According to the World Wide Fund for Nature the Chihuahuan Desert may be the most biologically diverse desert in the world as measured by species richness or endemism. The region has been badly degraded, mainly due to grazing.[4] Many native grasses and other species have become dominated by woody native plants, including creosote bush and mesquite, due to overgrazing and other urbanization. The Mexican wolf, once abundant, was nearly extinct and remains on the endangered species list.[5]
Climate
The desert is mainly a rain shadow desert because the two main mountain ranges which bound the desert, the Sierra Madre Occidental to the west and the Sierra Madre Oriental to the east, block most moisture from the Pacific Ocean and the Gulf of Mexico, respectively.[3] Climatically, the desert mostly has an arid, mesothermal climate with one rainy season in the late summer and smaller amounts of precipitation in early winter, the mean daily temperature of the coldest month warmer than 0 °C (32 °F).[3] The majority of rain falls between late June and early October during the North American Monsoon when moist air from the Gulf of Mexico and the Sea of Cortez penetrates into the region, or much less frequently, when a tropical cyclone moves inland and stalls.[3][6]
Owing to its inland position and higher elevation than the Sonoran Desert to the west, mostly varying from 480 to 1,800 m (1,575 to 5,906 ft) in elevation,[7] the desert has a slightly milder climate in the summer (though usually daytime June temperatures are in the range of 32 to 40 °C or 90 to 104 °F), with mild to cool winters and occasional to frequent freezes.[6] The hottest temperatures in the desert occur in lower elevation areas and valleys, including near the Rio Grande from south of El Paso into the Big Bend, and the Bolson de Mapimi.[7] A subtropical temperature regime describes lower elevations in the Texas Big Bend region up to the Presidio and Candelaria areas, then southward into similar elevations, while a warm temperate temperature regime describes higher elevations and further north. The average annual temperature in the desert varies from about 13 to 22 °C (55 to 72 °F), depending on elevation and latitude.
The mean annual precipitation for the Chihuahuan Desert is 235 mm (9.3 in) with a range of approximately 150–400 mm (6–16 in), although it receives more precipitation than most other warm desert ecoregions.[3] Nearly two-thirds of the arid zone stations have annual totals between 225 and 275 mm (8.9 and 10.8 in).[8] Snowfall is scant except at the higher elevation edges. Northern and eastern portions have more definite winters than southern and western portions, receiving a portion of winter precipitation as snowfall most winters.[6]
The creosote bush (Larrea tridentata) is the dominant plant species on gravelly and occasional sandy soils in valley areas within the Chihuahuan Desert. The other species found with creosote bush depend on factors including the soil type, elevation, and degree of slope. Viscid acacia (Vachellia vernicosa), and tarbush (Flourensia cernua) dominate northern portions, while broom dalea (Psorothamnus scoparius) occurs on sandy soils in western portions. Yucca and Opuntia species are abundant on slopes and uplands in most areas, while Arizona rainbow cactus (Echinocereus polyacanthus) and Mexican fire-barrel cactus (Ferocactus pilosus) inhabit portions near the US–Mexico border.
Desert or arid grasslands comprise 20% of this desert and are often mosaics of shrubs and grasses. They include purple three-awn (Aristida purpurea), black grama (Bouteloua eriopoda), and sideoats grama (Bouteloua curtipendula). Early Spanish explorers reported encountering grasses that were "belly high to a horse"; most likely these were big alkali sacaton (Sporobolus wrightii) and tobosa (Pleuraphis mutica) along floodplain or bottomland areas.[4]
The United Nations Environment Programme reported in 2006 that nearly half of the bird, mammal, and butterfly species in the Chihuahuan Desert are expected to be replaced by other species by 2055 due to climate change.[9]
