Crossing the ecological threshold: Spatio-temporal dynamics of blue-green space ecosystem health in Delhi using a VORR framework

Delhi, one of the fastest-urbanising megacities in the Global South, has undergone severe fragmentation of blue-green spaces (BGS) over three decades, driving deterioration of urban ecosystem health through impervious-surface expansion, habitat loss, and the collapse of ecological connectivity. This study assessed BGS ecosystem health evolution across Delhi's metropolitan landscape from 1991 to 2023 using a Vigor-Organisation-Resilience-Recoverability (VORR) framework, integrating structural, biotic, and functional dimensions into a spatially explicit composite Ecosystem Health Index (EHI). Multi-temporal Landsat imagery provided land-use and vegetation inputs; landscape fragmentation metrics characterised structural decline; and resilience diagnostics quantified the loss of ecological recovery capacity, enabling district-scale identification of tipping zones, resilience hotspots, and restoration priorities. The EHI was validated against bird species richness data, yielding high spatial correspondence and confirming the framework's capacity to capture biologically meaningful variation in ecosystem condition. Analysis revealed that ecologically poor zones expanded by 50.8%, from 541.47 km2 to 816.36 km2, between 1991 and 2023, with a 53% decline in median EHI values, a 50% contraction of persistence surfaces, and sustained reductions in ecological vigour, structural integrity, and resilience across BGS categories. Spatial correspondence between impervious surface expansion, landscape fragmentation, and EHI decline confirmed that this trajectory reflects governance failure rather than inevitable megacity growth. The VORR framework provides a replicable diagnostic tool for ecosystem health assessment applicable to rapidly urbanising contexts across the Global South, where evidence-based governance of blue-green infrastructure is essential for sustaining ecological function under climate and developmental pressures.