Desert Biome

Abstract

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.