Delhi witnesses a travesty of ecological restoration [Commentary] – Mongabay India

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Delhi’s Central Ridge, a mosaic of forest and open savannah with patches dominated by species such as dhok (Terminalia pendula), is a landscape that would be a rare gem anywhere. The location of this remnant of the ancient Aravallis — spanning a little over 850 hectares in the heart of a sprawling urban metropolis — makes it even more special. Yet, for over a century, it has remained misunderstood and misconstrued.
In the early 20th century, the planners of New Delhi sought to create a forested backdrop for the Government House (now Rashtrapati Bhavan). Successive waves of tree planting were carried out, but very few of the introduced trees survived because they were poorly suited to local conditions. The exception was the South American tree Neltuma juliflora (formerly Prosopis juliflora). Better known as vilayati keekar, it is now a widespread invasive alien tree across arid and semi-arid regions of India. Since its introduction to the Central Ridge, it has come to dominate the landscape, along with other invasive species such as gaajar ghaas (Parthenium hysterophorus), lantana (Lantana camara), and subabool (Leucaena leucocephala), which arrived more recently.
Invasive alien species must meet two criteria. First, they must be non-native to a region. Typically, these are species intentionally introduced by people, though some introductions occur inadvertently.
In India, many non-native plants originated in South America and were introduced as crops (such as potato, tomato, and papaya), ornamentals (such as lantana and water hyacinth), or for forestry and agroforestry (such as vilayati keekar and subabool). Others, like gaajar ghaas, which is thought to have been a contaminant of imported food grain, arrived unintentionally.
Not all non-native plants become invasive. Invasive species meet a second criterion: They establish, spread rapidly, and threaten biodiversity, ecosystem services, and human wellbeing. They can drive native species declines, alter wildlife habitats, increase human-wildlife conflict, reduce crop productivity, and negatively affect human health, making them a major environmental and socio-economic concern.
Invasive species spread can be facilitated by people, and vilayati keekar and subabool are good examples of this, since they have been widely planted as part of “greening” activities throughout the country. Other invasive species’ spread can be facilitated by the alliances they form with native seed dispersers. Lantana, for instance, has benefited greatly from a number of fruit eating birds that feed on its fruit and spread its seeds far and wide. Other invasive species are prolific fruiters and have fruits or seeds dispersed by wind or water.
While we know that vilayati keekar was deliberately introduced to Delhi’s Central Ridge in an effort to make it appear perennially green, there is no record of other invasive species being planted there. It is likely that they arrived from other parts of the city aided by birds, or the wind. Lantana, for instance, was widely used as a hedge plant across New Delhi in the 1970s and gaajar ghaas occurs along roadsides all over the city.
Today, vilayati keekar is by far the most widespread and dominant of the invasive plant at Delhi Central Ridge, and has over time replaced much of the native woody flora. There is even evidence that it has driven some species, like Hibiscus surattensis, to local extinction during the time that the botanist, J.K. Maheshwari, compiled The Flora of Delhi (published in 1963). The impacts of other invasive species on biodiversity of the Central Ridge have not been as well documented. However, both subabool and lantana can form dense thickets, edging out native species.
The growing recognition of vilayati keekar’s transformation of the habitat and its negative impacts on native biodiversity have focused attention on the need for its removal and habitat restoration.
An earlier attempt to remove the plant, in the 2010s, involved lopping the trees to let light into the understory and planting lianas at the trees’ base in the hope that they would strangle them. However, the lianas chosen for this included non-native species such as Bougainvillea and Rangoon Creeper, going against the tenets of ecological restoration. There is no precedent for such a method elsewhere; nor is there any evidence that it worked. What is more, a great deal of collateral damage was done, since it involved the clearing of pre-existing native vegetation to make room for the lianas to be planted. Ultimately, the vilayati keekar rapidly coppiced, lianas notwithstanding, and much harm ensued to the remaining native biodiversity of the Ridge in the process.
The latest working plan for the NCT (National Capital Territory) of Delhi (2026-27 to 2036-37) has proposed a seemingly different method to remove vilayati keekar as part of a restoration effort. On the surface, there appears to be merit in the plan. First, it speaks of uprooting the trees, which would be more effective than the strangulation method attempted previously. Second, it speaks of doing it in a phased manner, suggesting that it has factored in the need to minimise damage to the existing habitat and remnant native biodiversity. Finally, it speaks of reintroducing native species, indicating that there has been some consideration of working towards recreating the original vegetation of the native Aravali thorn scrub, as exemplified by Mangar Bani — one of the best surviving remnants of the original vegetation, and a potential reference site for any well-done effort to ecologically restore the Central Ridge.
In its execution, however, the recently initiated restoration as per the revised working plan falls short of these aims. Instead of the gradual and careful uprooting of vilayati keekar, earth movers have been brought in to carry out the task. Large trees are being excavated, gouging out the soil in the process.
Such large-scale mechanised removal can severely destroy local environmental conditions. Heavy machinery causes soil compaction, which, in turn, could reduce rainwater infiltration through cracks and fissures in the underlying rock, greatly reducing ground water recharge. Significantly, this compaction and soil disturbance also destroys fragile biocrusts — the intricate web of lichens, cyanobacteria, mosses, and desiccation-tolerant colonisers like the grass, oropetium — that form a skin on the surface of soils in arid and semi-arid regions such as the Central Ridge. Together, this network of organisms helps bind soil, retains moisture, traps nutrients, and creates an environment for other plants to grow. Biocrusts can take many decades to form, but can be destroyed in no time at all, drastically setting back restoration by many decades.
To make way for this heavy machinery to be brought in, native shrubs like heens (Capparis sepiaria), gangeti (Grewia tenax) and kair (Capparis decidua) are also being summarily cleared.
With much of its original woody flora having been replaced by Vilayati Keekar, it is these shrubs that represent the remaining perennial diversity of the Central Ridge. This native flora supports a host of native biodiversity like the Pioneer butterflies that lay their eggs on the heens. Being slow-growing, resilient, and adapted to grow in the Aravallis’ dry, rocky environment, these cleared individuals are likely to be many decades old.
How, then, are invasive species on the Central Ridge to be tackled? Invasive species can be completely eradicated from small, high-priority regions of interest (such as the Central Ridge) in a targeted manner, followed by the reintroduction of native biodiversity. Such interventions have been successfully carried out in India in places such as Rao Jodha Desert Rock Park in Rajasthan, or in rain forest patches in the Anamalais in Tamil Nadu.
Importantly, such removal of invasive species needs to be done in a manner appropriate to the local context, and minimising damage to the environment or co-occurring species. Removal also needs to be accompanied by continued monitoring, to prevent recolonising by invasive species, along with planting of native species characteristic of that particular ecosystem.
Successful examples of restoration, both from India and elsewhere, teach us that restoration is the well-earned fruit of patient, humble, and intelligent tinkering. This should not come as a surprise. Trees take many decades to grow to maturity, and ecosystems can take much longer than that to develop the complex inter-linkages that constitute functioning and self-sustaining ecological systems. In the case of Delhi’s Central Ridge, the colonisation and degradation by species such as vilayati keekar has taken the better part of a century. There is hubris in thinking that we can reverse this in a matter of a decade or two.
 
Banner image: Earth-moving equipment clears vegetation along the Central Ridge in Delhi. Image by Pradip Krishen.
Ankila Hiremath is a plant ecologist who has worked on invasive species, experiments to evaluate their removal methods and post-removal restoration interventions. Vikram Iyer is a PhD student at the Indian Institute of Science Education and Research (IISER), Mohali, where he works on topics in evolutionary biology and genomics.
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