A study in Nature Reviews Biodiversity reveals that alien plant species are rapidly invading tropical ecosystems, replacing native flora and causing irreversible changes.
Climate change and human activities exacerbate this spread, threatening biodiversity and livelihoods in the Greater Tropics.
The study calls for urgent interdisciplinary research and restoration efforts to mitigate these impacts.
Alien plant species, introduced by humans directly or indirectly in different geographical regions of the world, are now spreading at an unprecedented pace, reshaping tropical ecosystems globally.
These novel plant communities are rapidly replacing rich, diverse and stable ecosystems. They are causing irreversible transformation, changing the fundamental characteristics of the native ecosystems.
New research published in the journal Nature Reviews Biodiversity has revealed that invasive plant species have established themselves in many parts of the world. In multiple places, they are more dominant than native species.
The study also noted that climate change-driven anthropogenic activities further exacerbate the spread of alien plant species, catalysing novel ecosystem dynamics.
The authors studied the tropics and subtropics, collectively termed as the ‘Greater Tropics’. These are spread across nearly 60 per cent of Earth’s land surface and support a majority of its biodiversity. A billion humans are dependent on the Greater Tropics for livelihood.
The review focusses on terrestrial vascular alien species, comprising 75 per cent of all reported alien species.
Alien species initially spread as a result of humans moving them into areas outside their native range. They eventually spread due to biotic and abiotic drivers, with multiple levels of influence at various stages of invasion.
The analysis observed that since the 1950s, the spread of alien plant species increased exponentially and at present, at least four per cent — nearly 13,939 to 18,543 species — have settled outside their biogeographic ranges.
“The Greater Tropics contains around 9,831 established alien plants. Although the distinction between established alien plants and invasive plants is not always clear, many alien plants might still be in the initial phase of invasion and their ranges could subsequently expand,” the study noted.
For instance, the researchers said that around 26 per cent of islands around the world have more established alien plants than native species. “Island systems such as Guam constitute 66.5 per cent, while Tahiti consists of 73.8 per cent alien flora,” the study said. Other endemic hotspots such as Hawaii, Madagascar and the Caribbean have been affected by dense plant invasions, it noted.
Though the pattern of invasion hotspots remains unclear, the study pointed that invasions tend to increase with moderate climate, high productivity and anthropogenic land use.
For example, Mediterranean regions and South Asian systems are typically invaded more. Plant species including Lantana camara, Sonchus oleraceus and Chromolaena odorata that prefer such environments have hence become pan-tropically invasive, it said.
In India, settlements and agriculture have led to fire and herbivory regime change. Land use modifications have resulted in invasions of about 66 per cent — 750,000 km — of presently existent natural areas.
By 2050, introduction of alien species would increase by approximately 21 per cent (669 species) across South America, 12 per cent (503 species) across Africa and 10 per cent (227 species) in the tropical regions of Asia.
Plant invasions in South America, West Africa, coastal Asia and Southwest China are predicted to notably increase under some projections, it noted. Such increase will lead to homogeneous ecosystems, which will have negative impacts compared to rich, diverse native ecosystems.
According to the scientists, these projections are modest as the weakening of native alien plant species control mechanisms and rapid global changes amplify the risk of spread.
Citing the reasons for such concerning spread, the researchers said climate change, fuelled by anthropogenic activities, has provided conducive conditions for the growth of alien plant species.
Increased number of extreme hot days and deadly heat are collectively leading to forest dieback in wet biomes such as Amazon and amplifying droughts.
“These global changes and disruptions often interact with alien plants. For example, alien C4 grasses such as Brachiaria decumbens are rapidly invading along the fragmented, logged and warming forests of the Amazon, increasing fuel load for intense fire regimes — an indicator of large-scale degradation,” the study observed.
This degraded biome further disrupts inherent biotic resistance, encouraging the growth of other alien plants, further fuelling additional fires.
The scientists project that degradation of Amazon forests could impact over 50 per cent of dependent fauna and rapidly transform the future forest to a carbon source rather than a carbon sink, ultimately increasing atmospheric CO2 and contributing to a warming climate.
Such changes would directly impact billions of humans, non-human species and diverse ecosystems, the scientists warned.
The document also noted that increased atmospheric CO2 levels may be promoting plant biomass growth across the Greater Tropics.
“Woody domination or thickening is often met via invasion of alien plants such as Prosopis juliflora and Lantana camara in Africa and Asia and Ziziphus mauritiana and Vachellia nilotica in Australia. Additional complexity in community composition arises from introducing plants from different ecosystems within the same region, known as native invaders,” the study said.
Twenty-one per cent of alien species in African countries originate from other parts of the continent, according to the study. Similarly, native woody plants such as Vachellia karroo and Terminalia sericea are rapidly colonising African savannas due to historical plantations and rising CO2 levels.
In India, the study mentioned that woody thickening in savannas and open forests have intensified forest fires, which in turn promote future invasions. “These societal changes form a positive feedback loop between global drivers and the woody transition of savannas, facilitated by alien plants, probably accelerating future plant invasions,” it noted.
Further, these alien plant species are interacting with native species. This has complex and negative implications. The study observed that invasive Prosopis juliflora depletes native forage plants for herbivores as its pods provide a critical food source for blackbuck (Antilope cervicapra) during dry seasons in semi-arid India. Such herbivores disperse alien plants and facilitate germination, in turn, contributing to the resultant loss of native plants.
It noted that for a country like India, as per 2023 estimates, controlling the spread of invasive plants would require 36 times — about US$13.5 billion — of its total environment budget.
The scientists have now called for a long-term and interdisciplinary study to understand the impacts across the Greater Tropics and strengthening documentation and awareness among communities, especially in the Global South, along with ecosystem restoration.