Multiple ecological mechanisms drive effects of fertilization and herbivore exclusion on species richness in an alpine grassland community

Fertilization and herbivore exclusion are major anthropogenic drivers of biodiversity change in grassland ecosystems, yet their effects and underlying mechanisms remain insufficiently understood. We conducted a full-factorial field experiment in an alpine grassland on the Qinghai–Tibet Plateau in 2021 and 2023 to examine how fertilization, herbivore exclusion, and their combination influence species richness, and to identify the ecological pathways mediated by litter, soil nutrient availability, and light-related plant functional traits. We found that both fertilization and herbivore exclusion significantly reduced species richness, with a synergistic decline that was stronger under the combined treatment. Moreover, fertilization increased living biomass, whereas herbivore exclusion promoted litter accumulation, and both treatments reduced light availability. Structural equation modelling (SEM) revealed that the decline in species richness was jointly driven by increased soil nutrient availability, litter biomass, and community-weighted mean (CWM) plant height, which together explained 66% of the variation. In contrast, specific leaf area (SLA) showed limited explanatory power. These results indicate that biodiversity loss arises from the combined effects of above- and below-ground processes, integrating functional trait-mediated competition for light, litter accumulation, and nutrient-induced changes in soil conditions. Our findings highlight that effective management of natural grasslands should avoid both excessive fertilization and complete herbivore exclusion, and instead promote balanced nutrient inputs and appropriate grazing regimes to sustain biodiversity.