Rise of the invaders

It is natural for plants and animals to move across their habitats and ecosystems. But when they do it at a greater rate—either with humans for profit or through inadvertent transport through global shipments—a “biological invasion” is initiated. Invasive species breed profusely in places far away from their original ecosystem, occupy more areas and soon alter their new territories while displacing the native species.

Biological invasions have grown to such an extent that globally, alien species are now one of the five major drivers of change in nature along with land- and sea-use change, direct exploitation of organisms, climate change and pollution. There is no place on the planet that remains untouched by biological invasion, which is also driving thousands of species to extinction.

In September 2023, the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES), an independent body comprising more than 140 member-states, released an assessment—the most comprehensive account so far—on invasive alien species and their impacts on the planet’s people and biodiversity. The “Assessment Report on Invasive Alien Species and their Control” was helmed by 86 experts from 49 countries, who dove deep into some 13,000 studies over four-and-a-half years. Its findings acknowledge the alarming fact that invasive species will continue to conquer new geographies, with deep and irreversible impacts on the natural ecosystem.

According to the new assessment report, human activities have caused translocation of some 37,000 alien species across the planet, with 200 new alien species recorded every year. “Studies with evidence of negative impacts exist for more than 3,500 of these species, which are categorised as invasive alien species. The proportion of established alien species known to be invasive varies among taxonomic groups, ranging from 6 per cent of all alien plants to 22 per cent of all alien invertebrates,” says the IPBES assessment. “Invasive alien species are a major threat to biodiversity and can cause irreversible damage to nature, including local and global species extinctions, and also threaten human wellbeing,” says Helen Roy, co-chair of the IPBES assessment.

This comes as the last warning for the planet. In 2019, IPBES released the first-ever “Global Assessment Report on Biodiversity and Ecosystem Services” that warned, “Nature is declining globally at rates unprecedented in human history—and the rate of species extinctions is accelerating with grave impacts on people around the world now likely.” It said one million animal and plant species are under extinction, with thousands likely to disappear within decades. While the 2019 assessment flagged the threat from invasive species, it did not quantify the risk like the latest report.

“Invasive alien species have contributed solely or alongside other drivers to 60 per cent of recorded global extinctions, and are the only driver in 16 per cent of the documented global animal and plant extinctions,” says the new assessment. It adds, “the majority of documented global extinctions attributed mainly to invasive alien species have occurred on islands (90 per cent), and local extinctions account for 9 per cent of documented impacts of invasive alien species on islands.” In a quarter of the world’s islands, alien plant species outnumber native ones. “At least 218 invasive alien species have been responsible for more than 1,200 local extinctions. In fact, 85 per cent of the impacts of biological invasions on native species are negative,” says Anibal Pauchard, co-chair of the IPBES assessment report.

Species have been moving to alien lands and becoming invasive since 1500, when colonisation and global trade tentatively started. But the growth of global trade and large-scale transport systems—“fivefold increase in the size of the global economy over the last 50 years”—have resulted in alien species being recorded at an “unprecedented” rate. For instance, a bit more than one-third of the 37,000 known alien species have been recorded since 1970. Scientific studies referred to for the latest assessment of IPBES forecast that this rate is going to be higher in the future, with more areas under occupation of alien species. Says Roy, “Under ‘business-as-usual’ conditions, we project that total numbers of alien species will continue to increase in this way.” It is estimated that by 2050, the total number of alien species will be 33 per cent more than the number in 2005.

There are humongous economic costs to these biological invasions. Invasive species do not just alter the local ecosystems but also afflict the local water, biology and soil security that make livelihood difficult, particularly for those who depend for survival on ecosystems. It is estimated that in 2019, such invasions cost the world a whopping $423 billion. “The vast majority of global costs (92 per cent) accrue from the negative impact of invasive alien species on nature’s contributions to people or on good quality of life, while only 8 per cent of that sum is related to management expenditures of biological invasions,” says the assessment. The economic cost is increasing by fourfold every decade since 1970. A study titled “Massive economic costs of biological invasions despite widespread knowledge gaps: a dual setback for India”, published in the journal Biological Invasions in April 2022, says that in the last six decades invasive alien species cost India $127.3 billion.

These costs are born disproportionately by the already poor and disadvantaged population. “Invasive alien species can add to marginalization and inequity, including, in some contexts, gender and age-differentiated impacts,” says the assessment. Across the world, the poor tend to reside in degraded environment, and these areas are also heavily infested by alien species. For example, the IPBES assessment finds that more than 2,300 invasive alien species have been recorded in lands used by indigenous communities.

