All deserts have two things in common: they are dry, and they support little plant and animal life. If a region receives an average of fewer than 10 inches (25 centimeters) of rain each year, it is classified as a desert. Contrary to what most people believe, not all deserts are hot. Some deserts near the North and South Poles are so cold that all moisture is frozen—these are called polar deserts. Tropical desert areas are near the equator. Temperate desert areas are between the tropics and the North and South Poles.
True deserts cover about one-fifth of the world’s land area. With the addition of polar deserts, the total rises to 30 percent. Another 25 percent of Earth’s land surface possesses desertlike characteristics. In all, deserts constitute 33 million square miles (86 million square kilometers). Most deserts lie near the tropic of Cancer and the tropic of Capricorn, two lines of latitude about 25 degrees from the equator. The area between these two lines is called the Torrid Zone (torrid means very hot).
Deserts are generally caused by the presence of dry air. The average humidity (moisture in the air) is between 10 and 30 percent. In some cases, mountain ranges prevent moisture-laden clouds from reaching the area. Mountains can cause heavy, moisture-filled clouds to rise into the colder atmosphere. There, the moisture condenses and falls in the form of rain, leaving the air free of moisture as it crosses the range. In other cases, certain wind patterns along the equator bring air in from dry regions. Cold-water ocean currents can cause moist air to drop its moisture over the ocean. The resulting dry air quickly evaporates (dries up) ground moisture along the coastal regions as it moves inland.
Deserts have always existed, even when glaciers covered large portions of Earth during the great Ice Ages. Although geological evidence is scarce,
|WORDS TO KNOW|
|Arroyo: The dry bed of a stream that flows only after rain; also called a wash or a wadi.|
|Butte: A small hill.|
|Deforestation: The cutting down of all the trees in a forest.|
|Desertification: The changing of fertile lands into deserts through destruction of vegetation (plant life) or depletion of soil nutrients. Topsoil and groundwater are eventually lost as well.|
|Dormant: A suspension of growing (plants) or activity (animals) when conditions are harsh.|
|Estivation: An inactive period experienced by some animals during very hot months.|
|Mesa: A flat topped hill.|
|Oasis (plural is oases): A fertile area in the desert having a water supply that enables trees and other plants to grow there.|
|Wadi: The dry bed of a stream that flows after a rain; also called a wash, or an arroyo.|
|Xeriscaping: Landscaping method that uses drought tolerant plants and efficient watering techniques.|
scientists tend to agree that some desert areas have always been present, but they were probably smaller than those of today. Fossils, the ancient remains of living organisms that have turned to stone, can reveal the climatic history of a region. For example, scientists believe that the Arabian Desert, which covers most of the Arabian Peninsula to the east of North Africa, once included wetlands because fossils of a small species of hippopotamus have been found there. In the Sahara Desert of North Africa, rock paintings made 5,000 years ago show pictures of elephants, giraffes, and herds of antelope that are no longer present.
Desertification (DES-aurt-ih-fih-KAY-shun; desert formation) occurs continuously, primarily on the edges of existing deserts. It is caused by a combination of droughts (rainless periods) and human activity such as deforestation (cutting down forests) or overgrazing of herd animals. When all the grass is used and rain is scarce, plants do not grow back. Without plants to hold the soil in place, wind blows away the smaller and finer particles of soil, exposing the less compacted layer of sand. This leaves a barren, unprotected surface. Eventually, even groundwater disappears.
Scientists measure a region’s aridity (dryness) by comparing the amount of precipitation (rain, sleet, or snow) to the rate of evaporation.
Evaporation always exceeds precipitation. Deserts can be classified as hyperarid (less than 1 inch [2.5 centimeters] of rain per year); arid (up to 10 inches [25 centimeters] of rain per year); and semiarid (as much as 20 inches [50 centimeters] of rain per year, but are so hot that moisture evaporates rapidly). Most true deserts receive fewer than 4 inches (10 centimeters) of rain annually.
Except for those at the North and South Poles, which are special cases, deserts are classified as hot or cold. Daytime average temperatures in hot deserts are warm during all seasons of the year, usually above 65°F (18°C). Nighttime temperatures are chilly and sometimes go below freezing. Typical hot deserts include the Sahara and the Namib Desert of Namibia. Cold deserts have hot summers and cold winters. At least one month during the year the mean temperature is below 45°F (7°C). Cold deserts include Turkestan in Kazakhstan and Uzbekistan, Gobi (GOH-bee)
in China and Mongolia, and the Great Salt Lake Desert in Utah. These deserts usually get some precipitation in the form of snow.
Deserts can be further characterized by their appearance and plant life. They may be flat, mountainous, broken by gorges and ravines, or covered by a sea of sand. Plants may range from nearly invisible fungi to towering cacti and trees.
Although desert climates vary from very hot to very cold, they are always arid (dry).
In hot deserts, days are usually sunny and skies are cloudless. During the summer, daytime air temperatures between 105° and 110°F (43.8° and 46.8°C) are not unusual. A record air temperature of 136.4°F (62.6°C) was measured in the Sahara Desert, in a place called El Azizia, on September 13, 1922. The absence of vegetation exposes rocks and soil to the sun, which may cause ground temperatures in the hottest deserts to reach 170°F (77°C). Nights are much cooler. The lack of cloud cover allows heat to escape and the temperature may drop 25 degrees or more after the sun sets. At night, temperatures of 50°F (10°C) or less are common, and they may even drop below freezing.
The Sands of Time
When living things die, moisture in the air aids the bacteria that cause decay. Before long, tissues dissolve and eventually disappear. Desert air is so dry that decay does not take place or occurs extremely slowly. Instead, tissues dry out and shrink, turning an animal or human being into a mummy.
In ancient Egypt around 3000 BC, the dead were buried in shallow graves in the sand. The very dry conditions mummified the bodies, preserving them. Later, for those who could afford it, Egyptian burials became more complex. Internal organs were removed, and the bodies underwent special treatments designed to preserve them. They were then placed into tombs dug into rocky cliffs or, in the case of certain pharaohs (kings), placed within huge pyramids of stone. In most cases, bodies of the ancient Egyptians are so well preserved that much can still be learned about what they ate, how they lived, and what caused their deaths.
Graves discovered in the Takla Makan (TAHK-lah mah-KAN) Desert of China have also given scientists important information. (The name Takla Makan means “the place from which there is no return.”) Well-preserved mummies as much as 3,800 years old have been found in the graves. The mummies have European features and some are dressed in fine woolens woven in tartan (plaid) patterns commonly used by the ancient Celts and Saxons of Northern Europe. Scientists believe these mummies were the first Europeans to enter China, which was officially closed to outsiders for thousands of years. Evidence exists that these people rode horses using saddles as early as 800 BC, and they may have introduced the wheel to China. Their descendants, who have intermarried with the Chinese, still live in the Takla Makan.
Winters in cold deserts at latitudes midway between the polar and equatorial regions can be bitter. In the Gobi Desert, for example, temperatures below freezing are common. Blizzards and violent winds often accompany the icy temperatures.
Rainfall varies from desert to desert and from year to year. The driest deserts may receive no rainfall for several years, or as much as 17 inches (43 centimeters) in a single year. Rainfall may be spread out over many months or fall within a few hours. In the Atacama Desert of Chile, considered the world’s driest desert, more than half an inch (1.3 centimeters) of rain fell in one shower after four years of drought. Such conditions often cause flash floods, which sweep vast quantities of mud, sand, and boulders through dry washes, gullies, and dry river beds (called wadis or arroyos). The water soon evaporates or disappears into the ground. The Atacama Desert is the site of the world’s longest known drought; no rain fell for 400 years (from 1571 until 1971).
In coastal deserts, fog and mist may be common. Fog occurs when cold-water ocean currents cool the air and moisture condenses. The Atacama Desert lies in a depression behind mountains, so most of its precipitation is received in fog form.
Some deserts, such as the polar deserts, experience snow rather than rain or fog.
The geography of deserts involves landforms, elevation, soil, mineral resources, and water resources.
Desert terrain may consist of mountains, a basin surrounded by mountains, or a high plain. Many desert areas were once lake beds that show the effect of erosion and soil deposits carried there by rivers. Wind
helps shape the desert terrain by blowing great clouds of dust and sand that break down rock, sometimes sculpting it into strange and magnificent shapes. In the Australian Desert, unusual pinnacles (tall mountain shapes) of limestone rock, formed over thousands of years by the wind, stand on the desert floor. These limestone rock formations may be divided into two types. A mesa, the Spanish word for plateau, is a steep-sided, flat-topped hill. The mesa may be reduced to a butte (BYUT), the French word for hill or knoll, by erosion.
Disappearance of Lady Be Good
Becoming lost in the desert can often end in tragedy. In 1943, an American bomber, called Lady Be Good, went off course and crashed in the Sahara Desert. The surviving crew saw a line of hills in the distance and mistook them for the hills around the Mediterranean, where they hoped to find human settlements. At night they walked and they rested by day, covering 75 miles (120 kilometers) in a week. As it turned out, the hills were not those around the Mediterranean, and the crewmen were still 375 miles (600 kilometers) from the sea. The entire crew died from sun exposure and lack of food and water. Their bodies were not found until 1960—seventeen years later.
Bare rock, boulders, gravel, and large areas of sand appear in most desert landscapes. Vast expanses of sand dunes, sometimes called ergs, are not very common. Sand dunes make up less than 2 percent of deserts in North America, only 20 percent of the Sahara, and 20 percent of the Arabian Desert. The Empty Quarter (Rub’ al-Khali) in the Arabian Desert is the largest sandy desert in the world, covering about 250,000 square miles (647,500 square kilometers).
Unless anchored by grass or other vegetation, sand dunes migrate constantly. Their rate of movement depends upon their size—smaller dunes move faster—and the speed of the wind. Some dunes may move over 100 feet (30 meters) in a year and can bury entire villages. In the Sahara, dunes created by strong winds can achieve heights of 1,000 feet (300 meters). Scientists estimate that dunes in the Namib Desert are the largest in the world.
Dunes take different shapes, depending upon how they lie in respect to prevailing winds. When the wind tends to blow in one direction, dunes often form ridges. The ridges may lie parallel to the wind forming seif (SAFE), or longitudinal dunes; or at right angles forming transverse dunes. Seif dunes are the largest, with some in the Sahara approaching 100 miles (160 kilometers) in length. At desert margins where there is less sand, dunes may assume crescent shapes having pointed ends. These are called barchan (bahr-KAN) dunes, and the wind blows in the direction of their “points.” In the Sahara, stellar (star-shaped) dunes are commonly found. Stellar dunes form when the wind shifts often, blowing from several directions.
Desert soils tend to be coarse, light colored, and high in mineral content. They contain little organic matter because there is little vegetation. If the area is a basin or a catch-all for flash-flood waters, mineral salts may be carried to the center where concentrations in the soil become heavy. If the area was once an inland sea, like the Kalahari (kah-lah-HAHR-ee) Desert of Botswana, eastern Namibia, and northern South Africa, exposed bottom sediments (matter deposited by water or wind) are very high in salt.
Most desert sand is made of tiny particles of the mineral quartz. Placed under pressure for long periods, grains of sand may stick together forming a type of rock called sandstone.
Some deserts have little soil, exposing bare, wind-polished, pebbly rock, called desert pavement. Rocks are often broken due to contraction and expansion caused by extreme temperature variations. Basin areas scoured by winds show surfaces of gravel and boulders, and on steep slopes, whipping winds may leave little soil.
Dust devils, columns of dust that spin over the desert landscape, are carried by whirlwinds. Dust storms can produce clouds thousands of feet high. In the Sahara, up to 200 million tons (180 million metric tons) of dust is created each year. Saharan dust has occasionally crossed the Atlantic to the United States. It has been known to travel as far north as Finland.
Two long-lasting chemical reactions affect desert rocks and soil. One, called desert varnish, gives rocks, sand, and gravel a dark sheen. Desert varnish is believed to be caused by the reaction between the moisture from overnight dew and minerals in the soil. The second reaction is the formation of duricrusts—hard, rocklike crusts that form on ridges when dew and minerals such as limestone combine, creating a type of cement.
Desert soils offer little help to plant life because they lack the nutrients provided by decaying vegetation and are easily blown away, exposing plant roots to the dry air. Some deep-rooted plants can exist on rock, where moisture accumulates in cracks. Other plants remain dormant during the driest periods, thriving and blooming after brief rains.
Soil reveals much about a desert’s geological history. In Jordan, a Middle-Eastern country, the Black Desert takes its name from black basalt, a rock formed from volcanic lava.
Deserts exist at many altitudes. North American deserts are partly mountainous, but Death Valley, a large basin in California, is 282 feet (86 meters) below sea level at its lowest point. The main plateau of
the Gobi is 3,000 feet (914 meters) above sea level, and the Sahara extends from 436 feet (130 meters) below to 10,712 feet (3,265 meters) above sea level. Temperature, plant, and animal life, are all influenced by elevation.
Valuable minerals like gold and oil (petroleum) are often found in desert regions. In the Great Sandy Desert of Australia, miners hunt for gold nuggets. “Black gold,” as oil is often called, is found beneath the desert regions of the Middle East, where it formed over time from the sediment of prehistoric oceans. Iron ore is mined in portions of the Sahara. Borax—a white salt used in the manufacture of such products as glass and detergent—was once mined in Death Valley, California.
Water sources in the desert include underground reserves and surface water.
In addition to occasional rainfall, deserts may have reserves of underground water. These reserves, often trapped in layers of porous rock called aquifers, were formed over thousands of years when rainwater seeped underground. Reserves close to the surface may create an oasis, a green, fertile haven where trees and smaller plants thrive. The presence of water may allow a completely different biome to form like an island in the desert.
People who live in the desert dig wells into aquifers and other underground water sources to irrigate crops and water their animals. As
|WELL-KNOWN DESERTS OF THE WORLD|
|Arabian||Hot; extremely arid and arid||Arabian Peninsula||900,000 square miles (2,330,000 square kilometers)|
|Atacama||Hot; extremely arid and arid||Chile||70,000 square miles (181,300 square kilometers)|
|Australian||Hot; arid and semiarid||Australia||529,346 square miles (1,371,000 square kilometers)|
|Death Valley||Hot; arid||California (United States)||3,012 square miles (7,800 square kilometers)|
|Gobi||Cold; arid and semiarid||China, Mongolia||500,000 square miles (1,300,000 square kilometers)|
|Kalahari||Hot; arid||Southern Africa||10,038 square miles (260,000 square kilometers)|
|Mojave||Hot; arid||California and Nevada (United States)||25,000 square miles (65,000 square kilometers)|
|Namib||Hot; arid||Botswana, eastern Namibia, and northern South Africa||52,000 square miles (135,000 square kilometers)|
|Negev||Hot; arid||Israel||4,700 square miles (12,170 square kilometers)|
|Patagonian||Cold; arid||Argentina||260,000 square miles (673,000 square kilometers)|
|Sahara||Hot; extremely arid and arid||North Africa||35,600 square miles (92,200 square kilometers)|
|Thar||Hot; extremely arid||India and Pakistan||92,163 square miles (238,700 square kilometers)|
desert populations grow, water sources shrink and cannot be replaced fast enough. There is a real danger that groundwater reserves will one day be depleted.
Water may be found in desert areas in the form of rivers or streams. Some streams form only after a rain, when water sweeps along a dry river bed in a torrent (violent stream) then quickly sinks into
the ground or evaporates. Moisture sometimes remains under the surface, and plants can be seen growing along the path of streams.
Permanent rivers are found in desert regions. The Colorado River is one example. Over a million years ago, the Colorado River began cutting a path into the plateau of limestone and sandstone rock in northern Arizona, ultimately forming the Grand Canyon, which is 1.2 miles (1.9 kilometers) deep and 277 miles (446 kilometers) long. Perhaps the most famous desert river is the Nile, which bisects Egypt. Since ancient times, Nile floods have brought enough rich soil from countries farther south to turn Egypt’s river valley into fertile country well known for its agricultural products, such as cotton.
Permanent lakes rarely occur in desert regions. Two exceptions are the Great Salt Lake of Utah, which is all that remains of what once was a great inland sea, and the Dead Sea of Israel and Jordan. The Dead Sea is actually a salt lake that was once part of the Mediterranean.
One of the most important characteristics of any biome is its plant life. Not only do plants provide food and shelter for animals, they recycle gases in the atmosphere and add beauty and color to the landscape. Deserts support many types of plants, although not in large numbers.
