Think of an invasion, and the picture that comes to mind is usually that of planes and tanks, machines and men, shells and gunshots. But animal and plant species also run amok, especially when they enter an ecosystem where they didn't previously exist. After habitat destruction, NITIN SETHI tells us, invasive alien species are the biggest threat to world biodiversity
Invasive Alien Species
70 years ago, a species called the yellow crazy ant (Anoplolepis gracilipes) turned up at the Christmas Islands off Australia, perhaps stuck to a piece of timber. For a long time, it remained dormant. Then in the mid 1990s, its population began to explode. Yellow crazy ants began to form super colonies, some as grotesquely large as 700 hectares (ha). The species swarmed the island, colonising -- by 2003 -- 2000 ha of tropical forest. Literally chewing up all the litter and canopy fauna (spiders, insects, birds, mammals and reptiles), in the last five years the crazy yellow ant has also devoured 20 million red crabs. Now, Christmas Island's red crabs are a decimated species; the forest, robbed of all its pollinators, is going the way of dead wood. And the crazy ant is a mere metaphoric drop in the roiling ocean of biological invasions swamping countries.
A biological invasion occurs when an animal or plant species expands into an area it hasn't previously occupied.Such invasions alter the ecosystem, usually for the worse. Invasive species — defined as those that produce fertile offspring in large numbers at a considerable distance from the ecosystem they originally belong to — are today the second biggest threat to biodiversity; after habitat destruction. While the movement of plants and animals across habitats and ecosystems is a natural process — ecosystems are inherently dynamic, losing some species, gaining others — it is the constantly accelerating rate of invasions that today looms menacingly over the world’s economy. Increased human movement, domestic and trans-national, has caused such acceleration. If many species are inadvertently carted off to new habitats, others are deliberately introduced, for profit or even vanity: US nursery catalogues, for example, offer nearly 60,000 plant varieties for sale, often to the global market via the Internet. A study conducted by David Pimentel and others at the College of Agriculture and Life Sciences, Cornell University, US shows that over 120,000 non-native species of plants, animals and microbes have invaded just six countries — the US, the UK, Australia, South Africa, India and Brazil. Extrapolating this figure, the study concludes that 480,000 alien species have been introduced into ecosystems the world over. While not all alien invasives are harmful — only think of rice or wheat — the study calculates such species today cause more than US $314 billion worth of damage every year (see table: The alien invasive bill). It is an approximate cost, for it is impossible to quantify, or substantiate, damage by invasives. This is also because only some 1.5 million species of the estimated 14-45 million species on earth have been identified. What all has been lost? It is not known.
What we do know
The loss to agriculture and soil productivity is well known. According to A S Vastrad of the University of Agricultural Sciences, Dharwad, the serpentine leafminer (Liriomyza trifolii) — introduced into India during 1990-91 through imported chrysanthemum cuttings — feeds on several economically important crops: cucumber, snake gourd, bottle gourd, pumpkin, watermelon, beans, palak, gerbera and marigold. Then there is the spiralling whitefly, (Aleyrodicus disperses). A native of Central America that rapidly spread westward across the Pacific and southeast Asia, and entered India from Sri Lanka in 1994, it predates upon more than 150 species including fruit plants, vegetables, avenue trees and hedge plants. It has already wreaked havoc, Vastrad notes, on 72 plants belonging to 38 genera. The invasion of the coconut mite (Aceria guerroronis) causes damage worth Rs 200-250 crore annually in Kerala. Forests lined by lantana (Lantana camara), waterbodies taken over by the water hyacinth (Eichhornia crassipes), fallow lands colonised by the mile-a-minute weed (Mikania micrantha), even houses infested with rats…the list is endless, and knowledge of the impact of alien invasives is limited. Nevertheless, ecological scientists have begun to crack a hitherto puzzling question: what makes a species invasive in a new habitat?
What makes a species invasive?
