The Mojave Desert is the smallest of the four American deserts. While the Mojave lies between the Great Basin Shrub Steppe and the Sonoran desert, its fauna is more closely allied with the lower Colorado division of the Sonoran desert. Dominant plants of the Mojave include creosotebush (Larrea tridentata), all-scale (Atriplex polycarpa), brittlebush (Encelia farinosa), desert holly (Atriplex hymenelytra), white burrobush (Hymenoclea salsola), and Joshua tree (Yucca brevifolia), the most prominent endemic species in the region (Turner 1994).
The Mojave is bounded to the north by the Great Basin Shrub Steppe, to the west by the Sierra Nevada and California montane scrub, and to the east by the Colorado Plateau. To the south, the Mojave blends into the Sonoran Desert, with a combination of species typical of the two deserts plus species most often found in the scrublands or conifer woodland of the Great Basin. Mojave species generally favor the colder plains, while Sonoran species are found on hillsides.
The Mojave’s warm temperate climate defines it as a distinct ecoregion. Species that serve to separate the Mojave from the Sonoran desert include such widespread Sonoran species as ironwood (Olneya tesota), blue Palo Verde (Cercidium florium), and chuparosa (Justica californica). Mojave indicator species include spiny mendora (Mendora spinescens), desert senna (Cassia armata), Mojave dalea (Psorothamnus arborescens), and goldenhead (Acamptopappus shockleyi) (Turner 1994).
The Mojave supports numerous species of cacti, including several endemics, such as silver cholla (Opuntia echinocarpa), Mojave prickly pear (O. erinacea), beavertail cactus (O. basilaris), and many-headed barrel cactus (Echinocactus polycephalus). The region is also rich in ephemeral plants, with perhaps 80 to 90 endemics (Turner 1994).
The Mojave desert contains a range of elevations not found in other North American deserts. Elevation ranges from below sea level in Death Valley (-146 m) to over 1,600 m on some mountains. Most of the region lies between 610 and 1,220 m, giving rise to the term "high desert." The Mojave receives little precipitation (between 65 and 190 mm annually) and dry lakes are a common feature of the landscape.
While the Mojave Desert is not as biologically distinct as the other desert ecoregions, distinctive endemic communities occur throughout it. For example, the Kelso Dunes in the Mojave National Preserve harbor seven species of endemic insects, including the Kelso Dunes jerusalem cricket (Ammopelmatus kelsoensis) and the Kelso Dunes shieldback katydid (Eremopedes kelsoensis). The Mojave fringe-toed lizard (Uma Scoparia), while not endemic to the dunes, is rare elsewhere (Schoenherr 1992).
The native range of California’s threatened desert tortoise (Gopherus agassizii) includes the Mojave and Colorado deserts. The desert tortoise has adapted for desert existence by storing up to a liter of water in its urinary bladder. The tortoise feeds on ephemeral plants in the spring and accumulates enough reserves of water to carry it through the remainder of the year (Schoenherr 1992).
Other endemic fauna include the Mojave ground squirrel (Spermophilus Mojavensis) and Amargosa vole (Microtus californicus scirpensis),
The Mojave desert is rich in ephemeral plants, most of which are endemic. Of the approximately 250 Mojave taxa with this life form, perhaps 80-90 are endemic (Sheve and Wiggins 1964). During favorable years, the region supports more endemic plants per square meter than any location in the United States. Most of these species are winter annuals. Flowering plants also attract butterflies such as the Mojave sooty-wing (Pholisora libya), and the widely distributed painted lady (Vanessa cardui) (Schoenherr 1992).
A majority of the fauna found in the Mojave desert also extends into the Sonoran or Great Basin deserts as well. However, the following avifauna and herpetofauna are characteristic of the Mojave region in particular: LeConte’s thrasher (Toxostoma lecontei), banded gecko (Coleonyx variegatus), desert iguana (Dipsosaurus dorsalis), chuckwalla (Sauromalus obesus), and regal horned lizard (Phrynosoma solare). Snake species include the desert rosy boa (Lichanura trivirigata gracia), Mojave patchnose snake (Salvadora hexalepis mojavensis), and Mojave rattlesnake (Crotalus scutulatus) (Brown 1994).
Habitat Loss and Degradation
Roughly half of the Mojave desert remains as intact habitat, and the remaining half has not been heavily altered by human activity. The main reasons for habitat loss in the region include urbanization and suburbanization from Los Angeles and Las Vegas, the increasing demand for landfill space (Los Angeles and San Diego are proposing a large landfill in the region), agricultural development along the Colorado River, grazing, offroad vehicles, and military activities. Areas under particular pressure include Ward Valley (near Mojave National Park) and Riverside Country, west of Joshua Tree National Monument. A falling water table also threatens Death Valley National Park.
Remaining Blocks of Intact Habitat
The most important remaining habitat blocks include:
•Death Valley National Monument - eastern California
•Desert National Wildlife Range - southern Nevada
•Joshua Tree National Monument - southeastern California
•Lake Mead - southeastern Nevada and northwestern Arizona
•Nevada Test Site - southern Nevada
Degree of Fragmentation
Habitats in the Mojave desert are generally contiguous, with a high degree of connectivity. Roads, however, have fragmented habitat for certain species, such as desert tortoise and some species of snakes. Big horn sheep migration routes also are not adequately protected between reserves.
Degree of Protection
By historical accident and the California Desert Protection Act, the Mojave desert is the one of the best protected ecoregions in the United States. The full range of habitats are included in reserves, although riparian areas need more protection. In addition to the habitat blocks listed above, important protected areas include:
•Mojave National Park - southeastern California
•Sheephole Wildlife Area - southeastern California
•Lands covered under the California Desert Protection Act
Types and Severity of Threats
Conversion threats to the Mojave are concentrated in the southwest and east central portions of the ecoregion. Lower elevation valleys are largely in private hands and lack protection. Off-road vehicles and development threaten these valleys, and development also is harming creosotebush areas. Tamarisk is invading springs, and grazing is damaging mid-elevation pastures. Wildlife trade threatens chuckwallas, gila monsters, and desert tortoises.
Suite of Priority Activities to Enhance Biodiversity Conservation
The most important conservation activity in the Mojave desert is to protect riparian areas and low elevation valleys.
•California Desert Protection League
•California Native Plant Society
•The Wilderness Society
Relationship to other classification schemes
This ecoregion roughly corresponds to Bailey’s Mojave Desert Section, which falls within the American Semi-desert and Desert Province, though the Bailey section extends somewhat further to the east, into the Sonoran desert and Colorado Plateau, and to the west into the Southern California Montane scrub. Omernik places the Mojave in the Southern Basin and range, a region which also includes much of the area we classify as the Sonoran Desert. We feel the communities within these desert ecosystems are distinct enough to warrant separate classifications and conservation planning.
Prepared by: B. Holland, G. Orians, J. Adams