"We don't like to mention those snails here," says Vrinda, who owns an organic farm on the outskirts of Mysore. "They're a menace and they leave nothing untouched," she adds - referring to Giant African Snails (Achatina fulica) that have been playing havoc with the farms on the other side of Mysore. They breed in the neighbouring riparian areas, and stop over at the farms to feed. And feed they do. "No grains, no herbs survive. They eat leaves and even fruit. One day there is the coriander plant - the next morning, gone." Vrinda's farm has escaped the scourge so far. But it probably is only a matter of time.
Giant African Snails are among the 100 worst invasive species anywhere in the world. Transported across the world as escargot and as a source of protein during World War II, the species was then left behind with no controls over its habitation. Its population tends to explode soon after establishment in an area, and unless controlled immediately it can become highly invasive. In India, this species is now a pest, infesting broad leaf forests, grasslands, farms and gardens. Especially vulnerable are seedlings of herbs and vegetables, falling prey overnight to the voracious hermaphroditic mollusks.
The scourge of invasive species is probably the second biggest threat to biodiversity, after habitat destruction. Being mainly generalists, invasive species tend to be hardy, long-lived, voracious (in the case of animals), aggressively pervasive, and very resilient. Without natural predators or controls in the new land, they take over the ecosystem and compete with native species for nutrition, pollination, and survival. In the bargain, the native species could eventually be replaced by the non-natives which could not only severely alter, but may eventually take a whole ecosystem down. And as human travel and trade intensifies across the world, the risks from invasive species have also grown greatly. Animal and plant species are turning up in places far from their native homelands - either transported intentionally (as in the case of the giant African snails) or by accident - like a clutch of eggs that is transported with a sack of grain, or a species that clings to the hull of a ship. Weeds in one country reach another with imported wheat, and then take root and become all-pervasive.
"Invasives swamp a forest, depress reproductive output by robbing pollinators of native trees," he adds. Lantana for example is pollinated by butterflies, but so are other native species. The abundance of lantana - which tends to grow in clumps and crowds out native species even physically in our forests - results in a high recruitment of the pollinators and hence a lessening of the native species.
A walk through a portion of the Cauvery Wildlife Sanctuary near Malavalli in Karnataka makes the lantana scourge abundantly clear. But the very presence of invasives is a symptom of a larger problem - that of habitat disturbance. Invasives rarely pervade intact areas. It is when forests are cleared or fires come through, that invasives waiting for their chance to proliferate take over. And restoring the lost biodiversity often requires a long battle. "This problem of invasives has only been recently recognised in India," Shaanker adds. The impact is both hidden and apparent. Each displaced species could have enormous value. And a displacement by invasives could come at a tremendous cost.
At what cost?
Quantifying the cost of invasive species is not easy, this exercise has, moreover, not been carried out in India. We do not know how much biodiversity or forest products we have lost to lantana. We do not know how destructive the giant African snail has been - how much agricultural produce has been lost to the mollusk - and neither can we put a dollar value to the invasion of parthenium into our arable land.
The growth of the aquatic weed water hyacinth (Eicchornia crassipes) has led to loss of water in water bodies devoid of oxygen. In Bangalore alone, a study over 13 years has shown that the area covered by hyacinth has more than doubled, at the cost of agricultural, open and scrub lands, with about 60% of the water area reduced. Hyacinth has traveled from its native Brazil to over 50 countries, colonising water bodies and drying them up. Its luxuriant and rapid growth - apparently 2 plants could multiply to 120,000 over just 4 months - increases water loss through transpiration. This increase is about two to eight times that from free water surfaces. Apart from the obvious loss of a precious resource to these colonisers, the amount of money that municipalities spend on clearing it up all is also significant.
A mapping project using temporal remote sensing data has documented water hyacinth proliferation in 112 tanks around Bangalore. The water hyacinth areas have changed over the years - mostly increased - while the water in the lakes has drastically decreased. From December 1988 to March 1989 alone, for example, water hyacinth added 10-12 hectares to its name in Rachenahalli, Yelakanha, Hebbal and Nagavara lakes. Manual and mechanical removal has slowed the hyacinth growth (but not arrested or reversed it) at a huge cost to the water area that is soaked up by the weed. To a drought prone area relying on surface water and ground water, such invasions come at a devastating cost.
Only 2% of the original landscape in the Nilgiris has not been taken over - a 2-by-16 km stretch on a ridge. This thin edge will not be able to stand the onslaught of wattle gorse and broom, feels Seshan. Moreover, as the climate warms and the coolest places get warmer, these species from lower altitudes will invade the last remaining strongholds of the natives.
