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The Desert Environment
An area having an annual rainfall of 10 in. (25 cm) or less is considered to be a desert. Some deserts have no rain for intervals of several years. Deserts and semideserts exist in some regions having up to about 20 in. (50 cm) of rainfall where evaporation is very high and loss by runoff is great. The largest desert regions of the world lie between 20° and 30° north and south of the equator, either where mountains intercept the paths of the trade winds or where atmospheric high-pressure areas cause descending air currents and a lack of precipitation. Other factors contributing to the formation of deserts include the amount of sunshine, rate of evaporation of water, and range of temperature. Temperature ranges in deserts are often extreme.
Plants of the desert have leaves and stems adapted to lessen their loss of water, and individual plants are more widely spaced than those in more humid regions; their roots form a spreading network sometimes penetrating to 50 ft (15 m) underground. Among the animals living in deserts of North America are species of squirrels, mice, bats, foxes, rabbits, and deer; reptiles, e.g., the Arizona coral snake, species of rattlesnakes, the desert tortoise, and the horned toad, gila monster, and many other lizards; a number of birds, e.g., the cactus wren, the road runner, species of owls, sparrows, and hawks; and spiders, scorpions, termites, and beetles. See dune; oasis.
The Deserts of the World
Europe is the only continent without deserts; there are, however, semiarid portions around the Black and Caspian seas, in parts of Ukraine and the N Caucasus. In Asia a great desert, the Gobi, exists in the middle latitudes chiefly because of its remoteness from water. Also in central Asia are the Kara Kum and Kyzyl Kum deserts. Farther south there are desert areas in NW India and through S Pakistan, Afghanistan, Iran, Iraq, and Arabia; these are largely the result of their situation in a subtropical high-pressure belt and of the distribution of pressure areas that produce cold, dry winds in winter and hot, dry winds in summer.
The Sahara, the largest desert in the world, is in Africa. Second only to the Sahara in area is the desert region of central and W Australia, lying in a high-pressure belt and in the path of the trade winds (which lose much of their moisture on the windward slopes of the east-coast mountains). South America has deserts on the coast and interior of Chile and E of the Andes in Argentina and Patagonia. In North America, deserts are found from N Mexico northward through parts of the SW and W United States. Extreme desert conditions exist in the Mojave Desert, the Imperial Valley, and Death Valley. The northern plateau region of Mexico and the adjacent portions of Texas, Arizona, and New Mexico have less extreme desert conditions with a quite abundant growth of mesquite, greasewood, creosote bush, yucca, and various species of cactus. Middle-latitude deserts are found in parts of the Great Basin.
See J. W. Krutch, The Voice of the Desert (1955); D. F. Costello, The Desert World (1972); G. L. Bender, ed., Reference Handbook on the Deserts of North America (1982); G. N. Louw, Ecology of Desert Organisms (1982); B. Spooner and H. S. Mann, Desertification and Development (1983); A. Grainger, Desertification (1986); L. Berkofsky and M. G. Wurtele, ed., Progress in Desert Research (1987); studies by E. C. Jaeger on desert flora and fauna (1957, 1961, 1965).
No precise definition of a desert exists. From an ecological viewpoint the scarcity of rainfall is all important, as it directly affects plant productivity which in turn affects the abundance, diversity, and activity of animals. It has become customary to describe deserts as extremely arid where the mean precipitation is less than 2.5–4 in. (60–100 mm), arid where it is 2.5–4 to 6–10 in. (60–100 to 150–250 mm), and semiarid where it is 6–10 to 10–20 in. (150–250 to 250–500 mm). However, mean figures tend to distort the true state of affairs because precipitation in deserts is unreliable and variable. In some areas, such as the Atacama in Chile and the Arabian Desert, there may be no rainfall for several years. It is the biological effectiveness of rainfall that matters and this may vary with wind and temperature, which affect evaporation rates. The vegetation cover also alters the evaporation rate and increases the effectiveness of rainfall. Rainfall, then, is the chief limiting factor to biological processes, but intense solar radiation, high temperatures, and a paucity of nutrients (especially of nitrogen) may also limit plant productivity, and hence animal abundance. Of the main desert regions of the world, most lie within the tropics and hence are hot as well as arid. The Namib and Atacama coastal deserts are kept coot by the Benguela and Humboldt ocean currents, and many desert areas of central Asia are cool because of high latitude and altitude.
The diversity of species of animals in a desert is generally correlated with the diversity of plant species, which to a considerable degree is correlated with the predictability and amount of rainfall. There is a rather weak latitudinal gradient of diversity with relatively more species nearer the Equator than at higher latitudes. This gradient is much more conspicuous in wetter ecosystems, such as forests, and in deserts appears to be overridden by the manifold effects of rainfall. Animals, too, may affect plant diversity: the burrowing activities of rodents create niches for plants which could not otherwise survive, and mound-building termites tend to concentrate decomposition and hence nutrients, which provide opportunities for plants to colonize.
