Location and General Description
The Sonoran Desert in the United States reaches from extreme southeastern California across the western two-thirds of southern Arizona. In Mexico, the desert stretches south to encompass much of the state of Sonora, as well as the eastern shore of Baja California to the town of Loreto. The Colorado River passes through this ecoregion as do the Yaqui, Salt, Verde and Gila Rivers.
The highest area in the western part of the ecoregion, which reaches 1,206 m in elevation, was formed when intense volcanic activity adjacent to a portion of the Gulf of California formed a lava spill and a variety of cinder cones surrounding the Pinacate area. The rest of the western section is composed of plateaus and sand dunes reaching no more than 200 m above sea level. The south-central part of the Mexican state of Sonora is dominated by the foothills of the western Sierra Madre Occidental. These mountains reach elevations between 1,000 and 2,000 m, resulting from a system of faults and generalized volcanic activity during the Cenozoic era (Ferrusquía-Villafranca 1993). Soils are sandy and alkaline in the dunes, but toward the Pinacate and mountainous regions they are made of igneous or metamorphic material.
The climate of the Sonoran Desert ecoregion varies slightly due to its large size. In the Arizona upland section climate is more mesic, with bi-seasonal rainfall between 100-300 mm annually. Climate is subtropical dry near the Gulf of California. Near the Colorado River Valley and all remaining parts of the ecoregion temperatures are high year round with infrequent, irregular rainfall creating an arid dry climate. The Desierto de Altar, in the western Sonoran ecoregion, is one of the driest areas in North America, with periods of drought that can last for 30 months (Ezcurra & Rodrigues 1986). In general, the ecoregion is very dry receiving less than 90 mm of annual rainfall.
Creosote (Larrea divaricata) and white bursage (ambrosia dermosa) vegetation (McNab 1994) characterize the lower Colorado River Valley section. The Arizona upland section to the north and east is more mesic, resulting in greater species diversity and richness. Lower elevation areas are dominated by dense communities of creosote and white bursage, but on slopes and higher portions of bajadas, subtrees such as palo verde (Cercidium floridum, C. microphyllum) and ironwood (Olneya tesota), saguaros (Carnegiea gigantia), and other tall cacti are abundant (Turner et al 1995). La gobernadora (Larrea tridentata) and hierba de burro (Ambrosia dumosa) form the scrub that dominates the northwest part of the Sonoran Desert. It grows on deep, sandy soils in the flatlands. Where the dunes allow for slight inclination of the slope, species of mesquite (Prosopis), Cercidium, palo fierro (Olneya tesota), Candalia, Lycium, choya (Opuntia), Fouquieria, jécota (Hymenoclea) and Acacia are favored. The coastal plains of Sonora are composed of an almost pure Larrea scrub (Rzedowski 1988). Away from the Gulf influence in the area surrounding the Pinacate Encelia farinosa, Larrea tridentata, and cacti species such as Olneya, Cercidium, Prosopis, and Fouquieria again dominate the desert. Epiphytes are very scarce but not absent, as are moss and lichens.
The Sonoran Desert has the greatest diversity of vegetative growth of any desert in the world. (Nabham & Plotkin 1994). 560 plant species grow in the extremely harsh conditions of drought and heat, and interact in a variety of ecological relationships that add to the complexity of the community. More than 160 plant species, including six threatened succulents, depend upon legumes such as ironwood and mesquite for their regeneration in the Sonoran Desert (Nabham & Plotkin 1994).
The saguaro cactus (Cereus giganteus) is by far the most recognizable Sonoran Desert species. The largest of all cacti, it is only one of many cactus species found in this ecoregion. Cholla cactus (Opuntia fulgida), organ pipe (Lemaireocereus thurderi), silver dollar cactus (Opuntis chlorotica), and jojoba (Simmondsia chinensis) are a few of the numerous cacti in this ecoregion. The Sonoran coastal plain area houses the palo fierro or ironwood (Olneya tesota) which is the oldest desert tree (Búrquez & Quintana 1994; Tewksbury & Petrovich 1994). It has also been demonstrated that ironwood (Olneya tesota) and palo verde (Cercidium floridum) nursing relationships promote diversification and increase richness of other plants in arid ecosystems (Búrquez & Quintana 1994; Tewksbury & Petrovich 1994). Another remarkable plant found in the Sonora desert is the ocotillo (Fouquieria splendens), which may remain leafless during the coldest months of winter but experiences five or six leafy periods throughout the year. The brilliant red conical flowers are often triggered by the first cool-season rain in the spring, flowering within as few as 48 hours and attracting hummingbirds and other nectar feeders.
Many wildlife species, such as Sonoran pronghorn antelopes (EN) (Antilocapra sonoriensis), desert bighorn sheep (Ovis canadensis) and the endemic Bailey's pocket mouse (Perognathus baileyi) use ironwood, cacti species and other vegetation as both shelters from the harsh climate and a water source. Other mammals include predators such as mountain lions (Felis concolor), coyotes (Canis latrans) and prey such as black-tailed jackrabbits (Lepus californicus), and round-tailed ground squirrels (Spermophilus tereticaudus). Other mammals able to withstand the extreme desert climate of this ecoregion include California leaf-nosed bat (Macrotus californicus) and ring-tailed cat (Bassasiscus astutus).
