DESERT

any large, extremely dry area of land with sparse vegetation. It is one of the Earth's major types of ecosystems, supporting a community of distinctive plants and animals specially adapted to the harsh environment. Desert environments are so dry that they support only extremely sparse vegetation; trees are usually absent and, under normal climatic conditions, shrubs or herbaceous plants provide only very incomplete ground cover. Extreme aridity renders some deserts virtually devoid of plants; however, this barrenness is believed to be due in part to the effects of human disturbance, such as heavy grazing of cattle, on an already stressed environment. Figure 1: Worldwide distribution of hot and temperate deserts. According to some definitions, any environment that is almost completely free of plants is considered desert, including regions too cold to support vegetationi.e., frigid deserts. Other definitions use the term to apply only to hot and temperate deserts, a restriction followed in this account. The distribution of hot and temperate deserts is shown in Figure 1. Jeremy M.B. Smith any large, extremely dry area of land with fairly sparse vegetation. It is one of the Earth's major types of ecosystems, supporting a community of distinctive plants and animals specially adapted to the harsh environment. The level of aridity commonly used to delimit desert areas is a mean annual precipitation value equal to 250 mm (10 inches) or less. Approximately 5 percent of the Earth's land surface receives such limited rainfall. Deserts are concentrated in high-latitude, circumpolar areas and in two discontinuous, Earth-girdling bands at middle and low latitudes. The high-latitude cold deserts include the perennial ice deserts of Antarctica and Greenland and portions of the seasonally snow- and ice-free tundra of North America and Eurasia. Precipitation is meagre in these polar regions, with various areas receiving only up to about 127 mm (5 inches) annually. The desertlike, rocky, barren lands within the tundra, however, result from complex causes that are partially but not entirely related to aridity, because most tundra areas are adequately supplied with water during the short, cool growing season. Hot, arid regions furnish examples of the most familiar desert features. These regions have little physical or biological relationship to cold polar deserts. Tropical deserts, occurring between about 15 and 30 north and south of the Equator, include the Sahara of northern Africa, the Thar Desert of the Indian subcontinent, the Victoria of Australia, and the Kalahari of southwestern Africa. These deserts owe their origin and location largely to aridity that results from the high moisture-holding capacity of air warmed by compression as it descends from the high-pressure belt of the horse latitudes to the belt of tropical trade winds. Decreased precipitation and extraordinarily high rates of evaporation are the two main factors that combine to desiccate the land, particularly on the western sides of the continents. The Gobi and Takla Makan deserts of central Asia are deep within continental interiors far from moisture-laden winds, whereas the deserts east of the Andes in southern Argentina exemplify the rain-shadow effect created by high mountains lying across the path of moist winds. Climatic conditions are characteristically extreme in all deserts, and variations are considerable from one to another. Differences in seasonal temperature regimes sharply differentiate tropical from mid-latitude deserts. Seasonal as well as diurnal temperature ranges are greatest in the latter. For example, in the Gobi, winter winds are violent and severe blizzards common, and parts of this area may have mean temperatures below freezing for as many as six months each year. But during the hottest months the air temperature may average 41 to 43 C (105 to 110 F). A distinguishing characteristic of most arid climates is variability of precipitation. Several years may elapse without any measurable rainfall, only to be followed by a deluge. In general, such variability is inversely related to the mean annual precipitation, increasing as the total rainfall decreases. Desert terrain may consist of rugged mountains, high plateaus, or plains. A large number of deserts occupy broad mountain-rimmed basins, which are known as bolsons in certain regions. Surface materials in deserts include bare bedrock, plains of gravel and boulders, and vast tracts of shifting sand. Wind-blown sands, commonly thought to be typical of deserts, make up only about 2 percent of North American deserts, 10 percent of the Sahara, and 30 percent of the Arabian desert. Outside of the polar regions, desert landscapes are primarily shaped by weathering, eolian, and fluvial processes. Rock fragmentation occurs principally as a result of expansion and contraction induced by wide temperature fluctuations. Winds transport enormous clouds of dust and impel large quantities of sand along the ground, all particles behaving as abrasive tools that carve, facet, and polish rocks. Continued removal of fine materials produces deflation basins with surfaces of residual gravel and boulders. Infrequent rains of high intensity and short duration cause flash floods, which rush from highlands as sheet floods or as torrents raging through usually dry arroyos, or wadis. The streams flow at high velocity and transport large quantities of mud, sand, and rock debris short distances into the desert basins before the water is dissipated by penetration into the ground and by evaporation. The centripetal drainage of bolsons often produces pediments consisting of upper eroded bedrock surfaces and lower undulating slopes of coalescing alluvial fans. Mineral salts leached from the highlands are carried to the centre of the basins, where they accumulate in ephemeral lakes that, lacking outlets to the sea, become increasingly saline. Plant and animal populations tend to differ qualitatively and quantitatively in various deserts, but all possess physiological and behavioral traits that favour obtaining and conserving a meagre water supply. Plants are primarily low-growing, thorny, small-leaved or leafless, grayish to light green in colour, with extensive and, in most cases, deep roots. Water-conservation adaptations in perennial species include reduced surface areas, daytime closure of stomata, water-impervious cuticles and waxy coatings, and succulent organs that accumulate large quantities of water. Drought-evading, ephemeral annuals, dormant during dry periods, appear in profusion after rains. The dominant xerophytic plants (i.e., those adapted to arid conditions) include agaves, cacti, composites, and yuccas. Typical desert fauna consists of insects (and other arthropods), reptiles, birds, many rodents, and a few larger mammals. Nocturnal habits are well-developed characteristics among the rodents, but most desert birds and reptiles are diurnal. The majority of animals drink water when it is available, but in its absence they depend on fluids obtained by eating succulent plants or the blood and other tissues of their prey. Many insects and some rodents (e.g., the kangaroo rat and pocket mouse) utilize metabolic water, and even the camel derives a significant water supply from the oxidation of fats accumulated in its hump. Water conservation is accomplished in diverse ways. Reptiles and some insects have water-impervious integuments, mammals concentrate urine by reabsorbing water before excretion, and reptiles excrete nitrogenous waste as uric acid crystals and thus retain water. Nocturnal habits reduce water loss, and some animals, such as the ground squirrel, estivate (enter a torpid state) during extreme heat and drought. Additional reading Tropical deserts are described and put into their vegetational context in an old but still informative book by Heinrich Walter, Ecology of Tropical and Subtropical Vegetation (1971; originally published in German, 2nd ed., 1964). Deserts are more explicitly and broadly described and explored in Michael Evenari, Imanuel Noy-Meir, and David W. Goodall (eds.), Hot Deserts and Arid Shrublands, 2 vol. (198586). Jeremy M.B. Smith