“Asuccessful invasion is a rare event,” says Suresh Babu, a researcher at the School of Environmental Studies, University of Delhi. An accepted thumbrule is that only one in 10 introduced species become naturalised, and only one in 10 among the latter actually turn invasive . This is because, as Babu puts it, “A species arriving in a new community faces a series of filters.” (see flowchart: What it takes to be invasive) Invasion usually occurs in three stages. A species is first transported to a new habitat. It must then establish itself there: a base population must manage to successfully reproduce itself. Naturalised in this way, the species population explodes and so turns invasive. Even so, what makes for a successful invasion? Out of 250 species in the genus Mikania, how is Mikania micrantha capable of smothering new habitats? Imported and introduced, reportedly, in West Bengal in the early 20th century, it was initially used in Assam to cover airstrips during World War II. It then moved from there and now is the biggest threat to tea plantations, apart from badly affecting pineapple, banana, ginger, acacia and rubber plantations. How is it able to do so? For one, this mile-a-minute weed could possesses what scientists call phenotypic plasticity. As O R Reddy, joint director, plant pathology, plant protection quarantine, Union ministry of agriculture, explains, “It helps the invader to have resilience to environmental stress, the ability to adjust to different environmental conditions like prosipis juliflora displays. It shows wide range of spread across India.” Greater genetic variability also helps. As Babu outlines, a plant must have the following traits to become invasive: It has long-lived seeds for discontinuous germination It grows rapidly from vegetative to reproductive stage It is capable of very high seed output when environmental conditions are favourable It produces seed continuously throughout the growth period and in a range of environmental conditions It is built to disperse seeds over short and long distances It has a strong potential to compete with other species Ecological paradox Usually, if a species is present in small numbers, its breeding options are low. This genetic bottleneck might cause it to become extinct. But invasive species manage the bottleneck with flourish, and after establishment can explode in numbers. New research published in the journal Conservation Biology suggests a factor called propagule pressure might be crucial to “invasiveness”: the number of individuals introduced, and the number of times a population is released into the new habitat. But how is a species able to outperform locally adapted, indigenous species? R M Callaway, in a paper published in the journal Science, provides some answers. First, some species may be intrinsically better competitors because they evolved in a more competitive environment. Second, the absence of enemies — say, herbivores in the case of plants — gives the non-indigenous species more resources to grow, reproduce and eventually out-compete native species. In this context, the case of the Chinese tallow tree (Sapium sebiferum) is extremely interesting: as it evolved in its introduced range, it began to show competitive ability far greater than it exhibited in its original habitat. Indeed, invasive species can create and manage brilliant strategems to beat the competition. When a pathogen’s host is already there, as mosquitoes were for the West Nile virus, it can tip the odds in favour of the invader, says Daniel Simberloff, professor at the University of Pennsylvania, USA.
But no two invasions are alike
A species may thrive because its natural enemies don’t exist in the habitat it has arrived into. It may be adept at colonising a disturbed area, or quickly take over the fresh dirt turned up at a construction site, or a roadside. Some species sit around innocuously for years, only to then unleash themselves upon the ecosystem. The rogue Lantana camara exudes poisons through its roots, killing off its competitors. It also uses the colours of its flower petals — yellow and red — to advantage. As Uma Shanker, professor at the University of Agricultural sciences, Bangalore, explains, “Butterflies have a very high fidelity in moving from one flower type to the other. A butterfly visiting a red flower may not visit a yellow flower, but would only visit another red flower. Adapting to avoid this phenomenon, different lineages of Lantana have evolved.” Other invasives go piggybacking. In the eastern US, non-native variety of fish aid bullfrog invasion by eating native dragonfly nymphs that would normally prey on bullfrog larvae. Indeed, evidence of such “aiding and abetting” between two nonnative vertebrates is conclusive, according to a US Geological Survey study published in the journal Ecology Letters. “Gauging the potential invasiveness of species is usually based upon comparing geographic origin, climatic and latitudinal ranges, taxonomic relations, past invasive performances, and assessing their reproductive biology and general ecology. But [such studies] are often erroneous because we do not appreciate the role of mutual interactions between species,” says Uma Shanker. Shanker’s colleague K N Ganeshaiah has recently completed a study to predict the movement of the deadly woolly aphid (see box: Wanted: more predictive research).
