In the course of their evolution, humans have developed many strategies to enhance their chances of survival. In his discussion of one such strategy, nomadic pastoralism, Spooner (1973:3) states that "all populations live in a particular relationship with their environment and this relationship is mediated by their subsistence pattern". There are two common subsistence adaptations to grasslands. Since humans cannot digest grass directly, energy has to be filtered through grass-eating animals by either hunting and eating wild animals or managing domesticates in a grassland environment. The difference between the two is the degree of control over animals and environment. A pastoral ecosystem has three main elements: people, animals and environment, that under certain conditions combine in a dynamic relationship to achieve a balance among all three (Swift 1977:273). How this balance is achieved is largely dependent on decisions taken by the herd-owners who, in coming to these decisions, are constrained by the environment in which they live and the biology of the animals on which they depend.