Although water is the major resource controlling primary productivity, the nutrients N and P particularly exert a strong influence on the functioning of these systems (Breman and de Wit, 1983). There are two critical aspects of nutrients. The first is that nutrients are extracted from large soil volumes by plant roots and then concentrated by litter fall in the vicinity of perennial plants, particularly shrubs. Thus the distribution of general soil fertility (organic matter, nitrogen, phosphorus as well as microbiological activity), which is universally low in the soil at large, becomes spatially and vertically concentrated in the uppermost layers of the soil (e.g. Belsky,1986; Noy-Meir,1985; Ruess,1987). This spatial patterning, 'fertile islands in a sterile desert', may persist long after the plants have been removed (Noble and Tongway,1986; Walker, 1987).

   The second aspect of nutrients is that while turnover is relatively slow it is periodically pulsed by rain. Pool sizes may therefore fluctuate rapidly (Crawford and Gosz,1982). The combined effect of this spatial and temporal concentration of nutrients is that any disturbance--grazing, fire, or soil erosion--which removes the perennial plants (the nutrient pumps) and redistributes the uppermost (< 5 cm) soil layer can dramatically reduce both the nutrient capital of an ecosystem and, as a consequence, its resilience or capacity to recover from that disturbance (Belsky,1986; Ruess,1987).