Chihuahuan Desert
Chihuahuan Desert (outlined in yellow) from satellite. Source: NASA
The Chihuahuan Desert encompasses one of the most biologically diverse arid regions on Earth. This ecoregion extends from within the United States south into Mexico. This desert is unique, as it has been sheltered from the influence of other arid regions such as the Sonoran Desert by the large mountain ranges of the Sierra Madres. This isolation has allowed the evolution of many endemic species; most notable is the high number of endemic plants; in fact, there are a total of 653 vertebrate taxa recorded in the Chihuahuan Desert. Moreover, this ecoregion also sustains some of the last extant populations of Mexican Prairie Dog, wild American Bison and Pronghorn Antelope.
Severe conservation pressures are on the Chihuahuan Desert. On the Mexico side, there are destructive activities from illegal poaching, habitat destruction from illegal drug trafficking and large scale movements of illegal immigration. On the USA side of the border there is widespread habitat destruction from encampments of illegal immigrants, as well as wildlife poaching and accumulation of large amounts of uncollected refuse. Until the late twentieth century much of this damage on the USA side was mitigated by the presence of hikers, campers and ranchers conducting routine recreational and commercial activities in southern Texas, New Mexico and southeast Arizona. However, presently park rangers in the USA discourage recreational use of considerable expanses of federal and state lands, due to the dangers posed from drug trafficking from Mexican sources as well as extensive use of the borderlands for unauthorized encampments and trail use associated with illegal immigration.
Location and general depiction
Near Coahuila, Mexico. (Photograph by Edward Parker WWF) The Chihuahuan Desert stretches from the southeastern corner of Arizona across southern New Mexico and west Texas to the Edwards Plateau in the USA. 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. Some of the northern parts of the Chihuahuan Desert are drained by the Gila River.
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 1100 to 1500 meters (m). Due to its generally higher elevation, the Chihuahuan Desert is cooler; the mean annual temperature is 18.6 Co. 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 [[ecoregion]s], with precipitation ranging from 150 to 400 millimeters (mm).
The majority of the region is composed of sedimentary rocks of marine origin, although some portions of the mountains are of igneous origin. The region is influenced hydrologically by four river basin systems: the Rio Bravo (Río Grande) that establishes the political boundary between Mexico and the USA; the Río Casas Grandes, the Mapimí Bolsón and the Mayrán. Many small and medium-sized rivers serve as tribitaries within these catchment basins, but only a few transport significant volumes 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, L. tridentata can occur in association with other species. More generally, an association between L. tridentata, American Tarbush (Flourensia cernua) and Viscid Acacia (Acacia neovernicosa) dominates the northernmost portion of the Chihuahuan Desert. The meridional portion is abundant in Yucca and Opuntia, and the southernmost portion is inhabited by Mexican Fire-barrel Cactus (Ferocactus pilosus) and Mojave Mound Cactus (Echinocereus polyacanthus). Herbaceous elements such as Gypsum Grama (Chondrosum ramosa), Blue Grama (Bouteloua gracilis) and Hairy Grama (Chondrosum hirsuta), among others, become dominant near the Sierra Madre Occidental. In western Coahuila State, Lecheguilla Agave (Agave lechuguilla), Honey Mesquite (Prosopis glandulosa), Purple Prickly-pear (Opuntia macrocentra) and Rainbow Cactus (Echinocereus pectinatus) are the dominant vascular plants.
