Ecological threshold and the mechanisms of plateau pika (Ochotona curzoniae) bioturbation regulating ecosystem multifunctionality and stability

Plateau pika (Ochotona curzoniae, hereafter pika) is a keystone species and allogenic engineer on the Qinghai-Tibet Plateau (QTP). However, excessive pika density adversely affects alpine grasslands. Yet the critical threshold of pika density that triggers a shift in ecosystem functioning and stability remains unclear, which hinders the implementation of precise pika control and grassland restoration. To address this knowledge gap, we conducted a large-scale field survey combining aerial photography and field sampling at 91 fixed observation sites across the northeastern QTP from 2023 to 2025. We used the effective pika hole (EPH) to quantify pika disturbance intensity at plot scale, and further evaluated the responses of ecosystem multifunctionality (EMF) and ecosystem stability (ES) along disturbance levels to clarify the critical ecological threshold for pika control. Our results showed that EMF and ES exhibited divergent unimodal responses to pika disturbance. Functional niche volume peaked under low disturbance (9–150 EPH·ha⁻¹), whereas EMF peaked at moderate disturbance (150–420 EPH·ha⁻¹). Compared with undisturbed plots, functional niche overlap decreased and centroid distance increased gradually as disturbance levels rose. Below 420 EPH·ha⁻¹, pika disturbance enhanced EMF by strengthening positive synergistic regulation of plant (standardized effect, se = 0.34*) and soil functions (se = 0.02). Above this threshold (>420 EPH·ha⁻¹), those positive interactions of plant (se = −0.68*) and soil (se = −0.16) were disrupted and EMF declined significantly. Notably, the drop threshold of soil EMF (420 EPH·ha⁻¹) preceded that of plant EMF (560 EPH·ha⁻¹), suggesting that soil-mediated EMF collapsed at a lower pika disturbance. In contrast, plant functional niche (community level) was more sensitive and rapidly narrowed under low disturbance (9–150 EPH·ha⁻¹) before EMF declined. Those indicates that soil function can serve as an early warning indicator for grassland degradation, while plant community restoration should be prioritized. Our results identified that EPH > 420·ha⁻¹ as the ecological threshold for pika control. Meanwhile, strengthening monitoring within the range of 150–420 EPH·ha⁻¹ was proposed to prevent irreversible grassland degradation by pika disturbance. Our findings confirm the dual ecological roles of pika, and thus we call for an ecological management strategy shifting from blanket culling to threshold-based proactive regulation.