Invasive plants are rapidly changing the tropics
In tropical parts of Oceania, India and Brazil invasive plants are altering food chains and damaging traditional ecosystem services. The tendency is only exacerbated by climate change.
Aarhus University
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The invasive water hyacinth has taken over a communal pond in a small village in northern India. All other plants have been pushed out.
view moreCredit: Ninad Avinash Mungi
Invasive species are reshaping ecosystems and local people’s relationship with nature in the tropics, a study led by Danish researchers Ninad Avinash Mungi and Jens Christian Svenning from Aarhus University shows.
The researchers found roughly ten thousand alien plants in the greater tropics – which is an area composed of both the tropic and sub-tropic parts of the world. Especially islands, are invasion hotspots and some have more alien plants than native ones, explains Ninad Avinash Mungi.
But even though it sounds alarming, most alien species are not a problem.
- Many alien species are imported for their usefulness and value, but few escape and become invasive with undesirable impacts, he says.
In collaboration with scientists from Brazil, Ethiopia, India and the US, the two Danish researchers brought together their experience and a rich set of scientific evidence that offers critical insight into the rapidly changing greater tropics.
In India for example, large areas have been overrun by the invasive plant Lantana camara – or Lantana, as it’s popularly known.
- In the last century, large areas in the Western Ghats of India became overrun by Lantana. This triggered the native forest-dependent Soliga people to abandon their traditional livelihood and migrate in search of alternatives, he says.
A worldwide problem
Lantana originally comes from the tropical Americas. It was brought to Europe by Dutch explorers in the late 1600s. In Europe it quickly became popular as an ornamental garden plant – and not before long the Portuguese brought it with them to their colonies, including that of India.
Today Lantana has spread to large parts of the greater tropics. In Australia it’s estimated that 4 million hectares of land are infested with the weed. In Hawaii that number is 160.000 hectares. And in India, Lantana has invaded 30 million hectares. The invasive plant is, in other words, a huge problem around the world, Ninad Avinash Mungi explains.
- Invasive plants like Lantana competitively suppress native food plants in forests and on savannas. Scarcity of food makes herbivores decline, which further affects the carnivores. We’ve seen examples of tigers preying on livestock when herbivores decline.
- Often this happens in very poor areas, where people are closely dependent on ecosystems. Invasive plants not only weaken the ecosystems, but can also create wildlife conflicts, thereby changing people’s relationship with nature.
A problem as old as agriculture
Introducing new plants into ecosystems around the world is not a modern thing. It’s been going on since the dawn of agriculture. But it has accelerated in recent years, some studies show.
- The tropics have historically been changed by people for thousands of years. But the pace changed during the colonial era. People brought species from colony to colony spreading plants and animals all over the tropics. Global trade after WW2 has even further accelerated the process. Today species are being exchanged all the time, explains Ninad Avinash Mungi.
Even though the phenomenon is old, it has only recently been mapped and studied. And the new studies gives an even broader view of the problem.
- Human-caused deforestation, wildlife extirpation, pollution and land-use transformations are important drivers of invasive plant spread. These changes weaken the natural control over alien plants, facilitating their spread. Thus, invasive plants are initially passengers of disturbances, but eventually become drivers of disturbances, he says.
Climate change is leading to widespread ecosystem collapse and degradation, making the ecosystems more vulnerable to invasive species. In the study it’s highlighted how wildfires, rising temperatures and logging are contributing to trees and plants wilting and dying in the Amazon.
- In the Amazon invasive grasses are catalyzing this collapse by fueling wildfire, preventing native regeneration, eventually contributing to higher CO2 emissions, he explains.
- Paradoxically, increasing levels of CO2 in the atmosphere supports woody plants in place of grasses across savannas in the greater tropics. Here, woody invasive plants are benefitting from CO2, fire-suppression, and loss of large herbivores.
The good, the bad, and the novel
According to the two main researchers behind the study it is important to separate alien species with neutral and positive impacts from those that have undesirable impacts – which we often call invasive.
- While invasive species need focused management, other more neutral alien species can sometimes offer a positive ecological role, particularly when native species are struggling amidst a changing climate or have been removed by people, professor Jens-Christian Svenning explains.
Global climate changes combined with these alien species are leading to new ecosystems developing around the world. These ecosystems may be more stable in a climate of rising CO2, heatwaves and pollution. And alien species in these ecosystems can be prevented from becoming invasive by bringing more biodiversity, which is able to naturally control alien populations.
- Alien plants are often seen only as a threat, but in reality it’s more complex. Most alien plants are neutral, and some can even play positive roles – especially as climate change pushes ecosystems toward collapse. Understanding both the risks and the buffering potential is key for smart management, explains Jens-Christian Svenning.
No easy fix
But managing forests and savannahs in a smart way isn't easy. In many areas where invasive plants take hold, it’s often caused by human activity such as land-use change – and that is not always possible to control, explains Ninad Avinash Mungi.
- The problem with managing widespread invasive plants is that it’s very costly to completely eradicate them, and the outcomes are uncertain, as the species can often re-invade.
- Since invasive plants have spread across the natural areas, they simply can’t just be bulldozed, as that can have several undesirable impacts on the ecosystems. A sustainable alternative is urgently needed.
One such solution is what the two Danish researchers call Nature-based Solutions for Biological Invasions. A way of managing nature that they recently proposed in the scientific journal People and Nature.
Here they list several mechanisms across ecosystems to utilize to naturally control invasive plants. One example is to introduce large herbivores like buffaloes and elephants into floodplains and savannas. They will naturally restrict many invasive plants from spreading.
In some places though, invasive plants may have already replaced native species – or management resources are unavailable. In such areas, local communities are often compelled to accept invasive plants and adapt to take advantage of the invasive plants.
- In many places across the greater tropics, the locals have started using invasive plants like Lantana to build craft and furniture, Prosopis juliflora for biochar production, and water hyacinth for commercial products. They have, in other words, adapted to a new reality, says Ninad Avinash Mungi and continues:
- Both our studies emphasize the importance of working in partnership with local communities when trying to manage invasive plants. Local communities are often more aware about the local ecosystem and about creative ways of management.
Method of Research
Systematic review
Subject of Research
Not applicable
Article Title
Alien plants and novel ecosystems in the Greater Tropics
