DESERT

Less than 25 centimeters of annual precipitation usually falls in the world's desert ar eas-so low an amount that water is the predominant controlling variable formosi biological processes and is also highly variable in quantity both during a given yeai and from year to year. In desert regions the vegetation is characteristically rather sparse. Such regions occur around 20 to 30 degrees north and souili latitude, where the warm air that rises near the equator falls and precipitation is limited. Deserts are most extensive in the interiors of continents, especially in Africa (the Sahara Desert), Eurasia, and Australia. Less than 5% of North America is desert.

Because the vegetation is sparse and the skies are usually clear, deserts radiate heat rapidly at night. This leads to large daily changes in temperature, sometimes exceeding 30° C, between day and night. Summer daytime temperatures in deserts are extremely hot, frequently exceeding 40° C. Indeed, atmospheric temperatures of 58° C have been recorded both in Libya and in San Luis Potosi, Mexico-the highest that have been recorded on earth. Annual plants often are abundant in deserts and simply bypass the unfavorable tin' season in the form of seeds. After the rainfall, they germinate and grow rapidly, sometimes forming spectacular natural displays. Exhibiting a similar seasonal rhythm, animals such as fairy shrimps often appear and breed rapidly in temporary ponds.
The trees and shrubs that live in deserts often have deep roots that reach sources of water far below the surface of the ground. Thus trees may grow, even in regions that essentially lack precipitation, such as in the Atacama Desert of northern Chile, The woody plants that grow in deserts may be either deciduous, losing their leaves during the hot, dry seasons of the year, or evergreen, with hard, reduced leaves: the creosote bush (Larrea) of the deserts of North and South America is an example of an evergreen desert shrub. Near the coasts, in areas where there is cold water offshore, deserts may be foggy, and the water that the plants obtain from the fog may allow them to grow quite luxuriantly.

Succulent plants are more common in desert regions than elsewhere. Like many other kinds of plants and animals that occur in desert regions, they often have evolved a life-form that is successful under desert conditions, Such a phenomenon, in which unrelated groups of organisms evolve to become superficially similar, is called convergent evolution. Such succulent plants often exhibit CAM photosynthesis. Their stomata open at night and close during the day, thus conserving water while cutting down on photorespiration during the day. Succulent plants that have this system are thus able to avoid the effects of the intense heat of the desert days and still photosynthesize efficiently.

Desert animals, too, have fascinating adaptations that enable them to cope with the limited water and high temperatures of the deserts. They often limit their activity to a relatively short period of the year when water is available, or even plentiful:they resemble annual plants in this respect. Many desert vertebrates live in deep, cool, and sometimes even somewhat moist burrows; and those that are active over a greater portion of the year emerge from these burrows only at night, when temperatures are relatively cool. Some, like camels, drink large quantities of water when it is available and then safely withstand the loss of much of it. Many animals simply migrate to or through the desert, where they exploit food that may be abundant seasonally; when the food disappears, the animals move on to more favorable areas.
Many arthropods combine several of these strategies with a tough exoskeleioc and colors suited to the absorption or reflection of heat, depending on their habits. The cuticles and exoskeletons of all arthropods constitute an excellent preadaptauon to desert conditions and are an important reason why the members of this phylum are so successful in desert. Desert survival depends on water conservation, achieved by structural, behavioral, or physiological adaptations. Plants and animals may restrict their activity to favorable times of the year, when water is present.
One of the means by which desert animals avoid seasonal extremes in heat and dryness in the desert is estivation. Estivation is a prolonged state of torpor that occurs under hot, dry conditions, in contrast to the hibernation of animals in cold climates, which is a more profound and prolonged condition of dormancy. Ground squirrels (Spermophilus) hibernate in the cold regions of North America, whereas [he species of the same genus that occur on the deserts estivate, each thereby avoiding the extremes of the most unfavorable seasons, Estivation occurs in a number of desert rodents and some other animals, including a few birds such as the whippoorwill.