Landscape ecological risk based on optimal scale and its tradeoff/synergy with human activities: A case study of the Nanjing metropolitan area, China

• The appropriate granularity and amplitude for landscape ecological risk are 60 m and 9 km, respectively. • The landscape ecological risk showed a downward trend. • The impact of precipitation, temperature, and NDVI on the tradeoff-/synergy between LER and human activities was significant. • The interaction between precipitation and NDVI was the strongest. • Propose suggestions for the future planning of the Nanjing metropolitan area. Exploring the tradeoff/synergy between landscape ecological risk (LER) and human activities is crucial for advancing sustainable development. Based on land use maps and nighttime light data from 2012, 2017, and 2022, this study used methods such as the landscape index, semivariogram function, and geodetector to determine the optimal spatial scale of LER in the Nanjing metropolitan area (NMA) and elucidated the spatiotemporal characteristics of LER and human activities. Furthermore, it explored the tradeoff/synergy between LER and human activities and their influencing factors. The results indicated that 60 m and 9 km were the appropriate granularity and amplitude, respectively, for LER analysis in the NMA. From 2012 to 2017, the human activities and LER mainly exhibited a tradeoff effect. However, from 2017 to 2022, the two mainly presented a synergy effect. The impact of precipitation, temperature, and the normalized difference vegetation index (NDVI) on the tradeoff/synergy between LER and human activities was significant, and the interaction between precipitation and NDVI was the strongest. This may be because suitable precipitation and temperature boost vegetation growth, enhancing livability, and fostering eco-friendly industries like services, high-tech, and tourism. The low explanatory power of population density may be attributed to the varying intensity of human activities across different regions. Minor elevation changes in the northern and central regions may reduce the explanatory power of Digital Elevation Model (DEM) and slope. The research findings offer valuable insights for advancing ecological civilization and promoting regional sustainable development in the NMA.