. Méx. 15: 29-46.
Prepared by: C. Williams, D. Olson, Alejandra Valero, Tom Allnutt, Jan Schipper, and Christine Burdette
Reviewed by: In process