Following Odum(1983,1985), ecosystems can be described by three elements of (static) structure: biotic and abiotic composition and distribution, and the abiotic environment; and three elements of (dynamic) function: energy flow, nutrient flow, and system characteristics. Not all of these elements are relevant to this analysis of desertification, and a restricted set are discussed below.

   Measures of the mean values (high temperature, low precipitation) and their variations are commonly used to illustrate the extreme and variable nature of the physical environment (e.g. Shmida et al.,1986). The variables of temperature and radiation are relatively stable and predictable, in sharp contrast to rainfall which is discontinuous, arriving in discrete pulses. Arid ecosystems also respond in pulses of activity, but the totality of the response is determined not by the total depth of rain but by the time sequence of the independent pulses (Noy-Meir,1973; Westoby,1980). Each spike of rain produces a pulse of soil water which, in turn, drives a wave of primary productivity.

  The second critical dimension of rainfall variability is spatial. There is a negative correlation between annual precipitation and its variability in time and space. As total rainfall declines its 'patchiness' increases (Noy-Meir, 1973). The spatial heterogeneity in the effectiveness of rainfall in driving a pulse of plant growth is further exaggerated at the landscape level by the factors of edaphic diversity and runoff redistribution. The lower the rainfall, the greater the spatial patterning of vegetation becomes, driven by small differences in topography and soils. The scale of this patterning is of the order of 1000 m or so in the African savannahs (e.g. Tinley, 1982), to 100 m or so in the sparse arid woodlands (e.g. Mabbutt and Fanning, 1987), to 10 m or less in the arid grasslands (e.g. in the African Kalahari--Werger,1986).

  The characteristics of the abiotic environment of arid ecosystems are critical. First because they determine the dimensions of habitat, and habitat is the template of life history strategy acting as a sieve for an organism's dimensions of space (dispersal and foraging range) and (generation) time (Southwood,1977). Secondly the space/time scaling of habitat, when matched with that of any disturbance, e.g. grazing, fire, erosion, etc., determines the mechanisms of impact of that disturbance.