Several larger mountain ranges include the Sierra Madre, the Sierra del Carmen, the Organ Mountains, the Franklin Mountains, the Sacramento Mountains, the Chisos Mountains, the Guadalupe Mountains, and the Davis Mountains. These create "sky islands" of cooler, wetter, climates adjacent to, or within the desert, and such elevated areas have both coniferous and broadleaf woodlands, including forests along drainages and favored exposures. The lower elevations of the Sandia–Manzano Mountains, the Magdalena–San Mateo Mountains, and the Gila Region border the Chihuahuan Desert at their lower elevations. There are a few urban areas within the desert: the largest is Ciudad Juárez with almost two million inhabitants; Chihuahua, Saltillo, and Torreón; and the US cities of El Paso and Albuquerque. Las Cruces and Roswell are among the other significant towns in this ecoregion. 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. 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.
Climate
The desert is mainly a rain shadow desert because the two main mountain ranges covering 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. Climatically, the desert has a dry climate with only one rainy seasonin the summer and smaller amounts of precipitation in early winter. Most of the summer rains falls between late June and early October, during the North American Monsoon when moist air from the Gulf of Mexico penetrates into the region. Owing to its inland position and higher elevation than the Sonoran Desert to the west, mostly varying from in altitude, the desert has a slightly milder climate in the summer and cool or cold winters with occasional frosts. The average annual temperature in the desert is, which varies with altitude. The hottest temperatures in the desert occur in lower elevation areas and in valleys. Northern areas have more severe winters than the southern portion and can receive snowstorms. The mean annual precipitation for the Chihuahuan Desert is with a range of approximately, although it receives more precipitation than other warm desert ecoregions. Nearly two-thirds of the arid zone stations have annual totals between. Snowfall is scant except at the higher elevation edges. The desert is fairly young, existing for only 8000 years.
Flora/fauna
The creosote bush is the dominant plant species on gravelly and occasional sandy soils in valley areas within the Chihuahuan Desert. The other species it is found with depends on factors such as the soil, altitude, and degree of slope. Viscid acacia, and tarbush dominate northern portions, as does broom dalea on sandy soils in western portions. Yucca and Opuntia species are abundant in foothill edges and the central third, while Arizona rainbow cactus and Mexican fire-barrel cactus inhabit portions near the US–Mexico border. Herbaceous plants, such as bush muhly, blue grama, gypsum grama, and hairy grama, are dominant in desert grasslands and near the mountain edges including the Sierra Madre Occidental. Lechuguilla, honey mesquite, Opuntia macrocentra and Echinocereus pectinatus are the dominant species in western Coahuila. Ocotillo, lechuguilla, and Yucca filifera are the most common species in the southeastern part of the desert. Candelilla, Mimosa zygophylla, Acacia glandulifera and lechuguilla are found in areas with well-draining, shallow soils. The shrubs found near the Sierra Madre Oriental are exclusively lechuguilla, guapilla, Queen Victoria's agave, sotol, and barreta, while the well-developed herbaceous layer includes grasses, legumes and cacti. Grasslands comprise 20% of this desert and are often mosaics of shrubs and grasses. They include purple three-awn, black grama, and sideoats grama. Early Spanish explorers reported encountering grasses that were "belly high to a horse;" most likely these were big alkali sacaton and tobosa bottomlands.
