Herbivores (excluding granivores) only utilize a small proportion of available plant production. In more arid environments the pulses of plant growth are unpredictable and transient or, in the wetter savannahs, the quality, rather than the quantity, of the herbage layer is limiting (Breman and de Wit,1983; Sinclair, 1975). Thus the overall utilization rates of vegetative biomass are low, 5-10% (Noy-Meir,1985).

   The strategies of harvesting this small fraction are critical, however. Considering all trophic levels we can simplify the many dimensions of habitat into just two by the nature of the resources that are offered; 'richness', i.e. quality and quantity, and 'reliability', i.e. frequency and predictability (Shmida et al.,1986; Southwood, 1977). As a consequence of these two habitat dimensions, consumers have evolved two main strategies to use these resources. Populations of short-lived organisms with an intrinsically high rate of increase (insects and rodents) erupt when resources are plentiful and decline when they are not. For long-lived species with a low intrinsic rate of increase (large raptors, mammalian herbivores), the only possible strategy is migration (nomadism) to smooth the variability of resources in space and time.