Stage-wise responses of ecosystem structure and function to human activities: A case study of the Qinghai-Tibet...

Understanding how ecosystem structure and function respond to increasing human disturbance is essential for ecological regulation in fragile regions. However, existing studies have often examined ecosystem structural risk or functional supply separately, leaving limited understanding of whether the two dimensions exhibit coordinated or divergent threshold responses along a unified disturbance gradient. To improve this understanding, this study developed an integrated "disturbance-structure-function" framework, using human activity intensity (HAI) as a common gradient to analyze the parallel responses of landscape ecological risk (LER) and total ecosystem services (TES) on the Qinghai-Tibet Plateau from 2000 to 2020. Response thresholds were identified by combining non-parametric detection with parametric validation, and an XGBoost-SHAP framework was used to examine stage-specific variations in dominant drivers. The results show that HAI increased overall during 2000-2020, LER exhibited significant spatial clustering, and TES declined across 69.36% of the study area, whereas 30.64% showed improvement. Both LER and TES showed pronounced nonlinear responses along the HAI gradient, with overlapping transition intervals within the HAI range of 0.15-0.25. Within this threshold range, LER declined and TES increased, whereas further HAI intensification was associated with rapid risk accumulation and declining ecosystem service supply. These findings indicate that ecosystem responses to human disturbance are not simply linearly cumulative, but shift across identifiable disturbance stages, thereby providing a quantitative basis for recognizing critical intervention windows and supporting stage-specific ecological regulation in fragile regions.