March of the aliens

March of the aliens


INVASIVE SPECIES: The Indian gaur standing in an undergrowth of invasive parthenium. PHOTO Arati Rao

The vast deciduous jungles in the Nilgiris are green right now. In the middle of the monsoon, the undergrowth is thick, which is as it should be. Except that, the foliage is not what it once was. The understory today is nothing but dense parthenium, eupatorium and lantana. The latter branched, arched, and reached way above our heads at times, swallowing up even an elephant that was lurking in there. All the three plant species are invasive species in India – that is, they are non-native and ones that grow so thick and fast as to crowd out any native species. 

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 with increased human travel and trade 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 once purely ornamental lantana) 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. Why, not just in forests, but the extremely aggressive parthenium (parthenium hysterophorus) has run roughshod over all untilled arable land and disturbed ecosystems in the Deccan.

Scourge of lantana

Lantana (lantana camara) is native to tropical America, but about 650 different sub-species have colonised various parts of the world, becoming a significant weed. The British introduced lantana to India in 1807 as an ornamental plant for the Calcutta Botanical Garden. Then in 1850, it was introduced in Dehradun’s cantonment area. It is an extremely adaptable plant and can grow from just a bare stick. Today, it has spread all over India except in the Thar desert. Lantana 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 forests, results in a high recruitment of the pollinators and hence a lessening of the native species.

Symptomatic of a bigger problem

But the presence of invasive species is merely a symptom of a larger problem. For, invasive species rarely pervade intact areas where native species thrive. In the Vazhachal Reserved forest in Kerala, for example, there is still much primary growth. The banks along the single-lane roads are still fern-covered. There are no signs of lantana or parthenium here – yet. But there is a proposal to make that road a four-lane highway through the forest. It is when forests are cleared, roads are hewn, or fires come through, that invasive species waiting for their chance to proliferate, take over. Once lost, restoring the biodiversity often requires a long battle. 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.
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 or eupatorium, or how much arable land has been degraded further by parthenium.  

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 several 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 per cent of the water area reduced.

Hyacinth has travelled from its native Brazil to over 50 countries, colonising water bodies and drying them up. Its luxuriant and rapid growth, apparently two plants could multiply to 120,000 over just four 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.

This problem is not particular to India. Invasive species all over the world are wreaking havoc and huge amounts of money are being sunk into its control. The Working for Water programme in South Africa, for example, was born out of a desperate need to control invasive plants and save the water resources. The government employed poor people to do this at a cost of over $3 million. The success saw the programme grow over seven years to involve an outlay of over $57 million. But costs can go deeper than the obvious cost of removing the offending flora and fauna.  

The invasion of the exotics holds another far-reaching consequence.
Take for example the highly invasive wattle that was planted by the British to simulate Europe in the pristine grasslands of the high Western Ghats. Wattle dries out ground water, marshlands and wetlands. Recognising this as a serious problem, in some parts, the Forest Department in some places in the Western Ghats, has taken to removing the wattle. But even that has to be done carefully and needs skilled labour. If you cut it too high, it will come back stronger the next year, inflating costs and labour.  

Responding to invasive species

How one should respond to invasive species is a highly polarised debate. While one set of people believes that the ecosystem will sort the problem out, that over time, ‘species equilibrium’ will be attained, the other camp is not so sure. They believe that where native species are suppressed, the whole ecosystem is impacted.

Communities collecting forest resources are affected, as are carbon cycles and water resources. And, this camp believes, these perturbations are happening too fast to allow for the natural time for species equilibrium. But, maybe the question to ask ourselves is: do we want a landscape that is dominated by large swathes of monoculture – of invasive species, where once there was a biologically diverse landscape of hundreds of native species in just one sq km?

Both sides agree that it is an important question to ask and an urgent problem to address. 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. Efforts have shown that we can restore patches to native states with concerted thought and effort.

To sit by and watch our biodiversity exchanged for pests and weeds is clearly unforgivable; but whether we are up to the challenge to reverse what is happening and prevent further degradation, will determine our legacy.

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