The southeastern portion of the desert scrub is gradually transformed from a strict Larrea-Flourensia scrub to one in whichOcotillo (Fouquieria splendens), St. Peter's Palm (Yucca filifera),Lecheguilla Agave (Agave lechuguilla) and Cordia boissieri are the most prevalent species. In areas with shallow soils of good drainage, a scrub dominated by Candelilla Wax (Euphorbia antisyphilitica),Vachellia glandulifera and Mimosa zygophylla develops. Near the Sierra Madre Oriental, the scrub becomes a strict combination of Agave victoria-reginae with Lechuguilla Agave, Guapilla (Hechtia glomerata), Barreta (Helietta parvifolia), Sotol (Dasylirion spp.) and a well-developed herbaceous stratum composed of gramineae, leguminosae and cacti. The grasslands, about twenty percent of the desert cover, are often mosaics of grass and scrub. They include Side-oats Grama (Bouteloua curtipendula), Black Grama (Chondrosum eriopodum), and Purple Three-awn Grass (Aristida purpurea). Bottomlands of Tobosa (Hilaria mutica) and Big Alkalai Sacaton (Sporobolus wrightii) were probably the grasses early Spanish explorers encountered when they reported clump-grasses that were "belly high to a horse".
Biodiversity characteristics
Mexican prarie dog (Cynomys mexicanus) in the Chihuahua Desert, Mexico. (Photograph by Colby Loucks) The Chihuahuan Desert is one of the three most biologically rich and diverse desert [[ecoregion]s] in the world, rivaled only by the Great Sandy Tanmi Desert of Australia and the Namib-Karoo of southern Africa. Approximately 3500 plant species live in the Chihuahuan Desert, and estimates of endemism state that there could be up to 1000 endemic species (29 percent), and at least sixteen endemic plant genera. 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 a 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 nineteen species of the family Heliantheae can be found. 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. The easternmost region of the desert (Coahuila, Nuevo León and San Luis Potosí states in Mexico) 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.
The ecoregion is widely recognized for its diversity and high levels of endemism in Cactaceae. Moreover, it also contains the largest assemblage of endangered cacti in America. Perhaps one-fifth of all the world’s cacti - as many as 350 of the 1500 known species - occur in the Chihuahuan Desert. Two cacti genera, Coryphanta and Opuntia, are among the five most speciose in the entire flora. There are numerous special status taxa that are found in the Chihuahuan Desert, denoted as Near Threatened (NT), Vulnerable (VU), Endangered (EN), or Critically Endangered (CR).
Mammals
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), Robust Cottontail (Sylvilagus robustus EN); Mule Deer (Odocoileus hemionus), Grey Fox (Unocyon cineroargentinus), Jaguar (Panthera onca), Collared Peccary or Javelina (Pecari tajacu), Desert Cottontail (Sylvilagus auduboni), Black-tailed Jackrabbit (Lepus californicus), Kangaroo Rats (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 highly endangered taxa that inhabits this desert. 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 Black-tailed Prairie Dog (Cynomys ludovicianus).
Birdlife
Common bird species include the Greater Roadrunner (Geococcyx californianus), Burrowing Owl (Athene cunicularia), Merlin (Falco columbarius), Red-tailed Hawk (Buteo jamaicensis), and the rare Zone-tailed Hawk (Buteo albonotatus). Geococcyx californianus), Curve-billed Thrasher (Toxostoma curvirostra), Scaled Quail (Callipepla squamata), Scott’s Oriole (Icterus parisorum), Black-throated Sparrow (Amphispiza bilineata), Phainopepla (Phainopepla nitens), Worthen’s Sparrow (Spizella wortheni), and Cactus Wren (Campylorhynchus brunneicapillus). In addition, numerous raptors inhabit the Chihuahuan Desert and include the Great Horned Owl (Bubo virginianus) and the Elf Owl (Micrathene whitneyi).
Reptiles
The Chihuahuan Desert herpetofauna typifies this ecoregion.Several lizard species are centered in the Chihuahuan Desert, and include the Texas Horned Lizard (Phrynosoma cornutum); Texas Banded Gecko (Coleonyx brevis), often found under rocks in limestone foothills; Reticulate Gecko (C. reticulatus); Greater Earless Lizard (Cophosaurus texanus); several species of spiny lizards (Scelopoprus spp.); and the Western Marbled Whiptail (Cnemidophorus tigris marmoratus). Two other whiptails, the New Mexico Whiptail (C. neomexicanus) and the Common Checkered Whiptail (C. tesselatus) occur as all-female parthenogenic clone populations in select disturbed habitats.
