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We examined how disturbances by Siberian marmots (Marmota sibirica) and associated spatial heterogeneity of foraging patterns and soil properties affect trait variations in five dominant perennial grasses (including sedges) in a Mongolian grassland. Using four continuous traits (leaf height, leaf area, leaf mass per area, and root length) of each grass species, we compared species and plot mean trait values and species’ niche breadth (calculated on the basis of species’ traits) between sites with and without marmots. At sites with marmots, investment in leaves was not favored, probably because of the prevalence of foraging, with the result that plot mean values of leaf height and area were smaller than at control sites. Niche breadth values for leaf area and leaf mass per area were greater at marmot sites, probably due to the spatially heterogeneous patterns of foraging. We observed greater values of species and plot mean root length values at marmot sites. We suggest that the modification of soil physicochemical properties by marmot burrowing, defecation, and urination might enhance root growth for ensuring physical stability of plant bodies and increasing the rate of nutrient acquisition. Niche breadth value for root length was greater at marmot sites, probably due to the spatial heterogeneity of soil properties. Quantification of trait distributions among plant species may help to explain the different plant adaptive mechanisms in relation to external drivers, such as disturbance.