India under threat

Another recent study estimates that about 66 per cent of the country’s natural systems are threatened with invasive species. The study, published in the Journal of Applied Ecology in October 2023, is the first account indicating the distribution status of high-concern invasive plants spread across the country. The findings are a result of a national-level survey conducted in India, and note that 158,000 plots in 358,000 sq km of wild area are invaded by alien species. The 11 high-concern invasive plant species that showed presence in 20 states of the country include Lantana camara, Prosopis juliflora and Chromolaena odorata. The study estimates that loss due to biological invasions would cost the Indian economy up to $182.6 billion.

“The sampling effectively covered 31 per cent of savannas, 51 per cent of dry deciduous forests, 40 per cent of moist deciduous forests, 29 per cent of semi-evergreen forests, 44 per cent of evergreen forests and 33 per cent of moist grassland savannas,” the report says. Findings from the core tiger-populated regions are significant, says Qamar Qureshi from the Wildlife Institute of India (WII), a co-author of the study. This is because “the survival of apex predators like tigers depends on abundance of herbivores, which in turn depend on habitats free from the negative impacts of plant invasions. Proliferation of invasive plants jeopardises these delicate ecosystems, with far reaching impacts on species and people dependent on these ecosystems,” he adds. “Project Tiger was envisaged to serve as a flagship to monitor the changing status of carnivores, herbivores, and their habitat. Plant invasions alter habitats and reveal intricate ecological changes across biomes,” Qureshi says in a statement issued by WII.

Some 53 per cent of plots have recorded at least one of the 11 high-concern invasive plants that have invaded a total of 254,880 sq km of land. The analysis reveals the distinctive relationship between the average invasion cover and independent variables. The data also indicates that the invasive cover increases up to a threshold with rise in temperatures, and then declines in conditions of increasing rainfall, seasonal vegetation opening and human modification index (a set of 13 anthropogenic sensors that provide a cumulative measure on the impact of human modification on terrestrial lands across the planet). Savannas are reported to have the highest susceptibility (87 per cent) to invasions, followed by moist grasslands and dry deciduous forests (72 per cent each). Evergreen forests were found to be least suitable for invasive species, at 42 per cent susceptibility. However, the suitability and drivers varied for each species.

The study points out that Lantana camara had the largest expanse, spread across 574,186 sq km, covering 50 per cent of natural areas occurring across all natural systems. Mikania micrantha had the least expanse, stretching over an area of 148,286 sq km and covering 13 per cent of the natural area, mainly moist grasslands and forests.

“Most species depict the eco-climatic affinity of these plants. For example, 94 per cent invasion of Prosopis juliflora coincided with dry grassland savannas and dry deciduous forests. Species like Senna tora, Xanthium strumarium and Mesosphaerum suaveolens were predominant in dry savannas and deciduous forests, whereas Mikania micrantha and Ageratina adenophora were distributed in moist grasslands and evergreen forests,” the report says. The Nilgiri Biosphere Reserve in the Western Ghats is one of the largest impacted invasion hotspot areas, dominated by Lantana camara, Prosopis juliflora and Chromolaena odorata.

The southern Eastern Ghats are found to host the most densely invaded landscapes, with the maximum vegetation of Prosopis juliflora and Lantana camara. “Fragmented forests of Central Indian Highlands were also largely and densely invaded forest systems, typically along the ecocline between dry and moist systems. Savannas were largely invaded by woody Prosopis juliflora, particularly surrounding semi-arid protected areas,” the study says.

Human modifications, shifting soil moisture regime, historical propagation of invasive plants and altered cycles of natural disturbances are the main driving factors behind the invasions. The increasing work population densities and proportional increase of demand for food, infrastructure, energy and socio-ecological drivers further threaten to intensify and possibly escalate the already accelerating invasion, the authors note.

YV Jhala, former dean of WII and a co-author of the study, says, “Increase in invasive species means loss in ecosystem services from forests and increased propagules (a vegetative structure that if detached from a plant can give rise to a new plant) into agri-cultural areas will result in economic losses.” He says that an increase in the invasive plants which are unpalatable will translate into lack of food and result in reduced carrying capacity for herbivores, eventually causing decline and shortage of food for the apex predators. “Managing invasive species demands more than mere removal—it necessitates context-sensitive restoration, stakeholder participation, and adaptive holistic policies that can enable positive changes,” says Ninad Avinash Mungi from Aarhus University, Denmark, and co-author of the research.

This was first published in the State of India’s Environment 2024

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