Algae, fungi, and lichens
Algae (AL-jee), fungi (FUHN-ji), and lichens (LY-kens) do not fit neatly into either the plant or animal categories.
Most algae are single-celled organisms; a few are multicellular. Like plants, nearly all algae have the ability to make their own food by means of photosynthesis (foh-toh-SIHN-thuh-sihs). Photosynthesis is the process by which plants use the energy from sunlight to change water and carbon dioxide into the sugars and starches they use for food. Other algae absorb nutrients from their surroundings. Although most algae are water plants, the bacteria known as blue-green algae do appear in the desert. They survive as spores during the long dry periods and return to life as soon as it rains. (Spores are single cells that have the ability to grow into a new organism.)
Fungi are commonly found in desert regions wherever other living organisms are found. Fungi cannot make their own food by means of photosynthesis. Some species, like molds and mushrooms, obtain nutrients from dead or decaying organic matter. They assist in the decomposition (breaking down) of this matter, releasing nutrients needed by other desert plants. Fungi that attach themselves to living plants are parasites. Parasites can be found wherever green plants live, and may weaken the host plant so that it eventually dies. Other parasitic fungi actually help their host absorb nutrients more effectively from the soil. All fungi reproduce by means of spores.
Lichens are actually combinations of algae and fungi living in cooperation. The fungi surround the algal cells. The algae obtain food for themselves and the fungi by means of photosynthesis. It is not known if the fungi aid the algal organisms, but they may provide them with protection and moisture.
Lichens are among the longest living organisms. Some lichens in polar deserts are believed to have survived at least 4,000 years. Although lichens grow slowly, they are very hardy and can live in barren places under extreme conditions, such as on bare desert rock or arctic ice. Crusty types colored gray, green, or orange, often cover desert rocks and soil. During very dry periods, they remain dormant. When it rains, they grow and make food.
Most green plants need several basic things to grow: light, air, water, warmth, and nutrients. In the desert, light, air, and warmth are abundant, although water is always scarce. The nutrients—primarily nitrogen, phosphorus, and potassium—obtained from soil may be in short supply.
Earth’s Balancing Act
Many scientists recognize the many links among all life forms living on our planet. One biologist, James Lovelock, has theorized that life itself is responsible for changes in the land, water, and air. For example, until about 2 billion years ago, there was almost no oxygen in the atmosphere. Then blue-green bacteria, also called blue-green algae, began using energy from the sun for photosynthesis (a food-making process), which produces oxygen as a by-product. After enough blue-green algae and later plants got to work, the atmosphere eventually became, and is still maintained at, 21 percent oxygen, which is ideal for animal life. Lovelock believes that living things somehow work together in this way, instinctively providing a comfortable environment for themselves and one another.
To test his theory, Lovelock has produced a computer model. (Computer models enable scientists to more quickly study processes that take very long periods of actual time to show a result.) Suppose, for example, there are two species of flowers, one white and one dark blue. The white flowers reflect the sun’s heat and can survive in warm climates. The dark blue flowers absorb heat and do better in cooler climates. According to Lovelock’s model, the flowers help control the environment. The presence of blue flowers in a cool environment means heat is absorbed and the surrounding temperature is prevented from being too cool. The white flowers, on the other hand, reflect the heat and help keep a warm environment from being too warm.
Desert plants must protect against water loss and wilting, which can damage their cells. Large plants require strong fibers or thick, woody cell walls to hold them upright. Even smaller plants have a great number of these cells, which makes them fibrous and tough. Their leaves tend to be small and thick, with fewer surfaces exposed to the air. Outer leaf surfaces are often waxy to prevent water loss. Pores in the surface of green leaves allow the plant to take in carbon dioxide and release oxygen. The leaves of some desert plants may have grooves to protect their pores against the movement of hot, dry air. Other leaves curl up or develop a thick covering of tiny hairs for protection. Still others have adjusted to the dry environment by adapting the shape of their leaves. For, example cactus leaves are actually spines (needles). These spines have less surface area from which water can evaporate. As a result, more water is stored within the plant.
Common desert plants
Several species of plants grow in the desert, including cacti, shrubs, trees, palms, and annuals.
Cacti Cactus plants originated in southern North America, Central America, and northern South America. Instead of leaves, a cactus has spines, which come in many forms from long, sharp spikes to soft hairs.
A Prickly Compass
The stems of the barrel cactus of the southwestern United States grow in a curve. The curve always points south because the cactus grows faster on its northern side, which is in the shade.
Photosynthesis takes place in the stems and trunk of the plant. Nectar, a sweet liquid that appeals to insects, birds, and bats, is produced in the often spectacular flowers.
One of the largest plants in the desert is the giant saguaro (sah-GWAH-roh) cactus. Its large central trunk can grow as tall as 65 feet (20 meters) and have a diameter of 2 feet (60 centimeters). Ninety percent of its weight is water, which it stores in its soft, spongy interior. During very dry conditions, as the plant uses up this stored water, the trunk shrinks in size.
Like many desert plants, the saguaro has a wide, shallow root system designed to cover a large area. After a long dry period, its roots can take up as much as 1 ton (1 metric ton) of water in 24 hours. The trunk expands as it absorbs and stores the new supply.
Pores run in deep grooves along the saguaro’s stems. These pores open during the cooler nighttime hours to take in carbon dioxide and release oxygen. For protection from wind and animals, long, sharp spines run along the grooves. These spines reduce air movement, which conserves water and keeps grazing animals away.
Birds and bats love the nectar produced in saguaro flowers, and bats help to pollinate the saguaro. A bat’s head fits the shape of the flowers almost perfectly. As the bat drinks the nectar, its head gets heavily dusted with pollen grains, which it then carries to the next plant.
Woody shrubs and trees Small, woody perennials that flourish in the desert climate include sagebrush, salt-brush, creosote, and mesquite (meh-SKEET). They have small leaves and wide-ranging root systems. Most shrubs have spines or thorns to protect them against grazing animals.
The mesquite is a tree with roots that may grow 80 feet (24 meters) deep, with most of the root system in the upper 3 feet (1 meter). The long roots find a constant supply of water and it remains green all year. The mesquite produces long seedpods with hard, waterproof coverings. When the pods are eaten by desert animals, the partially digested seeds pass out of the animal’s body and begin to grow.
Joshua trees, a form of yucca plant, live for hundreds of years in the Mojave Desert of California. They can grow to 35 feet (11 meters) tall. Yuccas and treelike aloes store water in their leaves, not their stems. One
type of aloe, the kokerboom tree of southwest Africa, can survive several years without rain.
Palms Date palms, found at many oases in the Sahara and Arabian Deserts, can grow in soil with a high salt content. Only female trees produce dates, and only a few male trees are necessary for cross-pollination. The dates can be eaten raw, dried, or cooked and are an important food source for desert dwellers.
The Washingtonian fan palm is the largest palm in North America. Native to California, it requires a dependable supply of water to survive and sends out a thick web of tiny roots at its base. When its fronds (branches) die, they droop down around the trunk and form a “skirt,”
where animals like to live. The fan palm produces a datelike fruit that is eaten by many desert inhabitants.
Annuals Long grasses, such as alpha or esparto grass, often flourish in the desert after seasonal rains. Their stems can be used to make ropes, baskets, mats, and paper. Tufts of grasses sometimes become so entwined they form balls that drop their seeds as they spin across the landscape in the wind.
Almost every desert has its share of blooming annuals that add masses of color after a rain. Primroses and daisies adorn the California desert, while daisies, blue bindweed, dandelions, and red vetch beautify the Sahara.
The growing season in deserts is limited to the brief periods of rain that, in some cases, do not occur for several years. In some coastal deserts, certain plants absorb mist from the nearby ocean through their leaves. Where the soil is rich and rain is more regular and dependable, desert plants may flourish.
In cold deserts where real winters occur, plants are much like those in temperate climates. The portion above ground dies off, but the root system goes deep and is protected from freezing by layers of snow.
Deserts are home to both annuals and perennials. Annuals live only one year or one season, and they require at least a brief rainy period that occurs regularly. Their seeds seem to sprout and grow overnight into a sea of colorful, blossoming plants. This period of rapid growth may last only a few weeks. When the rains disappear, the plants die and the species
withdraws into seed form, remaining dormant (inactive) until the next period of rain.
Unlike annuals, perennials live at least two years or two growing seasons, appearing to die in-between but reviving when conditions improve. Those that live many years must be strong and have several survival methods. As young plants, they devote most of their energy to developing a large root system to collect any available moisture. Plants are often many yards apart, because their roots require a large area of ground in order to find enough water. The above-ground portion of young perennials is small in comparison to the root system because their leaves do not have to compete for sunlight or air as they do for water. Some perennials are succulents (SUHK-yoo-lents), which are able to store water during long dry periods. Some, like the century plant, store water in their leaves, while others store it in their stems or in large roots.
Except for occasional rivers and oases, water in the desert comes from the brief rains. Plants may grow in greater numbers in arroyos or wadis where moisture may remain beneath the surface. In coastal deserts, some plants absorb moisture from fog and mists that condense on their leaves. In cold deserts, spring thaws provide water from melting snow.
Pollination (the transfer of pollen from the male reproductive organs to the female reproductive organs of plants) is often a problem in deserts. The wind may carry pollen from one plant to another, but this
method is not efficient because plants usually grow far apart. Insect pollination is rare because there are fewer insects than in other biomes. As a result, many plants have both male and female reproductive organs and pollinate themselves.
Many desert plants, such as the candy cactus, saguaro cactus, and the silver dollar cactus, are threatened because of their popularity as house plants and for landscaping. They are available for sale in nurseries, but some people take them from the wild. Desert plants are sparse to begin with, so removal from their native home upsets the delicate balance of their reproduction.
All animals face the same problems in adapting to the desert. They must find shelter from daytime heat and nighttime cold, as well as food and water, which are often scarce. In spite of these extreme conditions, many animal species are represented in the desert environment, even some typically associated with temperate or wet surroundings.
Animals lacking backbones are called invertebrates. They include simple desert animals such as worms, and more complex animals such as the locust. Certain groups of invertebrates must spend part of their lives in water. These types are usually not found in deserts. One exception is the brine shrimp, an ancient species that lives in desert salt lakes. Other exceptions are certain species of worms, leeches, midges, and flies that live in the fresh water of oases and waterholes.
Most invertebrates are better adapted to desert life than vertebrates. Many have an exoskeleton (an external skeleton, or hard shell, made from a chemical substance called chitin [KY-tin]). Chitin is like armor and is usually waterproof. It protects against the heat of the desert sun, preventing its owner from drying out.
Common desert invertebrates
Termites, spiders, locusts and scorpions are all invertebrates found in the desert.
Termites Termites, found all over the world, build the skyscrapers of the desert. Their mounds, often more than 6 feet (2 meters) tall, are erected over a vast underground system of tunnels. These tunnels go as deep as the ground-water, providing a water supply readily available to the termite colony. The mounds, which are made of dirt, decaying plants, and termite secretions that dry rock-hard in the sun, have many air ducts. As the sun warms the mounds, they grow very hot. The hot air inside the mounds rises, drawing cooler air through the tunnels, creating a type of air-conditioning system.
Termites eat plant foods, especially the cellulose (substance making up a plant’s cell walls) found in woody plants.
Living Juice Boxes
Honeypot ants, which live in Africa, Australia, and the Americas, take the business of food very seriously. They maintain herds of aphids, insects that suck the juices of plants and then secrete a sugar-rich “honey.” Worker ants collect this honey and bring it back to the nest, where they feed it to a second group of workers. This second group takes in so much aphid honey that their stomachs swell and it is difficult for them to move. They then suspend themselves from the ceiling of the nest and wait. Later, when food becomes scarce, they spit the honey back up for other ants to eat. Some desert peoples like eating honeypot ants and seek out their nests.
Termites have an elaborate social structure. A single female—the queen—lays all the eggs and is tended to by workers. Soldier termites, equipped with huge jaws, guard the entrances to the mound. Soldiers cannot feed themselves, and the workers must tend to them, as well.
Locusts Locusts are found in the deserts of northern Africa, the Middle East, India, and Pakistan. As members of the grasshopper family, they have wings and can fly, as well as leap, considerable distances. For years they live quietly, nibbling on plants and producing a modest number of young. Then, for reasons not completely understood, their numbers increase dramatically. Great armies of locusts suddenly emerge, hopping or flying through the desert in search of food. Eating every plant in sight, these swarms may travel thousands of miles before their feeding frenzy ends. In a short time, the hordes die off and locust life returns to normal. The devastated landscape they leave behind may take years to recover. The most destructive species, the desert locust, lives wherever the average rainfall is less than 8 inches (20 centimeters). A swarm may contain as many as 40 million to 80 million insects.
Food and water
Many invertebrates are winged and can fly considerable distances in search of food. They eat plant foods or decaying animal matter. Some invertebrates are parasites, like the Guinea worm, that lurk at waterholes waiting for an unsuspecting animal to wander by.
Parasites attach themselves to the animal’s body or are swallowed and invade the animal from the inside.
Arachnids (spiders and scorpions), which are carnivores (meat eaters), seem well suited to desert life. They prey on insects and, if they are large enough, small lizards, mice, and birds. Scorpions use their pinchers to catch prey, then inject it with venom (poison) from the stinger in their tails. Their venom can be lethal, even for large animals and humans. Arachnids do not usually drink water but get what they need from their prey.
Most desert spiders live on the ground rather than on webs, hiding in holes or under stones to escape the heat. The large, hairy camel spiders are nocturnal, which means they rest during the daytime hours and hunt at night.
Scorpions hunt at night to avoid the heat of the day, taking shelter beneath rocks or burying themselves in sand or loose gravel. Several American and Australian species dig burrows more than 3 feet (1 meter) below the surface.
Some invertebrates, such as the scorpion, go through a mating ritual. Male and female scorpions appear to dance with their pinchers clasped together, as they do a two-step back and forth. After mating, the males hurry away to avoid being killed and eaten by the female.
Insects, the largest group of invertebrates, have a four-part life cycle that increases their ability to survive in an unfavorable environment. The first stage of this cycle is the egg. The egg’s shell is usually tough and resistant to long dry spells. After a rain and during a period of plant growth, the egg hatches. The second stage is the larva, which may be divided into several stages between which there is a shedding of the outer covering, or skin, as the larva increases in size. Larvae have it the easiest of all in the desert, often being able to spend a portion of their life cycle below ground where it is cooler and more moist than on the surface. Some larvae store fat in their bodies and do not have to seek food. The third stage of development is the pupal stage. During this stage, the animal often lives inside a casing, in a resting state, which may offer as much protection as an egg. Finally, the adult emerges.
Amphibians are vertebrates (animals with backbones) that usually spend part, if not most, of their lives in water. Unlikely as it seems, such animals can be found in a desert. Frogs and toads manage to survive in significant numbers in desert environments.
The short, active portion of their lives occurs during and immediately after the seasonal rains, when pools of water form. Mating, egg-laying, and young adulthood all take place in these pools. Offspring that survive into maturity leave the pools and take their chances on the desert floor, where they spend a few weeks feeding on both plants and insects. They must find shade or risk dying in the heat of the sun.
Avoiding Hot Sand
Several species of lizards have evolved methods for traveling over hot sand without burning their toes. The agamid lizard has long legs and keeps one foot raised and swinging in the breeze to cool it off while the other three support the animal. It does this in rotation, so that all its feet get an equal chance to cool off.
Frogs may spend up to ten months out of the year in their burrows, which are usually 3 feet (1 meter) under the desert floor. The Australian water frog can wait up to seven years for rain in a burrow made of its own shed skin. The Sonoran Desert toad is poisonous to predators and the largest toad in Arizona.
Frogs and toads feed on algae, plants, and freshwater crustaceans such as tiny shrimps that manage to survive in egg or spore form until brought to life by the rains. Frogs that eat the meat of crustaceans while they are tadpoles often become cannibals as they mature, eating their smaller, algae-eating brothers and sisters. If the rainy season is short, the cannibals have a better chance of survival because they have more food choices. If the rainy season lingers, the smaller tadpoles have a better chance because the cannibals cannot see their prey as well in muddy waters churned up by the rains, and the plant-eaters receive an increased supply of algae.
