For many people, the thought of the desert conjures up images of vast expanses of sand, shifting dunes, sparse vegetation, and little water. Such a landscape is one type of desert, but it is neither the only nor the most typical one. The Gobi desert in China and Mongolia, for example, is not a sandy desert. It is made up of nearly 1,300,000 square km (500,000 square miles) of mostly bare, flat rock. In contrast, the desert landscape found in the Great Basin in Nevada and Utah is largely made up of a series of mountain ranges and closed valleys, and the American Southwest is rich with distinctive saguaro cactus. In this book you will explore the unique deserts of the world, along with the different semiarid environments that often surround them.
Deserts are one of Earth’s major biomes, covering 20-30 percent of Earth’s land area, depending on which definition is used. They are surprisingly diverse in their landforms, vegetation, and animal life. Unlike temperate forests, which appear as wide swaths of continuous growth, each desert is an isolated environment. In other words, no two deserts share the exact same flora and fauna, although they may support similar species with similar adaptations to the harsh desert environment.
One characteristic that all deserts have in common is an extreme lack of rainfall. Some parts of deserts may go years without any rain at all. More typically, deserts experience long months of little or no precipitation, interspersed with short, heavy rains that fall on a few days a year. Desert rainfall generally amounts to less than 400 mm (16 inches) of annually. Some classification systems limit true deserts to those areas receiving less than 250 mm (10 inches) of water per year.
The lack of rainfall also leads to another characteristic common to all deserts, which is the sparseness of vegetation. Trees are generally absent, and much of the ground has no plant cover at all. The plants that do exist have adapted in a variety of ways to the lack of available moisture. Succulents, such as cacti, take in water when it’s available and store it in their tissues. During moist conditions, plants that produce seeds, which remain dormant until water is available, suddenly blossom and set seed again within days. Some plants in coastal desert regions even absorb water from fog and dew directly through their leaves.
The sparse vegetation of desert regions limits the number of animals that the desert environment can support. Most deserts have few large herbivorous species, although one group, the camels, are famously adapted to desert life. In Africa and Arabia, most camels are in domesticated herds. Wild camels, however, can be found in Asia and also in the Australian Outback, where they were introduced in the 1800s. Smaller mammals, such as porcupines, hares, and jerboas, are common in many deserts, and some deserts also support a wide variety of lizards and other reptiles. Kangaroos and related marsupials such as wallabies and bandicoots range through the Australian Outback. Desert carnivores include hyenas, foxes, and a variety of large cats, as well as many species of carnivorous birds such as vultures.
Deserts are not generally conducive to human habitation. The Sahara desert, the largest in the world, is approximately the same size as the United States, but supports only about 2.5 million inhabitants. The desert’s human residents are clustered in small settlements wherever reliable water sources allow for farming or the grazing of animals. The Gobi also supports a population of mostly nomadic livestock herders that occur in densities of about 1.3 persons per square km (about 1.1 persons per square mile). The Namib desert in southwestern Africa contains large areas set aside as conservation preserves, but it is almost completely uninhabited except along the coast. In the Arabian peninsula, the discovery of vast oil reserves under the desert have allowed significant development in that region, and many of the traditional Bedouin, a nomadic people, have settled into permanent habitations.
In many cases, intensive agriculture and the stresses of human settlement in or near desert regions have caused the spread of desert conditions into bordering semiarid grazing or agricultural areas. Such desertification is typically caused by the overuse of the land and its resources. Specifically, desertification may result from deforestation, overgrazing, unsustainable irrigation practices, or combinations of these factors. Desertification is of particular concern in the Sahel region of Africa just to the south of the Sahara, as well as areas adjacent to the Kalahari desert of southern Africa. It occurs in parts of Australia, in Central Asia near the Gobi and Takla Makan deserts, and even in parts of North America. The United Nations has declared desertification, along with its associated effects on human populations, to be one of the most severe environmental challenges the planet faces.
To understand the severity of this issue, it is important to explore the full range of dryland environments. Semiarid regions at the greatest risk of desertification generally include dry grasslands, or steppes, and scrublands. Although these types of environments have supported a variety of human activities for many thousands of years, they can be easily stressed by overpopulation and poor land management practices.
The steppe is described as any region that receives relatively low levels of precipitation with variable temperatures and is characterized by grasses as the predominant vegetation type. Steppe environments are typically found in the interior area of large continents far from the influence of moist maritime air currents. In North America, the Great Plains of the United States and Canada is one example of this climate type. Similar environments, such as the Pampas of South America and the Rajasthan Steppe in India, occur on other continents. The term “steppe” is more commonly applied, however, to the vast grasslands that extend more than 8,000 km (5,000 miles) across Eurasia from Hungary through Ukraine and Central Asia to Manchuria.
The Eurasian Steppe is divided into two main areas. The more fertile and temperate Western Steppe is primarily grassland, but the region also contains rivers and streams with trees along their banks. The higher, colder, and drier Eastern Steppe contains sparse vegetation. However, the Eastern Steppe still supports seasonal grasslands. The temperature variation between summer highs and winter lows is particularly extreme in the Eastern Steppe.
