DeserstFar from being barren wastelands, deserts are biologically rich habitats with a vast array of animals and plants that have adapted to the harsh conditions there. Some deserts are among the planet's last remaining areas of total wilderness. Yet more than one billion people, one-sixth of Earth's population, actually live in desert regions. Deserts cover more than one-fifth of Earth's land area, and they are found on every continent. A place that receives less than 25 centimeters (10 inches) of rain per year is considered a desert. Deserts are part of a wider classification of regions called “dry lands”. These areas exist under a “moisture deficit,” which means they can frequently lose more moisture through evaporation than they receive from annual precipitation. Despite the common conceptions of deserts as hot, there are cold deserts as well. The largest hot desert in the world, northern Africa's Sahara, reaches temperatures of up to 50 degrees Celsius (122 degrees Farenheit) during the day. But some deserts are always cold, like the Gobi desert in Asia and the polar deserts of the Antarctic and Arctic, which are the world's largest. Others are mountainous. Only about 20 percent of deserts are covered by sand. The driest deserts get less than half an inch (one centimeter) of precipitation each year, and that is from condensed fog, not rain. Desert animals have adapted ways to help them keep cool and use less water. Camels, for example, can go for weeks without water. Many desert animals, such as the fennec fox (Vulpes zerda), are nocturnal, coming out to hunt only when the brutal sun has descended. Some animals, like the desert tortoise (Gopherus agassizzi) in the southwestern United States, spend much of their time underground. Most desert birds are nomadic, crisscrossing the skies in search of food. And among insects, the Namib desert beetle (Stenocara gracilipes) can harvest fog from the air for water. Because of their very special adaptations, desert animals are extremely vulnerable to changes in their habitat. Desert plants may have to go without fresh water for years at a time. Some plants have adapted to the arid climate by growing long roots that tap water from deep underground. Other plants, such as cacti, have special means of storing and conserving water. Many desert plants can live to be hundreds of years old. Some of the world's semiarid regions are turning into desert at an alarming rate. This process, known as desertification, is not caused by drought, but usually arises from deforestation and the demands of human populations that settle in semiarid lands to grow crops and graze animals. The pounding of the soil by the hooves of livestock in ranching, for example, may degrade the soil and encourage erosion by wind and water. Global warming also threatens to change the ecology of deserts. Higher temperatures may produce an increasing number of wildfires that alter desert TEXT 3 landscapes by eliminating slow-growing trees and shrubs and replacing them with fast-growing grasses.
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