One important limiting factor on the power of chiefs was the mobility of the population. Individuals could and did move to other areas or join other leaders, and the chief who wished to maintain his influence over his followers could not carry out policy greatly opposed to their wishes. The mobility of the population is a function of several important facts. First, the bands were not corporate units in the sense of groups holding rights over strategic resources. As we have seen, there were no such limits within the general range of Shoshone-speaking people. Band territoriality would have been directly contradictory to the enormous distances traversed by the mounted people. The region, as a whole, presented the possibility of a balanced diet and annual subsistence cycle, but smaller subdivisions of it could not do so, though they provided overabundance of a limited number of foods in certain seasons. Even the area of Shoshone occupation, as compared with other peoples' territory, was vague and ill defined. Territory, as such, was not a matter of great concern in the relations of the Shoshone with hostile tribes. Rather, they vigorously defended their horses and their own lives during enemy invasions. The buffalo country east of the mountains was roamed over by several groups, as were the mountain areas of western Wyoming and Montana. And peaceful groups of the Basin-Plateau merged and intermingled with the Shoshone in the areas in which they had contact. The individual, the family, or the group of families that elected to change leaders within an area or to shift from one area to another did not give up vested rights and prerogatives. And, given the loose nature of Shoshone social structure and the diffuse and widespread network of social relationships, the person or group seeking a change could usually count upon acceptance elsewhere. The reservation system tended to tighten political organization and to define groupings more closely, since reservation membership did constitute a vested interest and government legitimation and stabilization of a central chieftaincy restricted the choices open to the individual. The principal item in the productive apparatus of the mounted Shoshone was the horse. Without sufficient horses to pursue the buffalo hunt, the Indian was relegated to "digger" status and had to remain within the Great Basin. But horses were individual, private property, and a man could locate under any leadership as long as he was mounted. His primary economic dependency was thus shifted from the larger social group to his own herd. Each man was to a large extent his own master and acted accordingly.