The story of bioinvasion for beginners
it is a biological free-for-all -- a bedlam of alien organisms colonising ever newer territory (in the oceans, on the islands, everywhere) decimating the local species and wreaking ecological havoc. These organisms range from protozoa to fishes, from viruses to weeds, from human and cattle pathogens to forest diseases. This is a multi-dimensional menace that reduces the biodiversity of the invaded areas, creates economic losses in billions of dollars every year and destroys the cultures of indigenous people.
The bio-mix that has resulted from the bioinvasion has created problems that are difficult to deal with. So far different species had survived and prospered in their own so-called 'cloistered worlds' protected by mountains, separated by continents and oceans. The islands had their own special mix of species. Everything has changed with the exploding volume of migration, travel and trade. The result is that species, which had developed some resistance to familiar pathogens, are suddenly face to face with new, unfamiliar pathogens against which they have no resistance.
Human pathogens travel quite as readily as crop pests or weeds "and entire branches of humanity have fallen away as a result." The diseases brought into the Americas by European colonists provide one example. In the century after the conquistadors' triumphant arrival, nearly two-thirds of the Western Hemisphere's natives could have died from small pox, malaria and other diseases brought from Europe to which the natives had no resistance.
Intensifying global trade and travel have compounded the problem. One example is the Asian tiger mosquito, which has spread throughout the world and is responsible for carrying 18 viral pathogens that create deadly diseases like encephalitis, dengue and yellow fever. The bioinvasion is not limited to the spread of human diseases alone. It is responsible for extensive forest disease, destruction of crops and fisheries. Forests pests in the us are responsible for damage worth us $4 billion every year.
Introduced species have a way of reducing the local bio-diversity over the years. Leidy's comb jelly, which travelled in a ship's ballast water tank from its native Atlantic coast of the Americas to the Black Sea, caused the collapse of its eco-system and is poised to move on to the Mediterranean. The invaders flourish because of a complexity of characteristics described as 'weediness'. They spread quickly, multiply fast and prosper especially well in disturbed eco-systems. Animal 'weeds' have a great adaptability in their diet which help them survive in all kinds of melee. Rats, house sparrows, zebra mussels and water hyacinths are the best examples of weediness.
How the weedy species destroy the ecology of the conquered area is evident from the sad saga of Lake Victoria where an alien plant (water hyacinth) and a fish called Nile Perch, have wreaked great havoc.
As the 'Green Revolution' led to the spread of many plant weeds and plant pathogens, the 'Blue Revolution' brought through culture of exotic fish and other aquatic animals has serious implications. This, like the Green Revolution, was driven by the greed for immediate returns on investment -- the long term implication of which are yet to be understood.
Today, pathogens are being carried all over the world on the wheels of aircraft landing in different countries, in ships' ballast tanks, by human travellers, and birds and animals hitchhiking on ships traversing the oceans.
In earlier times, European colonisers were responsible not only for introducing new human pathogens to unknown areas but also introducing new plant species that threatened the local biodiversity. The case of eucalyptus is quite intriguing. In many areas where it was introduced by colonisers, it absorbed all the ground water and starved other plant species. Its gluttony for large doses of water is quite well known.
The ever-conquering empire-builders from Europe were avid introducers of new species in conquered territories."We have given the sheep to Australia; why have we not taken the Kangaroo -- a most edible and productive creature?" wrote Isidore Geoffrey Saint-Hilaire, the first president of the French Societe Zoologique d'Acclimitation. "New conquests of animals and plants will serve as new sources of wealth," argued the empire builders, thus laying the foundations of the biotic bedlam we are witnessing today.
Like the path to hell being paved with good intentions, the path to the ecological disarray was paved with not only the mundane motive of profit, but with the nobler motive of civilising and feeding the colonised people. "The white people put the baby monsters in the lake to help us", an old woman who had lived all her life on the shores of Lake Victoria, told a Dutch biologist. She was referring to the introduction of the Nile Perch which grows up to 200 kilogrammes. At present the perch itself is in danger of extinction as the fish on which it feeds are disappearing (see box: The war in water ).
The introduction of exotic plants has been instrumental in nudging the local plants and trees into extinction. Entire tracts of forests have succumbed to plant pathogens that came from nearby plantations of introduced trees. The ever-growing need for pulp to feed the paper industry as well as for timber has led to a situation where forests are being cleared to make way for thousands of acres of plantations. The disturbed forests recede, their bio-diversity is reduced. The plant pathogens for which they had some resistance, now travel to the plantations and destroy them. After doing that, their new strains with mutant genes come back to the forests which have no resistance to these new strains. The results: these forest epidemics destroy entire forest ranges quickly (see box: Eucalyptopolis ).
Even islands like Hawaii, protected from such invasions by natural barriers, have not been safe. About 17,600 species are thought to be native to the islands, and at least 9,000 to 10,000 of them are endemic (that is, species that are found nowhere else). Ninety one per cent of all native flowering plants, 99 per cent of molluscs and 98 per cent of insects are endemic. What makes Hawaii distinctive in another sense is that the species are getting extinct at an amazing rate. Over the last couple of centuries, 263 of its native creatures have become extinct and about 360 are on the endangered list.
Agriculture and logging have undermined Hawaii's ecology but the pressure on more than 95 per cent of Hawaii's endangered species comes from direct bioinvasion. Home to some of the most beautiful and rarest of birds, these islands are no longer the ideal place for these creatures. The reason: Hawaii's earliest human colonisers brought in with them the Polynesian rat, pigs and dogs, species that thrive on birds and their eggs.
Then came the mongoose, imported by colonists in order to catch the rats. The barn owl, native to Eurasia, was brought in to do the same. The mongooses, rats, cats and owls, too, predate on birds.
The pressure on birds was compounded when in 1820 a ship's crew came to shore to clean and refill mosquito-infested casks from their ship. The mosquitoes picked up a couple of exotic bird diseases -- avian pox and avian malaria -- from the birds and spread it among the native birds which had no resistance to them. These diseases, raging among the native birds, brought their numbers down sharply.
The above is just an overview of the situation which is desperate but not hopeless. The present rates of bioinvasion are as unsustainable as the present rates of deforestation or greenhouse gas emissions. Yet, it is possible to roll back the effects of bioinvasions over a period of time. Like the strategies to turn the tide of greenhouse gas emissions, a set of international laws have to be made and enforced.
In the struggle to halt bioinvasion and restore the disturbed ecology, cultural and attitudinal changes will be as crucial as technical skills. The value of the native flora and fauna has to be recognised by people and they have to learn to spot 'invaders'. Involvement of local groups in ecological conservation would be possibly the most significant factor in the end. Many people 'see' their neighbourhood, but few 'watch' it.
The idea is to 'read' the landscapes and identify the invaders. Conservation of the local flora and fauna and keeping a watch on bioinvasion could be the beginning of the counter-invasion strategy. Governments, as well as people, are entrusted with equal responsibility in this struggle against bioinvasion.
The article is based on the book,"Life out of bounds", written by Chris Bright, published by the Worldwatch Institute in 1998
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