Balancing Rapid Urbanization and Ecological Security on a Tropical Island: Multi-Objective Land Use Optimization and Simulation Based on Non-Linear Driving Thresholds

Subjected to high-intensity development, enclosed tropical islands frequently face severe spatial resource constraints and acute risks of ecological degradation. To seek a comprehensive solution to multidimensional spatial conflicts, this study selected Hainan Island—a region facing development pressure from the Free Trade Port construction—as the study area. By coupling the XGBoost-SHAP, NSGA-II, and PLUS models, we constructed an integrated modular spatial planning framework of “mechanism interpretation—macro optimization—micro reconstruction”. The main findings are as follows. (1) Over the past 15 years, the fragile ecosystem of the island has faced immense pressure. Lacking an external buffer hinterland, land-use transitions exhibited a distinct chain transmission characteristic of “built-up land occupying agricultural land, and agricultural land squeezing forest”. This internal spatial cascading substitution constituted the core mechanism of ESV decline. (2) The reconstruction of the spatial pattern is governed by dual thresholds of natural constraints and economic pulling. A rigid ecological defense baseline (NDVI ≥ 0.40) restricts disorderly urban sprawl, while spatial inflection points (8.46 km from the coastline and 271 m from roads) precisely delineate the coastal and traffic-oriented agglomeration boundaries for built-up land. (3) Among the predefined scenarios, the Sustainable Development (SD) scenario emerges as the optimal pathway. By proactively reducing inefficient agricultural land space at the macro level and strictly implementing identified threshold controls alongside “infill” urban agglomeration at the micro level, the SD scenario is expected to successfully achieve a Pareto-optimal synergy between socioeconomic growth and the restoration of regional ESV. This study not only establishes an objective benchmark for the refined multi-objective spatial governance of the Hainan Free Trade Port, but also provides a referential planning paradigm for vulnerable enclosed islands globally facing high-intensity anthropogenic stress.