Deserts and mining

Deserts are regions defined by the scarcity of a crucial resource, water. They are unique ecosystems with their own types of plant and animal life; a number of desert plants have been used by humans for thousands of years. Mining for minerals and petroleum also occurs in desert regions around the world.

Background

Although deserts are characterized by general aridity, there is no universal definition of a desert. Webster’s dictionary defines a desert as an “arid land with usually sparse vegetation; especially such land having a very warm and receiving less than 25 centimeters of sporadic rainfall annually.” Deserts are generally thought of as hot and dry, but heat is not necessarily a requisite of most definitions of a desert. Low precipitation does not, alone, characterize a desert. Arctic tundra landscapes often receive scant precipitation, yet tundra soils are often saturated from low evaporation and restrictive permafrost below a narrow active thawed layer during the summer. The common perception of a desert is a trackless expanse of sand such as the Sahara or the Arabian Desert. In reality, however, deserts are often fairly well vegetated. The characteristic that all deserts have in common is aridity. Aridity refers to a general dryness, not to short drought periods.

Characteristics of Deserts

Deserts are located on all continents; Antarctica is considered a desert. They occur on a variety of substrates, but often are characterized by historical patterns leaving alluvial fans on the foothills of small mountains or hills and isolated islands of more resistant material. These remnants are called buttes in the western United States. Sandy soils are common, but wind-blown soils and ash deposits from volcanoes are also prevalent in many deserts. Desert soils often lack structural aggregation and are subject to erosion. They often have surface crusts from raindrop impact and deposits from sediments that are left as water infiltrates into the soil.

In the United States, cool deserts (sagebrush grass and salt-desert shrub types) occur in old lake beds. Ancient Lake Bonneville in northern Utah and Lake Lahontan in Nevada once occupied much larger areas and dominated the landscape. These deserts are also influenced by the Sierra Nevada and Cascade Mountain ranges, which cast a rain shadow effect on their eastern valleys. Intermittent drainages (wadis or arroyos) often cut across desert landscapes; they contain running water only during or immediately after a rainfall event. These drainages support unique vegetation and serve as important habitats for birds and other animals.

Another major feature of deserts is that nutrients are often limiting. Soil nitrogen and organic matter are especially low in these ecosystems. Free-living and symbiotic nitrogen-fixing bacteria are scarce in deserts. Research indicates that lichens and algae, forming crusts on desert soils, may be the major source of soil nitrogen for plant growth.

Desert Adaptations

Rainfall events occur infrequently in deserts. Both plants and animals must have adaptations to take advantage of these episodic periods of available water to survive in these harsh environments. Thus one sees flushes of desert flowers during spring and summer months, especially in years when rainfall is abundant. These same flowers may not be seen again for several years. Desert plants exhibit several adaptations that allow them to exist successfully under these stressful conditions. Some plants, such as mesquite, have deep root systems that allow access to deep sources of water. They do not have to rely on rainfall during the growing season. Some plants have dense, shallow root systems that allow them to tap soil water in the soil surface from light showers. Some plants have both types of root systems. Cactus and other succulents have the ability to store water in their tissues for use during periods of low rainfall. Some plants, such as the creosote bush, shed many leaves to reduce transpirational requirements. Others have few stomata on their leaf surfaces, thereby reducing transpirational stress. These stomata tend to close and restrict transpiration. Desert soils are often high in salt content from surface evaporation of rainfall that does not penetrate far into the soil, and plants growing on saline soils have special adaptations for coping with these conditions.

Animalsalso have many adaptations to desert conditions. Some desert animals can live in dormant stages during unfavorable periods. Many exhibit no definite breeding season and can breed whenever conditions are favorable. Some, including birds, can conserve water by reducing loss through concentrated urine. Some desert animals obtain most of their water through the food they eat. Many are nocturnal, thereby avoiding the high temperatures of the day. Intermediate-sized and small mammals often burrow to escape the heat and find more favorable conditions.

Conservation and Prevention Issues

Desert plants and animals are frequently under threat from a variety of natural and human-related activities. Since water resources are so scarce, any external factor that changes the water cycle or the availability of water at critical times may threaten organisms. In some cases the existence of desert plants and animals is not obvious, and adequate information on their status is lacking.

