How the invasive Senna spectabilis poses a threat to the Nilgiri's ecosystem

As a result, this invasive plant, a species of the legume family (Fabaceae) in the subfamily Caesalpinioideae, was widely planted in the NBR areas to enhance its beauty. However, this decision soon started negatively impacting the reserve, as the plant could potentially disrupt the entire ecosystem, damaging the forest food chain and exacerbating human-animal conflicts.

The largest protected forest area in India, NBR, spans Tamil Nadu, Karnataka, and Kerala.

It includes the protected areas of Mudumalai National Park and Mukurthi National Park in Tamil Nadu; Nagarhole National Park and Bandipur National Park in Karnataka; and Silent Valley National Park, Aralam Wildlife Sanctuary, Wayanad Wildlife Sanctuary, and Karimpuzha Wildlife Sanctuary in Kerala. Sathyamangalam Wildlife Sanctuary and BR Hills Tiger Reserve in Karnataka act as its extensions.

In South America, these plants are commonly grown for their attractive bright yellow flowers, which bloom throughout the summer in front yards, parks, gardens, and buildings.

"A few Kerala forest officials who went on an official trip brought in the Weeping Cassia without knowing it would irreparably damage our ecosystem. It's spreading alarmingly now, forcing wild animals and people living in the forest fringes to weep," said P.A. Vinayan, biodiversity expert and president of Ferns, a nature conservation society based in Wayanad.

Over the years, it has spread to other parts of the NBR, disregarding state and geographical boundaries. In each place it spreads, senna has hindered the growth of local vegetation, preventing grass from growing underneath it and discouraging animals and insects from being attracted to it. Additionally, it has harmed the soil quality, disrupted water availability, and significantly impacted the forest food chain.

According to N. Badusha, president of Wayanad Prakruthi Samrakshana Samithi, a non-governmental organisation (NGO) committed to preserving forests and the environment, Senna has become the main cause of increasing human-animal conflicts in the region. Senna does not allow other plants, which are typically used as wild animal feed, to grow. It is also drying up water bodies. The forest departments in three states are currently struggling to contain this threat.

"Senna spectabilis has been introduced into many other countries in the past, but for a very different reason," says wildlife expert Tarsh Thekaekara. "Several African nations, including Kenya, Malawi, Tanzania, and Uganda, used it to supply inexpensive firewood. They have not planted them in forests," he said.

In 2010, the Kerala Forest Department officially recognised Senna spectabilis as an invasive species 25 years after being planted.

According to G. Ravikanth, a senior fellow at Ashoka Trust for Research in Ecology and the Environment (ATREE), the forest department planted this species in wildlife reserves over the years, unmindful of the impact it would have on wild animals. The plant has caused biodiversity loss in over 4800 hectares of forest across the three states.

According to environmental campaigner Veena Maruthoor, based in Thiruvananthapuram, Senna grows much faster than similar plants. The flowers bloom at various times throughout the year, and the seeds are dispersed widely. Cutting the plant does not help; it regrows quickly from the available roots. A tree can produce around 6,000 seeds per season, and each seed can survive for eight to nine years, even in harsh soil conditions. A sapling can grow into a mature tree and bloom within two years.

The Wayanad Wildlife Sanctuary has seen an increase in Senna spectabilis' presence. According to a 2021 report from the UK-based charity Rufford Foundation, Senna spectabilis covers over 23% of the sanctuary. However, official estimates from the forest department suggest that Senna's territory currently covers a much higher 42%. Making matters more complicated, most herbivores, except elephants, chitals, and porcupines, do not consume their leaves.

According to the Rufford Foundation, elephants are strangely the primary carriers of Senna, and their dung contributes to the plant's germination.

The Senna spectabilis is out-competing other plant species for resources in the woods of Kerala and the Bandipur-Nagarhole-Mudumalai region that connects Karnataka and Tamil Nadu. This is leading to an increase in conflicts between humans and wildlife. As food resources inside forests diminish, animals are increasingly raiding crops in villages near the forests, impacting food security.

These efforts are facing challenges because the plant is hard to eliminate. Traditional methods like uprooting and cutting the plant have been ineffective because the plant can easily grow back. The Department of Forest is considering a plan to remove the plants gradually. This plan might include drying out the plants and using chemicals to eliminate this invasive species.

``Senna is no longer a problem confined to just Wayanad; it has spread across the three southern states of Kerala, Tamil Nadu, and Karnataka. Joint efforts are required for its massive removal," observes K Mohanraj, a Coimbatore-based conservationist.

While there have been some sightings in Assam, mostly in orchards, gardens, and farms, the invasion of Senna has not been reported in other parts of the country. Researcher Ninad Mungi, one of the authors of the IPBES (Intergovernmental Platform on Biodiversity and Ecosystem Services) assessment of invasive alien species, has confirmed this.

Officials are concerned that Senna will significantly cover the entire NBR in the next 10 to 15 years unless urgent action is taken to contain its spread soon. During the last week of June, the Kerala forest department reached an agreement with the public sector Kerala Paper Products Ltd (KPPL) for the cutting and removal of senna trees from the Wayanad forests. KPPL will convert the trees into pulpwood for the manufacture of paper, as part of the plan. Forest Minister A K Saseendran stated that KPPL will purchase the trees from 5,000 hectares of forest at ₹350 per metric tonne.

The proceeds from the sale will be used for afforestation and the restoration of natural habitats, the minister added. However, environmentalists are objecting to the move, as KPPL may not have a scientific mechanism to remove the species. They argue that simply cutting and removing the trees may leave behind roots from which the species would regrow with renewed strength.