The Desert Biome
The defining characteristic of a desert is that it is dry. Depending on its geographical location, the annual precipitation in a desert varies from half an inch to as much as 15 inches. Rainfall is usually very localized, and although it is frequently seasonal, it is difficult to predict when or where it will occur. At times in the Atacama Desert in Chile, years have passed with no measurable rainfall at all. However, that is not generally the caseDeserts can be either hot such as the Australian Desert or cold such as the Gobi Desert.
As with all biomes, the desert climate is determined by geographic conditions. Geographic conditions such as location, high atmospheric pressure, and proximity of mountain ranges determine just what type of desert it is.Deserts may occur along the coast such as the Atacama and Namib deserts or in the interior of continents such as the Great Basin and Australian deserts, which are far from any source of water. Coastal deserts are located on west coasts of continents between 20° to 30° latitude. Prevailing winds blow in an easterly pattern and prevent the moisture from moving onto the land. Semiarid deserts, like the Great Basin Desert, are not only located far from moisture, but are frequently associated with high mountain ranges that produce a rainshadow effect. The rainshadow effect prevents available moisture from reaching the area. The great Gobi Desert of Mongolia has little rainfall because the Himalayan Mountains prevent rainfall from moving into this region.Because all deserts are dry, they have large daily temperature variations. Temperatures are high during the day because there is very little moisture in the air to block the Sun's rays from reaching Earth. Once the Sun goes down, the heat absorbed during the day quickly escapes back into space. High daytime temperatures and low nighttime temperatures make survival in the desert very difficult.
Animals
At first glance, deserts may appear to be without animal life. However, deserts are home to many reptiles, insects, birds, and small mammals. The kangaroo mice of North America and the bilby and red kangaroo of Australia are just a few examples of small mammals that live in the desert. Most large animals have not adapted to desert life. Their size prevents them from finding shelter from the Sun's heat and they are not able to store water for future use. Animals that do survive in the desert have developed a number of adaptations. Photo: Red Kangaroo.
Animal adaptations
The most universal behavioral adaptation used by small mammals, reptiles, and insects to deal with high temperatures is staying in the shadow (shade) of plants or rocks, thus avoiding the direct rays of the Sun. These animals also seek shelter by burrowing into the ground. Just as a basement room is cooler than an above-ground room, a burrow, even a few feet underground, can decrease the temperature by several degrees. Another behavioral adaptation used by desert animals is to remain inactive during the hot daylight hours. They hunt at night when temperatures are cool and when there is less risk of losing precious body water. Animals that use this adaptation are referred to as nocturnal. Some animals get all of the water they need from the insects, bulbs, and seeds they eat. They will not drink water even when it is available. Some animals have developed salt glands, a physical adaptation that allows the secretion of salt without the loss of water. The absence of sweat glands, and the concentration of urine are other physical adaptations made by desert animals. Because fat intensifies heat, a unique physical adaptation of some desert animals is the storage of fat in humps or tails, rather than throughout the entire body.
Plants
Short grasses, sagebrush, creosote bushes, and cacti are just a few of the plants that can be found in the desert. Plant abundance and variety are determined by the geographic location of the desert. Although short grasses can be found in nearly all desert locations, the saguaro cactus is unique to the Sonoran Desert, and the spiniflex is associated with the Australian Desert.
Plant adaptations
Because of the dry climate, plants have developed a number of different methods of capturing water. Some plants have developed long (20-30 foot) taproots that go deep into the ground and tap into groundwater sources. Other plants have developed extensive horizontal root systems. These horizontal root systems lie just below the surface and extend far beyond the plant canopy. When it rains the numerous tiny roots capture the water. The mulga tree's root system lies close to the base of the tree. The tree survives because it has developed its own unique system of collecting water. The tree's numerous tiny leaves grow upward. When it rains the leaves capture the water and funnel it down along the branches to the center of the tree. The water then falls to the ground near the trunk of the tree where tree roots are concentrated.Another common physical adaptation is the ability of desert plants to store water in their roots, stems, leaves, or fruit. Plants that store water in this way are referred to as succulents, and they include cacti.Desert plants retain moisture by limiting water loss through their leaf surface. Many plants accomplish this by adapting the size, sheen, or texture of their leaves. Small leaves or spines limit the amount of surface area exposed to the drying heat. Glossy leaves reflect the Sun's radiant heat reducing leaf temperatures and evaporation rates. Waxy leaves prevent moisture from escaping. Water escapes from leaves through the stomata, or leaf pores. A behavioral adaptation used by some plants is to only open leaf pores during the night when air temperature is cool and evaporation rate is low.
Desert Worksheet
Read the definitions below, then label the major deserts on the world map.
Arabian - covering most of the Arabian peninsula.
Atacama - running down the western coast of South America (in Chile).
Australian - hot deserts covering much of western and central Australia.
Iranian - northeast of the Arabian peninsula and west of the Thar desert.
Kalahari - covering much of the southeastern tip of Africa.
Namib - running down the southwestern coast Africa.
North American - rain shadow deserts in southwestern North America, in the USA and Mexico.
Patagonian - in southeastern South America.
Saharan - a huge desert covering much of northern Africa - the biggest desert in the world.
Takla Makan-Gobi - a continental desert in central Asia.
Thar - in northwest India.
Turkestan - in southern Russia .
Writing Activity: A Trip to the Desert
Write a report about a real or imaginary trip to the desert. What would you do in the desert? Would you ride a camel? What would the weather be like? How would you feel? Think about your five senses!