Desert INTRODUCTION Scientists define a desert as a region that receives less than 25 centimeters (10 inches) of rain per year. About onethird of Earth s land surface is desert. Deserts may be hot or cold, with great expanses of barren sand or snow or sandy gravel with hardy vegetation. In hot deserts precipitation falls as rain; in cold deserts, it falls as snow. Both types of desert have there is very little precipitation and very low humidity. Art Explosion Deserts have a wide variety of organisms that have adapted to live in extreme environments. Art Explosion Very few of the world's deserts are barren, windswept sand dunes. Only 30% of the Sahara is sand dunes like the ones shown here. The largest single desert is in Antarctica. It is a polar desert and very cold year-round. Subtropical deserts are the hottest and most numerous. They have hot summers, mild winters, and rapid evaporation. Cold winter deserts have hot summers and cold winters. They are often found at higher elevations. The rarest type of desert is cool coastal deserts. They are the same latitudes as subtropical deserts, but are cooler because of cold offshore currents. Page 1 of 6
Data from http://www.infoplease.com/ipa/a0778851.html Deserts are found throughout the world. Other deserts are caused by a rain shadow. As air moves over a mountain range, it gets colder and can t hold as much water vapor. Rain or snow falls on the mountaintops. When the air moves down the other side of the mountain, it warms and can hold more water vapor. No rain or snow falls; a desert forms. ABIOTIC DATA Deserts are be among the hottest and coldest places on Earth. Temperatures in hot deserts may reach 38 C (100 F) or more. At night the temperatures may fall to 0 C (32ºF) or less. The dry air and lack of a cloud cover that admits solar radiation during the day, allows for a rapid heat loss at night. Some examples of hot deserts are the Arabian Desert on the Arabian Peninsula; the Great Sandy, Victoria, and Gibson Deserts in Australia; the Chihuahuan, Mojave, and Sonoran Deserts in Mexico and southwestern United States; the Kalahari and Sahara Deserts in Africa; and the Thar Desert in India and Pakistan. Page 2 of 6
Data from Missouri Botanical Garden Deserts receive less than 25 centimeters (10 inches) rainfall per year. Cold deserts have short, moist, and moderately warm summers. The winters are long and cold. The average winter temperature is between 2 and 4 C (31 39 F); the average summer temperature is between 21 and 26 C (70 79 F). Examples of cold deserts are the Atacama Desert along the coasts of Peru and Chile, the Gobi in northern China and southern Mongolia, the Great Basin in the United States, the Namib Desert along the southwest coast of Africa, and the Turkestan Desert in the Middle East and southwest Russia. The polar desert on Antarctica has snow dunes instead of sand dunes. The small amount of snowfall does not melt in the summer and through the years has built up. Harsh polar winds blow the snow in this cold, dry climate. Art Explosion Sand dunes are constantly moving with the prevailing winds. All deserts have very low humidity, and any moisture that falls or flows into a desert evaporates quickly. Deserts may be rocky or sandy, flat or mountainous. Desert pavement covers much of the surface of hot deserts. This layer of rocks and gravel remains after smaller sand particles have blown away. Desert pavement can become very hard packed. Sand dunes form when prevailing winds move the smallest particles. They can become lodged in cracks and crevices Page 3 of 6
or next to plants. They are constantly on the move, one grain of sand at a time. It takes centuries for large sand dunes to form. BIOTIC DATA Deserts support fewer plant and animal species than do environments with more moisture. Desert plants and animals have adapted to harsh conditions in order to survive. Soils in the desert have very little humus or organic matter. What little organic matter is found in these sand and gravel soils is in the form of lichens, fungi, and cyanobacteria growing on the surface. This thin and fragile layer is called desert crust. The fibrous mats of microorganisms anchor and protect desert soils. Plants grow far apart in deserts. Their root systems often spread over a large area. This allows them to gather water and nutrients without competition from other plants. Cacti are one type of plant that have adapted to life in a desert. They store water in their thick leaves and stems and use it during the long dry periods. Courtesy of John and Karen Hollingsworth, U.S. Fish and Wildlife Service Plants in the yucca forest of the Desert National Wildlife Refuge in Nevada are widely spaced. Their roots may spread far from the plant to gather water. During spring in the Mojave Desert, wildflowers bloom when water is abundant. Ground between these creosote bushes is bare the rest of the year. The seeds of some flowering plants can lie dormant in soil for many years until rain provides the moisture they need to sprout. During these short, wet periods the desert will come alive with wildflowers. A number of animals live in the desert. These include insects, spiders, reptiles, birds, and small mammals. All animals living here have adapted to survive in harsh desert conditions. Some have unique ways of storing water. Many desert animals are nocturnal and come out only at night when the temperatures are relatively cool. With few plants to hide in, some animals have adaptations that make it difficult for predators to find them among sand and rocks. Color and body shape help to camouflage them. Page 4 of 6
Desert tortoises get most of their water from plants they eat and store it in their bodies. They can withstand long periods without water. Courtesy of John Game The horned lizard uses camouflage to hide among the dusty rocks and gravel in the desert. Few large mammals live in the desert, since most cannot store water and survive the heat. Some mammals, such as deer, coyotes, and wolves, visit deserts in search of food. ISSUES Human use of deserts is a concern to many environmentalists. Because water is so scarce, it takes many centuries for vegetation to grow in the desert. Any disturbance of the desert crust and shallow plant roots will kill many desert plants. These plants provide shelter and food for many animals. These motorcyclists race across the Mojave Desert between Barstow, California, and Las Vegas, Nevada. Overgrazing by livestock like these sheep can damage the fragile desert crust. The use of off-road vehicles puts plants, animals, and desert landscapes such as sand dunes at risk. Grazing by livestock may cause damage through soil erosion, water pollution, and destruction of desert crust. Once damaged, the crust can take centuries to recover. Mining, drilling, and resource exploration can also threaten desert habitats. Minerals in mine tailings and chemicals used in the mining process can leach into the soil. Remains of mines abandoned in the 19th century are found Page 5 of 6
throughout the deserts of the southwest United States. Damage caused by offroad vehicles in the Mojave Desert, California. Trails in desert crust will remain visible for hundreds of years. These are mine tailings, rocks that have been dug out of a mine. Are deserts likely to expand? Scientists at NASA have suggested that a worldwide expansion of deserts is likely. Soils are extremely dry in deserts because evaporation and transpiration is greater than the average rainfall. If global temperatures were to rise by 4 C, the potential evaporation could increase 30-40 percent, while precipitation would only increase 10-15 percent. As a result, deserts would expand toward the poles and toward the equator. Scientists are predicting decreased water supplies over continents as deserts expand and evaporation rates increase. Rivers could dry up as droughts intensify, and the higher evaporation rates might mean a decrease in groundwater supplies for plants. Page 6 of 6