Three decades ago, large parts of Central Delhi contained some of the healthiest ecosystems in the capital, featuring green spaces that supported biodiversity and regulated temperatures. However, by 2023, 73.8 per cent of these ecologically robust areas had disappeared.
A study analysing 32 years of satellite imagery found that the area in Central Delhi classified as having "excellent" ecosystem health at one point, shrank from 13.88 square kilometres in 1991 to just 3.63 square kilometres in 2023.
During the same period, land classified as having "poor" ecosystem health increased from 38.57 square kilometres to 54.90 square kilometres.
The study, titled 'Crossing the ecological threshold: Spatio-temporal dynamics of blue-green space ecosystem health in Delhi using a VORR framework' was carried out by a team from Jamia Millia Islamia comprising Priyanka Jha, Pawan Kumar Yadav, Md Saharik Joy and Taruna Bansal of the Department of Geography, Faculty of Natural Sciences, and Ajit Narayan Jha of the Department of Environmental Science, Faculty of Engineering and Technology.
For the study, researchers analysed Landsat satellite images from 1991, 2001, 2011 and 2023 and mapped six land-use categories -- dense vegetation, open green spaces, cropland, water bodies, wastelands and built-up areas. The land-use classification achieved an overall accuracy ranging from 92.2 per cent to 94.8 per cent, and the findings were validated using bird species richness data.
The findings point to a broader ecological decline across Delhi, where forests, wetlands, water bodies and green spaces have come under increasing pressure from urban expansion, shrinking natural cover and growing habitat fragmentation.
Across the city, the area classified as having poor ecosystem health increased by 50.8 per cent, from 541.47 sq km in 1991 to 816.36 sq km in 2023. The study also found that Delhi's median Ecosystem Health Index declined by 53 per cent during the period, indicating a widespread weakening of ecosystem functions.
The assessment measured ecosystem health using indicators such as vegetation productivity, ecological connectivity, resilience to environmental disturbances and the ability of landscapes to recover after stress.
Areas classified as "excellent" generally contain dense vegetation, healthier water systems and strong ecological connections, while "poor" areas are marked by extensive urbanisation, habitat fragmentation and limited recovery potential.
Researchers found that the decline was visible across most districts, although the pace and extent varied.
North Delhi recorded one of the sharpest deteriorations, with the area under poor ecosystem health more than doubling from 62.16 sq km in 1991 to 131.18 sq km in 2023.
Southwest Delhi witnessed a 132 per cent increase in poor ecosystem health areas during the period, while West Delhi recorded a rise of around 60 per cent.
East Delhi, Shahdara and Northeast Delhi emerged among the weakest-performing districts, with very little land remaining in the good or excellent ecosystem health categories. The researchers linked this to intense urbanisation, industrial activity, degradation of the Yamuna floodplain and shrinking green cover.
South Delhi remained relatively better off because of the Delhi Ridge and protected forest areas that continue to support biodiversity and ecological functions despite increasing development pressures.
The New Delhi district showed signs of partial recovery, with the area classified as having excellent ecosystem health declining sharply from 38.62 sq km in 1991 to 11.35 sq km in 2001 before increasing to 23.39 sq km in 2023, suggesting that conservation and restoration efforts may have improved conditions in some areas.
The study also indicated that the deterioration was not confined to the expansion of poor-quality ecosystems.
Citywide, the area under the "excellent" ecosystem health category declined by 51.5 per cent during the study period, while areas in the "good" category fell by 36.4 per cent and those in the "fair" category by 48.3 per cent.
Although the area categorised as "moderate" increased by 48.7 per cent, the researchers noted that this reflected landscapes under ecological stress rather than genuine ecological improvement.
The city's most stable natural landscapes also shrank steadily. Areas retaining continuous natural cover, including forests, wetlands, open green spaces and water bodies, declined from 49 per cent of Delhi's area in 1991 to 39 per cent in 2001, 34 per cent in 2011 and 30 per cent in 2023.
According to the study, this illustrates the gradual replacement of natural land by residential, commercial, and infrastructure development, as well as the fragmentation of remaining green spaces.
The researchers also documented a sharp decline in ecological resilience, the ability of ecosystems to withstand disturbances such as pollution, heat stress and land-use change while continuing to function.
Delhi's ecological resilience index fell from 0.61 in 1991 to 0.52 in 2001, 0.40 in 2011 and 0.36 in 2023.