Originally from Asia, 1,200 collar parakeets now live in the Dutch capital, descendants of a small group whose owner, driven mad by their constant chatter, set them free in the late 1970s.

The parakeets are among many species -- from Asia and elsewhere -- that have relocated as increased world trade has facilitated the movement of "invasive alien species".

This biological globalisation is recognised as a major threat to biodiversity, second only to habitat destruction, and conservationists and scientists are seeking ways to curb, or at the very least, monitor these movements.

Annie Simpson of Global Invasive Species Information Network (GISIN), says scientists estimate that up to 5 percent of the world's gross domestic product is being destroyed as a result of foreign pests and other invasive species.

The invaders can destroy habitats and drive endemic species to extinction. In Amsterdam, woodpeckers are being driven out of tree holes by the parakeets.

"A parakeet pushing out a woodpecker is peanuts compared with the economic damage done by other invasive species," said Amsterdam ecologist Remco Daalder.

Researchers have estimated the United States alone spends $137 billion each year fighting immigrant flora and fauna, a figure that rises to $336 billion when Britain, Australia, India, South Africa and Brazil are included.

The World Conservation Union says uncertainty exists about the total economic costs, but estimates of the costs of particular species to particular sectors show a serious problem.

Environment ministers will meet in Curitiba, Brazil, from March 20-31, for United Nations talks to protect animals and plants from threats like pollution, global warming and invaders.

HUMANS TO BLAME

Invaders are a global problem: the Nile perch has wiped out endemic fish in Africa's Lake Victoria; commercial fishing in the Black Sea collapsed after the arrival of plankton-eating North American jellyfish; and the European zebra mussel causes $5 billion of damage a year to American industrial plants.

In most cases, humans have themselves to blame.

In the 1960s, European oyster farmers introduced Japanese oysters to the cold North Sea after heavy frost killed off most of the native species. The Asian shellfish started to thrive.

Driven north by the currents and without a natural enemy because birds cannot open them, the oysters are squeezing out mussels, colonising sandbanks, and eating the mussels' larvae.

"This has gone from an opportunity to resuscitate oyster farming to a real threat for nature," says Hans van Geesbergen of the Dutch mussels producers organisation.

The deregulation of national and international markets has reduced both the barriers to trade and surveillance, causing more invasions, said Charles Perrings at the environment department of the University of York in Britain.

Ships transfer approximately 3 to 5 billion tonnes of stabilising ballast water internationally each year -- one of the main causes of globalisation of marine species.

The UN International Maritime Organisation (IMO) estimates at least 7,000 different species are being carried in ballast tanks around the world, including bacteria and other microbes.

Cholera bacteria have been discovered in ballast waters of cargo ships and poisonous microscopic red-tide algae are absorbed by oysters. When eaten by humans, contaminated shellfish can cause paralysis and even death, the IMO says.

BUT CAN HUMANS SOLVE THE PROBLEM?

Once established, organisms can spread at amazing speeds.

The first Japanese crab was found in the North Sea less than six years ago. Now they are crowding out the bigger local shore crab and pushing north at a rate of 100 km (62 miles) per year.

"It breeds three to four times a year, compared with once a year for the local crabs. I found a female with eggs in the middle of winter," said Reindert Nijland, a crab specialist doing a doctorate in molecular genetics at Groningen University.

In an attempt to tackle such a wide-ranging problem, GISIN is drawing up a global database of invasive species.

"I think there will be a prototype global network in the next year or so," said Michael Browne, manager of the Global Invasive Species Database of the World Conservation Union in Auckland, New Zealand.

"At first, it may simply integrate checklists of invasive species from around the world so that we can see for the first time which invasive species are where."