Noy-Meir(1973, 1974a, 1980) and Shmida et al. (1986) note the lack of evidence for the existence in arid ecosystems of regulatory feedbacks, e.g. competition and predation. However, there is evidence for such feedbacks operating for at least part of the time in the higher rainfall savannahs (Walker, 1987). Nonetheless, the simplest model that underlies all of the characteristics listed above is that arid ecosystems are controlled by water availability and pulses of activity. In very arid environments the pulses may be small and unpredictable, whereas in the higher-rainfall savannahs they are larger and far more predictable (e.g. rainy seasons). These pulses of activity are transmitted through successive trophic levels with little time lag or feedback. The causal network is largely unidirectional. Spatial heterogeneity and variability generated by climate, topography, soils and biota further uncouples potential feedbacks in these systems. Lastly, the abiotic environment is characterized by high variability and extreme values of the significant variables of rainfall and temperature. These extremes can generate changes in species composition that persist long after the event. The perennial components of arid ecosystems act as a memory.