During the hottest, driest seasons, amphibians go through estivation (ess-tih-VAY-shun), an inactive period. While the soil is still moist from the rain, they dig themselves a foot or more into the ground. Only their nostrils remain open to the surface. Normally, their skin is moist and soft and helps them absorb oxygen. During estivation the skin hardens and forms a watertight casing. All the animal’s bodily processes slow down to a minimum, and it remains in this state until the next rainfall, when it emerges. When water is scarce, Australian Aborigines (native peoples) dig up estivating frogs or toads and squeeze the animal’s moisture into their mouths.
Mating and egg-laying for amphibians takes place in water; the male’s sperm is deposited in the water on top of the female’s jellylike eggs. As the young develop into larvae and young adults, they often have gills and require a watery habitat. If there is not enough rain for pools of water to form, amphibian populations may not survive.
|HOW TEMPERATURES GROW COOLER UNDERGROUND|
|Burrowing animals escape the heat by going underground, where it can be up to twice as cool as above ground.|
|Surface land temperature||165°F (74°C)|
(31 centimeters) down
(61 centimeters) down
(91 centimeters) down
(122 Centimeters) down
Of all the animals, reptiles are perhaps most suited to living in the desert. Snakes, lizards, and some species of tortoises are the most common. The scaly, hard skin of reptiles prevents water loss, and their urine is almost solid, so no water is wasted.
Reptiles are cold-blooded, which means their body temperature changes with the temperature of the surrounding air. Early in the day, they expose as much of their bodies as possible to the sun for warmth. As the temperature climbs, they expose less and less of their bodies. During the hottest period of the day, they find shade or a hole in which to wait for cooler temperatures. During the chilly nights they become sluggish, because they do not have to use energy keeping their body temperatures up as do warm-blooded mammals and birds.
Besides being cold-blooded, some species of lizards have specially developed clear membranes in their lower eyelids that cover the entire eye and protect it from losing moisture.
Snakes have no legs. They move using special muscles that flex their flat belly scales forward and backward. Ridges on their scales grip the ground and pull them along. Some rattlesnakes, like the sidewinders of North American deserts, manage to move diagonally by coiling into a kind of S-shape, and propel themselves by pushing with the outside back portion of each S-shaped curve.
Common desert reptiles
Snakes are less common in deserts than lizards. Common desert snakes include the gopher snake, horned viper, Gaboon viper, rattlesnake, and cobra. The Egyptian cobra can grow up to 8 feet (2.5 meters) long and is found in Africa. The Western diamond-back rattlesnake, the most dangerous American snake, can grow to 6.5 feet (2 meters).
Common lizards include the gecko, the skink, the bearded dragon, the iguanid lizard, and the monitor lizard. The only two species of venomous lizards, the Gila monster and the bearded lizard, are related to the monitor. They are found in the southwestern United States and western Mexico.
Food and water
The diet of lizards varies, depending upon the species. Some have long tongues with sticky tips good for catching insects. Many are carnivores that eat small mammals and birds. The water they need is usually obtained from the food they eat. Geckos can survive long periods without food by living on stored fat.
While most lizards hunt food during the day, the Gila monster, found in the southwestern United States, looks for reptile eggs and baby animals after dark. Making the most of its opportunities, the lizard stuffs itself. During periods when food is scarce, its body draws on extra nutrients stored as fat in its tail, which can double in size after a big meal.
A Living Funnel
Most lizards obtain water from the food they eat. The thorny devil, a small lizard from Australia, is an exception. During the cool nights, dew condenses in the creases and folds of its skin. Most of these folds lead toward the lizard’s mouth, and the lizard is able to lap up the moisture.
All snakes are carnivores, and one decent-sized meal will last them days or weeks. In the desert, they use their eyes to hunt during the cool nights when their prey is most active. Snakes cannot close their eyes because they have no eyelids. A transparent covering protects their eyes from the dry air, dust, and sand. Snakes often hunt underground and they have adapted to detecting ground vibrations. Many desert snakes bury themselves in the sand so that only their eyes and flickering tongues are visible. Some snakes kill their prey with venom (poison).
Although commonly thought of as jungle dwellers, boa constrictors and pythons (a species of constrictor) live in the desert. Constrictors strike their prey, hold it with a mouthful of tiny teeth, and then wrap their body around it like a coil. The prey suffocates and the constrictor swallows its meal whole, gradually working it into its stomach with its hinged lower jaw and strong throat muscles.
Lizards and snakes hide in the shade during the hottest hours of the day to escape the sun. Only a few make their own burrows. Most take over the abandoned burrows of other animals, find shelter in rock crevices, or bury themselves in the sand.
Desert tortoises obtain some shade from the sun with their thick outer shells. Most of the time they escape the heat of the day by retreating to burrows they dig. In the spring and autumn, when the days are not excessively warm, the tortoise ventures out during the day to forage for food. During the summer they forage for food at night when it is cooler, remaining in burrows during the day. In the winter, tortoises hibernate (become dormant) in a second burrow they have dug.
The eggs of reptiles are leathery and tough and do not dry out easily. Some females remain with the eggs, but most bury
Rattlesnakes and other pit vipers have small pits on both sides of their face. These pits detect the body heat of prey, just like heat-seeking missiles. Pit vipers hunt at night, and their secret weapon allows them to find small animals hidden in the dark.
them in a hole. Offspring are seldom coddled and are left to hatch by themselves. Once free of the eggs, the offspring dig themselves out of the hole and begin life on their own.
All deserts have bird populations. Tropical and subtropical deserts are visited twice each year by hundreds of species of migratory birds traveling from one seasonal breeding place to another. These migrators include small birds such as wheat-ears, as well as larger species such as storks and cranes. Some species know the route and where to find food or water. Others fly at night, when it is cool. Migrators are not true desert birds. They cannot survive for long periods in the desert as can birds for which the desert is home.
Birds have the highest body temperature of any animal—around 104°F (40°C). They do not need to lose body heat until the desert temperature is greater than their own. This makes desert life easier for them than for mammals, which must lose heat regularly during the warmest months, usually by panting or sweating.
Feathers protect birds not only from the cold in winter but from the sun and heat. Air trapped between layers of feathers acts as insulation. Birds do
not sweat but, by flexing certain muscles, can make their feathers stand erect. This allows them to direct cooling breezes to their skin. Those having broad wing spans, such as eagles and buzzards, can soar at high altitudes and find cooler temperatures.
Common desert birds
Common desert birds include ground birds and birds of prey.
Ground birds Ground birds are not hunters or scavengers (animals that eat decaying matter) but obtain most of their food from plants and insects. They have strong legs that enable them to dart around on the ground without tiring.
The Flying Sponge
The sandgrouse, found in the deserts of Africa and Asia, is a water-drinking bird. Most members of this species live near waterholes, except during nesting periods, when they may be forced to remain in an area experiencing drought (extreme dryness). Male sandgrouse have evolved an unusual method of overcoming this problem. They fly to the nearest waterhole, perhaps 20 miles (35 kilometers) away, and turn themselves into sponges. They wade into the water and let their special belly feathers absorb the liquid—as much as twenty times their own weight. Then they fly back to the nest where the nestlings drink by squeezing the water out with their beaks.
In Asia, Africa, Australia, and the Arabian Peninsula, families of thrushes, called chats, are common. Varieties of chats live at many different altitudes, including those over 13,000 feet (4,000 meters). They are found in both arid and semiarid regions, and their diets and habits vary according to their location.
Wrens are common in desert habitats all over the world. They eat insects, and species in North American deserts eat seeds and soft fruits. Cactus wrens, as their name implies, live among prickly desert plants where they build their nests among the spines.
A small desert bird that is a popular pet is the parakeet. Originally from Australia, parakeets live in huge flocks containing tens of thousands of birds. During years when food is plentiful, a flock may number in the millions. Parakeets prefer seeds and are nomads, often traveling from habitat to habitat in search of the seeds of annuals that bloom after a rainfall.
The largest ground birds found in deserts are members of the bustard family. The houbara bustard is found in the Sahara and the deserts of central Asia. It is about 2 feet (60 centimeters) tall and weighs as much as 7 pounds (3 kilograms). Houbaras depend primarily upon plants for food, but will eat invertebrates and small lizards. Houbaras can run fast— up to 25 miles (40 kilometers) per hour—and seldom fly. Other birds include the pale crag martin and the swallow.
Birds of prey Birds of prey are hunters and meat eaters. They soar high in the air on the lookout for small animals to eat. Their eyesight and
The Eyes Have It
The eyes of all birds are large in relation to their body size. Some of the largest are the eyes of owls, which take up about one-third of the space in the skull. Owl eyes are specially adapted to see better at night. They have more rods, the cells that are most sensitive to low light levels, and fewer cones, the cells sensitive to bright light. An owl’s eyes are pear shaped and face forward, which gives the owl a kind of binocular vision and superior judgment of distances. An owl’s eyes cannot turn in their sockets because of their size and shape. To look around, the owl must swivel its entire head.
hearing are usually very sensitive and enable them to see and hear creatures scurrying on the ground far below.
The roadrunner, found in the southwestern United States, can scurry over the desert floor for long stretches at speeds of about 13 miles (20 kilometers) per hour. Roadrunners are carnivores and are known to come running at the sound of a creature in trouble.
Several species of falcons live in semiarid regions; one of the true desert falcons is the prairie falcon of North America. Prairie falcons hunt for other birds such as larks and quail, and small mammals such as rabbits and prairie dogs. When food is scarce, they eat insects and reptiles. A falcon attacks its prey by diving at the head and trying to seize the head in its talons. Prairie falcons do not build their own nests. Instead, they move into the abandoned nests of other birds or use a hollow in a rock.
Owls live on the edges of deserts. The eagle owl is the largest, measuring 27 inches (68 centimeters) in length with a wingspan of over 6.5 feet (2 meters). They have the power to attack small deer. The smallest owl, the elf owl, hunts invertebrates, mainly insects. This owl is about 5 to 6 inches (12 to 15 centimeters) long and weighs 1.25 to 1.75 ounces (35 to 55 grams). Most owls hunt in the evening or at night when their extraordinary eyes and superior hearing allow them to find their prey.
The bird most often pictured in the desert is the vulture, waiting patiently while a creature dies of thirst. Most vultures are not really desert birds, although they do spend much time hunting in desert regions. They soar high in the air, circling and looking for carrion (dead animals), which their excellent eyesight allows them to easily see. When one bird spots a potential food source it begins to descend, and the other vultures follow.
Food and water
Birds are found in greater variety and numbers around oases and waterholes where there is an ample supply of water, seeds, and insects. Some, like the Australian scarlet robin, often drink water. Birds that live in the desert itself are able to fly long distances in search of food or water. Some birds become nomads, following the rains from habitat to habitat. Birds usually require less than 10 percent of the amount of water needed by mammals. For this reason, many birds, like shrikes and some wheatears, can obtain enough moisture from the seeds, plants, and insects that they eat and do not need an additional water source. The same is true of vultures and birds of prey, which obtain water from the flesh of animals. Birds have kidneys that are very efficient in their ability to extract water, and their urine is not liquid but jellylike.
Sometimes animals are involved in relationships in which one animal is helped and the other is not affected. This is called commensalism. One example is a desert snake that takes over the abandoned burrow of a prairie dog.
During the hottest part of the day, most desert birds rest by roosting in the shade or in underground burrows. During excessively hot or dry periods, birds fly to more comfortable regions. It has been estimated that one-third of Australian birds are constantly on the move to escape the heat.
Many desert birds build nests in rock crevices, in abandoned burrows, or on the ground in the open because there are few trees in the desert. Those that build on the ground may put walls of pebbles around the nest that act as insulation and reduce the force of the wind.
Except in Australia, desert birds appear to breed as other birds do—according to the seasons. In Australia, they adapt their breeding habits to periods of rainfall, and breeding cycles may be years apart.
Birds are free to fly away from the heat most of the year, but during the breeding cycle they must remain in the same spot from the time nest building begins until the young birds can fly. This is usually a period of many weeks.
Normally, the parents sit on the nest to protect the eggs from heat or cold. During very hot weather, the parents may stand over the nest to give the eggs or the nestlings shade.
Many mammals live in the desert. More than 70 species live in North American deserts alone. Among all of these species, only monkeys and apes are rarely seen.
Mammals must prevent the loss of moisture from their bodies in the desert. The urine and feces of many desert mammals are concentrated, containing only a small amount of water. Small desert mammals, such as rodents, do not sweat. They manage to lose enough heat through their skin. During the hottest times, they burrow underground. Some estivate in summer months.
No Need for Sunscreen
The naked mole rat, sometimes called the sand puppy, is an animal that seems poorly designed for desert life. It has no fur, just its wrinkly skin for protection, and it is almost blind. To make matters worse, unlike most mammals, it cannot maintain a steady body temperature. Its name gives a clue as to how it survives in the Sahara Desert; it burrows. Naked mole rats are tunneling experts, and a clan of rats will create an entire apartment complex of nurseries, storage chambers, and bedrooms. They dig so fast with their sharp buck teeth most predators cannot catch them and get just a face full of dirt for their trouble. The clan leader is always a female. She is the only one who breeds. Other females do not mature sexually until the queen dies or they decide to leave the den to create their own clan elsewhere.
Medium-sized mammals, such as rabbits and hares, do not burrow or estivate; but they will use another animal’s burrow to escape from immediate danger. They have no sweat glands and cannot keep cool by sweating, although some heat escapes from their large ears. Hares and rabbits stay in whatever shade they can find during the hottest time of day. The quokka, a rabbitlike marsupial (a mammal that carries new offspring in a pouch) from Australia, copes with the heat by producing large amounts of saliva and licking itself. It is not known how the quokka makes up for this water loss.
Larger grazing animals, such as gazelles, can sweat, which helps them tolerate the heat. Some carnivores, such as coyotes, release body heat by panting (breathing rapidly through their mouths). This results in lost moisture. True desert dwellers, like the mongoose, the meerkat, and the hyena, avoid the midday heat by retiring to underground dens.
Common desert mammals
Other mammals commonly found in deserts include the kangaroo, the camel, and the hyena.
Kangaroos A well-known grazing animal of Australia is the red kangaroo. The kangaroo will hop long distances in search of food. Water is less of a problem as they get much of the moisture they need from grasses. They have efficient kidneys and produce a concentrated urine.
The Rabbit Plague
In the nineteenth century, English settlers introduced rabbits into the Australian Desert and grasslands. A few soon multiplied into half a billion strong. They ate everything growing in the pasturelands and then, when there was no more grass to supply moisture, the rabbits mobbed the water holes. Scientists got rid of vast numbers by introducing rabbit diseases in 1950. Some hardy animals survived and are still multiplying.
The kangaroo’s strong legs allow them to move as fast as 20 miles (30 kilometers) an hour. Each hop can carry them as far as 25 feet (8 meters). Kangaroos are marsupials, which means that females carry their young, called a joey, in a pouch on their abdomen. Females with young in their pouches are seldom able to travel fast. If the female is in danger and the joey is too heavy, the mother will dump the young out in order to escape. This may seem cruel, but if she is caught the joey will die anyway. If she escapes, she will breed again.
Camels Camels are the mammals most people associate with the desert. The Bactrian camel has two humps and lives in the deserts of central Asia. The one-humped Arabian camel, called a dromedary, lives in the Arabian and Sahara Deserts. Camels are well adapted for desert life. They can travel almost 100 miles (160 kilometers) a day, for as long as four days, without drinking. When they do drink, they can take in more than 20 gallons (100 liters) in just a few minutes.
At one time it was believed that camels stored water in their humps. This is not true. The hump contains fat, which is used up during long journeys and may shrink in lean times. Camels have strong teeth and the membranes that line the insides of their mouths are tough, allowing them to eat almost anything that grows in the desert, even the thorniest plants. They can drink bitter, salty water that other animals cannot tolerate.
Camels have thick fur that molts (falls out) during the hottest seasons and is replaced by new, thinner hair. Camels can sweat to reduce body temperature, but their body temperature is not constant and varies depending upon surrounding air temperatures. Their eyes have long protective lashes, and their nostrils can be closed to keep out blowing sand. Their padded, two-toed feet are well insulated against the hot desert floor.
Packing it Away
Packrats are rodents that collect almost anything—seeds, bones, rocks—and store it away in their dens. Some of those dens need housecleaning so badly that, over the years, the stuff becomes glued together. Many generations of packrats often live in the same spot, and the piles can grow for thousands of years. The packrats’ messy lifestyle may actually help scientists. When the burrows are studied, scientists can find out what an area looked like thousands of years ago.
Camels can no longer be considered wild animals. It is doubtful any wild groups still exist. They have been thoroughly domesticated (tamed) and are used by humans for transportation and other needs. Their ability to travel long distances and carry up to 600 pounds (270 kilograms) makes them useful.
