The desert biome covers approximately one third of the Earth´s surface, and occurs in regions where the annual average precipitation is less than 25 cm (10 inches) a year. There are regions which are extremely dry, and receive less than 2 cm (0.9 inches) per year. Temperatures in desert biomes can be very extreme. In subtropical deserts, temperatures may vary from over 38° C (100° F) during light hours to below 0° C (32° F) at night, while in cold deserts, temperatures may range from -2 to 4° C (28-39° F) in the winter and 20-25° C (68-79) in the summer.
Most hot and dry deserts are located in the tropics of cancer and Capricorn, 23.5° north or south of the equator. Other deserts are found in the continental mid-latitudes, and others further north or south of the equator in the arctic or Antarctic regions. Desert biomes are dependent upon average temperature and by the amounts of rainfall they receive annually. Deserts host a variety of plants, including desert holly, prickly pears, mesquites, yuccas, agaves, cacti plants, black bushes, and the brittlebush, among many others. These plants have adapted to desert conditions, and some have long roots that penetrate deep into the ground to absorb water. Almost all desert plants have spines instead of leaves, which allow them to conserve moisture.
Deserts are home to many burrowing animals, including rats, toads, rabbits, lizards, snakes, kangaroo rats, and badgers, who burrow underground to protect themselves from the desert heat, and only come out at night to feed. Most desert animals have developed camouflage, which allows them to remain hidden when hunting or avoid attack from predators. Deserts do not offer protection from the Sun for large animals; therefore, there are few large mammals because most are not capable of storing sufficient water to survive. According to National Geographic, there are five major types of desert biomes in the world: subtropical, coastal, rain shadow, interior, and polar deserts.
The Sahara desert, which is the world´s largest desert, is a subtropical desert in Northern Africa. Other subtropical deserts include the Tanami desert in Australia, and the Kalahari Desert in southern Africa. Their average temperature ranges from 38° C (100° F) during the day and below 0° C (32° F) at night. The average rainfall is approximately 15 cm (6 inch) a year. Plants are principally short woody trees, and ground hugging shrubs. Most animals are burrowers and kangaroo rats, although, there are also reptiles, birds, arachnids, and insects. Most animals remain inactive during the day, and only come out at night for hunting.
Coastal deserts are generally located on the western coasts of continents near the tropics. This desert biome is affected by ocean currents. Blowing winds interacting with cold ocean currents produce winter fogs, blocking solar radiation. Despite the high humidity, atmospheric changes responsible for producing rainfall are not present. Atacama, which is a coastal desert in Chile, is the Earth´s driest desert with 1 mm or above of rainfall every 5-20 years. Small patches of herbs and shrubs support animals and insects. Flocks of flamingoes can be seen feeding on red algae. The Namib Desert in South Africa receives 19 mm of rain per year, which is insufficient for plant growth. Temperature ranges from 12-18° C (54-64° F) all year round.
Rain shadow deserts are found on the lee back side of the slopes of mountain ranges. Mountains block the passage of rain, resulting in a shadow of dryness behind them. Death Valley in the U.S. is in the rain shadow of Sierra Nevada Mountains. Places west of the Colorado Front Range receive as much as 122 cm (40 inches) of rainfall throughout the year, while regions on the east only receive from 43-48 cm (17-19 inches). The Gobi desert, which lies in the rain shadow of the Himalayas, receives little moisture, resulting in a very dry environment.
Interior deserts are formed inland of the continents. Most of the moisture that creates rain comes from the evaporation of ocean water. By the time coastal air masses reach the interior of continents, they have lost all their moisture, and little moisture is left for precipitation. The Gobi desert in China and Mongolia lies well inside the interior of the continent, and lies in the rain shadow of the Himalaya Mountains, as well.
Polar deserts lie in the arctic and Antarctic regions. They contain large amounts of water; however, most water is contained within glaciers and ice sheets year round, leaving little water for survival of animals and plants. The largest polar desert in the world, Antarctica, has an annual precipitation of less than 20 cm (8 inches) a year and a mean temperature of less than 10° C during the warmest months. Temperatures change frequently, crossing the freezing point of water (0° C or 32° F).
Desert biomes vary according to altitude, amount of precipitation, and temperature. These factors determine the biodiversity found in them. The predominant plant biomass in most deserts is composed of perennial shrubs with deep roots and spine-like leaves. Therophytes (annual plants) can survive extremely dry periods as seeds, and remain viable in the soil for several years, until conditions to germinate occur. Some animals have adapted their metabolism well enough to survive for several days; however, both plants and animals are most able to survive in a biome where there are significant amounts of rainfall.
