Weeds like Mimosa, Siam are taking over Assam’s biodiversity

This is the fourth part of a series exploring the food crisis for wildlife. Read the first part, second part and the third part. 

Assam is facing a growing threat from invasive plants that are rapidly taking over its forests. Invasive plants have wreaked havoc on the region’s biodiversity, with several regions waging a battle against the spread of such plants, which has impacted the diet of wild as well as domestic animals.

Over the last few years, Kaziranga National Park and Tiger Reserve in Assam, a United Nations Educational, Scientific and Cultural Organization World Heritage Site and home of the one-horned rhinoceros, is battling the highly invasive Mimosa species.

Known as Touch-Me-Not, Shameplant or giant sensitive plant in the anglophone world, Mimosa is called Nilaji Bon or Lajuki Lata in Assamese because of its sensitive compound leaves that fold inward and droop on being touched or shaken. 

A climbing plant originating from the Americas, researchers point to the tea industry for bringing the invasive Mimosa to the region.

“The tea industry may have brought the plant as it is good for nitrogen fixation to enrich soils. Then it was not known as an invasive species. However, a host of climatic as well as geographical factors contributed to its spread, which choked the grasslands of Kaziranga within a few decades,” said Ishwar Chandra Baruah, a renowned botanist and senior scientist with Assam Agricultural University of Jorhat.

The 2002 study by the Wildlife Trust of India (WTI), titled Silent Stranglers: Eradication of Mimosa in Kaziranga National Park, Assam first identified the spread of Mimosa species near the rivers that flow along the boundaries of the park. As the invasion continued for more than a decade, at its peak, 56 per cent of Bagori range of Kaziranga was covered with the species. 

Researchers identified two varieties (Mimosa invisa and Mimosa invisainermis) spreading through the park, of which one is a thorny variety (Mimosa invisa). “The grasslands, consisting of elephant grass that served as a corridor for the rhino and other animals to move, soon became impenetrable,” says Rathin Barman, a wildlife biologist and joint director of WTI, who also heads the Centre for Wildlife Rehabilitation and Conservation in Kaziranga.

Between 2011 and 2013, Baruah investigated the impact of Mimosa species on cattle and discovered alarming findings. The thornless variety, which spread rapidly in the sugarcane-producing belt of Golaghat district, where two of Kaziranga’s major ranges are located, claimed the lives of several cattle. 

“The cattle developed an allergy, which led to their death. However, I found that the dried or cooked Mimosa herbs could be an important feed for cattle. We are yet to study the impacts on the wild animals in Kaziranga,” added Baruah.

The WTI identified several factors that facilitated the spread of Mimosa, including the annual occurrence of floods, human interactions closer to the park’s peripheries, seeds sticking to the tyres of vehicles passing on the Asian Highway 1 that runs through the park’s southern periphery, cattle straying into the park and the presence of several beels (waterbodies) and small streams within the park, which provide moist conditions for seed germination.

WTI suggested that only manual removal of the roots of Mimosa varieties could stop its spread. “For five years, beginning in 2005, we tackled the labour-intensive task of removing the invasive shrub. We then handed over the project to Park officials. While there were chemical alternatives, we could not use them because they would have harmed the animals,” Barman explained. 

According to Barman, Mimosa’s spread has been limited to approximately six square kilometres of Kaziranga National Park. 

Siam weed, also known as Chromolaena odorata, is a flowering shrub native to the Americas that was introduced as an ornamental plant in Dhaka, Bangladesh and escaped a herbarium before spreading throughout South Asia.

The weed rapidly spread in the plains of northeastern India, specially protected areas of Assam. A study by Assam-based non-profit working on conservation, Aranyak, found that in 2004, the density of Siam weed was found to be highest among invasive species in Manas National Park’s grasslands — ranging from 9.4 to 15.1 plants per square metre.

The invasive plant saw a 28 per cent increase in Golaghat district between 2013 and 2023, botanists from Assam Agricultural University found in a study. 

Interestingly, Baruah believes that Siam weed can be used as an insect repellent, despite the fact that the shrub is toxic to cattle.

Rivers, streams and wetlands in Assam are also facing threats from new alien species, such as Ludwigia peruviana, also known as Peruvian water primrose. Baruah along with a Tinsukia-based non-profit in Assam, Evergreen Earth, conducted a study in the catchment area of Dhansiri and Kopili, two southern tributaries of Brahmaputra.

The researchers found the aquatic plant has already covered an area of 500 square kilometres in 2022. “This has impacted the biodiversity of fish in the river streams and wetlands in the region,” Baruah said.

The agricultural scientist stated that among the 100 alien invasive plants in the world, 10 such shrubs or plants are now actively spreading across Northeast India.

“We keep updating the government and the forest department from time to time. In some parts of Assam, we have involved students and youth and some of these programmes have been really successful. The only way to protect the local biodiversity is through awareness programmes and outreach,” Baruah said.

Water hyacinth, considered an invasive aquatic plant that posed big threats, especially to wooden bridges across Assam, has now been successfully tamed. Baruah maintained that Assam government, through funding institutions such North East Development Finance Corporation, has encouraged local artisans to use fibre from the water hyacinth. 

“These efforts have been paying off, as water hyacinth, even in a small way, is now being tapped for fibre to make various useful and artistic objects,” the scientist said.