The relationship between high-quality development and ecosystem health in China's urban agglomerations

China's rapid urbanization and economic development have significantly improved living standards and industrial capabilities. However, these advancements have also exerted pressure on the environment and ecosystem health (EH). Urban agglomerations (UAs)-clusters of interconnected cities-play a crucial role in driving high-quality development (HQD) and serve as hubs for economic growth, innovation, and social progress. Understanding the relationship between HQD and EH is essential for ensuring ecological security and promoting regional sustainable development. This study examines 17 UAs in China, each at different stages of development, and analyzes their HQD and EH levels from 2000 to 2021. The HQD index shows a steady upward trend over this period, increasing from 0.086 in 2000 to 0.236 in 2021, a cumulative rise of 174.42%. In contrast, EH levels remained relatively stable throughout this period, with moderate variations across different regions. On average, local-level UAs exhibited higher EH levels than national-level UAs, with regional-level UAs falling in between. A U-shaped relationship between HQD and EH was observed in most UAs. The coupling coordination degree between HQD and EH has gradually improved, with many areas achieving high or intermediate levels of coordination. Over time, the number of regions with serious or slight maladjustment in the coupling coordination has decreased. High-level coordination hotspots were primarily located in eastern and northern UAs, while cold spots were mainly found in the coastal areas of southeastern China. These findings provide valuable insights for policymakers to better balance economic development and environmental protection.