Ankila Hiremath writes from over 20 years of experience of working in invaded landscapes across different ecoregions in India.
Tamarind and potato, pineapple and apricot, and even the rubber tree—plants that are familiar (and some, even essential) parts of our everyday lives—are amongst the 2000 or so alien plant species found in India. Alien plant species are those that have arrived on our shores or been carried across high mountain passes over the past many centuries. Many of them were brought by people because of their familiarity with them (as crop plants, for fruit or fibre, or as ornamental plants). Others probably arrived accidentally, as contaminants stowed away with deliberate introductions.
The list of alien plants also includes Lantana (Lantana camara), Congress Grass (Parthenium hysterophorus), and Vilayati Kikar (Prosopis juliflora)—plants that are again familiar to us, but for a very different reason, their ubiquity. They can be seen along roadsides and in empty city lots, fields and pastures, forests, and almost anywhere in the country. This second category of alien plants is known as invasive alien plants and makes up about a tenth of our alien flora.
What distinguishes an invasive plant from a non-invasive plant, and why do we care?
Invasive plants are a growing concern because of their impact on biodiversity, ecosystem services, and human well-being. They tend to be widespread, and this spread is typically at the expense of other native biodiversity. Many of these plants are unpalatable or even toxic for herbivores. As they spread, they alter habitats for wildlife—reducing the availability of palatable forage and altering the habitat structure. An example is Lantana, which has been linked to increased negative interactions between people and wildlife, because large herbivores in Lantana-invaded habitats are driven to seek forage in crop fields.
Invasive plants such as Alligator Weed and Water Hyacinth choke the surface of water bodies. In the process, they can negatively affect fish populations and interfere with navigation, thus affecting livelihoods and the well-being of traditional fisherfolk.
Then there are other invasive plants that alter ecosystem services. The Black Wattle, which has replaced the unique species-rich high-elevation grasslands in the Western Ghats, affects eco-hydrological processes, increasing flood risk during high rainfall and reducing streamflow during other parts of the year.
How do some plants become invasive?
Most introduced plants remain confined to gardens, plantations or farmlands—the purposes for which they were originally introduced. Occasionally, a plant might escape cultivation if its seeds are borne away by the wind or by a bird or animal attracted to its fruits. But it may take many such seeds to escape before a few can establish elsewhere without human help. Even so, these plants need to form the right alliances, for example, with native pollinators to be able to reproduce in the wild and with native frugivorous dispersers who could then help them spread out.
Not all plants are able to do all of this. But some may be better endowed than others. Those that are unpalatable to native herbivores or those that are able to resprout in addition to reproducing from seeds may be better able to establish wild populations. Some others may be genetically predisposed to evolve rapidly and adapt to the local conditions.
This process, from escape to invasiveness, can take decades. Therefore, it is perhaps not surprising that many of the plants we know as invasive species today arrived a century or two ago during the colonial period. But that is not always the case. Senna spectabilis, for example, which has rapidly become an invasive species in the forests of the Western Ghats over the past decade, was only introduced to India as an ornamental avenue tree relatively recently, post-independence. And plants continue to arrive even today—a majority as garden ornamentals, which is a dominant pathway for plant introductions in India and elsewhere.
What can we do about invasive plants?
Given this time lag between a plant’s introduction and its emergence as an invasive species, most invasive plants go undetected until they are suddenly everywhere. They may remain inconspicuous for decades before they start getting noticed. A study some years ago used herbarium records to reconstruct the history of Lantana’s spread across the Western Ghats. It showed how there were only a few scattered records of Lantana in the Western Ghats for well over a century after it first arrived, and then, within a few short decades, it had spread all across the region.
Knowing what we do about invasive plants, one way to try and detect them before they become widespread is to monitor and map them systematically. A recent citizen science effort to map invasive plants attempts to do just that. An added benefit of such mapping can also be to help us prioritise habitats that are already affected by widespread invasive species and are in need of management.
But knowing which are the priority invasive species and habitats to manage is only the first step. What then? How do we remove invasive species and restore habitats? This is easier said than done for several reasons.
One, in some landscapes, especially human-dominated landscapes, people have adapted to the invasive species, finding ways to use it to mitigate its negative impacts on their livelihoods. An example comes from Ashoka Trust for Research in Ecology and the Environment (ATREE)’s work with artisans in South India, who have been trained to make furniture using Lantana. Another example is large-scale charcoal-making using Prosopis juliflora in several arid regions of the country, where the invasive tree has taken over agricultural lands or replaced grasslands. In such situations, given these novel livelihoods that have emerged from the use of the invasive plant, its removal may need to be done selectively—in the case of Prosopis, for example, one potential option might be to restore the best grassland areas in the landscape while retaining some Prosopis-dominated areas from which people could continue to make charcoal.
Two, invasive species removal needs to be done gradually, especially where they have become widespread and dominant. Drastic removal could cause large-scale disturbance in the form of soil erosion or by disrupting populations of birds or butterflies that have come to rely on the plant’s fruits or nectar.
Three, many invasive plants re-grow rapidly, either from stem and root fragments that get left behind or from seeds buried in the soil. Thus, invasive species removal is seldom a one-time effort. It needs monitoring and repeat removals over several years, making it time and resource-intensive. The Shola Trust and The Real Elephant Collective have been exploring ways to make invasive species pay for their own removal, in a sense. Working with tribal Lantana artisans, they have crafted herds of life-size Lantana elephants. Apart from providing livelihoods to communities, these elephants have travelled the world, drawing attention to the impacts of Lantana on native habitats, especially on people's coexistence with elephants and other wildlife, and raising resources to mitigate these impacts. They are also making efforts to see whether there could be a market for the tons of Lantana biomass removed from invaded landscapes (e.g., to make fuel briquettes).
Finally, if we systematically apply ourselves to the task, it may be feasible to completely remove invasive plants and restore habitats where the invader is not yet widespread. An example would be the case of Senna spectabilis, which is still only confined to a certain section of the Western Ghats, even though a fairly large section. But in the vast majority of invaded habitats that we are concerned about, the particular invasive species are now widespread, and restoration can be localised, at best. Beyond these localised efforts, we may have to accept that these entirely new assemblages of native and alien plants—so-called ‘novel ecosystems’—are here to stay. This is part of a great global churning that we have brought about, and continue to do, which has vastly accelerated the movement of species across the globe by orders of magnitude more than would be expected in the natural course of biogeographic events. Climate change is likely to exacerbate this in times to come as species migrate to newer, more hospitable environments.
It is for these reasons that we need to redouble our localised efforts at restoration in alien-species-invaded habitats to which we accord high value, whether for conservation or the communities that depend on them or both. Alien plant invasions are often a function not just of a species’ invasiveness but also of an environment’s vulnerability to invasion. Invasive species have a competitive edge over native species—many are liberated from the herbivores or insects that would have eaten them and kept them in check in their native environments.
However, the presence of invasive plants in a habitat may also be a sign of an underlying malaise. The cause could be the overgrazing of a habitat, undermining the ability of native species to regenerate, or the fragmentation of a landscape by multiple linear intrusions that provide easy conduits for the arrival of invasive species. The cause could also be a change in local hydrology due to the damming of rivers, making it less hospitable for native plants and creating an opportunity for alien species to establish. Or it could be the cessation of fires in savannas that have historically burnt, where fires helped to keep out invasive species. Restoration, then, needs to be about removing invasive species, and equally, understanding the possible underlying causes that may have aided in their establishment and spread, and trying to address them to make habitats less invade-able.