In arid lands, water availability (precipitation vs. evaporative demand) imposes severe constraints on biological activity. Precipitation is characterized by unpredictability in time and space as well as by low total amounts (Noy-Meir 1974). Semi-arid ecosystems differ from true arid areas in structure and in the rate and regulation of ecosystem processes. We differentiate between these two, and highlight cases of conversion from semi-arid to arid lands (desertification; Verstraete and Schwartz 1991). Biodiversity, reflected in species richness, is moderately high in semi-arid regions and declines with increasing aridity for most taxa (Shmida 1985; Pianka and Schall 1981; O'Brien 1993). Certain taxa are diverse relative to other biomes (e.g. predatory arthropods, ants and termites, grasshoppers, snakes and lizards, rodents, annual plants), but there is substantial variation in the richness of particular taxa among the deserts of different continental areas. The abundance and activity of desert organisms are 'pulsed' in correspondence with episodes of high moisture availability (Noy-Meir 1973; Louw and Seely 1982); while the prevalence of dormancy, cryptobiosis, aestivation and other modes of escaping harsh conditions means that most of the biodiversity of arid regions can be impossible to census or sample during most time periods.