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)
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).
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