Desert biomes cover about one fifth of the Earth's surface and are defined to occur where rainfall is less than 50 centimeters (cm) per year. Although most deserts, such as the Sahara of North Africa and the deserts of the southwestern U.S., Mexico, and Australia, occur at low latitudes, another kind of desert, cold deserts, occur in the basin and range area of Utah and Nevada and in parts of western Asia. Most deserts have a considerable amount of specialized vegetation, as well as specialized vertebrate and invertebrate animals. Soils often have abundant nutrients because they need only water to become very productive and have little or no organic matter. Disturbances are common in the form of occasional fires or cold weather, and sudden, infrequent, but intense rains that cause flooding.
There are relatively few large mammals in deserts because most are not capable of storing sufficient water and withstanding the heat. Deserts often provide little shelter from the sun for large animals. The dominant animals of warm deserts are non-mammalian vertebrates, such as reptiles. Mammals are usually small, like the kangaroo mice of North American deserts.
Hot and dry desert
The four major North American deserts of this type are the Chihuahuan, Sonoran, Mojave and Great Basin. Others outside the U.S. include the Southern Asian realm, Neotropical (South and Central America), Sahara and Sahel (including Ethiopian Desert) and Australian.
The seasons are generally warm throughout the year and very hot in the summer. The winters usually bring little rainfall.
Temperatures exhibit daily extremes because the atmosphere contains little humidity to block the Sun's rays. Desert surfaces receive a little more than twice the solar radiation received by humid regions and lose almost twice as much heat at night. Many mean annual temperatures range from 20-25° C. The extreme maximum ranges from 43.5-49° C. Minimum temperatures sometimes drop to -18° C.
Rainfall is usually very low and/or concentrated in short bursts between long rainless periods. Evaporation rates regularly exceed rainfall rates. Sometimes rain starts falling and evaporates before reaching the ground. Rainfall is lowest on the Atacama Desert of Chile, where it averages less than 1.5 cm. Some years are even rainless. Inland Sahara also receives less than 1.5 cm a year. Rainfall in American deserts is much higher — almost 28 cm a year.
Soils are course-textured, shallow, rocky or gravely with good drainage and have no subsurface water. They are coarse because there is less chemical weathering. The finer dust and sand particles are blown elsewhere, leaving heavier pieces behind.
Flora and fauna
Canopy in most deserts is very rare. Plants are mainly ground-hugging shrubs and short woody trees. Leaves are "replete" (fully supported with nutrients) with water-conserving characteristics. They tend to be small, thick and covered with a thick cuticle (outer layer). In the cacti, the leaves are much-reduced (to spines) and photosynthetic activity is restricted to the stems. Some plants open their stomata (microscopic openings in the epidermis of leaves that allow for gas exchange) only at night when evaporation rates are lowest. These plants include: yuccas, ocotillo, turpentine bush, prickly pears, false mesquite, sotol, ephedras, agaves and brittlebush.
The animals include small nocturnal (active at night) carnivores. The dominant animals are burrowers and kangaroo rats. There are also insects, arachnids, reptiles and birds. The animals stay inactive in protected hideaways during the hot day and come out to forage at dusk, dawn or at night, when the desert is cooler.
The major deserts of this type include the sagebrush of Utah, Montana and Great Basin. They also include the Nearctic realm (North America, Newfoundland, Greenland, Russia, Europe and northern Asia).
The summers are moderately long and dry, and like hot deserts, the winters normally bring low concentrations of rainfall. Summer temperatures usually average between 21-27° C. It normally does not go above 38° C and evening temperatures are cool, at around 10° C. Cool nights help both plants and animals by reducing moisture loss from transpiration, sweating and breathing. Furthermore, condensation of dew caused by night cooling may equal or exceed the rainfall received by some deserts. As in the hot desert, rainfall is often very low and/or concentrated. The average rainfall ranges from 2-4 cm annually.
The soil can range from sandy and fine-textured to loose rock fragments, gravel or sand. It has a fairly low salt concentration, compared to deserts which receive a lot of rain (acquiring higher salt concentrations as a result). In areas such as mountain slopes, the soil is shallow, rocky or gravely with good drainage. In the upper bajada (lower slopes) they are coarse-textured, rocky, well-drained and partly "laid by rock bench." In the lower bajada (bottom land) the soil is sandy and fine-textured, often with "caliche hardpan." In each case there is no subsurface water.
Flora and fauna
The spiny nature of many plants in semiarid deserts provides protection in a hazardous environment. The large numbers of spines shade the surface enough to significantly reduce transpiration. The same may be true of the hairs on the woolly desert plants. Many plants have silvery or glossy leaves, allowing them to reflect more radiant energy. These plants often have an unfavorable odor or taste. Semiarid plants include: Creosote bush, bur sage (Franseria dumosa or F. deltoidea), white thorn, cat claw, mesquite, brittle bushes (Encelia farinosa), lyciums, and jujube.
During the day, insects move around twigs to stay on the shady side; jack rabbits follow the moving shadow of a cactus or shrub. Naturally, many animals find protection in underground burrows where they are insulated from both heat and aridity. These animals include mammals such as the kangaroo rats, rabbits, and skunks; insects like grasshoppers and ants; reptiles are represented by lizards and snakes; and birds such as burrowing owls and the California thrasher.
These deserts occur in moderately cool to warm areas such as the Nearctic and Neotropical realm. A good example is the Atacama of Chile.
The cool winters of coastal deserts are followed by moderately long, warm summers. The average summer temperature ranges from 13-24° C; winter temperatures are 5° C or below. The maximum annual temperature is about 35° C and the minimum is about -4° C. In Chile, the temperature ranges from -2 to 5° C in July and 21-25° C in January.
