Desert

A desert is generally a very hot, barren region on Earth that receives little rainfall. Most sources describe a region as being a desert if it receives less than 10 inches (25.4 centimeters) of rain a year. It has also been described as a place where more water evaporates than falls as precipitation. Despite being an extremely harsh environment, deserts support a diverse community of both plant and animal life. As one of the six terrestrial (land) biomes (particular types of large geographic regions), deserts cover between one fifth and one quarter of Earth's surface.

A desert is a stark, dramatic place whose topography (surface conditions) is almost immediately recognizable. Its miles of sand dunes or endless stretches of flat, featureless sand are not easily forgettable; nor are its strangely adapted plants (like cacti) apt to be confused with vegetation from some other region. It is easy to understand what makes a desert what it is. Any part of Earth that constantly experiences a water "debt" rather than a water "surplus" is so dry that the need to capture, conserve, and store water is not only overwhelming, but affects and determines everything living in that place. Despite the impression that a desert is a lifeless place, it is home to certain plants and animals who have adapted to its harsh conditions and who do very well there.

THE LOCATION OF DESERTS

Most of the world's deserts are located on two desert belts that wrap around Earth's equator (the circular band around Earth's middle which divides the Northern and Southern Hemispheres). The belt in the Northern Hemisphere is along the tropic of Cancer and includes the Gobi Desert in China, the Sahara Desert in North Africa, the deserts of southwestern North America, and the Arabian and Iranian deserts in the Middle East. The belt in the Southern Hemisphere is along the tropic of Capricorn and includes the Patagonia Desert in Argentina, the Kalahari Desert of southern Africa, and the Great Victoria and Great Sandy Deserts of Australia. Altogether, there are about twelve major deserts, the largest of which is the Sahara Desert which measures 3.5 million square miles (5.63 million square kilometers). This is an area as big as the entire United States.

THE CREATION OF DESERTS

In a way, deserts are made and not born, meaning that Earth's weather patterns created a desert in the first place and continue to work to keep it that way. These regular patterns, or moving currents of hot and cold air interact with each other so that descending currents of air pick up moisture and dry out the land. Mountain ranges also influence these currents, as dry air moving off their slopes evaporate even more moisture. The steady lack of moisture in the air above a desert region leads to extreme changes in temperature once the Sun goes down. In normally humid areas, the moisture in the air acts as an insulating barrier, and clouds keep some of the daytime warmth from the Sun trapped, thereby moderating temperatures. However in a desert, which has no moisture in the air above it, there are no clouds to act as a blanket, meaning that although daytime temperatures are extremely hot, they can be near freezing at night.

As with any biome, deserts vary considerably throughout the world, and they can be as diverse as the lifeless-looking and appropriately named Death Valley in California and Nevada, and the almost-lush looking Vazcaino Desert in Mexico when it bursts into flower following its annual spring rain. Even in as harsh an environment as Death Valley or the Sahara Desert, life can be found. Sometimes life is a dormant seed buried for years and waiting for a bit of moisture so that the seed can spring into existence as an aboveground plant. Other times desert life is a toad hibernating below ground and rushing to find a mate and lay its eggs as soon as it rains. Life in a desert is a constant challenge, and plant and animal inhabitants do not have the luxury of being wasteful that other organisms in more temperate climates might have.

PLANT LIFE IN THE DESERT

Desert plants have evolved many methods to obtain and efficiently use available water. Certain ones compress their entire life cycle into one growing season. The seeds or bulbs of some flowering desert plants can lie dormant in the soil for years until a heavy rain enables them to germinate (sprout), grow, and bloom. Woody plants may develop deep root systems—like the mesquite whose tap root can measure 50 feet (15.24 meters) down although the aboveground tree is only about 10 feet (3.048 meters) tall. They may also develop a network of shallow, spreading, hairlike roots that can take up water from dew and the occasional rain shower before it seeps below ground. For many plants, the answer to years of absolute drought is to drop leaves, allow the aboveground part of the plant to die, and keep the underground root alive in a state of dormancy (functioning slowly or not at all). Conserving and storing water becomes important for a plant once it has obtained moisture. Since all plants lose water by evaporation from their leaves, many desert species minimize this by having very small or rolled leaves, or by turning their leaves into spines or barbs. These thornlike leaves protect a plant's water supply from animals. The problem of storing water is solved by the cactus, which is a succulent and can store water in its leaves, stems, and roots. An amazing example of adaptation is the Saguro cactus of the American southwest. The trunk of this large cactus is folded or pleated like an accordion, which can unfold and expand as the plant absorbs water after a heavy rain. A Saguro that is 20 feet tall (6.1 meters) can hold more than one ton (1.102 metric tons) of water.

ANIMAL LIFE IN THE DESERT

Desert animals, like desert plants, have also evolved ways to cope with the desert's arid environment using avoidance and/or adaptation. Besides the highly adapted camel, most desert animals are small and do not have an extensive range. While their size limits their ability to leave, it does make easier their ability to remain in cool underground burrows during the day and emerge only after dark to feed. Animals that do this are called "nocturnal" or "crepuscular." Other small mammals and reptiles survive the most extreme times by a process called estivation, which is similar to the sleeplike state of hibernation. Other animals have adapted specialized body parts to help them cool off. A well-known example is the huge ears of the small Fennec fox of the Sahara Desert and the Kit fox of North America. Both have enormous ears whose dense network of tiny blood vessels run just below their skin and act as radiators, releasing excess heat. Larger desert animals developed broad hooves that allow them to move more easily over soft sand. Some animals can actually slow down their production of body heat by varying their heart rate, while others reabsorb the water in their urine several times before finally excreting a highly concentrated form of urine.

Just as the animal and plant population in deserts is not overly abundant because the desert's difficult conditions can only support small numbers, deserts cannot support humans in large numbers. People must, like animals and plants, make adjustments in order to survive in the desert's extremes, and in the past they have lived in mud houses that kept cool in the daytime and provided warmth at night. Long robes were often used in Africa and the Middle East for protection against the scorching sunlight and blowing sand. With today's technology, however, people can live comfortably in a desert if they have air conditioning and an adequate water supply. A good, steady source of water also allows humans to raise crops in a desert since they are usually naturally fertile regions because there is seldom enough water to leech away important nutrients. Crops can be grown on desert lands with irrigation, but farmers must be prepared to deal with a buildup of salts in the soil as a result of evaporation (which takes away most of the water they put down). Humans can also be responsible for creating deserts or allowing an existing desert to spread. This is usually the result of burning or overgrazing of animals. When a desert encroaches, or spreads, to arable land (land able to be farmed), that process is called "desertification."

[See alsoBiome ]