The Sonoran desert is recognized as an exceptional birding area within the United States. Forty-one percent (261 of 622) of all terrestrial bird species found in the United States can be seen here during some part of the year. The Sonoran desert, together with its eastern neighbor the Chihuahuan desert, is the richest area in in the United States for birds, particularly hummingbirds. Among the bird species found here are the sanguaro-inhabiting cactus wren (Calypte costae), black-tailed gnatcatcher (Polioptila melanura), phainopepla (Phainopepla nitens), Gila woodpecker (Melanerpes uropygualis), and Costa's hummingbird (Calypte costae). Perhaps the most well-known Sonoran bird is the roadrunner (Geococcyx californianus), distinguished by its preference for running over flying as it hunts scorpions, tarantulas, rattlesnakes, lizards, and other small animals. The Sonoran desert harbors two endemic bird species, the highest level of bird endemism in all of the United States. The rufous-winged sparrow (Aimophila carpalis) is fairly common in most parts of the Sonoran, but only along the central portion of the Arizona-Mexico border, seen in desert grass mixed with brush. Rare in extreme southern Arizona along the Mexican border, the five-striped sparrow (Amphispiza quinquestriata) lives predominantly in canyons on hillsides and slopes among tall, dense shrubs.
Fifty-eight species of reptiles, including six species of rattlesnake, are found in the U.S. portion of this rich desert. The threatened desert tortoise (Gopherus agassizii) makes its home in burrows and frequents desert oases, washes, and riverbanks throughout the Sonoran. Perfectly camouflaged for life in the Sonoran, the giant Gila monster (Heloderma suspectum) is one of several lizards inhabiting this region. The widespread tiger salamander (Ambystoma tigrinum), the world's largest land-dwelling salamander, is able to survive in the Sonoran by burrowing underground during the dry season and emerging during wet times. The tiger salamander is one of only a dozen amphibians able to live in this arid ecoregion.
About 60% of habitat in the US alone have been altered by agriculture, grazing, excessive groundwater pumping, and urbanization. Riparian woodland habitats have suffered, by far, the worst and are now one of the rarest habitats in North America. Residential development on bajadas is eliminating the habitat of bajadas-dependent species such as cholla cacti (Opuntia) and columnar cacti such as saguaro (McMahon 1988). Rocky habitat areas preferred by gila monsters and bighorn sheep are prime real estate for development. The last population of Sonoran pronghorn is confined to Cabeza area in southern Arizona, isolated by lack of connectivity with other habitat areas.
The dry, inhospitable nature of the Sonoran Desert has favored the preservation of large areas with original vegetation. Four protected areas have been established in the Mexican Sonoran Desert in the last two decades, but they occupy less than 5% (9,540 km2) of the original desert area. In the United States approximately 17% of the Sonoran Desert is protected. The Cabeza Prieta National Wildlife Refuge and Organ Pipe National Monument Biosphere Reserve, both in Arizona, and the four protected areas in the Mexican part of the Sonoran Desert ecoregion form the second largest protected area of xeric scrub in North America (Challenger 1988). Lack of connectivity between all protected areas within and across borders also remains a serious problem. Other areas that include wildlife refuges, national monuments and/or military installations are located in the Kofa Mountains, Lower Colorado River, Arrastra Mountains, Chuchwilla Mountains, Eagletail Mountains, and Turtle Mountain. In addition, there are large areas of intact habitat within the Tohono O'odham Indian Reservation.
Types and Severity of Threats
Threats to the ecoregion include cattle grazing, agriculture, excessive ground water pumping, urbanization, and mineral extraction. The urban and suburban areas of Phoenix and Tucson in the U.S. continue to expand rapidly. This in turn is pushing agricultural operations further into the desert and along riparian areas such as the Gila River, with tremendous impacts on wildlife habitat. Riparian habitats are threatened by trampling, grazing and fouling by domestic livestock; water diversion and dam building; conversion for agriculture; and introduced species such as the Tamarisk tree (Tamarix chinensis) (Noss and Peters 1995). Furthermore, the introduction of invasive animal and plant species could displace native fauna and flora through direct competition.
The introduction of buffel grass (Cechrus ciliaris) for cattle grazing has been particularly harmful; it accumulates, forming combustible litter that causes the complete burning of ironwood and other plants. As a result, arid grasslands replace the xeric scrub, thereby lowering or preventing the recruitment of perennials. If this practice remains uncontrolled, the landscape of the Sonoran Desert may change and biodiversity may be lost (Búrquez & Quintana 1994). Another significant threat to the region is the abundant, illegal collection of plant and animal species of high biological value, and the uncontrolled hunting of endangered species such as mule deer, pronghorn antelope, and mountain sheep. Tourism activities in the region have become extensive (INE-SERMARNAP 1995), bringing increased danger of pollution and disturbance of flora. Long-term threats to the area include the extension of highways and the establishment of an industrial plant for toxic waste treatment, which could pollute the environment.
Justification of Ecoregion Delineation
The United States portion of the Sonoran Desert ecoregion corresponds to Küchler’s (1975) unit 36 (Creosote bush-bur sage) and 37 (Palo verde-cactus shrub). Omernik’s (1995) unit 14 (Southern Basin and Range) was split into the Mojave Desert and Sonoran Desert ecoregions based on differences in community structure. The boundary location was based on Bailey (1994) and MacMahon (1988). This ecoregion incorporates Bailey’s (1994) sections 322B (Sonoran Desert) and 322C (Colorado Desert). The Mexican portion was mapped according to INEGI (1996) vegetation maps, from which we lumped "desert vegetation", "microphyll desert matorral", "sacrocaulous matorral", and all enclose agriculture. Linework was reviewed by experts during ecoregional priority setting workshops (CONABIO 1996 and 1997) in Mexico.
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Prepared by: C. Williams, D. Olson, Alejandra Valero, Jan Schipper, Christine Burdette, and P. Hurley
Reviewed by: In process