Many desert landscapes were formed by erosion, but changes may be very slow and difficult to ascertain. For example, there has been much concern over the increase in desert areas in the world. The word “desertification” has been used to describe the degradation of arid and semiarid areas into desertlike environments. For some time, nearly all observers believed that the Sahara Desert was expanding, but this has become a subject of debate. Detailed analyses have suggested that the Sahara Desert may expand and contract, perhaps in response to climatic cycles. In many areas in Africa, such as the Sahel, livestock populations are increasing to support the expanding human population. During the dry season, woody plants supply needed high-quality forage for these animals. These same woody plant resources are also heavily exploited for fuel wood despite restrictions placed on their harvest. Depletion of these valuable woody plant resources could have serious consequences for these delicate ecosystems. Other examples of resource problems in desert areas include overhunting of native ungulates for animal products such as tusks.

Natural Resources

Deserts of the world hold most of the of oil, natural gas, and coal, and they therefore serve as a source of natural wealth for many countries in the Middle East. These resources remained sufficient to supply energy needs into the twenty-first century, but with escalating costs, alternative energy sources will become more important. Deserts also provide other minerals, such as silver, lead, diamonds (in Southern Africa), and copper, but many deserts, such as the Sahara, are not important for resources.

Desert plants are also used as food by many native people. In the southwestern United States, American Indians used cactus fruits as a staple food. In the Sahel in northern Africa, the following types of foods were available in substantial quantities: rhizomes and fleshy stems, the seeds of forbs, grass seeds, fruits, edible gums, and mannas. Fibers from desert plants are also used for basket weaving in many countries, and plant pigments are used as natural dyes.

Vegetational Dynamics

In desert regions of the United States and elsewhere, shrubby plants are increasing at the expense of grassland. Reasons for this change are not clear, but several factors are probably important. Some workers have suggested that climatic changes have favored a shift from grasses to shrubs. Increased carbon dioxide concentration of the from burning fossil fuels is one hypothesis. Others insist that the introduction of domestic livestock beginning in 1850 disrupted the ecological balance in favor of shrubs. Lack of fire to restrict the development of shrubs in has also been advanced as a possible cause. Several studies in southwestern deserts in the United States have shown that native mammals and rabbits can exert considerable influence on vegetation and can reduce grass cover and abundance. It is possible that all these factors, and probably others not considered, have together been responsible for changes in desert vegetation.

Increases in shrubs, such as mesquite in the United States, eventually alter nutrient distribution patterns. In relatively uniform grasslands, soil nutrients are distributed fairly evenly. As shrubs such as mesquite increase at the expense of grasses, however, the shrubs are able to take up nutrients from a much larger volume of soil. These nutrients then tend to become concentrated around the individual mesquite trees and to form islands of nutrient concentration. Interdune areas often suffer soil loss through wind erosion, and the soil accumulates around individual mesquite plants. If the soil is deep enough, mesquite dunes eventually form. Interdune areas with little surface soil are deficient in nutrients, especially nitrogen, and lack water-holding capacity. These changes also affect animal life. Some animals, such as bannertailed kangaroo rats and pronghorn antelope, utilize grassland habitat and are favored by grassland. Others, such as those that feed on mesquite (insects and arthropods), are favored by the mesquite dunelands.

Mining for coal and other minerals often disturbs desert ecosystems. Mines are limited in size and in the area affected, but they can leave conspicuous scars on the landscape, especially with deep open-pit mines. The area disturbed by the mines can be restored, but restoration is difficult and expensive.

Bibliography

Evenari, Michael, Imanuel Noy-Meir, and David W. Goodall, eds. Hot Deserts and Arid Shrublands. New York: Elsevier, 1985.

Goudie, Andrew. Great Warm Deserts of the World: Landscapes and Evolution. New York: Oxford University Press, 2002.

Laity, Julie. Deserts and Desert Environments. Hoboken, N.J.: Wiley-Blackwell, 2008.

Quinn, Joyce Ann. Desert Biomes. Westport, Conn.: Greenwood Press, 2009.

Naimark, Jared. "A Threat to Land, Air, and Water." Desert Report, 13 Sept. 2023, desertreport.org/lithium-mining-in-the-desert/. Accessed 21 Dec. 2024.

Sowell, John. Desert Ecology: An Introduction to Life in the Arid Southwest. Salt Lake City: University of Utah Press, 2001.

Ward, David. The Biology of Deserts. New York: Oxford University Press, 2009.

West, Neil E., eds. Temperate Deserts and Semi-Deserts. New York: Elsevier Scientific, 1983.