Hyenas Hyenas are members of the dog family and have one of the strongest sets of jaws of any animal. They range over the deserts of Africa and Arabia, hunting in packs for antelope and other game, or stealing the meals of other carnivores. They also eat carrion and, occasionally, plant foods. Hyenas are not particularly fast, but they do not give up easily and may simply wear out their prey, which collapses beneath the snarling pack. Hyenas live as part of a clan in dens they dig themselves.
Food and water
Some small mammals eat plant foods and insects; others, like the desert hedgehog, eat bird and reptile eggs and young. In Australia, a tiny mole no more than 8 inches (20 centimeters) long eats at least its own weight in insects and young lizards every day. Many small mammals do not need to drink water as often because they obtain moisture from the food they eat.
Large grazing animals need to drink and require a water source to replace the moisture lost in sweating. A few, such as the Arabian gazelle and the Nubian ibex, seldom drink water. They obtain what they need from the plants they eat. Grazing animals can travel to areas where rain has recently fallen. In temperate climates, most grazing animals live in large herds. The desert food supply will not support such numbers, and desert groups are usually very small.
Carnivores, such as mountain lions and coyotes, do not live in the deep desert but remain on the desert fringes where a supply of water is more readily found. Carnivores obtain much of their moisture requirements from the flesh of their prey, but they still need to drink.
Small mammals remain in burrows during the day. The air inside a burrow is up to five times more humid than the outside air. This helps the animal prevent moisture loss.
Medium-sized hares and rabbits do not live in burrows but seek shelter in the shade of plants or rocks and in shallow depressions. Grazing animals look for shade during the midday heat. Young animals may lie on the ground in the shade created by the adults.
Carnivores dig their own dens, which may consist of many tunnels to accommodate a family clan. The males mark their territories with scent, usually that of urine.
Young mammals develop inside the mother’s body where they are protected from heat, cold, and predators. The extra weight can make it difficult for the pregnant females themselves to escape danger. Female mammals produce milk to feed their young. In the desert this presents a problem in lost moisture. Those that live in dens must remain nearby until the young can survive on their own, making survival for both mother and young during drought conditions more difficult.
As in other biomes, many desert animal species are threatened.
The addax, a desert antelope with beautiful horns, has been greatly reduced in numbers by Saharan nomads who believe its stomach contents have healing powers. Its skin is valuable, for it is believed to have the power to ward off attacks from snakes and scorpions.
After World War II (1939–45), when dichlorodiphenyl-trichloroethane (DDT) and other dangerous pesticides were in common use, aplomado falcons shrank in number. These small falcons lived in desert areas of Mexico, Texas, and Arizona, but are no longer found north of Mexico. The poisons were used to kill grasshoppers that were devouring grasses and other vegetation. Grasshoppers are part of the falcons’ diet, and as the birds ate the contaminated insects, the poison killed the falcons. The number of falcons will hopefully increase now that DDT is banned.
The California condor is one of the world’s rarest birds. When early settlers slaughtered deer herds for food, they reduced the condor’s food supply. Later, cattle ranchers set out poison baits to kill wolves and coyotes. The poison also killed the condors who ate the dead animals. California
The Web of Life: Natural Balance
Balance is important to every biome. Changes made by humans with even the best of intentions can create serious problems. During the nineteenth century, early settlers brought the American prickly pear cactus to Australia because they liked the plant’s appearance. In America the prickly pear is a useful desert shrub, often serving as fencing for livestock. In Australia, where it had no natural enemies, it was a disaster. By 1925, prickly pear cacti covered more than 100,000 square miles (260,000 square kilometers) of the Queensland and Victoria provinces.
Nobody wanted the job of cutting down all those plants. Instead, a natural predator, a little moth that in caterpillar form loves to munch on prickly pears, was brought to Australia. Five years later, there were no more prickly pears.
condors are now protected. As a result of captive breeding in 2008 there were about 300 condors, with about half of those in captivity.
The beautiful spiral horns of the Arabian oryx are prized by hunters looking for trophies. By 1960 the Arabian oryxes were reduced to only about a dozen animals. In 1961 they became protected and several hundred were bred in zoos. After 1982 many of the zoo-bred animals were reintroduced into the wild.
Human beings are able to adapt to many unfriendly environments. It is no surprise that they have learned to live in the desert and make it their home. Ten percent of the world’s population lives on arid (dry) land; they include a large variety of native peoples of all races. This number is growing because most desert inhabitants live in developing countries with high rates of population growth.
Impact of the desert on human life
Humans are able to maintain a safe body temperature in the desert by sweating. Under extreme heat, a human being may lose as much as 5 pints (3 liters) of moisture in an hour and up to 21 pints (12 liters) in a day. This water must be replaced or the person will die from dehydration. Dehydration occurs when tissues dry out, depleting the body of fluids that help keep it cool. Unlike the kidneys of many desert animals, human kidneys cannot concentrate urine to conserve water. The loss of salt through sweating is also a problem. Humans need a certain amount of salt to maintain energy production. If too much salt is lost, painful muscle cramps occur.
Humans can change very little physically to adapt to desert conditions, so they must change their behavior. This has been done in many ways, from the choice of lifestyle to the development of technologies such as irrigation (watering of crops).
Food and water
Until the mid-1900s, many native desert dwellers were nomads—either hunter-gatherers, like the Bushmen of the Kalahari and Namib, or herders, like the Bedouin of the Middle East. They moved regularly, usually along an established route, in order to seek food and shelter for themselves or their herds of animals. Most nomads return year after year to the same areas within a given territory. They know when the rains usually come, where to find food or pastures, and where a water supply is located. By the late twentieth century less than 3 percent of desert peoples lived nomadic lives, having been driven from their lands by ranchers or mining companies looking for mineral, gas, and oil resources. Since 1950, many nomadic peoples have moved to cities where they often live in poverty.
In some cases it might be possible to reverse desertification because nature fights back. Grasslands in Northern Uganda, Africa, had been lost when herds of cattle were allowed to overgraze the area. Then tsetse flies moved in and attacked the cattle. When the cattle herders moved to avoid the flies, the plants returned.
The diet of hunter-gatherers consists primarily of plant foods and game animals, although meat is usually scarce. Some, like the Aborigines of Australia, eat the grubs (larvae) of certain insects, which provide a source of protein.
Herding tribes depend primarily upon their animals for food, but they may also raise grains or trade for them. In the Sahara, dates from the date palm are an important food.
Many Native Americans found important uses for desert plants. The leatherplant, also called sangre de drago (blood of the dragon), contains a red juice used as a medicine for eye and gum diseases. Wine and jelly were made from the fruit of the saguaro cactus, the fruit of the prickly pear was made into jam, and ocotillo branches were useful as building materials.
The homes of hunter-gatherers tend to be camps, not houses. The Bushmen of the Kalahari, for example, make huts from tree branches and dry grass.
Traditionally, the nomadic tribes of the Sahara, whose shelters had to be portable, used tents made from the hides or hair of herd animals such as goats. The tents of the Bedouin are an example. Village houses in the desert are often made of mud bricks dried in the sun. The bricks do not need to be waterproof because there is so little rain.
In the desert country of Mongolia, desert dwellers commonly live in yurts—tentlike structures made from felt created from sheep’s wool. The felt is stretched over a wood frame, and a hole at the top of the yurt allows smoke from cooking fires to escape.
Some Native Americans of the southwestern desert, such as the Hopi and Zuni tribes, built homes called pueblos from mud, wood, and stone.
The thick walls and small windows kept the interior cool. The Anasazi, who lived around 1100 AD, built their homes in the sides of cliffs. The homes were reached by ladders, which could be pulled up for defense. The Navajos made hogans—houses of logs and mud. Modern-day Native Americans live in the same kinds of homes as other Americans.
One advantage humans have over other animals is clothing, which is a substitute for fur. Unlike fur, clothing can be put on or taken off at will. Most traditional desert peoples wear layers of loose-fitting garments that protect the body from the heat. A naked person absorbs twice as much heat as a person in lightweight clothes. Loose clothing absorbs sweat and, as the air moves through, it produces a cooling effect. As a result, the person sweats less, conserving water. Many desert tribes, especially in the Middle East, have at least partially adopted western clothing styles.
The only desert peoples to go naked were the Bushmen of the Kalahari and the Aborigines of Australia. When nights were chilly they occasionally wore “blankets” made from bark, but more often their warmth came from a campfire. Those Bushmen and Aborigines who still live a traditional lifestyle continue to go without clothes.
Some kind of headgear is usually worn by people in the desert to shield their face from the sun and blowing sand. The Fulani, who live on the edge of the Sahara in West Africa, wear decorated hats made from plant fibers and leather.
For traditional hunter-gatherers, possessions are almost meaningless. If they favor a particular stone for sitting on, they might carry it along. However, a stone, as well as other possessions, gets very heavy after a few miles. Their economy tends to be simple and little trading is done.
A century ago, desert herders were self-sufficient. Their wealth moved with them in the form of herd animals, jewelry, tents, and other possessions. Commerce usually involved selling goats, camels, or cattle. In the Middle East, the discovery of oil made important changes in the economy. In some cases, sweeping modernization made irrigation and food production more stable. Nomads settled in one spot and became farmers. In other cases, the wealth fell into the hands of a few people, while the large majority lived in poverty.
Impact of human life on the desert
The fact that the desert is so unfriendly to human life has helped preserve it from those who could destroy its ecological balance.
Use of plants and animals
As long as traditional lifestyles remained in effect, the human impact on the desert was not severe. Desert dwellers understood the need to maintain balance between themselves and their environment. While animals and plants were used for food, they were not exploited (overused), and their numbers could recover. Since the introduction of firearms and the rapid growth of human populations, however, many plants and animals have become threatened.
Desertification, the deterioration of land until it becomes desertlike, continues in spite of efforts by conservationists (people who work to preserve the environment) to stop it. Several popular desert plants such as cacti, and animals such as lizards, are sold at high prices to collectors. Many of these species are threatened as a result.
Overpopulation has diminished many natural resources. The digging of wells has caused the water table (the level of ground water) to drop in many desert regions. Supplies of oil and minerals are being removed from beneath the desert surface and cannot be replaced.
For thousands of years, crops have been grown in desert soil with the aid of irrigation (mechanical watering systems). Furrows were dug between rows of plants, and water pumped from wells was allowed to run along the furrows. In modern times, dams and machinery are used to control the rivers or pump groundwater for irrigation, allowing many
Green lawns and many popular trees and shrubs require constant watering in the summer. This puts a strain on water supplies. Some desert communities ask residents to forget the lawns and xeriscape instead. Xeriscaping is a kind of landscaping using desert plants that need less water.
former nomads to become settled farmers. To irrigate large cotton farms in the Kara Kum Desert of central Asia, water is brought from the Amu Darya River by means of a canal 500 miles (800 kilometers) long.
Irrigation must be controlled. If too much groundwater is pumped, it may be used up faster than it can be replaced. When the water table drops in regions near the ocean, the land may slump. Salt water may enter aquifers (underground layers of earth that collect water), destroying the fresh water. Another problem caused by irrigation is an accumulation of salt in the soil. All soils contain some salts and, if irrigation water is used without proper drainage, the salt builds up within the surface layers and prevents plant growth.
Quality of the environment
People have impacted the desert environment in several ways. Drilling for oil and mining for other resources requires roads. The people who operate the drills need houses. Most of these changes have occurred along the Mediterranean in mineral-rich countries on the edges of the desert. However, the deep centers of deserts have usually not been disturbed. Roads remain tracks and have escaped being blacktopped.
The world’s climate may be changing because of human activity. If so, the climate of the desert will change with unknown consequences to the plant and animal life.
The Bushmen and the Tuareg are two groups of desert peoples commonly found living in the desert.
The Bushmen of the Kalahari and Namib Deserts of Africa live in clans consisting of several families. A clan’s territory is about 400 square miles (1,036 square kilometers). Clans move according to the rains or the seasons, returning to familiar campgrounds year after year. Their territory includes good waterholes. Bushmen live off the land, eating berries, roots, and wild game. Plants, which make up the greatest share of their diet, are gathered by the women. Bushmen are expert trackers and use these skills to hunt game for food with bows and poisoned arrows. The meat is cut into strips and dried so that it will not spoil. Bushmen are not tall, ranging between 55 and 63 inches (140 and 160 centimeters) in height. This may be partially due to a diet deficient in some nutrients.
Overnight shelters built from grass and branches provide protection from the wind. In the winter, clans may break up into smaller groups and build stronger huts to keep out the rains. The Bushmen population numbers approximately 20,000. Probably fewer than half of those still live as hunter-gatherers.
The Bushmen and the Aborigines are excellent huntsmen and trackers. Bushmen can follow animal signs over the hardest ground and can distinguish the signs of one individual animal from those of another. During World War II (1939–45), when pilots whose planes had crashed were lost in the Australian deserts, Aborigines could find them by following footprints no one else could see.
The northern Tuareg of the Sahara depend upon the camel for their livelihood, grazing their animals on what little pasture exists on the desert fringes. Camels may be killed for meat or their milk used to make butter and cheese. They are also the primary means of transportation. Since the Tuareg are traders, camels are essential to carry goods such as cloth and dates.
The southern Tuareg tribes are more settled. As a result of modern technology that allows the digging of deep wells, the Tuareg have established cattle ranches on the edges of the Sahara. Unfortunately, the land is often overgrazed, and more territory is lost to the desert every year.
Tuareg men wear a characteristic blue veil wound into a turban (head covering) on their heads. Both men and women wear loose robes for protection against the sun. An indigo (blue) dye is used to color some clothing. The Tuareg have been known as the “blue men” because the dye often rubs off on the skin.
Homes are usually low tents made of animal hides dyed red. The tent roof is supported by poles and the sides are tied to the ground with ropes to keep out the wind and sand. During the hottest parts of the year, the Tuareg build a zeriba, a large, tall hut made from grasses attached to a wooden frame.
The Tuareg are known for their ability to fight. Before firearms were introduced during the eighteenth century, they made impressive weapons such as daggers and swords. They now prefer high-powered rifles. Historically, the Tuareg have had a tendency to raid other tribes, and wars between one tribe and another may last for years.
It is estimated that the Tuareg number as many as 1 million people. This figure is only approximate, since about 700,000 of the Tuareg are nomads and rarely remain in one place long enough for a reliable count to be made.
The transfer of energy from organism to organism forms a series called a food chain. All the possible feeding relationships that exist in a biome make up its food web. In the desert, as elsewhere, the food web consists of producers, consumers, and decomposers. The following shows how these three types of organisms transfer energy to create the food web within the desert.
Green plants are the primary producers. They make organic materials from inorganic chemicals and outside sources of energy, primarily the sun. Desert annuals and hardy perennials, such as cacti and palms, turn energy into plant matter.
Animals are consumers. Plant-eating animals, such as locusts, gazelles, and rabbits, are primary consumers in the desert food web. Secondary consumers eat plant-eaters. They include the waterhole tadpoles that consume smaller, plant-eating family members. Tertiary consumers are meat-eating predators, like mongooses, owls, and coyotes. They will eat any prey small enough for them to kill. Humans, like the Bushmen, fall into this category. Humans are omnivores, which means they eat both plants and animals.
Decomposers feed on dead organic matter and include fungi and animals like the vulture. In moister environments, bacteria aid in decomposition, but they are less effective in the desert’s dry climate.
The Gobi Desert
In the eastern part of central Asia, extending into Mongolia and western China, is the great Gobi Desert. It is part of a chain of deserts, including the Kara-Kum, the Kyzyl-Kum, the Takla Makan, the Alashan, and the Ordos. Gobi is a Mongolian word meaning “waterless place.”
The Gobi Desert
Location: Mongolia and western China
Area: 500,000 square miles (1,300,000 square kilometers)
Classification: Cold; arid and semiarid
Surrounded by mountains—the Pamirs in the west, the Great Kingan in the east, the Altai, Khangai, and Yablonoi in the north, and the Nan
Shan in the south—the Gobi is a high, barren, gravelly plain where few grasses grow. It is so flat that a person can see for miles in any direction.
Except for the polar deserts, the Gobi is the coldest because of its altitude—about 3,000 feet (900 meters) in the east and about 5,000 feet (1,500 meters) in the south and west. Arctic winds blow down from the north so that, in the winter, some areas become snow covered. Any rainfall occurs in the spring and fall. Temperatures average between 27°F (-3°C) in January to 86°F (30°C) in July.