The invasion of the exotics holds another far-reaching consequence. "Wattle dries out ground water, marsh lands and wetlands. There is no spring water coming out of these parts of the Nilgiris anymore," warns Seshan. She lives and breathes these issues everyday in a bid for conserving native species and natural landscapes. It is only now that the forest department is recognising invasives as a real problem, and has invited Seshan's team to help hack out the invasives. But even that has to be done carefully, stresses Seshan, saying "Wattle is being removed cheaply, employing Sri Lankan refugees and other menial labourers, but they lack the knowledge and right expertise. They hack much too high. Next year, the same wattle comes back, behaving much worse than before and we have more work on our hands."
Responding to invasive species
Why should we really care? Shaanker believes that the ecosystem will sort the problem out with a new equilibrium until another perturbation occurs. He says, "There is nothing static about an ecosystem. If alien invasive species are there in one area, you will also see the ecological succession of species over time. Darwinian fitness will accommodate for these changes." This school of thought believes that over time, 'species equilibrium' will be attained.
However, Dr. Ankila Hiremath, a Fellow at ATREE, one of two people at that organization working on invasives, is not sure if such equilibrium can be reached. "The bottom line is that the rate at which the transformation is happening is much faster than historical and natural geological rates, and could have tremendous costs," she says. "Where native species are suppressed, we impact a whole ecosystem. Communities collect forest resources, carbon cycles are affected, water resources are affected. These perturbations are happening too fast to allow for the natural time for species equilibrium."
Seshan, who has done extensive work in Mukurthi National Park, agrees, adding a new dimension to the problem. "With highly fragmented 'pristine areas' the native species are even more vulnerable. Invasives take over any disturbed area and with the fragmentation the vulnerability increases manifold. Our native 'pristine areas' have been fragmented by hydel power projects and the plantation economy. What we are left with are pockets of natural or even just semi-natural areas. And even the forest department land surrounding Mukurthi NP can be denotified for hydel projects," says Seshan. The Nilgiri Wildlife and Environment Association's efforts to increase the NP for about 10-15 years have met with resistance, since the department wants to reserve the right to denotify the zone. She worries that once we lose the native landscape to invasives, they are gone. Do you want a landscape of soil-drying eucalyptus and wattle, or a natural one? "This question is not being asked hard enough," says Seshan.
But what can be done about this? Again, the opinion is divided. Shaanker believes that making the invasives work to the benefit of the local communities could be one way out. "Lantana cannot be controlled by physical or chemical methods. Those have been tried, and they have failed. If the invasive is not friendly to native biodiversity, you can control it by using it for livelihood," he says. Shaanker is pushing for this paradigm shift - if you cannot beat them, use them. "Hyacinth, for example, is a resource of immense value in Vietnam. It is not frowned up now." His suggestion to this effect was vetoed at a Cape Town conference but, he says, at the end of the 3-day conference some people were open to giving it a try. Hiremath, however, is not on board with this strategy. "Personally, I think the logic is flawed," she says. "If you value a species, it is tough to eradicate. Frankly, that is not the right way."
Picture: Lantana thrives in the Cauvery Wildlife Sanctuary, near Malavalli.
She makes an important point. Using water hyacinth for livelihood does not solve the fact that the plant sucks up all the water from catchment areas. How would you eradicate giant African snails from already infested areas, even if you wanted to use them as escargot? These are not easily answered questions, and definitely not easily implemented either. Shaanker agrees that going forward no other approach is feasible except disallowing further invasions. But how would you eradicate existing and aggressive invasives?
For some time, bio-control was thought to be the panacea. But it comes with its own set of problems. A case in point was the control of the giant African snail in the Pacific and Indian Ocean islands. A rosy wolf snail (also called a 'cannibal snail') was imported from the US (Florida), with the idea that it would act as a predator for the giant African snail. But the whole experiment went awry; the rosy wolf snail not only ignored the giant African snails, it preyed instead on native species of snails. In French Polynesia the native snails were wiped out, and in other places where the rosy wolf snail was introduced, like Hawaii, the extermination of the local species is nearly complete.
Such unexpected outcomes are a big risk; for bio-controls and effective integrated management to work well, extensive data and knowledge about the problems and the various vectors is necessary. But, in India at least, there is precious little data to go on - either to quantify losses, extent or assess the controls needed. Policy around this is not enforced, and indeed, even the few checks and controls that exist today are mostly only on paper. For example, one of the biggest causes for the movement of species is the exchange of ballast water from ships at ports. There is no monitoring of these exchanges and no enforcement of any regulations which could prevent the exchange of ballast water at ports (and instead do it at sea before docking which is a proposed rule in the US).
The Convention on Biological Diversity (CBD) makes eradication of invasives a priority area. What has already been done by way of these invasions is in many cases irreversible. What we have left is still precious. To sit by and watch our biodiversity exchanged for pests and weeds may be unforgivable; concerted eradication efforts and further prevention of this scourge of aliens is clearly an imperative. Whether we are up to this challenge will determine the habitats and species we leave behind for future generations.