Species turnover drives contrasting effects of large and small herbivores on aboveground biomass in alpine meadow ecosystems

Large and small herbivores often exert contrasting influences on plant communities, but their combined effects on community assembly and biomass allocation remain unclear. A three-year field experiment was conducted in alpine meadows of the Qinghai-Tibetan Plateau to evaluate how yaks (Bos grunniens) and plateau pikas (Ochotona curzoniae), the dominant large and small herbivores, affect plant community structure, soil physicochemical properties and aboveground biomass. Four herbivory treatments were established: control (both excluded), yak only, pika only, and both co-occurring. Community composition, functional group dynamics, and species-level contributions to biomass were quantified using rank–abundance curves, functional group analysis, and the Price equation. Grazing and burrowing disturbance from yaks and pikas significantly reduced the biomass of sedges and grasses, while increasing legumes and poisonous plants, primarily through species turnover driven by species gain. Co-occurrence of yaks and pikas amplified species gain and mitigated species loss, indicating complementary and synergistic roles in community restructuring. Functional group responses were divergent: grasses were suppressed by yaks, sedges were sensitive to pikas, and legumes and poisonous plants expanded under combined herbivory. Variations in aboveground biomass were predominantly explained by plant community attributes (82.23%) rather than soil properties (17.68%). Structural equation modeling revealed that aboveground biomass in alpine meadows was mainly governed by herbivore-induced top-down trophic cascades rather than bottom-up control from soil nutrient availability. These findings demonstrate that herbivore identity and interaction are key determinants of alpine plant community dynamics, operating through species turnover mechanisms.