Several rivers flow into the Gobi from the surrounding mountains, but the desert has no oases (waterholes). Dry river beds and signs of old lakes indicate that water once existed here. The only surface water that remains is alkaline (containing too many undesirable minerals). Fresh water is obtained primarily from wells.
The few trees found here are willow, elm, poplar, and birch. In general, the largest plants are the tamarisk bush and the saxoul. Grass, thorn, and scrub brush survive, and bush peas, saltbush, and camel sage are abundant. During the brief spring, annuals (plants that live only one season) flourish.
Few reptiles are present because they do not favor the cold winters, but several species of snakes live in the Gobi. Many birds, including sand grouse, bustards, eagles, hawks, and vultures, thrive here. Many of these are migrators (animals that have no permanent home). Small mammals include jerboas, hamsters, rats, and hedgehogs. Larger mammals include gazelles, sheep, and rare Mongolian wild horses.
The Mongolian people living in the Gobi include settled farmers, who cultivate grains on its fringes, and herders who prefer life on the windy plain. Herders tend to be nomads (wandering tribes) who raise horses, donkeys, camels, sheep, goats, cattle, and, in upland areas, yaks. The two groups often trade; the farmers provide grain and the herders supply meat. Since the 1940s, irrigation has been used to make cotton and wheat crops possible, and many nomadic peoples have settled on farms.
Famous Venetian explorer Marco Polo (1254–1324) reached the Gobi during his travels in the thirteenth century. Modern explorers, including archaeologists and anthropologists, still visit in search of dinosaur bones and eggs commonly found there. Such fossils are evidence that the Gobi was once a friendlier environment supporting diverse animal life.
The Thar Desert
The Thar (TAHR) Desert (also called the Great Indian Desert) begins near the Arabian Sea and extends almost to the base of the Himalaya Mountains. It contains a variety of landforms and differing climates. It is considered an arid, lowland desert characterized by extreme temperature variations. January temperatures average 61°F (16°C), while June temperatures average 99°F (37°C).
The Thar Desert
Location: India and Pakistan
Area: 77,000 square miles (200,000 square kilometers)
Classification: Cold; arid
The plain of the Thar slopes gently from more than 1,000 feet (305 meters) in the southeast to fewer than 300 feet (91 meters) in the northwest near the Indus River, broken only by areas of sand dunes—primarily seif (SAFE) dunes that run parallel to the wind. Some more or less permanent dunes may soar to heights of 200 feet (61 meters). Several salt lakes lie on the desert’s margins. The salt covering the lake beds is mined and sold.
Rainfall is unreliable and drops to about 4 inches (100 millimeters) a year in the west. In the summer, the Thar benefits from seasonal monsoons (rainy periods) that arrive in July and August. During monsoon season, the largest salt lake may cover 90 square miles (35 square kilometers) and be 4 feet (1.22 meters) deep, only to disappear during the dry period.
Wheat and cotton have been grown on the irrigated Indus plain in the northeast since the 1930s, but most areas support only grass, scrub, jujube, and acacia, which are eaten by domesticated camels, cattle, goats, and sheep. Birds, such as the great bustard and quail, are found here. Wild mammals live in or on the fringes of the Thar, including hyenas, jackals, foxes, wild asses, and rabbits.
Nomads raise sheep, cattle, and camels. Villages are located in areas where grass will grow after a rain and support herds of livestock. Small industries are based on wool, camel hair, and leather.
About 5,000 years ago, the Indus Valley was home to the great Indus civilization. The decline and disappearance of this civilization is a mystery, but it may be partly due to climatic changes that caused the Thar to spread.
The Australian Desert
About three-quarters of the Australian continent is desert or semiarid land, a mixture of stone, rock, sand, and clay. January temperatures average 79°F (26°C), and July temperatures average 53°F (12°C). The central region is a basin, the western area a high plateau. The Australian desert region is composed of six different deserts—the Simpson, the Great Victoria, the Sturt Stony, the Gibson, the Great Sandy, and the Tanami. They blend into one another and are part of the same whole.
The Australian Desert
Area: 600,000 square miles (1,500,000 square kilometers)
Classification: Hot; arid and semiarid
The basin region is a flat land, where a visitor can gaze for miles without the view being interrupted by hills. This makes it a difficult place in which to navigate, for there are no landmarks. The terrain varies, changing from semiarid grassland to rocky stretches, to areas with sandy dunes. Strange, massive, solitary rock formations have spiritual meaning to the Aborigines, such as Ayers Rock or the Olgas.
The lowest point of the Australian Desert is near Lake Eyre, which lies 60 feet (18 meters) below sea level. The lake, situated close to sand dunes, is about 50 miles (80 kilometers) long and so salt-encrusted that during dry periods it appears white. It fills with water only once or twice during a ten-year period. When full, it supports abundant wildlife, including birds, frogs, and toads.
Mulga, a type of thorny acacia bush, and mallee, a species of eucalyptus, grow in the most arid regions of the Australian Desert. At the desert’s edges, spinifex grass grows.
Australia is home to a number of animals found nowhere else in the world, but their ways of adapting to the desert are the same as animals elsewhere. Insects, such as locusts and termites; snakes, such as the bandy bandy and the brown snake; and lizards, such as the blue-tongued skink are commonly found in the Australian Desert. Birds include the emu, a large, flightless bird that resembles an ostrich, parrots, quail, cockatoos, and kookaburras.
Kangaroos are Australia’s largest mammals. They prefer to live in grasslands but can travel long distances in the desert without water. Kangaroos are marsupials and carry their young in a pouch. Other marsupials unique to Australia are the wallaby and the wombat.
Until the middle of the twentieth century, most Aborigines of Australia still lived as hunter-gatherers. Traveling in small groups, or clans, they moved over an established territory. They gathered plant roots, bulbs, termites, and grubs; and hunted kangaroos and wallabies with spears and boomerangs. When young Aborigine men attained a certain age, they went on a “walkabout.” During this time they had to leave their clan to wander in the desert, perhaps for years, learning about life and survival.
The Arabian Desert
The Arabian Desert covers most of the Arabian Peninsula, from the coast of the Red Sea to the Persian Gulf. It measures 900,000 square miles (2,330,000 square kilometers). Except for fertile spots in the southeast and southwest, the peninsula is all desert. During prehistoric times, volcanic cones and craters formed along its western edge. On the eastern side, sedimentary rocks and prehistoric sea life formed the world’s richest oilfields. Inside some of these same rocks are vast supplies of underground water, captured during ages past when the area included wetlands.
The Arabian Desert
Location: The Arabian Peninsula
Area: 900,000 square miles (2,300,000 square kilometers)
Classification: Hot; extremely arid
The desert plateau consists of bare rock, gravel, or sand having a characteristically golden color, with the exception of deep ravines in the south. Wind and occasional flooding has carved the rocks into fantastic shapes. The Empty Quarter—about one-third of the southern part of the peninsula—is a vast sea of sand dunes. Treacherous quicksand is found here, its particles so smooth they act like ballbearings, drawing unwary creatures below the surface to their deaths. Another area of dunes, the Great Nafud, is found farther north and contains so few waterholes even camels find it difficult to cross. Along the coasts, changing sea levels have resulted in large salt flats, some as much as 20 miles (30 kilometers) wide.
A monsoon season (rainy period) occurs along the southeast; rainfall elsewhere is only 1.4 inches (35 millimeters) per year. Flash floods are common during the infrequent rains, and hailstorms sometimes occur. Droughts may last several years. January temperatures average 65°F (18°C).
In the southern portion of the Arabian Desert annuals spread a colorful blanket over the soil right after a rain; otherwise hardy perennials are the most common plant life, including mimosas, acacias, and aloe. Oleanders and some species of roses also thrive. Few trees can survive. The tamarisk tree helps control drifting sand, and junipers grow in the southwest. Date palms grow almost everywhere except at high elevations, and coconut palms are found on the southern coast. Plants that can be cultivated with the aid of irrigation include alfalfa, wheat, barley, rice, cotton, and many fruits, including mangoes, melons, pomegranates, bananas, and grapes.
Swarms of locusts move over the land periodically, causing much destruction. Other common invertebrates include ticks, beetles, scorpions, and ants. Horned vipers and a special species of cobra make their home in the Arabian Desert, as do monitors and skinks. Ostriches are now extinct there, but eagles, vultures, and owls are common. Seabirds, such as pelicans, can be seen on the coasts. Wild mammals include the gazelle, oryx, ibex, hyena, wolf, jackal, fox, rabbit, and jerboa. The lion once lived there but has long been extinct in the area.
For centuries, the Arabian Desert has been home to nomadic tribes of Bedouin. The camel makes life in the desert possible for them. A camel’s owner can live for months in the desert on the camel’s milk. The camel is also used for meat, clothing, and muscle power, and its dung (solid waste) is burned for fuel. Domestic sheep and goats are raised, as well as donkeys and horses.
The Sahara Desert
The Sahara Desert ranges across the upper third of Africa, from the Atlantic Ocean to the Red Sea, and is about 1,250 miles (2,000 kilometers) wide from north to south. It is the world’s largest hot desert, covering an area of 3.5 million square miles (9 million square kilometers). Its landforms, which tend to have a golden color, range from rocky mountains and highlands (some as high as 10,712 feet [3,265 meters]) to stretches of gravel and vast sand dunes. Erosion has shaped the sandstone rocks in some areas into unusual shapes and deep, narrow canyons.
The Sahara Desert
Location: North Africa
Area: 3,500,000 square miles (9,000,000 square kilometers)
Classification: Hot; extremely arid
Millions of years ago, volcanic activity occurred here. The region was the site of shallow seas and lakes, which contributed to the vast reserves of oil deposits found there. Around 150 BC when the Romans controlled North Africa, the northern Sahara was a rich agricultural area. Over the centuries sand has claimed the once fertile landscape.
The Sahara is the hottest desert, with a mean annual temperature of 85°F (29°C). Nights are cool and occasionally fall below freezing in the winter months. Except along the southern fringe, rain is not dependable and may be absent for as many as 10 years in succession. When it does rain, it tends to fall in sudden storms.
In some areas, such as the Tanezrouft region, nothing appears to grow. In other areas, annuals bloom after the unpredictable rains and provide food for camels and wild animals. At one time, the only perennial vegetation at oases were tamarisks and oleander bushes. The date palm has since been introduced and now citrus fruits, peaches, apricots, wheat, barley, and millet are all cultivated.
Animals such as gazelles, oryx, addax, foxes, badgers, and jackals live in the wild. Domesticated animals include camels, sheep, and goats.
Three main groups of people now live in the Sahara: the Tuareg, the Tibbu, and the Moors. Two-thirds of the population live at oases, where they depend upon irrigation and deep wells that tap underground water.
The Patagonian Desert
Along the entire length of Argentina, between the Andes Mountains in the west and the Atlantic Ocean in the east, lies the Patagonian Desert, which is 300,000 square miles (777,000 square kilometers). It owes its existence to a cold ocean current and the Andes Mountains, both of which cause dry air to form. The terrain is a series of plateaus, some as high as 5,000 feet (900 meters) near the Andes, which slope toward the sea. Deep, wide valleys made by ancient rivers have created clifflike walls, but only a few streams remain. These are usually formed by melting snow from the Andes.
The Patagonian Desert
Area: 300,000 square miles (777,000 square kilometers)
Classification: Cold; arid
In some areas, the soil is alkaline or covered by salt deposits and does not support plant life. Elsewhere it is primarily gravel. Mineral resources, including coal, oil, and iron ore, have been found in several places but quantities are too small to be important.
Rainfall, which occurs in the summer, is usually less than 10 inches (25 centimeters) annually. Although arid, the Patagonian Desert does not experience extremes of temperature, primarily because it is so close to the ocean. Temperatures range from 45°F (7°C) to 78°F (26°C). During the colder months, subzero temperatures and snow are common. Mean annual temperatures range from 43° to 78°F (6° to 26°C). Winds of 70 miles (112 kilometers) per hour are not uncommon.
Plants, mostly saltbushes and other members of the amaranth family, cover 15 percent of the desert. In moister areas, cushion plants, shrubs, feather grass and meadow grass cover almost half the ground.
Many reptiles are found here, but small mammals are the most numerous animals. Rabbits and hares are widespread. The rhea, an American ostrich, lives throughout the region. Other common animals are the mara and the guanaco, a llamalike animal prized for its long, fine hair. The guanaco was hunted almost to extinction because of the fine quality of its coat.
The most unusual animal of the Patagonian Desert is the armadillo. Its skin consists of plates of tough armor, including one that covers its face. It can travel very fast on its short legs and, when threatened, digs a hole to escape.
Patagonia means “big feet,” and refers to the Tehuelche Indians when they were first seen by Portugese explorer Ferdinand Magellan (c. 1480–1521) in 1520. The Tehuelche were nomadic hunters whose size and vigor caused Europeans to consider them giants. After the Europeans introduced the Tehuelche to horses, their ability to seek new territory and intermarry with other tribes increased. By 1960, the race was virtually extinct. Modern settlements in and around the Patagonian Desert were established during the twentieth century after oil was discovered on the coast.
The Atacama Desert
The Atacama Desert is a long, narrow, coastal desert 600 miles (965 kilometers) long, parallel to the coast of Chile. The Pacific Ocean lies to the west, where sheer cliffs about 1,475 feet (450 meters) high rise from the sea. Beyond these cliffs lies a barren valley that runs along the foothills of the Andes Mountains.
The Atacama Desert
Location: Chile and parts of Peru
Area: 54,000 square miles (140,000 square kilometers)
Classification: Hot; extremely arid
Cold ocean currents are responsible for the dry conditions that make the Atacama the world’s driest desert. Droughts may persist for many years, and there is no dependable rainy season. Although it is classified as a hot desert, mean temperatures seldom exceed 68°F (20°C).
Much of the Atacama consists of salt flats over gravelly soil. Sand dunes have formed in a few areas. In the south, a raised plateau nearly 3,300 feet (1,006 meters) high and broken with volcanic cones takes on an otherworldly appearance. In the southeast, a plateau bordering the Andes reaches a height of 13,100 feet (4,000 meters).
The area is rich in boron, sodium nitrate, and other minerals, which contaminate any underground water, making it unusable both by plants and animals. Plant life consists primarily of coarse grasses, mesquite, and a few cacti.
Animal life is rare. The most commonly found animals are lizards, but even they are not numerous. The rather timid giant iguanas, some growing over 6 feet (2 meters) long and resembling dragons prowling, are scavengers. Huge Andean condors that feed on carrion can be seen soaring overhead. Wherever cacti are found, cactus wrens feed and breed. Ovenbirds build their own little adobe (mud) huts instead of nests, and live in the less arid regions. The only mammals living in the Atacama are small rodents, such as the chinchilla, with guanaco and vicuña inhabiting higher elevations.
Indians may have lived in or on the fringes of the Atacama at one time; however, they may have been killed by European settlers during the eighteenth century. Those people who still live in the area are descendants of the European immigrants.
The Atacama was once important to the fertilizer industry for which sodium nitrate was mined. Another fertilizer, bird droppings called guano (GWAN-oh), was collected on the offshore islands, where seabirds would breed. In the 1920s synthetic fertilizers became popular, and the mining settlements in the Atacama were abandoned.
The Namib Desert
A coastal desert, the Namib is part of the vast tableland (flat highland) of southern Africa. Although it borders the Kalahari Desert to the east, the two are very different. Some scientists do not consider the Kalahari a true desert, while the Namib certainly is one. The desert is dry, but not very hot, with temperatures between 66°–75°F (19°–24°C). It has been nick-named the world’s oldest desert, dating back 55,000,000 years. The terrain is gravelly in the north, a rich area for gemstones, particularly diamonds. Sand prevails in the south, creating enormous dunes as much as 980 feet (300 meters) high in some areas.
The Namib Desert
Location: Eastern Namibia
Area: 52,000 square miles (135,000 square kilometers)
Classification: Hot; arid
Rain is rare and years may pass between showers. When rain does come, it creates flash floods and then disappears as quickly as it came. The winds that blow in from the Atlantic do not bring storms. Instead, dense fog rolls in almost nightly, creating very humid air.
Unusual life forms exist in the Namib, particularly in the southern dunes where plant life depends upon the fog and has evolved to draw moisture from it. The strange welwitschia plant is an example.
The Hartmann’s zebra that lives in the Namib is able to sniff out small pools of water that may lie in gullies or dry stream beds. The zebras dig, sometimes 2 or 3 feet (0.6 or 1 meter), into the ground until the water is uncovered. The Namib is also inhabited by elephants, rhinos, giraffes, and lions.