It isn’t only species traits
Sucessful invasion depends not only on the traits species have, but also on how susceptible the habitat is. Generally, islands face the highest risk. “Three generally accepted models to understand this susceptibility have emerged — the fluctuating resource availability model, the enemy release hypothesis and Niche opportunity model,” points out Babu. The first proposes that invasiveness depends on excess resources (energy) being available to be consumed. Such availability fluctuates over time; so, therefore, does a habitat’s vulnerability to invasion. The enemy release hypothesis suggests that when a plant species is introduced to an alien habitat, herbivores and other natural enemies pay it a little less attention. So it is able to literally bloom. This hypothesis is predicated on three principles: natural enemies are important regulators of populations; they impact native species more than exotics; and plants capitalise on less regulation by the enemy. Lastly, the Niche opportunity model defines conditions that promote invasions: interactions between resources, natural enemies and the physical environment, and how these vary in time and space.
Whatever the path, an invasive plant invariably alters the habitat, affecting its health. It draws in more nutrients and moisture, so out-competing native species. It paves the way for further invasions by microorganisms, agricultural pests and human pathogens. The worst cases result in large-scale environmental transformation. Chemicals released from their roots or leaf litter may change the soil’s chemistry; the chemical nature and decomposition rate of the leaf litter and other dead parts may alter the habitat’s nutrient cycle. “Invasion by fast-growing tree species in catchments as well as aquatic weeds in water bodies drastically reduce the flow of streams and rivers, as in the case of water hyacinth which has infested water bodies in Assam alone,” says R Ramani, senior scientist, Directorate of Biological Control, Bangalore. Animals can be as incapacitating. The zebra mussel is today considered one of the worst aquatic invaders in the US. Originally found in Russia, it came to the US floating in the ballast water of a transatlantic freighter. In less than 10 years, it has spread to over 100 lakes. Able to live and feed in different aquatic habitats, this mussel is a prolific breeder (each female produces one million eggs per year). Its larva is the size of the diameter of a human hair, and so can spread easily via water currents. Adult zebra mussels are larger than two inches in length, and attach themselves to hard objects. They often stick to boats, and so travel from one water body to another, clogging municipal, industrial and power plant water supply systems, damaging boat engines and fouling boat hulls, killing native molluscs and eliminating fish spawning beds. Damage by invasives is both ecological and economic. It’s a challenge the world is waking up to, slowly.
Than regret loss that happens in millions
Last year, a cricketing row broke out between New Zealand and India, when a member of the Indian team was fined for carrying soiled shoes. For India, it was cricketing pride at stake. But New Zealand customs authorities were merely following quarantine regulations. The shoes could have carried a possible invasive. For such species, the cliché — prevention is better than cure — works. It’s something countries like Australia, New Zealand and the US have learnt after bitter experiences. They now accept that intercepting potential invasives is the key to a more cost-effective and efficient way of controlling these noxious invaders. India, on the other hand, has just taken off the starting line. It has only recently updated its rules to monitor the import of seeds, plants and plant products for invasive pests and weeds — the Plant Quarantine (Regulation Of Import Into India) Order 2003. In this order, the government has also introduced a mandatory Pest Risk Analysis for any new product or an already imported product being sourced from a new country. The analysis tries to predict the possibility of a consignment carrying a potential pest or weed entering the country may of the product. “The depth to which the analysis is done depends upon how great the economic benefit is from the product,” explains Reddy.“For example import of pulses is necessary because India is not self-sufficient. So we right away make fumigation mandatory. But if you are importing strawberry, we shall conduct detailed analysis because many viruses, fungal pests, bacteria are involved.” Such analysis forms the bedrock of invasive detecting techniques in alert countries like New Zealand. But rules alone do not work. India’s quarantine workforce comprises a mere 200 technical staff, only 5 per cent of that in the US and about 7 per cent of the Chinese (a country which has very recently begun to secure its borders against invasive species). Reddy admits such a tiny team cannot monitor India’s large territorial and marine borders, and the airways. The laws, too, are lax. “We are not even allowed to monitor the discharge of ballast waters from ships, unlike in other countries,” he says. While it is aware of the threat, India does not guard its environment against invaders.