In early human history, the lack of reliable water sources combined with the ease of traveling across the open grasslands led to the growth of a nomadic civilization in these regions. Nomadic cultures are more dependent on livestock grazing than on settled agricultural farming, and it is likely that these steppe dwellers were the first to domesticate horses. Such horse domestication also allowed the growth of a warrior culture that periodically overran more settled agricultural areas, culminating in the cavalry of Genghis Khan and the Mongol empire. In areas of the steppe more conducive to settled farming, agricultural communities developed. Today, much of the native steppe vegetation has been replaced by cultivated crops, and these same regions provide significant amounts of Russia’s agricultural grains such as winter wheat and barley.
The Great Plains of North America is another vast area of native grasslands, extending from Texas in the south to the Canadian provinces of Manitoba, Saskatchewan, and Alberta in the north. Like the Eurasian steppe, it has now been largely converted to agriculture and ranching. The Dust Bowl of the 1930s provided a graphic illustration of how fragile these semiarid lands are. As settlers moved onto the Plains, they typically plowed under the native grasses and planted annual crops such as corn instead. During the winter months after the crops had been harvested, the soil lacked vegetative cover and a root system to anchor the soil and retain water. After several years of severe drought in the 1930s, tremendous amounts of topsoil were swept away by the prairie winds. Fortunately, the subsequent adoption of soil restoration, the introduction of more sustainable agricultural practices, and the replacement of intensive crop farming with less-intensive livestock has helped the region become more productive.
Other parts of North America face desertification caused by irrigation. At first glance, irrigation provides the land with a regular source of water, which allows marginal areas to become productive farmland. However, over the long term dissolved salts in the water can build up in the soil, slowly rendering the soil unsuitable for any vegetation other than salt-resistant desert varieties. Salt buildup in the soil has already led to a loss of over 70 percent of Iraq’s irrigated farmland, and some agricultural areas in Russia are expected to remain productive only until the middle of the 21st century.
Scrublands, the third type of dryland environment, can be found on every continent except Antarctica. Scrublands cover much of Australia, and large areas of scrubland can also be found in Africa bordering the continent’s deserts. (In all, dryland environments including deserts, scrublands, and dry grasslands account for some 65 percent of Africa’s land area.) In South America, the region of Patagonia is largely scrubland. In North America, scrubland is characteristic of many parts of California as well as other areas of the Southwest. In Europe and the Middle East, scrublands are predominant along the entire Mediterranean coast.
Scrublands typically occur in regions with mild winters and long, dry summers. These environments are characterized by the predominance of small woody shrubs as the primary vegetation type. Scrublands are most commonly found between 20 and 40 degrees latitude on both sides of the Equator. The combination of low rainfall and high evaporation rates during the warm summer months often makes these areas unsuitable for larger tree species. Many of the dominant native shrubs revert to dormant state during these dry periods, coming back to life in spring after the winter rains. Ecologically, fire is an important balancing force in scrubland environments, because it may serve to limit the number of larger tree species. Scrublands are also known for poor soils and high levels of salt-laden winds.
Not all scrublands are natural, however. Anthropogenic scrublands, that is, those arising from human activities, may be at least as widespread as natural ones. Scrubland creation may occur after the deforestation of an area previously populated by larger trees. The loss of the trees creates opportunities for shrubs to enter and dominate the environment. This phenomenon is most likely to occur in areas that are already marginal for tree growth. However, the process of long-term tree clearing, livestock grazing, and other human activities may also cause soil erosion and nutrient depletion to the extent that the environment can no longer support trees.
Human-generated scrublands can also occur when human activity changes the ecological balance of native grasslands. Within the last two hundred years, since the settlement of Australia by Europeans, large areas of semiarid grasslands have turned to scrub, and other areas of scrubland that have been used for grazing have become significantly less productive. These changes largely result from the large-scale introduction of sheep farming. As the sheep eat the grasses, they create the conditions by which shrubs can enter and slowly take over the landscape. As food, the invading shrubs are less favoured by the livestock. In addition, the sheep grazing within existing scrublands prefer more palatable, tender shrubs, leaving the woodier shrubs alone to flourish. Other Australian scrublands have been cleared for farming, removing native shrub species. The introduction of fertilizers to make the soil more productive for agriculture can also damage native scrublands.
Scrublands also carry a particularly high risk of desertification. The soil quality in these areas is typically poor, and environmental stresses, in the form of low moisture levels and high temperatures, are high. Beyond the effects caused by the overconsumption of vegetation by livestock mentioned previously, livestock herds may trample and kill vegetation by walking to and from water sources. The trampling process may also compact the soil, which inhibits the growth of plant roots and disrupts underground biological communities. Scrublands may also succumb to desertification through the clearing of woody shrubs for firewood. As these plants are cut, soil erosion increases and soil quality is reduced. This problem is particularly significant in Africa and parts of Asia.
Since drylands make up almost half the Earth’s land surface, their long-term health bears directly on the future of humanity. As you read about the many unique regions in the pages that follow, you will be exploring some of the most fascinating and most fragile of Earth’s ecosystems.