Bushmen live principally in the Kalahari, but they occasionally frequent the Namib.
The Mojave Desert
Deserts in North America are all part of the Great Basin, a vast territory stretching from just north of the Canadian border into parts of Mexico. In the north, cold semiarid deserts have formed, ringed by the Sierra Nevada and Rocky Mountains. Further south lie the hot deserts, the Mojave and the nearby Sonora Desert.
The Mojave Desert
Location: Southeastern California, Nevada, Arizona, Utah
Area: 25,000 square miles (65,000 square kilometers)
Classification: Hot; arid
The Mojave Desert, named for the Mojave Indians who once lived along the Colorado River, consists of salt flats, barren mountains, deep ravines, high plateaus, and wide, windy plains of sand. Elevations range from 2,000 to 5,000 feet (600 to 1,500 meters), and it measures 25,000 square miles (65,000 square kilometers). During prehistoric times the Pacific Ocean covered the area until volcanic action built the mountain ranges, leaving saltpans and mudflats as the only remaining signs. The desert is rich in minerals. Borax, potash, salt, silver, and tungsten are mined here.
Annual rainfall is less than 5 inches (13 centimeters). Frost is common in the winter, and snow occasionally falls. One major river, the Mojave, crosses the desert, running underground for part of its length. Summer temperatures often rise above 100°F (38°C), and winter temperatures often drop below freezing.
Vegetation characteristic of North American deserts includes different species of cacti such as organ pipe, prickly pear, and saguaro. Although nothing grows on the salt flats, plants, such as the creosote bush, can find a foothold in other areas. Joshua trees are protected in the Joshua Tree National Park. On the highest mountains, piñon and juniper grow.
A wide variety of invertebrates, amphibians, reptiles, birds, and smaller mammals make a home in this region. The rare Mojave ground squirrel may spend nine months estivating (being dormant in hot weather) in its burrow. Larger mammals include the puma, jaguar, peccary, prong-horned antelope, and bighorn sheep.
Death Valley is a basin in the Mojave Desert of which approximately 550 square miles (1,430 square kilometers) of the total 5,313 square miles (13,813 square kilometers) lie below sea level. Near its center is the lowest point in North America, 282 feet (86 meters) below sea level. Prehistoric salt flats exist in the lowest areas where nothing grows.
Area: 5,312 square miles (13,812 square kilometers)
Classification: Hot; arid
Death Valley is the hottest place on the continent. Average summer temperatures are 117°F (47°C) with a record temperature of 134°F (57°C) on July 10, 1913. Rainfall is seldom more than 2 inches (50 millimeters) per year. Many species of plants are found here. Annuals, such as poppies, appear in late winter and early spring; perennials, such as cacti and mesquite, survive all year. Lizards, foxes, rats, mice, squirrels, coyotes, bighorn sheep, wild burros, and rabbits live here, as well as many birds, such as ravens.
The name Death Valley came from the numbers of gold-seekers who died there during the gold rush in the mid-1800s. Although gold, silver, lead, and copper have been mined in the area, Death Valley became known for borax (compound used as an antiseptic and cleanser), discovered in 1873 and brought out by mule teams. Mining ghost towns now draw tourists. Death Valley became a national monument in 1933 and a national park in 1994.
Walter Edward Scott (1872–1954), known as “Death Valley Scotty,” was a cowboy in Buffalo Bill’s Wild West Show. A mansion approximately 32,000 square feet (2,973 square meters) was built in 1927 by Albert Johnson, a wealthy Chicago executive, in the Grapevine Canyon of Death Valley. Johnson met Scott at one of his shows and the two become friends. Through their friendship, the ranch became known as Scotty’s Castle, and is one of the most popular tourist attractions in Death Valley National Park.
The Antarctic Desert
Polar deserts are like other deserts only in the dryness of the air. Here, all moisture is frozen. During the warmest season in the polar desert, temperatures never rise above 12.8°F (-3°C), with the mean temperature ranging from -30° to -4°F (-34° to -20°C). In the cold season, the mean temperature is -52.9°F (-47.2°C) and ranges between -94° and -40°F (-70° to -40°C).
The Antarctic Desert
Area: Approximately 360,000 square miles (936,000 square kilometers)
Scientists know very little about the landforms in Antarctica because they are buried beneath such deep ice—over 14,000 feet (4,270 meters) deep in places. It is believed that the area is a series of islands connected by great ice sheets. Western Antarctica is mountainous, and eastern Antarctica is a plateau (high, flat land). It measures approximately 360,000 square miles (936,000 square kilometers). The Antarctic desert is the largest desert in the world, followed by the Sahara.
Tiny communities of microbes (microorganisms) have been found in the ice desert of Antarctica. During the summer, the sun warms little pockets of dirt and grit and turns them to slush. In the dim light, bacteria suspended in the slush can photosynthesize.
Plant growth is limited to the bordering tundra regions where lichens and mosses are the largest plant forms. Algae, yeasts, fungi, and bacteria are found in some areas. Grasses and a few other seed plants may grow on the fringes. The driest areas support no known plant life.
The only land animals are about 100 species of invertebrates, half of which appear to be parasites that live on birds and mammals. These include lice, mites, and ticks.
Seabirds, such as petrels and terns, frequent the coast where they feed on fish. The only true Antarctic bird is the penguin, and about eighteen species populate the area. Penguins live in large flocks and nest in the autumn, living on fish they catch in the coastal waters. They range in size from the fairy penguin, which is 16 inches (41 centimeters), to the emperor penguin, which is about 4 feet (1.2 meters) tall. Leopard seals, the largest of all seals, are 10 feet (3 meters) long and weigh 770 pounds (300 kilograms). They inhabit the Antarctic waters and are the natural predators of penguins.
Marine mammals, such as whales and seals, live in the coastal waters, but they do not enter desert areas. (Polar bears live only at the North Pole; they do not live in Antarctica.)
Temporary year-round human settlements were established in Antarctica around 1900, primarily for exploration purposes. Any industry is centered on the surrounding sea, primarily whaling and seal hunting.
Allaby, Michael. Biomes of the Earth: Deserts. New York: Chelsea House, 2006.
Hodgson, Wendy C. Food Plants of the Sonoran Desert. Tucson: University of Arizona Press, 2001.
Irish, Mary. Gardening in the Desert: A Guide to Plant Selection & Care. Tucson: University of Arizona Press, 2000.
Johnson, Mark. The Ultimate Desert Handbook: A Manual for Desert Hikers, Campers and Travelers. Camden, Maine: Ragged Mountain Press/McGraw Hill, 2003.
Oliver, John E., and John J. Hidore. Climatology: An Atmosperic Science. Upper Saddle River, NJ: Prentice Hall, 2nd ed. 2004.
Sowell, John B. Desert Ecology. Salt Lake City: University of Utah Press, 2001.
El-Bagouri, Ismail H.M. “Interaction of Climate Change and Land Degradation: the Experience in the Arab Region.” UN Chronicle. 44. 2 June 2007: 50.
Greer, Carrie A. “Your local desert food and drugstore.” Skipping Stones. 20. 2 March-April 2008: 34.
Lancaster, Pat. “The Oman experience.” The Middle East. 384 December 2007: 55.
Levy, Sharon. New Scientist: Last Days of the Locust. February 21, 2004, 48-49.
Chihuahuan Desert Research Institute, PO Box 905, Fort Davis, TX 79734, Phone: 432-364-2499; Fax: 432-364-2686, Internet: http://www.cdri.org
Friends of the Earth, 1717 Massachusetts Ave. NW, 300, Washington, DC 20036-2002, Phone: 877-843-8687; Fax: 202-783-0444; Internet: http://www.foe.org
Greenpeace USA, 702 H Street NW, Washington, DC 20001, Phone: 202-462-1177; Fax: Internet: http://www.greenpeace.org
National Center for Atmospheric Research. http://www.ncar.ucar.edu (accessed on August 17, 2007).
Desertion and related lesser offenses, such as going AWOL (absent without leave), bedeviled both the Confederate and Union armies during the American Civil War. Estimates of the total number of desertions vary depending on historic sources and individual definitions of desertion, but historians generally put the number of Union desertions from military duty somewhere between 200,000 and 260,000 troops and the number of Confederate deserters at somewhere in excess of 100,000 troops. The North, however, had a far larger military in the first place, and a greater pool of potential replacements to replenish it with. As a result, desertions never threatened to cripple the overall Union war effort. By contrast, in the South the military margin for error was much less, and the pool of replacements much shallower. So while the number of deserters as a percentage of the total Confederate army was not that much greater than the percentage of deserters within the Union ranks, Southern desertion had a much more severe impact on military operations and morale. As the war turned decisively against the South in the last two years of the conflict, rates of desertion soared in many Confederate units, and historians cite desertion as a leading factor in the South's military collapse.
Answering Desperate Calls from Home
The great majority of Civil War soldiers—even those who endured horrendous battles and deadly skirmishes on multiple occasions—never abandoned their military obligations, even in their darkest hours of doubt and fear. Soldiers who rallied to the cause in the opening months of the conflict had a particularly low rate of desertion—and a correspondingly high intensity of loathing for those soldiers, whether volunteer or conscript, who slipped out of the line before the war was over. For these veteran soldiers, notions of honor and duty sustained their motivation throughout the years (McPherson 1997, p. 168).
Tens of thousands of other soldiers, however, left the ranks of the North and South before they had fulfilled their military obligations. The factors behind these premature departures were legion. Some slipped away out of cowardice—a failure to control and manage the fear that afflicted all Civil War soldiers. Many other soldiers deserted for more complicated reasons, however. For example, numerous soldiers reluctantly slipped away for home under cover of darkness or in the chaos of battle out of concern for loved ones. This was especially true of some Confederate soldiers, who knew in the war's latter stages that much of the South was being overrun by enemy troops and sought to protect their families back home.
This agony of divided loyalties was further deepened by beseeching letters from home. "It is useless to conceal the truth any longer," wrote one North Carolina soldier in early 1865. "Most of our people at home have become so demoralized that they write to their husbands, sons and brothers that desertion now is not dishonorable" (Robertson 1988, p. 136).
Some letters from loved ones even warned soldiers that failure to immediately set off for home meant certain doom for family members. One Alabama soldier, for example, received a letter from home in 1864 informing him that "if you put off a-coming, 'twont be no use to come, for we'll all hands of us be out there in the grave yard with your ma and mine" (Martin 2003 , p. 172).
One of the better-known desertion trials of the Civil War concerned Confederate Private Edward Cooper, whose defense was based in part on one of these "please come home" letters—from his wife. This letter was reportedly a major factor in convincing authorities to spare Cooper's life. "I have been always proud of you, and since your connection with the Confederate army, I have been prouder of you than ever before," his wife's letter stated. "I would not have you do anything wrong for the world, but before God, Edward, unless you come home, we must die. Last night, I was aroused by little Eddie's crying. I called and said, 'what is the matter, Eddie?' And he said, 'O Mamma, I am so hungry.' And Lucy, Edward, your darling Lucy; she never complains, but she is growing thinner and thinner every day. And before God, Edward, unless you come home, we must die" (Moore 1880, p. 237).
This peril to loved ones—whether real or imagined— has been cited by historians as a contributing factor in the higher desertion rates among married soldiers than unmarried ones, as well as the higher desertion rates among privates than officers. Many of the latter came from comparatively affluent backgrounds and thus had families that were better able to sustain and protect themselves during the war.
Erosion of Morale
Myriad other factors contributed to soldiers' decisions to desert, either for home or for destinations that promised anonymity or opportunities to construct new lives. The Confederate Army's intensifying difficulties in procuring basic food and supplies for its soldiers undoubtedly played a role in rising desertion rates. In addition, the South had growing difficulty meeting its payroll obligations as the war went on. Both armies, meanwhile, experienced greater problems with desertion when they tried to transfer soldiers far from home. In the case of Confederate troops, the desire to be close to home increased as the war progressed and Northern troops pushed further and further into Southern territory, possibly endangering family members. In some cases, opposition to transfers to distant locales was so strong that large-scale desertions occurred.
Another contributor to diminished morale—and thus higher rates of desertion—in both Union and Confederate units was the decision by each side to build up its military units in response to mounting casualties. As gaps in regiments and divisions were filled with conscripts and other replacements, the esprit de corps that had predominated in the all-volunteer force was supplanted by tensions between the new arrivals and veteran volunteers who viewed the former as useless and untrustworthy. In some instances, the hostility of fellow regimental members was enough to make already unenthusiastic conscripts want to desert.
Other desertions stemmed from a growing sense among the infantry rank and file that the Civil War was "a rich man's war and a poor man's fight." This conviction, which could be amply supported by even a cursory glance at the socioeconomic inequities contained within the conscription acts of the Federal and Confederate governments, was further underscored by the furlough programs that both militaries instituted, which made it easier for wealthy soldiers to periodically return home. The failure of military authorities to grant deserved furloughs was especially commonplace in the increasingly soldier-strapped South. Confederate authorities tried to assuage the anger of troops with promises of future compensation, appeals to duty, and assorted excuses, but to little avail: Thousands of frustrated troops simply went home without permission (Wiley 1992, p. 139).
Punishments for Desertion
From the opening months of the Civil War, both the North and South recognized that desertion posed a potentially serious threat to their respective causes. With this in mind, the administrations of Jefferson Davis and Abraham Lincoln, as well as leading military officers from both armies, kept up a steady a drumbeat of entreaties and threats to keep their men from slipping away. Washington, DC, and Richmond, VA, even resorted to proclamations that promised pardons and general amnesties to deserters willing to return to military duty and thus remove the "stain" upon their honor.
"THE EXECUTION OF DESERTERS"
In September of 1863 Harper's Weekly published an illustration depicting the execution of five deserters, drawn by staff artist Alfred Rudolph Waud (1828–1891). Waud appended some brief remarks on the necessity of capital punishment for deserters:
The crime of desertion has been one of the greatest drawbacks to our army. If the men who have deserted their flag had but been present on more than one occasion defeat would have been victory, and victory the destruction of the enemy. It may be therefore fairly asserted that desertion is the greatest crime of the soldier, and no punishment too severe for the offense. But the dislike to kill in cold blood-a Northern characteristic-the undue exercise of executive clemency, and in fact the very magnitude and vast spread of the offense, has prevented the proper punishment being applied. That is past; now the very necessity of saving life will cause the severest penalties to be rigorously exacted. The picture represents the [five] men who were sentenced to death in the Fifth Corps for desertion at the moment of their execution. Some of these had enlisted, pocketed the bounty, and deserted again and again. The sentence of death being so seldom enforced they considered it a safe game. They all suffered terribly mentally, and as they marched to their own funeral they staggered with mortal agony like a drunken man. Through the corps, ranged in hushed masses on the hill-side, the procession moved to a funeral march, the culprits walking each behind his own coffin. On reaching the grave they were, as usual, seated on their coffins; the priests made short prayers; their eyes were bandaged; and with a precision worthy of praise for its humanity, the orders were given and the volley fired which launched them into eternity. They died instantly, although one sat up nearly a minute after the firing; and there is no doubt that their death has had a very salutary influence on discipline.
rebecca j. frey
SOURCE: "The Execution of Deserters." Harper's Weekly, September 26, 1863, p. 622.
These official efforts met with some limited success, but punishment (and the threat thereof) quickly emerged as the primary officially sanctioned means of addressing the desertion issue. Punishments for desertion ranged greatly, depending on the perceived severity of the offense and the personal characteristics of the authorities imposing the sentence. For example, soldiers found guilty of being absent without leave usually were punished with some combination of pay forfeiture and increased manual labor. Those who were found guilty of the far more serious crime of desertion, however, might be sentenced to branding (often with a C to denote a Coward or a D to denote a Deserter), public flogging, extended imprisonment, or even death by execution.
According to historian Jeffrey Rogers Hummel in Emancipating Slaves, Enslaving Free Men (1996), the Union and the Confederacy executed a total of five hundred of their own troops during the course of the Civil War. This total exceeds the total number of executions in all other American wars combined. Two-thirds of the executions that took place during the Civil War were for the crime of desertion. Almost invariably, they were staged publicly, so as to send a harsh warning to anyone contemplating leaving ranks.