Once established and naturalised, detection and prevention before the species becomes invasive is next to impossible. The only hope is to control contamination, or completely eliminate the threat. Options to curtail spread are limited. In the case of weeds, mechanical removal is the simplest way. But it rarely works. You can keep uprooting Lantana; it will come back. Even controlled fire does not work for many weeds. Indeed, the effect is often a fire-cleared field, now made ready for the invasive to take over. If the pest is not a small insect but a large mammal, the tactic is simple — shoot to kill off. Take the case of spotted deer, introduced to the Andaman Islands between 1915 and 1930. In the absence of a predator, their spread has been spectacular and debilitating, They have browsed the forest floors so rampantly that the ecosystem has changed. The undergrowth is missing and some tree species have stopped regenerating. “The changes could be irreversible even if the feral deer is now eliminated, studies conducted in other countries on the deer show,” says Rauf Ali, an ecologist with the Foundation for Ecological Research, Advocacy and Learning, Pondicherry. But India’s wildlife laws put the deer into schedule I — a list of animals most worth protecting. Ali and other ecologists have been begging the Indian government to allow the deer to be hunted, but the animal cannot be touched.
What about biocontrol?
There are two more complicated and sophisticated methods to uproot invasives — spraying chemicals or using biological control agents. The former is today a huge global industry; the latter is finding its feet in scientific labs. Most countries have accepted chemical spray in the case of agriculture to control weeds and pests today. Its indiscriminate use has had horrific health impacts. Pests and weeds have mutated and become more persistent. Biological control is touted as a less dangerous option. But it has not been successful in actual field conditions. The biological control idea is simple: find a predator of a pest or weed in its original habitat, and introduce it in the invaded region. But some tough parameters have to be met. The predator needs to be host-specific, for instance, and not predate on other plants or animals. In 1889, the Australian vedalia lady beetle was brought to California, USA, to control the cottonycushion scale insect threatening citrus orchids. The beetle completely eradicated the pest. In India, too, the introduction of Dactylopius opuntiae to control the prickly pear has been a great success (see: India: Bio-control). But ascertaining such host-specificity is difficult. Numerous examples exist of biocontrol agents themselves becoming pests. The most infamous one is that of the release of the predatory snail Euglandina rosea to control the alien African giant snail in the Hawaiian Islands. E rosea ate up most of the 41 indigenous snail species; the African giant snail still crawls over Hawaii’s forest floors. “India lacks the ability to monitor the release of biocontrol agents. We can experiment, but unless people are involved in the process of recovery and monitoring the real impact of a biocontrol agent in the heterogeneous environment of a tropical country like India, it is difficult for biological control to actually work,” says Reddy.
What will work?
India cannot replicate the governance and monitoring that developed countries like New Zealand and the US practice. The latter now has a federal umbrella agency to sniff out invasives, one empowered with a huge budget and stringent regulations. New Zealand, especially vulnerable because it is an island, has tight quarantine regulations and constant monitoring. “In countries like New Zealand and Australia an entire area can be sealed off if a contamination is reported,” explains S Ramani. India, heterogenous and complex, can hardly afford to take such measures. Thus prevention becomes paramount for this country, and therefore prediction methods, risk analyses and evaluations of potential invasives in tropical environments. But where are the taxonomists to do all this? Where is the original research on invasive alien species in India? The scattered bits and pieces of research carried out in different universities and some government laboratories is neither focussed nor organized. India needs a central and easily accessible reservoir of data, a directed campaign that builds on existing knowledge and does not repeat itself. “We have to generate our own data,” says Shanker. The same pest or weed can act differently in different climes: New Zealand’s research priorities cannot be India’s, too. Also, India cannot ignore the option of seizing some invasions as opportunities. Lantana basket weavers, of the Korava community of Pudhuputi village in Tamil nadu’s Dindgul district, have been using at least five more weeds besides lantana for more than 60 recorded years. While science from the top can assist, it is finally the people suffering the impacts of invasives who can create a credible force to fight this kind of terrorism. India needs to arm its communities, and must do it yesterday.
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