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Vegetation coverage plays an important role in hindering the erosion in grassland. The grasslands around the world are suffering from the overgrazing which usually causes coverage decrease. Thus, an urgent objective of ecology is to understand how the vegetation coverage varies with overgrazing in the grassland. Although overgrazing has been studied for a long time, its influence on the change of coverage is not well understood. This work modified Noy-Meir’s model of stability in grazing systems by modeling vegetation coverage instead of biomass. The grassland of Maqu (Gansu, China), which is located on the eastern Tibetan Plateau, was chosen as the case study site. The modified model and the introduced parameters were tested, and the relationship between coverage change and the overgrazing sheep units was discussed. The modified model proved to be more suitable to describe the change of vegetation coverage under overgrazing than the published classic models. The results reveal the variation of coverage is determined corporately by grazing intensity, natural conditions and property of grassland. Besides, these results give a reasonable explanation on the difference of coverage change under various grazing situations in previous works. Moreover, the spatial and temporal heterogeneity were studied through introducing some parameters, i.e., the ideally discontinuous grazing period, the proportion that the time of overgrazing occupies in the period, homogeneous factor and restraining constant. Also, the potential positive impact of grazing on the change of coverage was discussed through increasing the growth rate, based on its role in nutrient deposition. (C) 2012 Elsevier B.V. All rights reserved.