Loss of density dependence underpins decoupling of livestock population and plant biomass in intensive grazing systems.

Across the world, social-ecological rangeland systems have been transformed from a preindustrial extensive status to intensive exploitation, often leading to long-term livestock population booms, overgrazing, and rangeland degradation. To understand the regulatory mechanisms involved in such historical social-ecological transformations, we collected population data on the native sheep of the last nomadic county in the Inner Mongolia Autonomous Region (1961-2005). We detected changes in internal feedbacks (e.g., density-dependent effects) and external disturbance (e.g., winter harshness, rainfall, harvest) between the extensive and intensive management periods using regression models of sheep population growth rate and counterfactual analyses. We found that, in the extensive period, sheep populations were regulated during harsh winters by climate, while they were regulated during mild winters by negative density dependence. In the intensive period, the negative feedback of density dependence was removed through the provision of additional forage and shelter, and only winter climate and growing season rainfall regulated sheep populations. Counterfactual analyses also confirmed the irreplaceable role of density-dependence in maintaining a sustainable rangeland ecosystem. Although herders attempted to adapt to the removal of negative feedbacks by improving livestock harvest, overgrazing and grassland degradation remain a challenge in this system. We conclude that internal feedbacks within social-ecological systems should be carefully anticipated and accounted for when managing rangelands for sustainability.