Quantitative assessment of ecological security and its influencing factors in the Danjiangkou Reservoir based on a health–risk–service framework

The Danjiangkou Reservoir (DR) serves as the primary water source for the Middle Route of China's South-to-North Water Diversion Project (SNWDP), and its ecological security (ES) is critical to water supply safety in North China and to broader regional sustainability. However, systematic assessments of ES in the DR region remain limited. In this study, a health-risk-service framework was developed to evaluate the evolution of the ecological security in DR across three benchmark years (2003, 2013, and 2023). Furthermore, the XGBoost-SHAP model was employed to uncover the dominant natural, anthropogenic, and landscape influential factors behind ES variation. The results indicate that: (1) The proposed framework effectively captures the ES status of DR, with a strong correlation between ecological security index (ESI) and remote sensing ecological index (RSEI) (R² > 0.8, P < 0.001); (2) ESI exhibited a fluctuating upward trend over time, with over 95% of the area classified as Medium or above in terms of ecological security. The ESI hotspots were primarily distributed in the northern and southern regions, which are dominated by forest cover, whereas the cold spots were mainly concentrated in the central region, characterized by cropland and built-up land; (3) Results from the XGBoost-SHAP model revealed that ESI is influenced by multiple factors in a nonlinear fashion. NDVI and LPI were the primary positive contributors, whereas HDI and urbanization had negative impacts, with all these relationships exhibiting nonlinear threshold effects. Notably, threshold effects were identified within specific ranges of these variables. This framework provides a practical approach for evaluating ESI in reservoir regions and offers a scientific foundation for ecological protection and source-area ecological security management in cross-basin water diversion projects such as the DR.