Complementary effects of disturbance by livestock and marmots on the spatial heterogeneity of vegetation and soil in a Mongolian steppe ecosystem

In the Mongolian steppes, livestock and burrowing rodents are the main animal modifiers of the habitat. Although grazing lands and rodent habitats overlap, the combined effects of livestock and rodent disturbance on spatial heterogeneity of plants and soil have rarely been evaluated. We established study plots at each of four sites: sites heavily grazed by livestock, with and without marmots, and ungrazed sites, with and without marmots. We subdivided each plot into quadrats to survey the plant species composition and soil nutrient properties, and calculated the spatial heterogeneity of vegetation at three spatial scales using non-metric multidimensional scaling analysis. We also calculated the coefficient of variance among the soil samples. The vegetation's spatial heterogeneity did not differ significantly between grazed and ungrazed plots; however, it was higher under marmot disturbance than in the absence of marmots at a fine scale, but lower under marmot disturbance at a coarse scale, irrespective of livestock grazing. At a fine scale, unique habitats were formed by each combination of livestock grazing (presence/absence) and marmots (presence/absence). In addition, the plant species composition in the grazed plots was distinct from that in the ungrazed plots at the coarser scale. The occurrence of degradation-indicator plant species depended on the presence of grazing rather than on the additive effect of grazing and marmots. Marmots increased the abundance of degradation-indicator species only in the ungrazed plots. Each herbivore group increased the spatial heterogeneity of soil nutrients at coarser scales, but these influences were lost when both herbivore groups coexisted. These results show that the ecological roles of livestock and marmots are complementary, not functionally equivalent. That is, livestock modified the overall vegetation composition, and thereby modified spatial heterogeneity at the landscape scale, whereas marmots modified spatial heterogeneity at the local scale. Proper manipulation of the livestock grazing regime can maintain species-rich communities without serious land degradation in the Mongolian steppe.