India needs a single comprehensive law, nodal agency to deal with invasive species
Rhinoceros in Kaziranga National Park, Assam are also impacted by thorny invasive plants, which are pushing them out of the park to look for food, leaving them vulnerable. Photo: iStock This is the sixth part of a series exploring the food crisis for wildlife. Read the first part, second part, third part, fourth part and the fifth.
An insidious threat has been quietly unfolding in India’s forests — invasive alien plant species have displaced native vegetation, posing significant danger to wildlife by disrupting natural habitats and depriving animals of essential food sources. Down To Earth’s series showed how forests are being taken over by non-native plants, driving a scarcity of food for wildlife.
Forests in Karnataka, Kerala, Tamil Nadu, Assam and Uttakahand are all facing scarcity, showed the series, compelling wildlife to venture into human settlements and intensifying conflicts between communities and animals.
Almost 66 per cent of the country’s natural systems are infested by 11 invasive species that are of high concern to the country, suggested a study published in the Journal of Applied Ecology on August 16, 2023. This means that about two-thirds of the food available to wild animals in the country are either compromised or under threat, said Ninad Avinash Mungi, one of the authors of the study, titled Distribution, drivers and restoration priorities of plant invasions in India.
The study also found that while open and deciduous ecosystems were the most invaded, areas with extreme climates and less anthropic pressure were the least invaded. “This is because human actions remain at the heart of the current crisis,” said Mungi.
Species like Lantana camara, introduced around 200-250 years ago as an ornamental plant, and plants like Prosopis juliflora (mesquite), extensively propagated for their year-round greenery in arid and semi-arid regions of India, exemplify this trend.
Human interventions, including horticultural practices such as crossbreeding for increased resilience, adaptability and prolific flowering, have given invasive plants a competitive edge over native counterparts. As a result, species like Lantana camara manifest in diverse forms, such as shrubs, trees or climbers, depending on the habitat.
As invasive plants gained dominance, native ecosystems were constantly disturbed for timber, biomass, fruits, and other values, coupled with forest fragmentation due to land-use conversion, mining and linear infrastructure. This created niches that invasive species readily exploited, Mungi explained, adding that hydrological manipulation in dry forests can affect existing vegetation adapted to dry conditions and facilitate invasive plants.
Herbivores, both wild and domestic, rely on these landscapes for their very survival and thus are most affected. Megaherbivores (herbivores that weigh more than 1,000 kilogrammes) are also heavily impacted — in Kaziranga National Park, thorny inavsive Mimosa is pushing out rhinoceroses, who to stray out of the safety of the park, making them vulnerable to poaching.
In Gujarat, invasive species are threatening sloth bears, as per an audit report released in September 2023 by the Comptroller and Auditor General of India. It found that the nutrition requirements of sloth bears at two state sanctuaries, Ratanmahal and Shoolpaneshwar, were not being met due to invasive species.
These alien species remain a menace even at Kuno National Park in Madhya Pradesh, where cheetahs were reintroduced in September 2022. The Cheetah Action Plan, released in January 2022, identifies five invasive species — Prosopis juliflora, Cassia tora (sicklepod), Lantana camara, Ageratum conyzoides (goat weed) and Eupatorium spp (boneset) — that can reduce the grasslands.
This food shortage in forests has already started to alter the dietary patterns of wild herbivores, as they either start foraging invasive plant species or rely on farms and plantations.
“We have found the presence of invasive species in the stool samples of Kashmir stag or hangul,” said Manzoor Shah, who is part of a team at the University of Kashmir, Srinagar, that is currently researching the impact of alien species on the state animal of Jammu and Kashmir.
Even Karnataka’s forest minister Eshwar Khandre in December 2023 informed the Legislative Assembly that 38 per cent of the state’s forests has been taken over by invasive species such as Lantana camara, Senna spectabilis, Chromolaena odorata (siam weed) and Mikania micrantha (bitterwine) and that they had restricted the movement of wild animals.
Pogostemon benghalensis or Bengal shrub-mint, an erect herb abundant in the Himalayan foothills and Terai forests, creates a visual hindrance for many herbivores and the dense and rapid growth makes it almost impenetrable, restricting movement in patches of the forest.
“Multiple invasions exert combined magnified effects on native plants, soil nutrients and alters the plant-herbivore interaction in dry tropical forest,” said a study published in Forest Ecology and Management in July 2023. It found that in areas where “thicket-forming alien plants is more than 40 per cent, the presence of megaherbivores was less.”
Megaherbivores help native species fight against alien plant dominance and a restricted movement means further forest degradation, suggested a paper published in Nature Ecology and Evolution on July 26, 2023.
There is “a positive relationship between megaherbivore abundance and native plant richness and abundance, and a concomitant reduction in alien plant abundance,” said the study, which found the relationship strongest in protected areas with a mid-productive ecosystem and high megaherbivore density but weakest in areas where thicket-forming alien plants predominated.
A study Megaherbivores provide biotic resistance against alien plant dominance published in journal Nature Ecology & Evolution pointed out that restricting high densities of megaherbivores to small, pervasively invaded protected areas can probably deplete native plants and aggravate the invasion, providing a period of reprieve during which plants can regenerate.
To add to their woes, as modern protected areas become surrounded by anthropogenic activities, making them almost impermeable, the megaherbivores’ ranging patterns become restricted and their effects inside widespread invaded protected areas are “prolonged and amplified”.
These changes force wild herbivores to either feed on unpalatable invasive plant species, change dietary habits entirely and move out of forest areas in search of food, inevitably increasing human-animal conflict.
Mungi, who authored this study as well, stated that lower nutritional value in the alien plants and detrimental chemicals reduce their relative palatability. “Native plants that are already depleted due to the deleterious effects of alien plants face inflated herbivory pressure, compounding the impact,” he added.
Eventually, the forage depletion forces the herbivores to disperse and find more resources, often leading to crop depredation in neighbouring agricultural fields, the study noted.
Animals like elephants and rhinos may have higher health tolerance levels towards consuming invasive plant species, but high alkaloids and tannins in them weaken the physiology of smaller herbivores, Mungi said.
“In India, there have been cases of chital deaths due to excess consumption of invasive plant species,” he added.
Karnataka forest department is encouraging the planting of local fruit and berry-yielding trees to retain wildlife inside the national parks. However, experts warned that such artificial greening of forests might not be a good idea. Altering natural forest composition by propagating fruiting trees is not sustainable and can inflate the populations of certain species while depleting others.
“It would not be an exaggeration to imagine that humans are now trying to farm the wilderness to increase the survival of wild herbivores by creating artificial waterholes and plantations. But these well-intended efforts need to be replaced with science of restoration and rewilding, where sustainable ecosystems can be achieved,” said Mungi.
India, however, needs a single comprehensive legislation and a nodal agency to deal with invasive species, said Ankila Hiremath, adjunct senior fellow with the Ashoka Trust for Research and the Environment in Bengaluru. Currently, the country invokes a number of existing legislations and policies to deal with invasive species, for example, the Environment Protection Act, 1986 and the Livestock Importation Act, 1898.
But some of these existed before invasive species were ever recognised as an issue. Also, responsibility is divided across many different government departments and agencies, which hinders a comprehensive approach to the prevention and management of invasive species, says Hiremath.
There are isolated efforts to rein in the invasion by alien plants, like in Tamil Nadu, but India does not have a national policy. The issue is global — according to Intergovernental Science Policy Platform on Biodiversity and Ecosystem Services, only 17 per cent countries have national legislation addressing the control and management of invasive alien species.
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