Two small burrowing mammals commonly co-occur and play important functional roles in grassland ecosystems worldwide.Understanding the interactive effects of these two groups of herbivores where they coexist is important for effectively guiding conservation of regional biodiversity and management.On Tibet Plateau,plateau pika Ochotona curzoniae and Himalayan marmot Marmota himalayana are considered both pests and keystones of grassland ecosystems,but litter is known about their coupling roles in alpine system where they co-occur.To determine whether plateau pikas and Himalayan marmot have interactively effects on plant biodiversity and vegetation structure,we conducted a field study where the geographic distributions of pikas and marmots overlap.Plateau pikas alone had negative effects on the plant biodiversity,vegetation cover and height.However,when they co-occurred with Himalayan marmots,plant biodiversity and vegetation cover were surprisingly increased except vegetation height.Plateau pikas alone significantly reduced the vegetation cover of pikas preferred comparing with control,but when two herbivores co-existed,no significant differences were found compared with control.Our results demonstrate marmots co-exist with pikas and marmots partially offsets the negative effects of pikas on the alpine meadow ecosystem,and have important implications for understanding the interactive and coupling effects of two herbivores.