Urban-Rural Gradient in Biodiversity Intactness across Global Cities

Identifying the urban-rural gradient in biodiversity variation is crucial to better understand the response of biodiversity dynamics to global urbanization. However, there is a paucity of knowledge regarding the urban-rural gradient in biodiversity variation in global cities, largely due to the limitations of ground-based experiments. In this study, through statistical analyses of global gridded 100 m Biodiversity Intactness Index (BII) data, we find that the global mean BII is 0.40 ± 0.07 and 0.60 ± 0.11 for the urban and rural areas, respectively. Notably, this urban-rural BII difference is significant in Africa and Asia. There is a notable and pervasive pattern of increasing BII along the urban-rural gradient in global cities. The global mean BII demonstrates an increase from 0.37 ± 0.07 in urban center to 0.63 ± 0.12 in rural periphery. The difference in BII between the urban center and the rural periphery can reach 0.20 and 0.21 in Africa and Asia, respectively, representing 50% and 54% of the total BII in the urban center. In addition, generalized additive models (GAM) revealed nonlinear relationships between biodiversity intactness and key drivers: urban green space (UGS) exhibited threshold effects, with BII increasing significantly when vegetation indices exceeded critical levels (NDVI >0.40, EVI >0.20, VCF >60%; R²=0.58–0.72, p<0.001). Conversely, economic activity showed a strong negative association with BII (R²=0.87, p<0.001), stabilizing at higher GDP densities (>40 million/km²). The findings contribute to a deeper understanding of the large-scale urban-rural gradient in biodiversity variation and may prove beneficial for the conservation of urban biodiversity.