One of the greatest threats to the world's biodiversity - the richness of its wildlife - is the spread of alien species that have been deliberately or unwittingly introduced into new habitats where they can thrive.
In many ways, the movements of plants and animals around the world by human activity has been an inevitable consequence of the globalised economy in which we now live. Without such movement, we would not enjoy many of the benefits of modern life - but the consequences have been severe for fragile ecosystems.
What makes an alien species able to invade and then damage the biodiversity of its new home ultimately comes down to its ability to outperform resident lifeforms. Alien animals and plants become invasive and damaging because they can grow faster, breed more profusely, disperse more widely, tolerate a wider range of conditions or simply grow bigger than the resident species of the invaded ecosystem.
"They are called echizen kurage and they sound like monsters from the trashier reaches of Japanese science fiction.
They are 6ft wide and weigh 450lb (200kg), with countless poisonous tentacles, they have drifted across the void to terrorize the people of Japan. Vast armadas of the slimy horrors have cut off the country's food supply. As soon as one is killed more appear to take its place.
In some places jellyfish density is reported to be a hundred times higher than normal. Worst of all, no one yet understands why. One theory is that global warming is heating up the seawater and encouraging jellyfish breeding."
(Cyber Diver News Network)
But of course, there is another factor at work in deciding whether an introduced species becomes invasive. It is whether the animal or plant in question is able to form some sort of close association with man - arguably the most destructive invasive species on earth. After all, the oldest invasive species are the animals and plants that have been either domesticated by humans for many centuries, or, in the case of the mouse and rat, have lived in close proximity to man.
Cats, pigs and goats are especially destructive when introduced into remote islands, such as the Galapagos. Distant islands are vulnerable because life on them has evolved largely in isolation from the rest of the terrestrial world. The consensus is that island species tend to be naïve in terms of Darwinian competitiveness, having not been subjected to intense competition.
Islands exist on land as well, if you are a fish. Lake Victoria in Africa has been largely isolated from the rest of the aquatic world for thousands of years, which accounted for the 400 or so species of cichlid fish that had evolved there. It was the deliberate introduction of the Nile perch that abruptly upset a fine balance. The perch has eaten its way through half of the 400 or so species of indigenous cichlids.
Giant African snails Barbados
Snails are rarely regarded as the most agile of creatures but one species, the giant African snail, has proved to be a remarkably intrepid traveller. Originally from east Africa, this large gastropod has turned up in countries as far apart as Thailand and Brazil. Most recently it sneaked into the Caribbean island of Barbados, probably on a cargo ship, much to the consternation of local farmers who say it is wreaking havoc on the local cane-sugar and banana crops.
The secret of the giant African snail's ability to travel lies in the peculiar way it incubates inside its shell during food shortages, known as aestivation. The snail can incubate for years until the moist, warm conditions it favours return. Tourists to east Africa have often picked up seemingly empty shells and taken them back to their country, not knowing that once warm the snail will emerge from its shell, feeling particularly hungry.
Cane toad Australia
In 1935, 100 cane toads were introduced to Australia following reports about their role in controlling beetles in the sugar cane fields of central and South America. They quickly multiplied and now number more than 100 million - their leathery skin and high-pitched mating call can be seen and heard across New South Wales and the Northern Territory. However, the voracious appetite and ducts full of poisonous bile that make the cane toad such an effective predator have had a disastrous effect on indigenous fauna in Australia. A Northern Territory politician sparked controversy last year when he said a cricket bat was the best weapon against the toads. Animal rights groups were aghast, instead recommending they be frozen. Meanwhile, Western Australia has "active control and management" plans to stop the toads crossing the border. An ingenious entrepreneur is now selling assorted cane toad leather products, including purses, mobile phone cases and caps via the internet.
