Pastoral societies across the Sahel have until very recently exercised a nomadic strategy very similar to the large native ungulates. Cattle herds moved north into the more arid areas following the rain to graze annual grasses of very high abundance and quality, returning to graze the lower grasslands and agricultural stubble of the sedentary farmers (Breman and de Wit, 1983). Here, as with the large ungulates, the native grasslands and the agricultural lands experienced periods of no grazing.

   Sinclair and Fryxell (1985) assemble evidence which suggests that this nomadic system has been in operation for many centuries, possibly 2000-4000 years. The exact behaviour and reaction of this nomadic system under drought is not well known, but it is recognized that at least three major droughts have occurred in the past 100 years as severe as that which, in 1969-75, attracted attention to the Sahelian desertification (Gornitz and NASA, 1985).

     Direct and independent support for the importance of nomadism for the persistence of herbivore populations comes from the detailed investigations of energy flow in a Kenyan pastoral ecosystem (Coughenour et al. , 1985). They concluded that the key system parameter was the ratio of energy flow to maintenance level requirements and that this ratio must be high to stabilize the system in a variable environment, particularly during the stress of droughts. They concluded that the traditional patterns of operation, which included livestock diversity, nomadism, low energy efficiency and biomass maintenance, promoted the stability and persistence of this pastoral system.