EQUILIBRIUM OR NON-EQUILIBRIUM ECOSYSTEMS? SCALING EFFECTS IN RANGELAND ECOSYSTEMS OF WESTERN MONGOLIA

摘要

For many years, rangeland ecologists have debated about whether the state of semiarid and arid pastures is the expression of an ecological equilibrium dynamics reached in response to grazing livestock. Since the problem having been considered at different spatial scales, it is recognised that the competing concepts of equilibrium and non-equilibrium dynamics need to be integrated. The present study assesses grazing influence in Mongolia's arid desert steppe at three spatial scale levels. At the landscape level, the investigation focused on the impact of grazing on vegetation-determining ecological factors and on the occurrence of grazing-mediated plant communities. At the community level, the total number of species, the total vegetation cover, thepercentage of annual species, the cover of annual species, and the soil nutrient properties were assessed along gradients of grazing intensity. At the population level, the total number and weight of plants per plot, individual plant weight and the proportion of flowering plants of the dwarf semishrub Artemisia xerophytica were recorded along gradients of grazing intensity. The results show that grazing influence increases with decreasing spatial scale. At the landscape level, no grazing influence couldbe identified. At the community level, different vegetation parameters respond to grazing in different ways, and the responses of the same parameters vary between plant communities. At the population level, the results clearly reflect the grazing gradient in the way predicted by the equilibrium model. The study demonstrates that even arid desert steppes display equilibriai and non-equilibrial properties, depending on the observational scale. As a consequence, the assessment of vegetation dynamics and grazing impact in rangelands requires a multiple-scale approach that duly considers climatic differences. It is further suggested, that future research should draw comparisons between landscapes that co-evolved with herbivory, and those that did without.