Introduction

Abstract

For the purposes of this book, any land area outside of the polar regions, which receives 250 mm of rain or less per year will be called a desert. It has been customary to divide deserts into semi-arid (somewhere between 125 and 250 mm annual rainfall) and arid (less than 125 mm annual rainfall) (Maclean 1974c), and these subdivisions maybe loosely useful, though some so-called hot deserts, such as that of Madagascar, may have an annual precipitation of as much as 600 mm (Evenari 1985). Deserts cover about 25–30% of the land surface of the planet (Polis 1991); they have low and erratic rainfall, and loss of water by evaporation usually exceeds that gained by precipitation. Primary productivity is low: the lowest standing biomass ever recorded was in the Namib Desert, Namibia, after a drought (Seely and Louw 1980). However, desert ecosystems are not as simple as may first appear; they are relatively complex and biologically rich, though diversity decreases with increasing aridity (Polis 1991).