- Labelled a ‘High Concern Invasive Species’, invading moist grasslands and evergreen forests, the Crofton weed is rapidly establishing itself in the Himalayas.
- Also known as the Mexican devil as it is native to Mexico, but now found all over the world, this member of the daisy family has formed dense thickets in cool, high-altitude areas and is climbing higher with rising temperatures.
- While it has shown no significant ill-effects on plant diversity, researchers suggest that its rapid spread, characteristic of troublesome invasive species, warrants more attention and steps towards eradication.
During a field visit in the Kumaon region of the Central Himalayas, a team of researchers was surprised to find clusters of white flowers growing on dead tree trunks and even on the withered bark of a tree. Upon closer inspection, they identified the flowering plant as Ageratina adenophora.
“We were left fascinated by how fast this plant can exploit available resources and establish itself in a variety of environments, including a dead piece of wood,” Bhawna Negi, a researcher studying the invasion patterns of A. adenophora and its impact on native ecosystems of the Kumaon region of Central Himalayas at Kumaon University, Uttarakhand said.
Locally known as kaala bansa, the Ageratina adenophora (previously called Eupatorium adenophorum) is a herbaceous plant belonging to the sunflower family/daisy family. Flowering in late winter or early spring in the Himalayas, it forms dense thickets bearing triangular leaves, clusters of small white flowers and purplish stems that secrete a sticky, strong-smelling substance that often deters herbivores from eating it. Native to Mexico, but now found all over the world, it is also known as Crofton weed or, notoriously, the Mexican devil.
In India, invasive plants are negatively impacting native ecosystems, since they grow rapidly and are difficult to eradicate once established. While Lantana camara, considered one of the world’s most troublesome weeds, is widespread in India, other invasives like Senna spectabilis and Crofton weed have received little attention despite reports of being invasive.
Read more: What are invasive plants? Where do they come from?
In an October 2023 study, Crofton weed was labelled a ‘High Concern Invasive Species’ invading moist grasslands and evergreen forests.
Studies from the Indian Himalayan Region (IHR) have highlighted how rapidly Crofton weed is spreading in its cool, high-altitude habitats. In Central Himalaya’s Kumaon region, the plant has entered the native oak and pine forests and is beginning to show the effects of its presence.
“We observed that disturbed conditions, primarily of anthropogenic nature, were more prevalent in invaded forests compared to uninvaded ones. This suggests that disturbance, to some extent, contributes to creating chances for opportunistic species, such as invasive plants. Native vegetation of a smaller stature is at a higher risk than taller counterparts, as the overstory vegetation in invaded forests (including trees and saplings) remains unaffected by the presence of invasives. Similarly, for understory vegetation like shrubs, the presence of invasives does not have a significant impact; however, seedling presence was reduced,” Bhawna Negi, who is also a lead author of the study, said.
Previous studies done in the pine forests of China have shown that Crofton weed alters the plant diversity of other grasses and shrubs in areas that it invades, consequently affecting foraging. Negi’s study reports that as of now, Crofton weed has not significantly affected plant diversity in invaded areas, but the species was consistently present throughout and needs long-term monitoring.
Invasion begins with a successful spread
There are several stages a plant species goes through before it becomes invasive in an area it is introduced to. After introduction, it needs to overcome the unfamiliar biotic and abiotic conditions to successfully produce viable seeds and establish a self-propagating population, which can start to become dominant and disrupt the presence and growth of other plants in its vicinity. A certain portmanteau of traits, such as rapid vegetative growth and the ability to withstand frost and fire, coupled with anthropogenic disturbances, opens the door for invasive species to thrive.
“A very interesting trait of this plant that makes it so successful is its ability to reproduce without pollination and fertilisation. In other words, seeds develop without sexual reproduction, unlike ordinary plants. So, the plant can produce a very large number of viable seeds in the absence of pollinators as it does not depend on pollens. A single flower head can produce 50-60 seeds, and the adult plant can easily produce around 2000 flowering heads. High propagule pressure ensures reproductive success. The seeds of this plant are equipped with fine hairs that ensure that seeds are readily carried by the wind,” Arunava Datta, an assistant professor at the Department of Botany in St. Xavier’s College, Ranchi, told Mongabay-India.
Additionally, this plant is capable of growing under a wide range of climatic and soil conditions. “In the Himalayas, the plant ranges from warm subtropical regions to cool temperate regions, spanning a gradient of almost 2000 metres. This is a remarkable feat as very few native and even non-native plants have such a wide tolerance,” Datta said.
Experts say that the main reason this invasive species is not talked about could be related to its specific regional impact, as studies on the species remain regionally restricted. For example, most of the research on this plant has been reported from China, which is among the worst-hit countries in the world. In the IHR, most of the research has been from the Eastern Himalayas.
