Southwestern Deserts

A desert is an ecosystem with a biota constrained by the limited availability of moisture -- an area where annual precipitation is usually less than 500 mm and, in many places, is less than 250 mm (< 10 inches). During the day, temperatures can reach 55^oC (130^oF) in the shade, and temperatures at night can drop to near freezing. The relative lack of moisture in the air (low relative humidity) in deserts allows most of the energy from the sun to reach the surface, creating the hot daytime conditions. Likewise, the low relative humidity means that the air is unable to retain the heat as it is radiated from the surface, so nighttime temperatures drop rapidly. Although daytime heat is often associated with deserts, in the Great Basin and other northern deserts, plants and animals must deal with extended periods of cold temperatures in addition to the limited moisture.

Sonoran Desert, Tucson, Arizona
Photograph by Mark Eberle, March 1999

As if this environment wasn't challenging enough, in some areas, the aridity and local geology contribute to the formation of saline soils, where evaporation of water from the surface concentrates salts that were dissolved in the water. Water-impermeable, root-limiting caliche layers (deposits of calcium carbonate) also can form at the shallow depths to which precipitation carries this material. In general, desert soils (aridisols) are more alkaline and contain less organic matter (< 1%) than soils of adjacent grasslands and woodlands (3% or more). Yet, desert plants and desert animals have evolved to survive these extreme conditions. For individual species or communities, the patterns of desert biota on the landscape reflect their adaptations to these various environmental conditions. Linear patterns are often associated with gradients of water availability and temperature (as on mountain slopes). Patches are often associated with attributes such as water, soil chemistry, soil texture, and slope. Within these linear or patchwork patterns, relationships among organisms (for example, the presence or absence of a species of plant that provides shade, food, nesting habitat, etc.) also influence other organisms in the local communities. At a broader scale, the deserts of the western United States and northern Mexico are often divided into 4 or more desert regions, which are generally distinguished by their climate and the species in their communities.

Playa Lake and Mineral Crust, White Sands National Monument, New Mexico
Photographs by Matt Withroder, March 2003

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