14 invasive plants behind grassland loss in one of India’s first tiger reserves
Forest Department data from Assam's Bodoland Territorial Council show that grassland cover inside Manas Tiger Reserve has declined by 43.59% relative to its extent in 1990, over a 30-year period.
The report attributes the loss primarily to the spread of 14 invasive plant species that have colonised former grassland patches.
Manas is one of India's earliest tiger reserves and forms the core of a UNESCO World Heritage Site and Biosphere Reserve, making grassland integrity central to its conservation value.
Grassland decline directly affects habitat for grazing herbivores such as the greater one-horned rhinoceros, pygmy hog, and hispid hare, several of which are grassland specialists found in few other Indian landscapes.
Manas Tiger Reserve — Institutional and Conservation Status
Manas, located in Assam along the India-Bhutan border, was declared a Wildlife Sanctuary in 1928, brought under Project Tiger in 1973 as India's fifth tiger reserve (the first in Assam), and inscribed as a UNESCO World Heritage Site in 1985. It was designated a Biosphere Reserve in 1989, and additional reserve forests were added in 1990 to form the present-day Manas National Park.
Key Details
- Governed under the Wildlife (Protection) Act, 1972 — Manas is a Tiger Reserve (Section 38V) with a core-buffer structure, and portions are notified as a National Park (highest protection, no human activity permitted).
- Manas was placed on UNESCO's "World Heritage in Danger" list from 1992 to 2011 due to poaching and civil unrest, and was removed once conditions improved.
- Administratively, Manas falls within the Bodoland Territorial Council (formed under the Sixth Schedule-linked Bodoland Territorial Areas Districts arrangement), which explains why the Forest Department data cited comes through the BTC.
- Key grassland-dependent species: greater one-horned rhinoceros (Rhinoceros unicornis, IUCN Vulnerable), pygmy hog (Porcula salvania, IUCN Endangered, found almost nowhere else), Bengal florican (IUCN Critically Endangered).
The reserve's dual UNESCO/Project Tiger status means grassland loss inside it is tracked both by India's National Tiger Conservation Authority and by UNESCO's periodic State of Conservation reporting — making Manas a recurring case study for how invasive species erode protected-area management effectiveness.
Invasive Alien Species and Grassland Ecology
An invasive alien species is a non-native organism whose introduction and spread threatens biodiversity, ecosystem services, or native habitats. In India's grasslands and forests, invasion by fast-spreading exotic weeds converts open, grazing-friendly grassland into dense weed thickets unsuitable for native herbivores — a process distinct from deforestation, since the land remains vegetated but ecologically transformed.
Key Details
- Species commonly documented as driving grassland invasion in Manas and similar Terai grasslands include Chromolaena odorata (Siam weed) and Mikania micrantha (mile-a-minute weed), both native to tropical America and introduced to India in the 20th century; Lantana camara is another widely cited invasive across Indian forests.
- Several of these species feature among the IUCN's list of "100 of the World's Worst Invasive Alien Species."
- Invasive spread is compounded by altered fire regimes, reduced grazing/disturbance (which historically suppressed weed establishment), and floodplain dynamics from rivers originating in Bhutan.
- No single central Indian statute regulates invasive alien plants; management is handled administratively by state Forest Departments and NTCA guidelines under the Wildlife (Protection) Act, 1972 framework.
The 14 invasive species identified in the Bodoland Territorial Council's data represent the operational, on-ground picture of the ecological process by which India's tiger reserve grasslands are being lost — a theme with direct relevance for Mains answers on human-wildlife conflict, protected area management, and invasive species policy gaps.
Tiger Reserve Core-Buffer Framework and Grassland Management
Under the Wildlife (Protection) Act, 1972 (as amended in 2006), every tiger reserve has a notified "core" (critical tiger habitat, kept inviolate) and a "buffer" zone permitting regulated human use. Grasslands within the core are essential for prey-base species that sustain the tiger population, so their degradation has cascading effects up the food chain.
Key Details
- The National Tiger Conservation Authority (NTCA), established in 2005 under Section 38L of the Wildlife (Protection) Act following the Tiger Task Force report, oversees tiger reserve management plans, including grassland/habitat interventions.
- Restoration approaches used in comparable reserves include controlled/prescribed burning, manual uprooting of invasive weeds, and re-seeding with native grass species.
- Grassland loss is monitored through periodic remote-sensing/GIS-based habitat assessments compared against historical baselines (here, the 1990 baseline).
The 30-year, 1990-baseline comparison cited in the report reflects the standard NTCA/Forest Department practice of using historical satellite or ground-survey baselines to quantify habitat change inside core tiger habitats.
- Grassland cover loss in Manas Tiger Reserve since 1990: 43.59% (relative loss over 30 years)
- Number of invasive plant species identified as drivers: 14
- Manas declared a tiger reserve: 1973 (5th in India, 1st in Assam)
- UNESCO World Heritage Site inscription: 1985; Biosphere Reserve: 1989
- Manas was on UNESCO's "In Danger" list: 1992–2011
- Key grassland-dependent species: greater one-horned rhinoceros, pygmy hog, Bengal florican