The expansion of native poisonous plants in degraded Tibetan pastures enhances plant diversity via soil-mediated processes

• The native poisonous plant Ligularia virgaurea exerts density-dependent effects on plant community diversity. • Moderate densities of L. virgaurea enhance functional diversity by promoting niche partitioning among species. • High-density expansion of L. virgaurea suppresses plant diversity through trait dominance and resource depletion. Grassland degradation accelerates biodiversity loss and weakens ecosystem stability, particularly in alpine ecosystems. The expansion of native poisonous plants is commonly regarded as a sign of degradation. However, their ecological roles in shaping community dynamics within degraded ecosystems remain poorly understood. Here, we redefine the native poisonous species Ligularia virgaurea (Asteraceae) as a density-dependent ecological indicator that actively regulates biodiversity and ecosystem function. We integrated the concept of environmentally mediated density dependence with plant and soil trait measurements from 157 density patches across two spatial scales, community composition surveys, and structural equation modeling (SEM) to reveal the impacts of native poisonous plant expansion on plant communities. Our findings reveal a nonlinear facilitation-suppression relationship: at low to moderate densities, L. virgaurea enhanced community functional diversity and phylogenetic dispersion by increasing soil heterogeneity and promoting niche differentiation. However, at high densities, intensified competition and resource depletion suppressed diversity. These effects were primarily indirect, mediated through changes in soil water, pH, and nutrient availability. Our results challenge the conventional perspective that native poisonous plants act solely as degradative agents. Instead, we demonstrate that L. virgaurea plays a context-dependent regulatory role, with important implications for adaptive management of degraded ecosystems. Recognizing density thresholds and indirect soil-mediated pathways can improve restoration strategies and resilience-building in alpine grasslands under global environmental change.