Tiger mosquito Spain
The aggressive Asian tiger mosquito that invaded Spain two years ago has established itself in Barcelona where it is "spreading like an oil slick" and set to expand throughout the country, scientists warned yesterday. Aedes albopictus, 5mm long, with a white-striped body, long legs and vicious sting, "cannot be eradicated", admits Roger Eritja, a biologist from Barcelona's Mosquito Control Service. The creature travels by hiding under car seats, an ideal cool and sheltered habitat and at the end of its journey it seeks pools of stagnant water. Such haunts are the perfect humid habitat for eggs to grow into larvae and pupae, a process that takes about a week. Old tyres are a favourite incubation site, and a Barcelona tyre-recycling company was recently found to be infested. Catalans are being urged to discard containers that might harbour stagnant rainwater. But scientists warn that only the country's desert regions are likely to escape the plague.
Nomura's jellyfish Japan
They look more like something out of Doctor Who than the jellyfish seen in European waters. But since last year Japan's coastline was invaded by the massive sea creatures which can grow up to 6.5 feet (2 metres) wide and weigh up to 450lb (220kg). The creatures, which are normally found further south in the waters off China and Korea, have caused problems with local fishermen by clogging their nets and poisoning their catches. One theory is that the plague has been caused by heavy rains which swept the giants from China's Yangtze river delta to Japan, but others believe that, because of global warming, seas have been warmed and are better suited for breeding jellyfish. In addition to this, scientists blame over-fishing of natural predators that feed on jellyfish and pollution along the continent's coasts. Human injury is rare but a few unlucky swimmers have been killed.
For a long time, the name yellowjackets referred simply to Ohio's state basketball team. Now the mere mention of the insects is enough to bring Americans out in goosepimples. Giant yellowjackets, or German wasps (Vespula germanica), introduced by early European settlers, are invading the Deep South. In previous years, nests have been rarely larger than a basketball, but this new strain of yellowjackets are so adept at home improvements that nests the size of a Chevrolet, above, have been found. The largest nest to date was found in Alabama this summer. At first no bigger than a car tyre, seven weeks later, it had expanded to the size of the barn. Such a nest would house as many as 100,000 workers plus multiple queens. Mild winters are blamed.
Nile perch Africa
The introduction of the Nile perch to Lake Victoria in the 1950s has become an ecological cautionary tale, after the silver- skinned predator decimated hundreds of native fish species. Best chronicled in the award-winning documentary Darwin's Nightmare, the perch has stimulated a fishing industry worth more than £100m a year, but the arrival of cash and regular flights prompted an arms industry that has spread conflict within the region. The Nile perch (Lates niloticus), a species of freshwater fish in the Centropomidae family, is now found in Lake Chad, Congo, the Nile, Senegal, Volta, Lake Turkana and other river basins, but overfishing has driven stocks in Lake Victoria low enough for native species to make a comeback.
Water hyacinth Africa and Asia
Water hyacinth (Eichhornia crassipes) has caused havoc in more than 50 countries since being exported by Brazilian horticulturists in the late 19th century, initially to Africa and then Asia. The floating weeds, which flourish on tropical lakes and waterways, have one of the highest growth rates of any plant in the world and are able to double their population in just 12 days. Vast blankets of hyacinth starve the water below of oxygen and sunlight, decimating fish stocks. The thick, tightly entwined mats render shipping all but impossible and governments around the world have spent vast sums of money trying to control the weed with varying degrees of success. One of the most effective and inexpensive ways has been to introduce hyacinth-eating weevils.
Harlequin ladybird United Kingdom
First introduced to North America from Asia to control plant pests, the harlequin ladybird made its British debut in September 2004 and has since established colonies in Derby and the South-east of England. Harlequins are such effective aphid predators that they pose a serious threat to the survival of Britain's 46 resident species of ladybird. With a longer reproductive period than other species, their larvae can be found as late as October, long after most other ladybird species have gone into hibernation. Their propensity to tuck into grapes has caused havoc in French vineyards. The rotund brown-legged specimen is easily recognisable by its colour; it is most commonly orange with 15 to 21 black spots or black with two large red spots.
© 2006 Independent News and Media Limited