Guwahati: Over the past three decades, the spread of 14 invasive plant species has emerged as a key driver behind the loss of more than 43% of the grasslands in Assam’s Manas Tiger Reserve, threatening vital habitat for the greater one-horned rhinoceros, pygmy hog, hispid hare and several other rare wildlife species.
Data from the Forest Department under the Bodoland Territorial Council (BTC) show that grassland cover in the 2,837.31 sq km Manas Tiger Reserve declined from 53.61% in 1990 to 30.24% in 2019. This represents a relative loss of 43.59% and an absolute reduction of 23.37% in grassland area over the period.
The reserve, which shares an international boundary with Bhutan’s Royal Manas National Park, is one of India’s oldest tiger reserves and a UNESCO World Heritage Site.
Forest officials attributed the decline in grasslands to a combination of ecological and human-induced factors. These include prolonged civil unrest and insurgency between 1988 and 2004 that disrupted habitat management, uncontrolled forest fires, indiscriminate livestock grazing and the spread of invasive plant species.
Officials also said the decline of large herbivores reduced natural grazing and browsing, accelerating the conversion of grasslands into woodlands. Heavy poaching during the insurgency years, when rhinos were allegedly killed by extremist groups involved in illegal wildlife trade, led to the local extinction of the greater one-horned rhinoceros in Manas by the early 2000s. The species was later reintroduced through translocation programmes from Kaziranga National Park and other protected areas in Assam.
While tree cover has expanded in the reserve, officials said this has come at the cost of native grasslands that support a wide range of herbivores. Several invasive alien species, including Chromolaena odorata, Mikania micrantha, Lantana camara and Leea asiatica, along with woody species such as Bombax ceiba and Dillenia pentagyna, have spread extensively across the landscape.
“The woody invasive species have accelerated the succession of grasslands into woodlands,” a BTC Forest Department official said.
According to the data, woodland cover in Manas increased from 40.42% in 1990 to 60.62% in 2019, reflecting a significant shift in the reserve’s vegetation profile.
Officials warned that the shrinking grasslands have reduced suitable habitat for several grassland-dependent species, including the greater one-horned rhinoceros, pygmy hog, hispid hare, eastern swamp deer, Asian elephant, wild water buffalo, gaur, and hog deer.
The habitat degradation has also affected predators such as tigers, leopards and clouded leopards by reducing the availability of prey species.
Apart from habitat degradation, Manas Tiger Reserve continues to face encroachment challenges. Forest Department data indicate that around 3,709 hectares of land in the reserve’s core area have been encroached upon since the 1980s, primarily in the Bhuyanpara and Panbari ranges.