At first glance, a 1-metre tall parthenium plant looks innocuous. Its tiny cream flowers, borne on several frail, pale green stalks, dance in the wind. However, packed in each mature plant is the power to produce anywhere between 10,000 to 25,000 seeds. T+he wind will carry these seeds afar, ensuring the species’ future survival.

This and several other features have helped parthenium thrive – not just in its native range in tropical America, but on foreign shores as well. Accidental introductions more than 50 years ago helped the plant gain foothold in India as well, causing biologists to worry about its impact on native plants and assemblages.

A recent study does not paint a hopeful picture for India in the face of climate change. It predicts that as much as 65% of the country could be prone to the spread of parthenium. Three regions – the Western Himalaya, the North Eastern states and peninsular India – could be hotspots for invasion, says the study published in Biodiversity and Conservation.

Unwelcome alien

Parthenium hysterophorus, known as “Congress grass” in India, has spread to more than 40 tropical and subtropical countries, earning its place in the list of the world’s 100 worst invasive species. The alien has invaded almost all Indian states as well, as per estimates, its invasion has resulted in yield losses of up to 40% in several crops in the country and has also caused a 90% drop in forage production. It causes health problems too. While indirect or direct contact with parthenium can cause skin problems and allergies in people, the plant is toxic to livestock.

A leopard makes its way through a parthenium-infested undergrowth in the Nagarhole Tiger Reserve in Karnataka, peninsular India. Credit: Aathira Perinchery/Mongabay

According to Anzar A Khuroo, senior assistant professor at the department of botany at the University of Kashmir’s Centre for Biodiversity and Taxonomy, this unpalatability offers parthenium a competitive advantage over other native species. Combined with climate change – which could alter plant distributions, even create ideal conditions for the spread of many invasives – what could the future distributions of parthenium in the country be and what could this bode for India?

To answer this pertinent question, scientists including Khuroo and his colleagues at the University of Kashmir and the Ashoka Trust for Research in Ecology and the Environment in Bengaluru, used a combination of predictive modelling to identify areas vulnerable to parthenium invasion due to climate change and niche dynamics and to analyse changes in the niche or the position of the species in its native and invaded ecosystems.

First, they obtained 2,099 geo-referenced occurrence records on parthenium distribution, across its native and invaded ranges, from several sources. Sources included the Centre for Agriculture and Bioscience International, the India Biodiversity Portal, the Global Biodiversity Information Facility database and published literature and Indian herbarium records.

Using data from an international climate database, the team also sourced ten bioclimatic variables – including temperature and rainfall – between 1950 and 2000 to model the distribution of parthenium under current and future climatic scenarios. Their results reveal that both temperature- and rainfall-related factors govern the distribution of parthenium across India. Under current climatic scenarios, a staggering 65% of the total area of India is suitable for the potential invasion of this weed.

“Our ensemble model predicts three hotspots for its invasion – the Western Himalaya, North Eastern states and parts of peninsular India,” said Khuroo.

Southern Jammu and Kashmir and several areas in Himachal Pradesh and Uttarakhand would be more prone to parthenium invasion in the Western Himalayan region, show the results. In the North East, Southern Sikkim, Arunachal Pradesh and almost all of Assam, Meghalaya, Nagaland, Manipur, Mizoram and Tripura are vulnerable to parthenium invasion. Coastal Odhisa and Andhra Pradesh, Southern Karnataka and large parts of Tamil Nadu will face the brunt of parthenium invasion in peninsular India. Most other states, except Rajasthan and Gujarat, also face low to moderate levels of invasion.

Climate change

But there is some good news: the study finds that with future climate change – that is if by 2070, temperatures increase by 2.7 degrees Celsius to 3 degrees Celsius in the mid and late 21st century worldwide and continue to rise – there could be an overall decrease in habitat suitability for this invasive species. However, areas where parthenium has already invaded, like North East India, would remain vulnerable, while others such as the Western Himalaya may become highly vulnerable to its invasion with rising temperatures.

The team also compared the climatic niches of parthenium in its native range versus that in India. Their results reveal that its niche has shifted and the weed seems to have adapted to only specific environmental conditions in India. According to the scientists, this could be niche unfilling – a phenomenon where a species does not take over the niches it normally can. Some invasives do face such niche changes during the process of invasion.

A roadside clearance overtaken by profusely-growing parthenium plants. Credit: Aathira Perinchery/Mongabay

Detailed niche analyses also showed that though parthenium occurred in its native range in areas with an annual mean temperature of fewer than 18 degrees Celsius, this did not occur in India. This is a case of niche abandonment due to the non-availability of such habitats in India. Similarly, unlike in its native habitat, the weed occurred in India in areas which saw maximum temperatures of more than 41 degrees Celsius in the warmest months, cases of niche pioneering.

The study’s predictions are in line with several other studies that predict that not all invasive species will be benefitted by climate change, especially those that are range-restricted, or those that have narrow niche widths, according to Bharath Sundaram, assistant professor at Korea University, who has studied lantana, another invasive plant in India.

However, studies that model species distributions based on occurrence data should be ideally seen in unison with ecological studies that test predictions of the model at an experimental level, added Sundaram.

“Without such links, modelling studies tend to remain either inconclusive or speculative,” he said. “However, even with occurrence-based prediction, substituting space for time may allow for some deeper insights. For example, choosing plot subsets based on time-since-invasion or the distribution of invasives in the neighbourhood around focal plots might provide additional ecological clues.”

Their results on parthenium niche dynamics could help understand the nature of niche shifts in invasive species and predict the species’ invasion potential under climate change, say the authors. They advocate for “scientifically-informed management policies” that “take into consideration the interactions between the species, climate change, land cover changes and anthropogenic activities”, to prevent the introduction and establishment of parthenium in India.

But the response of state institutions to invasives tends to be quite ad hoc in India, said Sundaram.

“Additionally, most efforts are restricted in coverage in terms of spatial extent of eradication or mitigation,” he wrote in an email. “One-time activities generally tend not to work as far as invasive species control is concerned. Targeting well-established invasives would require intense activity over time and space and for now, this is lacking.”

According to Khuroo, it would be crucial to sensitise several stakeholders – including the horticulture, floriculture, forest and agricultural departments – about the introduction pathways through which parthenium could invade new areas in India under projected climate change. Even infrastructure development, which causes fragmentation of natural habitats, could potentially alter local environmental conditions and make an area conducive for parthenium to take over, he said.

“We need to bridge the communication gap between science, policy and society at large,” he added.

This story first appeared on Mongabay.