4.3.1 THE ECOLOGICAL ESSENCE OF ARIDITY

Arid ecosystems are classified in many ways. Most classifications are based on a climatological index of precipitation modified by temperature to produce an index of aridity. This index can be refined further by including measures of the variability and extremes of the key factors, rainfall and temperature (e.g. Shmida et al., 1986). However, it is useful to use the description of Noy-Meir (1973) as a definition. Arid ecosystems are characterized by precipitation so low that it is the dominant controlling factor for most processes.

     Precipitation is characteristically highly variable in space and time, occurring as infrequent and discrete events, and this variation has a large stochastic component. There is an emphasis on rainfall sequence in time as well as variability in time and space, because the growing conditions for plants cannot be characterized by averages, even with a measure of variability attached (Westoby,1980; Walker, 1987). Different plant growth forms and life strategies have evolved to exploit different rainfall sequences. The viability of a plant functional type or guild in any climate is determined not by an averaged measure of dryness, but by the temporal sequence of favourable growing conditions experienced, and the use of these relative to its competitors (Westoby,1980).

   This definition of arid ecosystems includes not only the systems of the desert margins but also the adjoining semi-arid woodlands or savannahs wherein the availability of soil water also strongly influences the ecological characteristics of these communities (Tinley, 1982; Walker, 1987). In structure the gradient is continuous but non-linear (Fig. 4.2). This wider definition is particularly appropriate because the process of desertification is reported to spread from the high-rainfall woodlands or savannahs to the marginal desert grasslands (Aubreville,1949; Lamprey, 1983; Sinclair and Fryxell,1985).