Abstract:Aneurolepidium chinense and Stipa grandis steppes are the two main community-types in the temperate,typical steppe zone of Eastern Mongolian plateau. The changes of their species diversity under grazing influence were studied on two representative grazing gradients, situated in the Xilin River Basin, Inner Mongolia. The results showed: The species richness of the two communities is reduced with the increase of grazing intensity,while their in dices of evenness and diversity on moderately grazed sites were higher than those on both ungrazed and heavily grazed sites. The diversity changes of the steppe community on grazing gradient depended mainly on the competitive exclusion of the species and the different effects of grazing on different species. These two functions could be integratively reflected by the synusia structure in communities. So the diversity of the synusia structure in a community was a measure of the degrees of differentiation of the ecological niches in it,and could be effectively used to interprete the species diversity changes. A moderate grazing intensity prevented the competitive exclusion of the dominant synusia,and at the same time,had no restrictions to the development of other synusia, which led to a diversified synusia structure with a high species diversity. Based on the species-area curve analysis, the measurement of large nested quadrats on homogeneous habitat was suggested as a suitable method for inventory and monitoring species diversity in the steppe area.