by Richard Ingham
PARIS, Aug 29, 2012 (AFP) - Some aliens arrived as stowaways. Others were brought in deliberately, for fun or profit. And others were so tiny that nobody noticed them until way too late.
They became a nightmare. They killed and devoured natives, stole their homes, sickened them with pathogens.
Sci fi? No: the alien invasion is happening right now.
It could be occurring in your garden. In the forest where you like to feel in harmony with Nature. It is almost certainly unfolding on the farms which produce your food.
It's the tale of species that Mankind brings to new habitats where they spread uncontrollably, ousting endemic wildlife and becoming major pests.
"Invasive species have a huge impact worldwide. In some countries, the cost is astronomical," says Dave Richardson, director of the Centre of Excellence for Invasion Biology at South Africa's University of Stellenbosch.
The International Union for Conservation of Nature (IUCN), staging a conference in South Korea next month, says foreign encroachment is the third biggest source of species threat.
Take the American grey squirrel -- "a rat with good PR," say enemies -- which is displacing Britain's scrawnier red squirrel. Or the Burmese python, gorging on small mammals in Florida's Everglades.
Invasive species inflict more than $1.4 trillion (1.12 trillion euros) in damage each year, or five percent of global GDP, according to an estimate made 11 years ago.
"Those numbers are controversial because it's difficult to put finite figures on these sorts of things," said Tim Blackburn, director of the Institute of Zoology at the Zoological Society of London.
"But the impacts are pervasive and affect so many aspects of life. The cost has the potential to escalate as we take more species to more areas where they don't naturally occur."
Many costs are indirect. For instance, US farmers use truckloads of pesticides to control foreign weeds, while in central Europe, their counterparts are surrendering tracts of land to the giant hogweed, a toxic Asian shrub.
There is the bill from the European rabbit, introduced for food by British settlers to Australia and New Zealand only to become cursed for ravaging grasslands and crops.
In the southern United States, Asian carp were imported in the 1970s to help clean up algae in commercial catfish ponds. Flooding washed the carp into the Mississippi River system, where they now threaten commercial and game fishing in the Great Lakes.
Invasive species have followed Man throughout his odyssey.
Polynesians wiped out innumerable bird species as they island-hopped across the Pacific over eight centuries, bringing in rats that had stowed away on their ocean-going canoes.
The trend accelerated in the early- to mid-19th century.
European species were shipped out to the colonies in Africa and Australasia to provide food or company, and exotics were brought back to Europe.
"There was a great flowering of 'acclimatisation societies', which were specifically set up to introduce new species to areas around the world," Blackburn recounted.
"In fact the Zoological Society of London, the organisation I work for, envisioned a golden age where we would have herds of elands roaming over the south of England."
Faster travel -- steamship, then jet plane -- accelerated the problem as global trade really took off.
Headaches include the zebra mussel, which has infested US waterways after hitching a ride from Europe in ships' ballast water. Another is the verroa mite, reported in countries in three continents, which is wiping out honey bees that pollinate many crops.
Even tinier is the chytrid fungus being spread to wild amphibians through the sale of pet frogs and frogs for food. As frog numbers plummet, insect populations surge -- another hidden cost.
In many countries, national border controls are often lax and laws are riddled with loopholes because of interest groups that trade in non-native species, said Richardson.
He said he knew of nowhere that firmly applies "polluter pays" principles, whereby someone who introduces a pest pays the bill to get rid of it.
As for international cooperation, there has been progress -- for instance, in marine conventions which oblige ships to exchange ballast water in mid-ocean.
But "in many instances, treaties and conventions do not have teeth," said Richardson.
Eradicating these threats is costly and often impossible, for it requires lots of manpower, sometimes over many years. Introducing a predator animal or insect to attack the invader sometimes makes things worse.
Jean-Philippe Siblet, director of Natural Heritage at France's Museum of Natural History, said eradication had to be "smart".
Conservationists must distinguish between useful, introduced species and those that become a burden.
"It's the globalisation of nature, and we're going to have a hard time stopping it," he said.
Following is a snapshot of some of the most notorious invasive species:
- Rat (Rattus rattus). Originally a native of Indian sub-continent, the black rat, also called the ship rat or house rat, has spread almost everywhere, usually by hitching a ride on ships. Rats that crept aboard ocean-going canoes decimated island bird species as the Polynesians spread across the Pacific. Plants and other small mammals are also victims.
- European wild rabbit (Oryctolagus cuniculus): Introduced by Europeans to Australia, New Zealand, South Africa and elsewhere as a food animal. Now a pest that causes billions of dollars in damages to crops and native plants.
- Nile perch (Lates niloticus): Voracious freshwater fish that can grow up to 200 kilos (440 pounds and two metres (6.5 feet). Introduced to Lake Victoria in East Africa in 1954 for game fishing, the carnivore has contributed to the extinction of more than 200 endemic fish species. The Nile perch starred in a 2006 Oscar-nominated documentary, Darwin's Nightmare.
- Cane toad (Bufo marinus): Central American toad introduced to Australia in the 1930s in the belief it would eradicate beetle pests, and instead became a pest in its own right. Weighing up to 1.3 kilos (three pounds), the toad has poisonous skin that can kill predators such as snakes and freshwater crocodiles. Current total estimated at more than 200 million.
- Water hyacinth (Eichhornia crassipes): Aquatic plant of tropical South America that flourishes in warm climates in Central America, North America, Africa, Asia, Australia and New Zealand. The hyacinth grows in thick rafts, deoxygenating the water for other species and impeding water flow and navigation. In Africa, the economic impact may be as much as $100 million annually.
- Asian tiger mosquito (Aedes albopictus): Black-and-white striped carrier of West Nile virus, dengue, St. Louis encephalitis and a painful disease of the joints called chikungunya. Now a major worry in the United States, and a source of concern in Europe's Mediterranean rim. In both cases, the insect is believed to have landed in shipments of old car tyres, which retained pockets of moisture enabling it to survive the sea trip from Asia.
- Zebra mussel (Dreissena polymorpha): Small stripey freshwater mollusc native to southern Russia and the Caspian which has spread to parts of North America, Britain, Ireland, Italy and Spain by hitchhiking a ride in the ballast water of ships. Mussel infestations are a major cost for power stations because they clog coolant pipes.
- Burmese python (Python molurus): First found in Florida's Everglades swamp in 1979, where it may have been abandoned by a pet owner, the Burmese python took only 21 years to become an established species there. Lacking natural predators, the snake snacks on native birds, deer, bobcats and other large animals. Current estimate of its population run as high as hundreds of thousands. In August, University of Florida scientists examined a record 5.36-metre (17 feet, seven-inch) specimen that had 87 eggs.
- Gorse (Ulex europaeus): Thorny shrub native to northern Europe that has been introduced to many countries by farmers seeking cheap enclosures for grazing animals. It becomes a pest by displacing native and cultivated plants and acidifying the soil. Gorse now grows in the Caribbean, Latin America, Australia, New Zealand, South Africa, the United States and elsewhere.
SOURCES: The Invasive Species Specialist Group (http://www.issg.org/index.html); International Union for Conservation of Nature (http://www.iucn.org/); US Geological Survey (http://diseasemaps.usgs.gov/wnv_us_mosquito.html); US National Invasive Species Information Center (http://www.invasivespeciesinfo.gov/index.shtml); New Zealand Department of Conservation (http://www.doc.govt.nz/conservation/threats-and-impacts/); news reports