Representative snakes include the Trans-Pecos Rat Snake (Bogertophis subocularis), Texas Blackhead Snake (Tantilla atriceps), and Sr (Masticophis taeniatus) and Neotropical Whipsnake (M. flagellum lineatus). Endemic turtles include the Bolsón Tortoise (Gopherus flavomarginatus), Coahuilan Box Turtle (Terrapene coahuila) and several species of softshell turtles. Some reptiles and amphibians restricted to the Madrean sky island habitats include the Ridgenose Rattlesnake (Crotalus willardi), Twin-spotted Rattlesnake (C. pricei), Northern Cat-eyed Snake (Leptodeira septentrionalis), Yarrow’s Spiny Lizard (Sceloporus jarrovii), and Canyon Spotted Whiptail (Cnemidophorus burti).
Mexican Treefrog. Source: Todd Pierson/ CalPhotos/ EoL Amphibians
There are thirty anuran species occurring in the Chihuahuan Desert: Chiricahua Leopard Frog (Rana chircahuaensis); Red Spotted Toad (Anaxyrus punctatus); American Bullfrog (Lithobates catesbeianus); Canyon Treefrog (Hyla arenicolor); Northern Cricket Frog (Acris crepitans); Rio Grande Chirping Frog (Eleutherodactylus cystignathoides); Cliff Chirping Frog (Eleutherodactylus marnockii); Spotted Chirping Frog (Eleutherodactylus guttilatus); Tarahumara Barking Frog (Craugastor tarahumaraensis); Mexican Treefrog (Smilisca baudinii); Madrean Treefrog (Hyla eximia); Montezuma Leopard Frog (Lithobates montezumae); Brown's Leopard Frog (Lithobates brownorum); Yavapai Leopard Frog (Lithobates yavapaiensis); Western Barking Frog (Craugastor augusti); Mexican Cascade Frog (Lithobates pustulosus); Lowland Burrowing Frog (Smilisca fodiens); New Mexico Spadefoot (Spea multiplicata); Plains Spadefoot (Spea bombifrons); Pine Toad (Incilius occidentalis); Woodhouse's Toad (Anaxyrus woodhousii); Couch's Spadefoot Toad (Scaphiopus couchii); Plateau Toad (Anaxyrus compactilis); Texas Toad (Anaxyrus speciosus); Dwarf Toad (Incilius canaliferus); Great Plains Narrowmouth Toad (Gastrophryne olivacea); Great Plains Toad (Anaxyrus cognatus); Eastern Green Toad (Anaxyrus debilis); Gulf Coast Toad (Incilius valliceps); and Longfoot Chirping Toad (Eleutherodactylus longipes VU). The sole salamander occurring in the Chihuahuan Desert is the Tiger Salamander (Ambystoma tigrinum).
Arthropods
The ecoregion is also an important center of insect endemism, constituting one of the top three such locales in Mexico, and it contains the highest diversity of bees in Mexico.
Ecological status
Agave species near Torreon in the Chihuahua Desert, Mexico. (Photograph by Edward Parker WWF) 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 [[soil]s] are those occupied by Yucca filifera and Algaroba mesquite (Prosopis juliflora), because these plants have the desired cultivable characteristics (e.g. deep soils that retain water). 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 dramatic alteration of native grasses.
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.
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 ha). 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.
Threat profile
The major conversion threats are urbanization, agricultural expansion, illegal immigration impacts and Mexican narcotics trafficking. Urban and suburban expansion around Cruces, New Mexico; El Paso, Texas; and other cities is threatening surrounding areas. Degradation and desertification threats include increasing off-road vehicle use in some areas, rubbish accumulation from illegal immigration encampments, invasions of alien 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; USA federal policies of offering below market grazing leases exacerbates the overgrazing threats. 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 ornamental species of desert plants are a threat for the region; accelerated loss of habitat through intensive human trampling is also reducing cacti populations and fragile cryptic surface cover.
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 increased 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.
References
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