Each desert has its own community of species, and these communities are repeated in different parts of the world. Very often the organisms that occupy similar niches in different deserts belong to unrelated taxa. The overall structural similarity between American cactus species and African euphorbias is an example of convergent evolution, in which separate and unrelated groups have evolved almost identical adaptations under similar environmental conditions in widely separated parts of the world. Convergent structural modification occurs in many organisms in all environments, but is especially noticeable in deserts where possibly the small number of ecological niches has necessitated greater specialization and restriction of way of life. The face and especially the large ears of desert foxes of the Sahara and of North America are remarkably similar, and there is an extraordinary resemblance between North American sidewinding rattlesnakes and Namib sidewinding adders. See Ecology, Physiological ecology (plant)
a type of landscape that developed in regions with a permanently or seasonally hot climate and that has very sparse and meager phytocenoses.
Depending on their soil, deserts are divided into sandy deserts, occurring on loose deposits of ancient alluvial plains; pebble and sandy pebble deserts on gypsum structural plateaus and piedmont plains; cobble gypsum deserts on plateaus and young piedmont plains; stony deserts on low mountains and areas of low, rounded, isolated hills; loam deserts on weakly calcareous mantle loams; loess deserts on piedmont plains; clay takyr deserts on piedmont plains and in ancient river deltas; clay badland deserts on low-mountain relief consisting of salt-bearing marls and clays; and solonchak, or salt, deserts in saline depressions and along seacoasts.
Desert vegetation is highly distinctive. Frequently, deserts were the sites of ancient speciation; these deserts contain many pre-Cenozoic endemics, among them Nitraria, Potaninia, and Ammodendron in Central Asia, Welwitschia and Acanthosicyos in South Africa, and Opuntia and Cereus in North and Central America. Some deserts have a frequent succession of plant associations characterized by complexity and therefore a variety of dominants. Such successional changes are related to the structure of the desert surface, variations in the soil, and frequently changing moisture conditions. On the other hand, in both distribution and ecology, the desert vegetation on the different continents has many common features because of the similarity of habitat—great sparseness, a poverty of species, and the constant presence of dominants, sometimes distributed over large areas. At the same time the composition of the dominants and their ecological appearance are distinctive in each desert.
Sclerophyllous plants, including leafless shrubs and semishrubs (Haloxylon, Calligonum, Ephedra, Salsola, Artemisia), are typical of inland deserts in temperate regions. Stem succulents are not found here, but herbaceous plants, both ephemerals and ephemeroids, occupy an important place in the phytocenoses of these deserts.
Xerophilic shrubs and perennial grasses predominate in the inland subtropical and tropical deserts of Africa and Arabia, although succulents also grow here. The vegetation is sparser than that of deserts in the temperate regions. The plant cover is richest in stony deserts and sparsest in sandy pebble deserts. Barchans (mobile dunes) and areas covered with a salt crust are completely devoid of vegetation.
The plant cover of subtropical deserts in North America and Australia is richer in species, and they are similar to the Middle Asian deserts in abundance of vegetation. There are almost no sterile stretches. Coarse xerophilic Spinifex grasses and Crotalaria are the dominants in the sandy deserts of Australia. Low-growing species of Acacia and Eucalyptus grow in the clay depressions between sand ridges. Semishrub halophytes such as A triplex and Vitex are characteristic of pebble and cobble deserts. Succulents prevail in subtropical and tropical coastal deserts—the Western Sahara, Namib, Atacama, California, and Mexican deserts.
Many of the same species grow on saline soils in temperate, subtropical, and tropical deserts, notably halophilic and succulent semishrubs and shrubs (Tamarix, Nitraria) and annual halophytes (Salsola, Suaeda). The phytocenoses of oases, tugais (gallery forests in the deserts of Central and Middle Asia), large river valleys, and deltas are considerably different from the basic vegetation of deserts. Groves of deciduous trees, including poplars (Populus diversifolia, Populas pruinosa), oleasters (Elaeagnus), and willows, are characteristic of valleys in the temperate deserts of Asia, and evergreens such as palms and oleanders grow in river valleys in the subtropical and tropical deserts.
M. P. PETROV
What does it mean when you dream about the desert?
Deserts may be fairly straightforward symbols for a sense of barrenness, poverty, lack, exhaustion, loneliness, or even death. On the other hand, as the unsettled “wilderness,” deserts often represent the unconscious, particularly the shadow self. Deserts have a wider range of meanings for someone from the desert Southwest than for someone from the East Coast. As with all dream symbols, the atmosphere and setting of the dream indicate which meaning is appropriate. (See also Sand).