Former dean and senior scientist at Wildlife Institute of India (WII) Dehradun, Gopal Singh Rawat, told Mongabay-India that during the past two decades, this species has replaced considerable ground vegetation in the Himalayan region. “It is indeed surprising that conservation agencies and foresters do not talk much about this species, possibly society at large is less sensitive to changes in our ground flora and herbaceous vegetation. However, with the pace at which this species is spreading, most of our native herbaceous plants, especially ground orchids and delicate flora, would go extinct from many parts of the Himalayas and Western Ghats,” he said.
Implications on local biodiversity and livelihood
According to experts, Crofton weed can potentially have a strong negative impact on the livelihood and economy of local people in the Himalayas. The leaves of the plant are not palatable to herbivores, but animals may resort to eating them during forage scarcity. Its frequent consumption may pose health risks to the livestock, which, in turn, might significantly affect livestock-based communities economically. Although no case of poisoning has been reported for cattle and sheep, it is fatally toxic to horses and causes the condition called “blowing disease” in Hawaii, “Numinbah disease” or “Tollebudgera Horse Disease” in Australia.
“Often villagers set fire to clear the dense stands/thickets formed by A. adenophora. However, uncontrolled fire is a hazard to forests and wildlife. Pastoral tribes such as Gaddis and Gujjars often need to travel longer distances in search of greener and more palatable fodder,” Datta explained.
Rawat elaborated that in the Kailash Sacred Landscape, their research studies reveal that Crofton weed occupies abandoned agricultural fields, stream courses and edges of the pine as well as oak forests, especially after the forest fires. “Rapid spread of A. adenophora in parts of Garhwal and Kumaon has serious economic and ecological consequences. First and foremost, its overabundance in the temperate belt reduces the forage availability in grasslands and forests for domestic as well as wild animals, it provides cover to certain species that damage agricultural crops such as porcupine and wild pigs, and it occupies much of the space which would be otherwise occupied by the native plants. Thus, its invasion in most areas is the indication of rapid loss of local biodiversity,” he said.
Impact of climate change
Global warming is causing Himalayan peaks to warm at a rate higher than the global average (0.6°C) and many species that were formerly restricted to the Himalayan foothills are expanding their ranges to higher altitudes.
In the Western Himalayas, Crofton weed is creeping upwards from its previously occupied foothills and middle elevations. “Several studies have projected that the distribution of A. adenophora will likely extend higher than its current range. Erratic weather events can create disturbance which might facilitate its expansion. This upward movement has implications for native ecosystems as a shift in richness and diversity can disrupt nutrient cycling and increase risks to species narrowly adapted to particular micro-habitat or altitude,” Negi said.
Datta detailed that their studies show that this invasive weed faces limitations due to low temperatures at higher elevations beyond 2500m. Specifically, the plant fails to overwinter (live through the winter) under very low temperatures. However, in a 2022 study, researchers looked at the spread of Crofton weed in the Sikkim Himalayas in 2018 and 2021 across elevations from below 600 m to 2700 m above sea level, reporting that the species had increased in spread and density across all elevations, and, most worryingly, managed to establish a gregarious population in the highest elevation within three years’ time.
On the other hand, some experts say that the spread of A. adenophora in the Himalayan region at an unprecedented rate cannot be directly attributed to climate change. Rawat says that Crofton weed was largely ignored and allowed to grow till it became obvious that it was a problem. “Now it has taken over a considerable area and its rate of spread has become exponential and extremely difficult to control, if not impossible,” he added.
Management strategies
Negi, who has been studying the species for years, said that various physical, chemical and even biological methods had been implemented for its eradication; but none have been that successful.
“In this situation, the only possible means is to realise its ecosystem services and utilise this freely available source for better use. The plant is reported to have several ethnomedicinal benefits. Utilising this plant as compost has shown promising results in increasing the growth and productivity of crops. The phytotoxic nature of A. adenophora can be exploited for possible utilisation as a biodegradable insecticide and herbicide,” she said.
In addition to this, the researchers said that biological control by the introduction of a gall-causing wasp and rust-causing fungus had been attempted with limited success. “Chemical control is possible with strong environmental costs. Mechanical control is possible in plain land, but in hilly terrains, it is very challenging. Ecological control by replacement with competitive native grass species seems to be possible, especially in pasture lands. We need to search for better control agents. Prevention is the best cure. So public awareness campaigns should be focused in regions that are suitable/vulnerable but have not been invaded yet,” Datta emphasised.
Read more: Ain’t no mountain high enough: alien invasive plants increasing in higher elevations
Banner image: While Lantana camara, considered one of the world’s most troublesome weeds, is widespread in India, other invasives like Senna spectabilis and Crofton weed have received little attention despite reports of being invasive. Photo by Bhawna Negi.