These executions undoubtedly had their intended effect in some cases. But in others, the brutal spectacles seemed to engender a deeper demoralization among some witnesses. A Rebel soldier from Florida, for example, was profoundly shaken after he witnessed the execution of a young deserter who spent the last moments of his life desperately begging for mercy. The soldier called the execution "one of the most sickening scenes I ever witnessed[;] … [it] looked more like some tragedy of the dark ages, than the civilization of the nineteenth century" (Dean 2002, p. 414). A Union soldier from Indiana expressed similar sentiments after witnessing an execution of a deserter from his army. "I don't think I will ever witness another such a horror if I can get away from it," he wrote."I have seen men shot in battle but never in cold blood before" (Dean 2002, p. 414).
Escalating Levels of Desertion
After the Civil War turned decisively against the South in mid-1863, rates of desertion from the Confederate forces rose dramatically. "In the wake of Gettysburg the highways of Virginia were crowded daily with homeward-bound troops, still in possession of full accouterments; and, according to one observer, these men 'when halted and asked for their furloughs or their authority to be absent from their commands, … just pat their guns defiantly and say, 'this is my furlough,' and even enroll-iing officers turn away as peaceably as possible" (Wiley 1992, p. 143–44).
As the months passed, entire garrisons and companies quietly left the Confederate ranks. Many of these deserters separated and returned to their far-flung homes. Others banded together into outlaw groups that sustained themselves by robbing local communities or military stores. In some areas of the South, these guerrilla bands became so powerful that they became a threat to the Confederate detachments that were sent to neutralize them. The need to send such detachments put a further drain on an army that was already groaning under the methodical, unrelenting pressure of a foe with superior resources.
By the time the final hours of 1864 were ticking away, desertion had reached epidemic levels in many Confederate units. Sentries walked away from their posts, infantrymen crept from their trenches under cover of darkness, and cavalrymen turned the heads of their mounts away from the front and toward home. Even Confederate General Robert E. Lee (1807–1870), the most respected and beloved military leader of the entire South, was powerless to stop some defections from the battered ranks of his Army of Northern Virginia. According to historian Bell Irvin Wiley's The Life of Johnny Reb (1992), the Confederate War Department reported that there were a total of 198,494 officers and men absent and only 160,198 present in the armies of the Confederacy on the eve of surrender (pp. 144–145). These figures confirm that although desertion constituted a problem for the North, its impact was far more crippling for the South.
Alotta, Robert I. Stop the Evil: A Civil War History of Desertion and Murder. San Rafael, CA: Presidio Press, 1978.
Dean, Eric T., Jr. " 'Dangled over Hell': The Trauma of the Civil War." In The Civil War Soldier: A Historical Reader, ed. Michael Barton and Larry M. Logue. New York: New York University Press, 2002.
Donald, David Herbert, Jean Harvey Baker, and Michael F. Holt. The Civil War and Reconstruction. New York: W. W. Norton, 2001.
Hummel, Jeffrey Rogers. Emancipating Slaves, Enslaving Free Men: A History of the American Civil War. Chicago: Open Court, 1996.
Martin, Bessie. A Rich Man's War, a Poor Man's Fight: Desertion of Alabama Troops from the Confederate Army. Library of Alabama Classics Series. Tuscaloosa: University of Alabama Press, 2003. Originally published as Desertion of Alabama Troops from the Confederate Army, New York: Columbia University Press, 1932.
McPherson, James M. For Cause and Comrades: Why Men Fought in the Civil War. New York: Oxford University Press, 1997.
Mitchell, Reid. Civil War Soldiers: Their Expectations and Their Experiences. New York: Viking, 1988.
Moore, John W. History of North Carolina: From the Earliest Discoveries to the Present Time, Vol. 2. Raleigh, NC: Alfred Williams, 1880.
Power, J. Tracy. Lee's Miserables: Life in the Army of Northern Virginia from the Wilderness to Appomattox. Chapel Hill, NC: University of North Carolina Press, 1998.
Robertson, James I., Jr. Soldiers Blue and Gray. Columbia, SC: University of South Carolina Press, 1988.
Robertson, James I., Jr., and the editors of Time-Life Books. Tenting Tonight: The Soldier's Life. Alexandria, VA: Time-Life Books, 1984.
Ward, Geoffrey C., with Ric Burns and Ken Burns. The Civil War. New York: Vintage, 1994.
Wiley, Bell Irvin. The Life of Johnny Reb: The Common Soldier of the Confederacy. Indianapolis, IN, and New York: Bobbs-Merrill Company, 1943. Reprint, Baton Rouge: Louisiana State University Press, 1992.
Williams, David. A People's History of the Civil War: Struggles for the Meaning of Freedom. New York: New Press, 2005.
A desert is an arid land area that generally receives less than 10 inches (250 millimeters) of rainfall per year. What little water it does receive is quickly lost through evaporation. Average annual precipitation in the world's deserts ranges from about 0.4 to 1 inch (10 to 25 millimeters) in the driest areas to 10 inches (250 millimeters) in semiarid regions.
Other features that mark desert systems include high winds, low humidity, and temperatures that can fluctuate dramatically. It is not uncommon for the temperature to soar above 90°F (32°C) and then drop below 32°F (0°C) in a single day in the desert.
Most of the world's desert ecosystems (communities of plants and animals) are located in two belts near the tropics at 30 degrees north and 30 degrees south of the equator. These areas receive little rainfall because of the downward flow of dry air currents that originate at the equator. As this equatorial air moves north and south, it cools and loses whatever moisture it contains. Once this cool, dry air moves back toward Earth's surface, it is rewarmed, making it even drier. Over the desert areas, the dry air currents draw moisture away from the land on their journey back toward the equator.
Deserts around the world
The vast Sahara Desert in northern Africa encompasses an area 3,000 miles (4,800 kilometers) wide and 1,000 miles (1,600 kilometers) deep. Sand composes just 20 percent of the Sahara, while plains of rock, pebble, and salt flats, punctuated by mountains, make up the rest. The Sahara can experience temperatures that rise and fall 100°F (38°C) in a single day. Decades can go by without rain. By contrast, the Gobi Desert, covering 500,000 square miles (1,295,000 square kilometers) in northcentral Asia, sits at a higher altitude than the Sahara. As a result, temperatures in the Gobi remain below freezing most of the year.
Words to Know
Arid land: Land that receives less than 10 inches (250 millimeters) of rainfall annually and has a high rate of evaporation.
Desert pavement: Surface of flat desert lands covered with closely spaced, smooth rock fragments that resemble cobblestones.
Desert varnish: Dark film of iron oxide and manganese oxide on the surface of exposed desert rocks.
Rain-shadow deserts: Areas that lie in the shadow of mountain ranges and receive little precipitation.
the Outback. Antarctica, the land mass at the southern pole of the globe, is a polar desert. One of the driest places on Earth, it receives only a dusting of snow each year. Warmest summer temperatures in Antarctica reach only 25°F (−4°C).
The deserts of the United States are located at higher latitudes and in higher altitudes than is typical of many other arid regions of the world. Death Valley in California is both extremely arid and extremely hot in the summer. South of it are the relatively cooler and wetter Mojave and Sonoran Deserts.
Rain-shadow deserts are those that lie in the shadow of mountain ranges. As air ascends on one side of a range, it releases any moisture it carries. Once on the other side, the air contains little moisture, forming deserts in the slope of the range. Among rain-shadow deserts are Death
Dunes, wind-blown piles of sand, are the most common image of a desert landscape. Wind constantly sculpts sand piles into a wide variety of shapes. Dunes move as wind bounces sand up the dune's gently sloping windward side (facing the wind) to the peak of the slope. At the peak the wind's speed drops and sends sand cascading down the steeper lee side (downwind). As this process continues, the dune migrates in the direction the wind blows. Given enough sand and time, dunes override other dunes to thicknesses of thousands of feet, as in the Sahara Desert.
Desertification refers to the gradual transformation of productive land into that with desertlike conditions. Desertification may occur in rain forests and tropical mountainous areas. Even a desert itself can become desertified, losing its sparse collection of plants and animals and becoming a barren wasteland.
Desertification occurs in response to continued land abuse, and may be brought about by natural or man-made actions. Among the natural forces are constant wind and water (which erode topsoil) and long-term changes in rainfall patterns (such as a drought). The list of human actions includes overgrazing of farm animals, strip mining, the depletion of groundwater supplies, the removal of forests, and the physical compacting of the soil (such as by cattle and off-road vehicles).
Almost 33 percent of Earth's land surface is desert, a proportion that is increasing by as much as 40 square miles (64 square kilometers) each day. The arid lands of North America are among those most affected by desertification: almost 90 percent are moderately to severely desertified.
Fortunately, scientists believe that severe desertification is rare. Many feel that most desertified areas can be restored to productivity through careful land management.
Sand carried by the wind can act as an abrasive on the land over which it flows. Rocks on the floor of a desert can become polished in this way. Closely spaced, smooth rock fragments that resemble cobblestones on the surface of flat lands are referred to as desert pavement. The dark film of iron oxide and manganese oxide on the surface of the exposed rocks is called desert varnish.
Life in the desert
The plants and animals that are able to survive the extremes of desert conditions have all evolved ways of compensating for the lack of water. Plants that are able to thrive in the desert include lichens (algae and fungi growing together). Lichens have no roots and can absorb water and nutrients from rain, dew, and the dust on which they grow. Succulent plants, such as cacti, quickly absorb rainwater when it comes and store it in their stems and leaves, if they have them. Other plants store nutrients in their roots and stems. Many desert shrubs have evolved into upside-down cone shapes. They collect large amounts of rain on their surfaces, then funnel it down to their bases.
Deserts are not lifeless, but are inhabited by insects, arachnids (spiders and scorpions), reptiles, birds, and mammals. Unlike plants, these animals can seek shelter from the scorching sun, cold, and winds by crawling into underground burrows. Many have adapted to the harsh desert environment by developing specific body processes. Some small mammals, such as rodents, excrete only concentrated urine and dry feces, and perspire little as a way of conserving body fluids. The camel's body temperature can soar to 105°F (41°C) before this mammal sweats. It can lose up to one-third of its body weight and replace it at a single drinking.
[See also Biome ]
In peacetime, desertion has been a continuing phenomenon in American military history, at least through the early twentieth century, although its extent has varied widely depending upon the circumstances facing the service people. Unlike European nations, the U.S. government had little control over its citizens, and deserters could escape relatively easily, particularly into the rural and frontier regions of the country. Low pay and poor conditions have contributed significantly to peacetime desertions.
The armed forces require enlisted men and women to serve tours of duty of specific duration. Unlike commissioned officers, enlisted personnel are not legally permitted to resign unilaterally. Thus, desertion constitutes an enlisted person's repudiation of his or her legal obligation.
A correlation has existed in peacetime between desertion rates and the business cycle. When the country experienced economic depression and high unemployment, fewer people abandoned the service. Yet in an expanding economy, with workers in demand and wage scales increasing, many more service men and women have forsaken the high job security but lesser monetary rewards of the military.
The highest peacetime desertion rates in American history were reached during periods of economic growth in the 1820s, early 1850s, early 1870s, the 1880s, early 1900s, and the 1920s, when the annual flow of deserters averaged between 7 and 15 percent of the U.S. Army. A peak of 32.6 percent was recorded in 1871, when 8,800 of the 27,010 enlisted men deserted in protest against a pay cut. (By contrast, the desertion rate in the British army was only about 2 percent.) Lured by higher civilian wages and prodded by miserable living conditions—low pay, poor food, inadequate amenities, and boredom—on many frontier western outposts, a total of 88,475 soldiers (one‐third of the men recruited by the army) deserted between 1867 and 1891.
The peacetime navy had its own desertion problems. In the nineteenth century, many of the enlisted men had grim personal backgrounds or criminal records or were foreigners with little loyalty to the United States. A rigid class system and iron discipline contributed to high rates of alcoholism and desertion. In 1880, there were 1,000 desertions from an enlisted force of 8,500 seamen.
During wartime, desertion rates in all the military services have varied widely but have generally been lower than in peacetime—perhaps reflecting the increased numbers of service people, national spirit, and more severe penalties prescribed for combat desertion. The end of hostilities, however, generally was accompanied by a dramatic flight from the military. After almost every war, the desertion rate doubled temporarily as many regular enlisted personnel joined other Americans in returning to peacetime pursuits. The variation in wartime desertion rates seems to result from differences in public sentiment and prospects for military success. Although many factors are involved, generally the more swift and victorious the campaign and the more popular the conflict, the lower the desertion rate. Defeat and disagreement or disillusionment about a war have been accompanied by a higher incidence of desertion.
In the Revolutionary War, desertion depleted both the state militias and the Continental army after such reverses as the British seizures of New York City and Philadelphia; at spring planting or fall harvesting times, when farmer‐soldiers returned to their fields; and as veterans deserted in order to reenlist, seeking the increased bounties of cash or land that the states offered new enlistees. Widespread desertion, even in the midst of battle, plagued the military during the setbacks of the War of 1812. In the Mexican War, 6,825 men, or nearly 7 percent of the army, deserted; and one unit of the Mexican Army, the San Patricio Artillery Battalion, was composed of American deserters.
The Civil War produced the highest American wartime desertion rates because of its bloody battles, new enlistment bounties, and the relative ease with which deserters could escape capture, particularly in the mountain regions. The Union armies recorded 278,644 cases of desertion, representing 11 percent of the troops. As the Confederate military situation deteriorated, desertion reached epidemic proportions. The Appalachian Mountains, Florida swamps, and Texas chaparral became the domain of armed bands of Southern deserters. In the final year of the war, whole companies and regiments, sometimes with most of their officers, left together to return to their homes. In all, Confederate deserters numbered 104,428, or 10 percent of the South's armies.
The brief and successful Spanish‐American War resulted in 5,285 desertions, or less than 2 percent of the armed forces in 1898. However, the rate climbed to 4 percent during the long and arduous Philippine War between 1900 and 1902. In World War I, because conscription regulations classified any draftee failing to report for induction at the prescribed time as a deserter, the records of 1917–18 showed 363,022 deserters, who would have been more appropriately designated draft evaders. Traditionally defined deserters amounted to 21,282, or less than 1 percent of the army in World War I.
In World War II, desertion rates reached 6.3 percent of the armed forces in 1944, and during the American reverses at the Battle of the Bulge, the army executed one American soldier, Private Ernie Slovik, for desertion in the face of the enemy as an example to other troops. Desertion rates dropped to 4.5 percent in 1945. During the Korean War, the use of short‐term service and the rotation system helped keep desertion rates down to 1.4 percent of the armed forces in fiscal year (FY) 1951 and to 2.2 percent or 31,041 in FY 1953.
The divisive Vietnam War generated the highest percentage of wartime desertion since the Civil War. From 13,177 cases—or 1.6 percent of the armed forces—in FY 1965, the annual desertion statistics mounted to 2.9 percent in FY 1968, 4.2 percent in FY 1969, 5.2 percent in FY 1970, and 7.4 percent (79,027 incidents of desertion) in FY 1971. Like the draft resisters from this same war, many deserters sought sanctuary in Canada, Mexico, or Sweden. In 1974, the Defense Department reported that between 1 July 1966 and 31 December 1973, there had been 503,926 incidents of desertion in all services during the Vietnam War.
The end of the draft and the Vietnam War, together with the enhancement of pay and living conditions in the All‐Volunteer Force, dramatically reduced desertions, although there was somewhat of another upsurge during the Persian Gulf War (1991).
[See also Military Justice; Morale, Troop.]
Ella Lonn , Desertion During the Civil War, 1928, 1966;
William B. Huie , The Execution of Private Slovik, 1954, 1991;
Russell F. Weigley , History of the United States Army, 1967;
Jack D. Foner , The United States Soldier between the Two Wars: Army Life and Reforms, 1865–1898, 1968;
Thomas L. Hayes , American Deserters in Sweden, 1971;
Robert L. Alotta , Stop the Evil: A Civil War History of Desertion and Murder, 1978;
Edward M. Coffman , The Old Army: A Portrait of the American Army in Peacetime, 1784–1898, 1986.
John Whiteclay Chambers II
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 ]
DESERTION from military service has been a continual phenomenon in American history although its extent has varied widely depending upon the circumstances that have confronted soldiers. The armed forces require enlisted men and women to serve tours of duty of specific duration and, unlike commissioned officers, enlisted personnel may not legally resign before the end of that period. Thus desertion—being absent without authorization for over a month—constitutes the enlisted person's repudiation of his or her legal obligation.
