Non-linear effects of multi-dimensional urbanization on ecosystem services in mega-urban agglomerations and its threshold identification

The rapid urbanization process of urban agglomerations had complex and non-linear effects on various ecosystem services (ESs), particularly within mega-urban agglomerations. Previous studies have focused on the non-linear impacts of urbanization on comprehensive ESs within single urban agglomerations. However, research on the non-linear relationship between multi-dimensional urbanization and different ES types across entire mega-urban agglomerations and the identification of impact thresholds remains limited. This lack of investigation has considerably hampered the formulation of targeted policies for urbanization development and ecological management. Addressing these research gaps, this study takes a holistic approach by taking the BTHUA, PRD-UA, and YRD-UA as a whole and employs a threshold regression model to measure the non-linear impacts of multi-dimensional urbanization on different ES types, along with identifying critical thresholds. The results are as follows: In terms of temporal evolution, urbanization has progressed most rapidly in the YRDUA and PRD-UA, contributing to varying degrees of ES decline. In contrast, despite the relatively low urbanization level in the BTH-UA, ESs are on the rise. Spatially, mega-urban agglomerations exhibited a "core-periphery" pattern in both urbanization and ESs distribution. High urbanization levels were found clustered in core regions, while elevated ESs levels were prevalent in the peripheries, indicating significant spatial disparities. The effects of both population urbanization (PU) and land urbanization (LU) on carbon storage (CS) exhibited an inverted U-shaped curve with thresholds of 272 people/km2 and 1.063%, respectively. The effect of LU on food production (FP) had a U-shaped curve with a threshold of 0.704%. The findings indicate that the impacts of PU and LU on ESs exhibit more complex characteristics in urbanization systems. The results of the study underscore the significance of regulating population density and urban expansion intensity to improve ESs within megaurban agglomerations. This research offers valuable support for policy formulation and sustainable development planning in these large urban clusters.