Urban multi-scale ecological network sequence and spatial structure optimization: A case study in Nanjing city, China

Rapid urbanization has had a segmented effect on the ecological land and natural environment in cities. Constructing an ecological network is of vital importance to protect ecological resources and maintain regional ecological security. However, ecological spaces at different administrative scales have their own ecological functions and elemental characteristics. Therefore, we formed the ecological network sequence to present the integrative connection of multi-scale with municipal area (MA), main urban area (MUA) and central urban area (CUA) in Nanjing. Ecological sources were identified through "landscape-function-structure" framework, and ecosystem services value (ESV) was used as an auxiliary method to identify supplementary sources. Ecological corridors and some key points were identified through circuit theory. The results show that the area of ecological sources in MA, MUA and CUA was 427.3 km(2), 62.5 km2 2 and 16.2 km(2), respectively. By integrating sources of three scales, we got 14 sources (442.7 km(2)) in the first sequence and 10 sources (7.7 km(2)) in the second sequence. Apart from that, we also got 4 new supplementary sources with area of 13.5 km(2). The length of ecological corridors in MA, MUA and CUA was 252.5 km, 263.9 km and 22.7 km, respectively. By integrating corridors of three scales, we found that the western corridors between the ecological sources had a higher gravitation. The ecological nodes are generally distributed in the landscape heterogeneity transition zone, ecological resource fringe zone and ecological spatial contiguous zone. The ecological pinch points are generally distributed along the water system. The ecological barrier points are generally distributed in the combination area among the urban block, village and road. In conclusion, this study made up for the research gap of finding the nesting and integration relationship among ecological networks at multi-scale.