Senna spectabilis planted with good intentions has become an ecological nightmare, swiftly overtaking large areas of the sanctuary. Its unchecked growth not only threatens biodiversity but also triggers a chain of ecological and economic repercussions, affecting both wildlife and local communities
Approximately one-fifth of the Earth's surface is threatened by invasive species. These biotic invaders, benefiting from rapid reproduction and a lack of natural predators, outcompete native species for vital resources like food, water, and habitat, posing a significant threat to biodiversity and ecosystem stability.
The Wayanad Wildlife Sanctuary (WWS), nestled in the heart of the Nilgiri Biosphere Reserve (NBR), is a biodiversity hotspot, brimming with a rich variety of flora and fauna. The sanctuary serves as a habitat for a substantial population of Asian elephants, along with over 3,700 documented species of plants and animals. Spanning an area of 344.44 km², the sanctuary is comprised of four distinct ranges: Sulthan Bathery, Muthanga, Kurichiat, and Tholpetty. Each range, with its distinct characteristics, adds to the rich tapestry of the WWS, making it a vital and vibrant hub of ecological diversity.
However, this ecological paradise is under severe threat from several invasive species, including Senna spectabilis, Lantana camara, and Acacia species. This is particularly noticeable in the case of Senna spectabilis, which has proliferated aggressively throughout the WWS and beyond, affecting the NBR and the corresponding ecological continuum of three southern Indian states of Kerala, Tamil Nadu, and Karnataka.
A decision made in good faith by the forest department in 1986 to plant Senna spectabilis, an ornamental tree with striking yellow flowers, has morphed into an ecological disaster. Little did one know, that this tropical American tree had a notorious history of invading continents, including parts of Asia, Africa, and Australia. By the time the mistake was realized, Senna had stealthily spread into many parts of the sanctuary. It expanded from covering less than 16 km² in 2013-2014 to 89 km² in 2019 and now stands at a staggering 123.86 km². Considering the total area of the sanctuary is 344.44 km², Senna has already engulfed 35% of it. If this trend is not curbed, the entire sanctuary could be swamped within a decade, turning it into a dense Senna forest.
A 2021 study has revealed that the Asian elephant, a significant inhabitant of the sanctuary, is aiding the spread of the invasive Senna spectabilis. The elephant’s dung provides a fertile environment for the Senna seeds to sprout, with a single pile of dung found to contain approximately 2000 seeds. The study also found other mammals, such as the chital and the Indian crested porcupine, contributing to the spread of Senna. The researchers observed that the fruiting period of Senna trees in the summer coincides with a high concentration of elephants in Wayanad due to their seasonal migration. This synchronicity could potentially expedite the plant’s dispersal across the landscape, particularly in micro-habitats commonly inhabited by elephants.
A single Senna pod harbours over a hundred seeds, and a medium-sized tree can yield at least one lakh seeds. The invasive nature of the Senna species has had a significant impact on the local ecosystem. It has displaced native vegetation by monopolizing the area and inhibiting the growth of species like grass, bamboo and several other minor forest produce.
The Senna tree employs a unique method of colonizing its surroundings. It releases specific substances, known as allelochemicals, either directly onto the target plants or indirectly into the surrounding soil. These allelochemicals interfere with the normal growth processes of other plants, providing the Senna tree with a competitive advantage. The dominance of the Senna tree has significant implications for the livelihoods of rural and tribal people who rely on non-timber forest produce, like amla or gooseberry, for sustenance. Many edible items like berries, mushrooms, and wild tubers, which were once a significant part of the tribal diet, are gradually disappearing from the forest.
