Desert Biome

by

David Ward

• LAST REVIEWED: 23 May 2012
• LAST MODIFIED: 23 May 2012
• DOI: 10.1093/obo/9780199830060-0044

Introduction

Deserts are defined by their arid conditions. However, deserts are not necessarily dry. It is the high evaporation relative to the precipitation that makes a desert such a harsh environment. Such evaporation occurs because deserts are often, but not always, hot, and because precipitation is low. A result of this aridity is that most of the area occupied by deserts is barren and monotonous. However, biologists perceive deserts to be laboratories of nature, where natural selection is exposed at its most extreme. Scientists have long considered the many unique adaptations of plants and animals for surviving the harsh desert environment. More recently, researchers have focused on the biotic interactions among organisms. Thus, the harsh abiotic environment defines the desert and imposes the strong selection pressures on organisms that live there. However, it is the relative simplicity of desert ecosystems that makes them frequently more tractable for study than more complex environments such as forests. According to Gideon Louw and Mary Seely’s Ecology of Desert Organisms (Louw and Seely 1982, cited under General Overviews) and John Sowell’s Desert Ecology (Sowell 2001, cited under Specific Deserts), most deserts have an average annual precipitation of less than 400 mm. A common definition distinguishes between true deserts, which receive less than 250 mm of average annual precipitation, and semideserts or steppes, which receive between 250 mm and 400 to 500 mm. Four factors influence the lack of rainfall in deserts: (1) the global atmospheric circulation maintains twin belts of dry, high-pressure air over the edges of the tropics, called Hadley cells; (2) marine circulation patterns contribute to aridity when cold coastal waters on the west coasts of North and South America, Africa, and Australia chill the air, reducing its moisture-carrying capacity; (3) rain shadows are created by mountain ranges; and (4) if the distances to the interior of a continent are too great (such as in the Gobi and Taklamakan deserts of China), then water is limited. Many of these four factors act in tandem. An additional type of desert is the polar desert, which occurs in the McMurdo dry valleys of Antarctica. This desert has extremely low humidity and no snow cover. Katabatic winds, which occur when cold and dense air is pulled down by gravity, heat as they descend and evaporate all moisture (see Doran, et al. 2002, cited under Defining the Desert Biome). These winds can reach speeds in excess of 300 km per hour. Here too, rain shadows are created by mountain ranges that are sufficiently high that the seaward-flowing ice is blocked from reaching the sea, thereby reducing humidity.

General Overviews

Louw and Seely 1982 serves as an effective introduction to the topic. In many respects, Whitford 2002 and Ward 2009 build on this introduction, although Whitford 2002 has a strong ecosystem approach while Ward 2009 focuses on an evolutionary one. Ward’s approach considers natural selection to be an important cause of the adaptations of desert organisms. Mares 1999 is a very large tome with many aspects that one can dip into. Ezcurra 2006 covers many aspects of desert ecology and geomorphology (with a useful introduction to the issue of humans in deserts) and is a very good modern introduction to the topic. Of all the books listed here, Ezcurra 2006 is the easiest to access, although Mares 1999 also covers many fundamental issues. Shachak, et al. 2005 is an edited volume and suffers from some of the problems of conference proceedings in that not all contributions strictly relate to deserts. The World Wide Fund for Nature (formerly the World Wildlife Fund) maintains a very useful website that includes references to many deserts.

• Ezcurra, Exequiel, ed. 2006. Global deserts outlook. Nairobi, Kenya: United Nations Environment Program.

  A superb introduction to deserts, including aspects of definitions of various deserts and interactions between people and deserts. Suitable for students and nonspecialists.

• Louw, Gideon N., and Mary K. Seely. 1982. Ecology of desert organisms. London: Longmans.

  An excellent introduction to the key adaptations of desert organisms, with a strong emphasis on adaptations and convergent evolution in desert animals.

• Mares, Michael A., ed. 1999. Encyclopedia of deserts. Norman: Univ. of Oklahoma Press.

  An important source of a wide diversity of desert information, although surprisingly lacking in information on certain key issues.

• Shachak, Moshe, James R. Gosz, Steward T. A. Pickett, and Avi Perevolotsky, eds. 2005. Biodiversity in drylands: Toward a unified framework. Oxford: Oxford Univ. Press.

  A wide array of topics is covered in this edited book. Unfortunately, some of the chapters have rather little specifically to do with arid systems.

• Ward, David. 2009. The biology of deserts. Oxford: Oxford Univ. Press.

  Has a strong evolutionary focus, although it also covers descriptions of major desert types and applied aspects such as desertification and conservation.

• Whitford, Walter G. 2002. Ecology of desert systems. San Diego, CA: Academic Press.

  An excellent coverage of the topic with a strong emphasis on an ecosystem approach.

• World Wide Fund for Nature WildFinder.

  Important references to many of the world’s deserts, with a focus on conservation. In the Search box on the startup page, enter the word “desert” or the name of a specific desert.