Ecosystem Service-Based Eco-Efficiency of Cultivated Land Use in Plateau Lake Regions: Spatial Dynamics and Nonlinear Drivers

Plateau lake regions face escalating conflicts between food production and ecosystem conservation under rapid urbanization and strict ecological regulation. However, existing evaluations often overlook the positive ecosystem services generated by cultivated land and fail to capture the nonlinear mechanisms shaping eco-efficiency of cultivated land use (ECLU). This study develops an ecosystem service-based framework to assess the ECLU of Kunming, a typical plateau lake-basin city in southwest China, from 2005 to 2022. Ecosystem service value (ESV) is incorporated as a desirable output within a super-efficiency SBM model, and an XGBoost–SHAP approach is applied to identify the intensity, nonlinear thresholds and interaction mechanisms. Results show an average ECLU of 1.12 with a fluctuating downward trend and widening spatial disparities. High-efficiency zones cluster in central–southern regions, while urbanizing cores experience ecological function degradation despite productivity gains. Cultivated land fragmentation is the dominant barrier, with a critical threshold of 31.90 mu, and fertilizer intensity turns detrimental beyond 0.19 t/ha. Urbanization exhibits an inverted-U pattern—initially suppressive (<35%), promotional (35–55%), and suppressive again (>55%)—with the promotion phase weakened by approximately 67% under severe fragmentation. Globally, threshold-based zoning and fragmentation mitigation must precede fertilizer optimization to ensure synergistic benefits.