The bitter leaves and irritating bark of Senna do not find favour with herbivores resulting in a decline in their numbers in the area. This has had a cascading effect on the region’s tiger population, which has decreased from 120 in 2018 to 84 in 2023. The scarcity of prey in the Senna-dominated areas is believed to be driving tigers to migrate in search of richer hunting grounds. Similarly, elephants are moving into areas outside the forests in their quest for food and water. Unfortunately, this has led to increased human-elephant conflicts, resulting in a backlash against the preservation efforts of elephants. The ecological disruption has had an unexpected benefit for local communities. With fewer wildlife incursions, settlements near the jungle have seen a significant drop in crop damage and cattle lifting incidents. The data from the past three years shows a downward trend in compensation claims by farmers for crop raids. In the fiscal year 2021-22, there were 734 cases of crop damage reported in the border agrarian hamlets of WWS. This figure fell to 475 in 2022-23, and in 2023-24, the number has further decreased to just 93 cases. Similarly, cattle lifting cases have also seen a decline, from 83 cases in 2021-22 to a mere 19 in 2023-24.
While the decrease in human-wildlife conflict might be viewed as a positive development by those seeking to address this issue, the underlying causes warrant serious concern. Locals like Mr. Stanley Augustine, a member of the Biodiversity Management Committee of Poothadi village, find these changes deeply troubling, particularly from the standpoint of the rural economy. The area is witnessing a worrying decline in agricultural productivity and an increasing trend of farmers abandoning their paddy fields. The forests too are under mounting pressure. The real challenge, therefore, lies in re-establishing equilibrium in the WWS while simultaneously securing the future of the human communities in the area.
In response to these challenges, the Forest Department has launched an initiative known as the ‘Senna Eradication and Forest Regeneration Project’. The project, supported by the National Bank for Agriculture and Rural Development, has a budget of Rs 5.31 crore. The initiative is currently being implemented in the Kurichiat and Muthanga forest ranges, to eradicate Senna from 1,672 hectares. To further support this initiative, the sanctuary authorities are exploring partnerships with the newsprint industry, taking inspiration from Tamil Nadu. The state government there has allowed the Tamil Nadu Newsprint and Papers Ltd (TNPL) to extract invasive trees like Wattles and Senna for paper production. In return, TNPL compensates the Forest Department and employs various extraction techniques, attempting to turn an ecological challenge into an opportunity for resource utilization.
In some areas, invasive plants, including Senna, are being repurposed to manufacture furniture. Senna, in particular, is valued as a timber source for making lightweight furniture and other wooden products. There is hope that policy changes could permit the extraction of Senna from forest areas and its subsequent sale outside. Such changes could potentially motivate the local community to actively participate in clearing the forest of this invasive plant.
The process of removing the Senna tree, however, is fraught with challenges. Traditional methods such as uprooting, girdling, and herbicide application for eliminating Senna from the area are labour-intensive, costly, and environmentally damaging. Uprooting large Senna trees could disturb the soil, leading to erosion, especially in hilly areas that receive high rainfall. Techniques like girdling and felling can inadvertently harm non-target species. Moreover, the use of herbicides may contaminate water sources and negatively impact the area’s biodiversity. Additionally, the resilience of the Senna tree is evident in its ability to regenerate from lateral roots, which complicates its eradication. Given these challenges, it is clear that innovative and sustainable strategies are needed to manage and ultimately remove the Senna species while simultaneously assisting the re-establishment of local species.
As we forge ahead, conservation measures must be holistic, encompassing not only the immediate environs of the sanctuary but also the larger watersheds and catchment areas that feed into the rivers. This holistic approach is particularly significant for Wayanad, the source of the Kabini River. The health of this river is essential for sustaining the lives and livelihoods of communities across Kerala, Karnataka, and Tamil Nadu. Notably, initiatives are in progress to increase the flow of the Kabini River—a major tributary of the Kaveri River—to address the issue of water scarcity in Bengaluru.
The situation in Wayanad is a stark reminder of the domino effect triggered by the introduction of invasive species. The ecological and economic ramifications are far-reaching, impacting not just the wildlife within the sanctuary but also the lives and livelihoods of communities dwelling in its vicinity and beyond. The fight to save Wayanad is a race against time. If immediate and decisive action is not taken, the future of this biodiversity hotspot and the well-being of the millions who depend on it hang in the balance. Wayanad's plight serves as a cautionary tale, urging us to prioritize rigorous scientific evaluation before introducing any new species in an area and to act swiftly and collaboratively in the face of ecological threats posed by invasive species.
(The writers are Former Principal Chief Conservator of Forests, UP and Maharashtra; views expressed are personal)
