Coupled social and ecological dynamics of herders in Mongolian rangelands

Motivated by a field study in a southern Mongolian rangeland, we developed a simple model that couples the social and ecological dynamics of herders who choose foraging sites for their animals in the dry season. If grazing pressure is very strong, the grass biomass becomes depleted and more herders choose to move their animals to an alternative rangeland. These herders may return to the focal rangeland when the quantity and quality of the grass improve. Our model assumes that herders tend to choose the foraging site that gives the higher payoff in a manner described by stochastic best response dynamics. In the model, this social dynamics is coupled with the dynamics of the grass biomass. The resulting system generates typical non-linear behaviors. For example, it may exhibit bistability, with two distinct locally stable equilibria suggesting a strong dependence on the initial condition, or perpetual large-amplitude fluctuation. Implications for rangeland management are discussed.