The average rainfall measures 8-13 cm in many areas. The maximum annual precipitation over a long period of years has been 37 cm with a minimum of 5 cm.
The soil is fine-textured with a moderate salt content. It is fairly porous with good drainage.
Flora and fauna
Some plants have extensive root systems close to the surface where they can take advantage of any rain showers. All of the plants with thick and fleshy leaves or stems can take in large quantities of water when it is available and store it for future use. In some plants, the surfaces are corrugated with longitudinal ridges and grooves. When water is available, the stem swells so that the grooves are shallow and the ridges far apart. As the water is used, the stem shrinks so that the grooves are deep and ridges close together. The plants living in this type of desert include the salt bush, buckwheat bush, black bush, rice grass, little leaf horsebrush, black sage, and chrysothamnus.
Some animals have specialized adaptations for dealing with the desert heat and lack of water. Some toads seal themselves in burrows with gelatinous secretions and remain inactive for eight or nine months until a heavy rain occurs. Amphibians that pass through larval stages have accelerated life cycles, which improves their chances of reaching maturity before the waters evaporate. Some insects lay eggs that remain dormant until the environmental conditions are suitable for hatching. The fairy shrimps also lay dormant eggs. Other animals include: insects, mammals (coyote and badger), amphibians (toads), birds (great horned owl, golden eagle and the bald eagle), and reptiles (lizards and snakes).
These deserts are characterized by cold winters with snowfall and high overall rainfall throughout the winter and occasionally over the summer. They occur in the Antarctic, Greenland and the Nearctic realm.
They have short, moist, and moderately warm summers with fairly long, cold winters. The mean winter temperature is between -2 to 4° C and the mean summer temperature is between 21-26° C.
The winters receive quite a bit of snow. The mean annual precipitation ranges from 15-26 cm. Annual precipitation has reached a maximum of 46 cm and a minimum of nine cm. The heaviest rainfall of the spring is usually in April or May. In some areas, rainfall can be heavy in autumn. The soil is heavy, silty, and salty. It contains alluvial fans where soil is relatively porous and drainage is good so that most of the salt has been leached out.
Flora and fauna
The plants are widely scattered. In areas of shadscale, about 10 percent of the ground is covered, but in some areas of sagebush it appoaches 85 percent. Plant heights vary between 15 cm and 122 cm. The main plants are deciduous, most having spiny leaves. Widely distributed animals are jack rabbits, kangaroo rats, kangaroo mice, pocket mice, grasshopper mice, and antelope ground squirrels. In areas like Utah, population density of these animals can range from 14-41 individuals per hectare. All except the jackrabbits are burrowers. The burrowing habit also applies to carnivores like the badger, kit fox, and coyote. Several lizards do some burrowing and moving of soil. Deer are found only in the winter.
Most of the world's deserts lie between latitudes twenty and forty degrees latitude, areas that are characteristically low in precipitation and yet relatively high in temperature. Equatorial zones are conversely more rich in rainfall and lack aridity required for the desert biome. It is surprising to many that endemism is quite high in many deserts and drylands, especially at higher taxonomic levels. For example one classic study indicated that endemism in South American drylands exceeded that of Amazonia.
Edaphic heterogeneities in deserts have profound effects upon species biodiversity, since variations in rock sizes and distributions allow more numerous microniches for specialist flora. These outcomes are particularly notable in the varied geological formations of the Namib Desert and for many desert mountains of the southwestern part of North America.
Shmida has classified the world's deserts into nine phytogeographic regions, although the high latitude cold deserts deserve a tenth:
- Sub-Saharan (Namib, Kalahari)
- Great Basin of the USA
- Southwestern North American (Sonoran, Mojave, Chihuahua)
- High latitude cold deserts (Antarctica, Greenland)
Orographic and ocean influences
Mountain barriers often contribute to rain shadow effects in defining low precipitation regimes driving desert formation. The most prominent examples of such orographic influences are in the case of deserts in South America, North America, Madagascar and Southern Africa. Cold water ocean currents can also be large scale contributors to desert regions; for example, the Benguela Current running north along the southwest African Coast brings cold water from the Southern Ocean, which is also a very dry atmospheric influence.Thus, when winds arrive onto the Namib Desert from the Atlantic Ocean, they are quite desiccating.
Desertification is the persistent degradation of dryland ecosystems due to human activities and variations in climate. While climate oscillations have historically had pronounced effects upon desertification, activities of man during the Holocene have had thegreatest impacts to induce desertification in the most recent 10,000 years. Home to a third of the human population in 2000, drylands occupy nearly half of Earth’s land area. Across the world, desertification affects the livelihoods of millions of people who rely on the benefits that dryland ecosystems provide.
Proximate chief drivers of desertification include overdrafting of groundwater, overgrazing and tillage practices in agriculture that place soils more vulnerable to wind and surface runoff scouring. In drylands, water scarcity limits the production of crops, forage, wood, and other services ecosystems provide to humans. Drylands are therefore highly vulnerable to increases in human pressures and climatic variability, especially sub-Saharan and Central Asian drylands.
- University of California Museum of Paleontology. Desert Biome
- Desert Biome, WorldBiomes.com.
- David Ward. 2009. The biology of deserts. Oxford University Press. 339 pages
- A.Shmida and R.H.Whittaker 1985. Trends of species diversity and growth forms in arid regions. Hebrew University of Jerusalem
- John C.Zak. ed. Susan Isaac. 1993. Aspects of tropical mycology: symposium of the British Mycological Society held at the University of Liverpool, April 1992. Cambridge University Press