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Arid and semi-arid ecosystems (here referred to as arid lands) are those in which water availability imposes severe constraints on ecological activity. Because water availability is a balance between precipitation and evaporative demand, no single value for precipitation is sufficient to define all semi-arid lands; thus the precise definition of arid lands, and their exact location on maps, varies by author or publication. A defining feature of an arid environment is the unpredictability of precipitation in time and space (Noy-Meir 1973). This distinguishes arid lands from Mediterranean climate regions, for example, where there is a predictable cool wet season. The unpredictability in moisture arises from differences between years in the precise timing of precipitation, longer-scale variations in moisture regime (e.g. El Nino -Southern Oscillation events, decadal drought cycles), and the patchy nature of convcclive storms, which supply a major portion of the precipitation in many summer-rain areas. In most temperate arid regions, precipitation can be characterized as falling primarily either in the hot season or in the cooler season. Large arid zones may show gradients in seasonality: e.g. the gradient from winter rainfall (Mojave) to summer rain (Chihuahuan) in southwestern North America, or the similar gradient within Australia.

Our discussion applies both to so-called hot or semi-tropical deserts and to cool or temperate arid zones; we exclude from discussion the polar deserts, where temperature constrains plant growth during most of the year. We recognize that aridity may arise from different combinations of regional climate, topography, and so on. For example, some deserts are rain-shadow deserts, caused by their location downwind of high mountain masses, while others are located in the center of large continents and thus are arid due simply to their distance from oceanic moisture sources. For this reason, various deserts will not respond uniformly to changes in global climatc; instead, each region will likely experience some unique alteration of tempera-

Functional Roles of Biodiversity: A Global Perspective .m,.

Edited by H.A. Mooney, J.H. Cushman, E. Medina, O.E. Sala and Ei.-D. Schube (fcvli C> 1996 SCOPE Published in 1996 by John Wiley & Sons Lid usr.r ture an particularly moisture regimes. Thus there is no simplistic uniform generalization to be drawn about future climatic patterns in today's arid regions.

Semi-arid ecosystems diifer substantially from arid areas or true deserts in structure and in the rate and regulation of ecosystem processes. The predictability, as well as the relative amount, of water resources arc greater in semi-arid than in truly arid lands. A process of increasing global concern is the loss or conversion of vegetation in semi-arid regions, leaving a landscape that resembles in structure and function more truly arid regions. This desertification, or loss of productivity, from semi-arid regions is ongoing worldwide (Jain 1986; Verstraete and Schwartz 1991), and it has prompted the UN to sponsor efforts to draft an international convention on desertification. Thus in our discussion we differentiate between semi-arid and arid ecosystems, and include discussion of desertification.

There have been numerous reviews of the structure and function of desert ecosystems, including Noy-Meir (1973, 1974), West and Skujins (1978), Evans and Thames (1981), West (1981), Louw and Seeiy (1982), Evenari et al. (1985) and Polis (1991). In this chapter we focus on a description of the nature and function of biodiversity, at several levels of organization, within these systems.

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