In peacetime there has been a direct correlation between desertion rates and the business cycle. When the country has experienced a depression and a labor surplus, fewer soldiers have abandoned the army. By contrast, in an expanding economy, with workers in demand and wage scales increasing, many more servicemen and women have forsaken the high job security but low monetary rewards of the army.
The highest peacetime desertion rates in American history occurred during the periods of economic growth in the 1820s, early 1850s, early 1870s, 1880s, early 1900s, and 1920s, when the flow of deserters averaged between 7 and 15 percent each year. A peak of 32.6 percent was reached in 1871, when 8,800 of the 27,010 enlisted men deserted in protest against a pay cut. Lured by higher civilian wages and prodded by the miserable living conditions of most frontier outposts, a total of 88,475, or one-third of the men recruited by the army, deserted between 1867 and 1891.
During wartime, desertion rates have varied widely but have generally been lower than in peacetime service, a tendency that perhaps reflects the increased numbers of troops, national spirit, and more severe penalties prescribed for combat desertion. A dramatic flight from military duty has generally accompanied the termination of hostilities. After almost every war the desertion rate has doubled temporarily as many servicemen and women have joined other Americans in returning to peacetime pursuits. The variation in wartime desertion rates seems to result from differences in public sentiment and military prospects. Although many factors are involved, generally the more swift and victorious the campaign and the more popular the conflict, the lower the desertion rate. Defeat and disagreement or disillusionment about a war have been accompanied by a higher incidence of desertion.
In the American Revolution, desertion depleted both the state militias and the Continental army after such reverses as the British seizure of New York City; at spring planting or fall harvesting time, when farmer-soldiers returned to their fields; and as veterans deserted in order to reenlist, seeking the increased bounties of cash or land that the states offered for new enlistees. Widespread desertion, even in the midst of battle, plagued the military during the War of 1812.Inthe Mexican-American War, 6,825 men, or nearly 7 percent of the army, deserted. Moreover, American deserters composed one unit of the Mexican army, the San Patricio Artillery Battalion.
The Civil War produced the highest American wartime desertion rates because of its bloody battles, new enlistment bounties, and relative ease with which deserters could escape capture in the interior regions. The Union armies recorded 278,644 cases of desertion, representing 11 percent of the troops. As the Confederate military situation deteriorated, desertion reached epidemic proportions. Whole companies and regiments, sometimes with most of their officers, fled together. In all, Confederate deserters numbered 104,428, or 10 percent of the armies of the South.
The Spanish-American War resulted in 5,285 desertions, or less than 2 percent of the armed forces in 1898. The rate climbed to 4 percent during the Philippine Insurrection between 1900 and 1902. In World War I, because Selective Service regulations classified anyone failing to report for induction at the prescribed time as a deserter, the records of 1917–1918 showed 363,022 deserters who would have been more appropriately designated draft evaders. Traditionally defined deserters amounted to 21,282, or less than 1 percent of the army. In World War II desertion rates reached 6.3 percent of the armed forces in 1944 but dropped to 4.5 percent by 1945. The use of short-term service and the rotation system during the Korean War kept desertion rates down to 1.4 percent of the armed forces in fiscal year 1951 and to 2.2 percent, or 31,041 soldiers, in fiscal year 1953.
The unpopular war in Vietnam generated the highest percentage of wartime desertion since the Civil War. From 13,177 cases, or 1.6 percent of the armed forces, in fiscal year 1965, the annual desertion statistics mounted to 2.9 percent in fiscal year 1968, 4.2 percent in fiscal year 1969, 5.2 percent in fiscal year 1970, and 7.4 percent in fiscal year 1971. Like the draft resisters from this same war, many deserters sought sanctuary in Canada, Mexico, or Sweden. In 1974 the Defense Department reported that there had been 503,926 incidents of desertion between 1 July 1966 and 31 December 1973.
Higginbotham, Don. War and Society in Revolutionary America: The Wider Dimensions of Conflict. Columbia: University of South Carolina Press, 1988.
Jessup, John E., et al., eds. Encyclopedia of the American Military: Studies of the History, Traditions, Policies, Institutions, and Roles of the Armed Forces in War and Peace. New York: Scribners; Toronto: Macmillan, 1994.
Weitz, Mark A. A Higher Duty: Desertion among Georgia Troops During the Civil War. Lincoln: University of Nebraska Press, 2000.
Whiteclay, John, et al., eds. The Oxford Companion to American Military History. New York: Oxford University Press, 1999.
John WhiteclayChambers/a. e.
A desert is an arid land area where more water is lost through evaporation than is gained from precipitation. Deserts include the familiar hot, dry desert of rock and sand that is almost barren of plants, the semiarid deserts of scattered trees, scrub, and grasses, coastal deserts, and the deserts on the polar ice caps of the Antarctic and Greenland.
Most deserts are the result of large-scale climatic patterns. As Earth turns on its axis, hot air rises over the equator then flows northward and southward. The air currents cool in the upper regions and descend as high pressure areas in two subtropical zones. North and south of these zones are two more areas of ascending air and low pressures. Still farther north and south are the two polar regions of descending air. As air rises, it cools and loses its moisture. As it descends, it warms and picks up moisture, drying out the land. This downward movement of warm air has produced two belts of deserts. 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 Kalihari Desert of southern Africa, and the Great Victoria and Great Sandy Deserts of Australia.
Coastal deserts are form when cold water moves from the Arctic and Antarctic regions toward the equator and comes into contact with the edges of continents. The cold water is augmented by upwellings of cold water from ocean depths. The air cools as it moves across cold water, carrying fog and mist but little rain. This pattern of air flow is responsible for coastal deserts in southern California, Baja California, southwest Africa, and Chile.
Mountain ranges also influence the formation of deserts by creating rain shadows. As moisture-laden air flows upward over windward slopes, it cools and loses its moisture. Dry air descending over the leeward slopes evaporates moisture from the soil, creating a desert. The Great Basin Desert in the western United States was formed from a rain shadow produced by the Sierra Nevada. Desert areas also form in the interior of continents when prevailing winds are far from large bodies of water and have lost much of their moisture.
Desert plants have evolved methods to conserve water. Some flowering desert plants are ephemeral and live for only a few days. Their seeds or bulbs can lie dormant in the soil for years, until a heavy rain enables them to germinate, grow, and bloom. Woody desert plants can either have long root systems to reach groundwater sources or spreading shallow roots to take up moisture from dew or occasional rains. Most desert plants have small or rolled leaves to reduce the surface area from which transpiration of water can take place, while others drop their leaves during dry periods. Some leaves have waxy coatings that prevent water loss. Many desert plants are succulents, which store water in leaves, stems, and roots. Thorns and spines of the cactus are used to protect a plant’s water supply from animals.
Desert animals have also developed protective mechanisms to allow them to survive in deserts. Most desert animals and insects are small, so they can remain in cool underground burrows or hide under vegetation during the day and feed at night when it is cooler. Desert amphibians are capable of dormancy during dry periods, but when it rains, they mature rapidly, mate, and lay eggs. Many birds and rodents reproduce only during or following periods of winter rain that stimulate vegetative growth. Some desert rodents (e.g., the North American kangaroo rat and the African gerbil) have large ears with little fur to allow them to sweat and cool down. They also require very little water. The desert camel can survive nine days on water stored in its stomach. Many larger desert animals have broad hooves or feet to allow them to move over soft sand. Desert reptiles such as the horned toad can control their metabolic heat production by varying their rate of heartbeat and the rate of body metabolism. Some snakes have developed a sideways shuffle that allows them to move across soft sand. Deserts are difficult places for humans to live, but some people do live in deserts, for example the Aborigines in Australia and the Tuaregs in the Sahara.
Desert soils are usually naturally fertile because little water is available to leach nutrients. Crops can be grown on desert lands with irrigation, but evaporation of the irrigation water can result in the accumulation of salts on the soil surface, making the soil unsuitable for further crop production. Burning, deforestation, and overgrazing of lands on the semiarid edges of deserts are enabling deserts to encroach on the nearby arable lands in a process called desertification. Desertification in combination with shifts in global atmospheric circulation has resulted in the southern boundary of the Sahara Desert advancing 600 mi (1, 000 km) southward. A desertification study conducted for the United Nations in 1984 determined that 35% of Earth’s land surface was threatened by desertification processes.
Deserts are environments shaped by aridity, or dryness. Aridity reflects the balance between precipitation and potential evapotranspiration (PET), or the air's ability to absorb water (determined by temperature and water content). In arid zones, precipitation may be 5 to 20 percent of PET; semi-arid regions receive more precipitation, and hyper-arid regions less, in relation to PET.
Features of a Desert
Roughly one-third of Earth's land surface is arid or semi-arid. The major desert regions are: Australia, western North America, western South America (Atacama), southern Africa (Namib), and Asia-northern Africa. There are so-called polar deserts; however, most arid lands are in the warm subtropics.
There are two primary causes of aridity. One is the subtropical highpressure belts, where high altitude air masses move away from the tropics. Tropical heat causes air to rise and cool, and therefore drop moisture as it moves away from the equator. The air then becomes more cool and dense. This air then sinks, warms as it nears the surface, and regains the ability to absorb water, thus creating zones of aridity. A second cause is the rainshadow effect caused by mountain ranges. Continental interiors are dry because most air masses have moved long distances or over mountains and in doing so have lost water.
Desert conditions may be quite harsh. Intense solar radiation and lack of shade cause surface temperatures as high as 50 degrees Celsius (130 degrees Fahrenheit). Limited precipitation and rapid evaporation greatly limit plant growth, and water is rarely available for animal consumption. Precipitation is predictable in some systems (such as winter rains in California's Mojave) but nonseasonal in others. Many sites experience long rainfree periods; in portions of the Atacama, rainfall has never been recorded.
Variability is another characteristic of deserts. Precipitation is episodic; rainstorms may be quite intense, with much of the annual total falling in just minutes. Similarly, resources may be spatially patchy. Arroyos or erosion channels and low spots may collect runoff from surrounding areas; rockiness and soil surface crusts contribute to runoff. Seeds and litter accumulate and support plant growth in low, relatively moist locations. Permanent water sources (desert springs or oases) are rare but important.
Evaporation draws water from the surface, leaving dissolved minerals as a salty crust. Sparse plant growth adds little organic material to the soil; thus the soil has limited capacity to retain water and minerals. Sparse vegetation also increases the erosional influences of high wind, runoff, and extreme temperatures. Sand dunes are accumulations of eroded materials; their instability makes them harsh environments for most organisms.
Desert organisms adapt to arid environments either by tolerating extreme conditions or by escaping them. Toleration is survival under stress. Many adaptations are related to water acquisition. Plants may have shallow, extensive root systems to absorb rainfall from the largest area possible. Animals obtain moisture from live food. Tenebrionid beetles of the Namib extract water from coastal fogs: The beetles do "headstands" on dune ridges, and moisture condensing on the beetle's textured carapace trickles down to the mouth. Kangaroo rats obtain virtually all of their water by oxidation of fats in dry seeds (metabolic water). Other adaptations involve water retention: storage of water in succulent tissues; specialized photosynthetic processes minimizing water loss; leaflessness, small leaves, or leaf loss during drought, also reducing plant water use; and animal use of burrows or shade. Finally, some organisms simply tolerate tissue dehydration.
Escape or avoidance results in activity only during favorable periods. Annual plants, completing their life cycle in a single year, are abundant in many deserts. They may spend years as dormant seeds; only after sufficient rainfall do they germinate and grow, reproducing quickly before the soil redries. Some invertebrates and amphibians remain dormant up to several years, the invertebrates as eggs or in the "suspended animation" of cryptobiosis , the amphibians as aestivating, or dormant adults, beneath the surface. When temporary ponds form after rain, these organisms hatch or awaken; feeding, reproduction, and growth of juveniles are all a race against time so that at least some mature before the ponds dry. Some organisms are nomadic or migratory, finding temporary patches created by local rainfall: These include large mammals such as antelope, birds, and even insects (for example, desert locusts or grasshoppers).
Arid and semiarid regions have been important for livestock grazing throughout history. As energy sources have made irrigation feasible, some regions have been converted to cultivation. Urban populations are increasing rapidly where groundwater or river water is available and affordable; the southwestern United States, for example, contains several rapidly growing metropolitan areas in desert, such as Phoenix, Arizona. Depletion of underlying groundwater is a major environmental consequence in such areas.
see also Biome; Grassland; Water Cycle
Laura F. Huenneke
Cooke, R. U., A. Warren, and A. S. Goudie. Desert Geomorphology. London: UCL Press, 1993.
Louw, G. N. and M. K. Seeley. Ecology of Desert Organisms. New York: Longman, 1982.
Mares, M. A., ed. Encyclopedia of Deserts. Norman: University of Oklahoma Press, 1999.
A desert is an arid land area where more water is lost through evaporation than is gained from precipitation . Deserts include the familiar hot, dry desert of rock and sand that is almost barren of plants, the semiarid deserts of scattered trees, scrub, and grasses , coastal deserts, and the deserts on the polar ice caps of the Antarctic and Greenland.
Most desert regions are the result of large-scale climatic patterns. As the earth turns on its axis, large air swirls are produced. Hot air rising over the equator flows northward and southward. The air currents cool in the upper regions and descend as high pressure areas in two subtropical zones. North and south of these zones are two more areas of ascending air and low pressures. Still farther north and south are the two polar regions of descending air. As air rises, it cools and loses its moisture. As it descends, it warms and picks up moisture, drying out the land. This downward movement of warm air masses over the earth have produced two belts of deserts. 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 Kalihari Desert of southern Africa, and the Great Victoria and Great Sandy Deserts of Australia .
Coastal deserts are formed when cold waters move from the Arctic and Antarctic regions toward the equator and come into contact with the edges of continents. The cold waters are augmented by upwellings of cold water from ocean depths. As the air currents cool as they move across cold water, they carry fog and mist, but little rain. These types of currents result in coastal deserts in southern California, Baja California, southwest Africa, and Chile.
Mountain ranges also influence the formation of deserts by creating rain shadows. As moisture-laden air currents flow upward over windward slopes, they cool and lose their moisture. Dry air descending over the leeward slopes evaporates moisture from the soil , resulting in deserts. The Great Basin Desert was formed from a rain shadow produced by the Sierra Nevada mountains . Desert areas also form in the interior of continents when prevailing winds are far from large bodies of water and have lost much of their moisture.
Desert plants have evolved methods to conserve and efficiently use available water. Some flowering desert plants are ephemeral and live for only a few days. Their seeds or bulbs can lie dormant in the soil for years, until a heavy rain enables them to germinate, grow, and bloom. Woody desert plants can either have long root systems to reach deep water sources or spreading shallow roots to take up moisture from dew or occasional rains. Most desert plants have small or rolled leaves to reduce the surface area from which transpiration of water can take place, while others drop their leaves during dry periods. Often leaves have a waxy coating that prevents water loss. Many desert plants are succulents, which store water in leaves, stems, and roots. Thorns and spines of the cactus are used to protect a plant's water supply from animals.
Desert animals have also developed protective mechanisms to allow them to survive in the desert environment. Most desert animals and insects are small, so they can remain in cool underground burrows or hide under vegetation during the day and feed at night when it is cooler. Desert amphibians are capable of dormancy during dry periods, but when it rains, they mature rapidly, mate, and lay eggs. Many birds and rodents reproduce only during or following periods of winter rain that stimulate vegetative growth. Some desert rodents (e.g., the North American kangaroo rat and the African gerbil) have large ears with little fur to allow them to sweat and cool down. They also require very little water. The desert camel can survive nine days on water stored in its stomach. Many larger desert animals have broad hooves or feet to allow them to move over soft sand. Desert reptiles such as the horned toad can control their metabolic heat production by varying their rate of heartbeat and the rate of body metabolism . Some snakes have developed a sideways shuffle that allows them to move across soft sand. Deserts are difficult places for humans to live, but people do live in some deserts, such as the Aborigines in Australia and the Tuaregs in the Sahara.
Desert soils are usually naturally fertile since little water is available to leach nutrients . Crops can be grown on desert lands with irrigation , but evaporation of the irrigation water can result in the accumulation of salts on the soil surface, making the soil unsuitable for further crop production. Burning, deforestation , and overgrazing of lands on the semiarid edges of deserts are enabling deserts to encroach on the nearby arable lands in a process called desertification . Desertification in combination with shifts in global atmospheric circulation has resulted in the southern boundary of the Sahara Desert advancing 600 mi (1,000 km) southward. A desertification study conducted for the United Nations in 1984 determined that 35% of the land surface of the earth was threatened by desertification processes.