Threshold Distinctions Between Equilibrium and Nonequilibrium Pastoral Systems Along a Continuous Climatic Gradient

Abstract

In equilibrium environments where rainfall is relatively stable, grazing animal–vegetation dynamics are density-dependent; it is therefore appropriate to calculate carrying capacities and use them to define sustainable animal populations. In contrast, nonequilibrium environments are characterized by fluctuations in characteristics such as rainfall, resulting in fluctuations in plant biomass and in the corresponding carrying capacity. Herders adapt to such environments by moving opportunistically to pastures with better conditions. Studies since the 1990s have significantly improved our understanding of the continuity and integration of equilibrium and nonequilibrium systems. However, it remains unclear how and where such continuous, integrated rangeland systems result in qualitatively different land use patterns by local herders along a climatic gradient. Here, we developed a simple model that uses key environmental factors to predict a threshold representing the boundary between equilibrium and nonequilibrium land use systems, and we used an area of Mongolian rangeland as an example. We found a threshold in the proportion of usable pasture that corresponded to a specific range of rainfall values. Comparison of our results with previous ones supported our hypothesis about this threshold. The threshold behavior suggested that it is important to identify and monitor the boundary between equilibrium and nonequilibrium land use systems so that managers can respond to climatic change. National governments and aid agencies must understand the threshold process before they can identify focal areas where management regime change is required and propose appropriate policies that will support herders in the long term. Our study provides a simple, low-cost tool to evaluate ecosystems in this context.