Antarctica, the Seychelles, Madagascar and areas which have the most endemic species are most at risk from the invaders.

"Rubbish at sea is much more dangerous than we had previously assumed. The problem of dumping at sea has got to be addressed," David Barnes, a marine biologist at the British Antarctic Survey, said in an interview.

The debris has doubled the spread of alien species in the subtropics and more than tripled it at high latitudes.

Marine organisms prefer to travel on discarded plastic containers, cigarette lighters and milk crates than on natural matter such as coconuts or logs. Because the man-made garbage is so abundant it is easy for the organisms to hop on and travel to virtually anywhere in the world.

"In the tropics you have up to 50 percent of debris being colonised," said Barnes.

GREATEST THREAT TO ANTARCTICA

Closer to the poles, humans have quintupled the amount of material floating on the ocean surface. Invading marine species could irrevocably change ecosystems in Antarctica, particularly if global warming weakens the natural seawater barrier that keeps out alien species.

"If freezing seawater temperature is the main barrier to alien organism invasion of Antarctica, polar warming could lessen this constraint," said Barnes. "Nowhere has the same levels of species which occur there and nowhere else."

Barnes' findings, which are reported in the science journal Nature, are based on a 10-year study of human litter washed ashore on 30 remote islands around the globe.

Plastic dominates the rubbish in most places. It is the most durable type of litter and is more readily colonised by organisms than other materials.

"When a new species arrives in a place like Galapagos it has the potential to displace the native species which are there, and only there," Barnes said.

"Once they are displaced from an environment which is the only place they occur, they are gone for ever."

Barnes is expanding his research and is looking for people who can conduct shore debris surveys on other islands including the Andamans, Ille Amsterdam, Bermuda, Chagos, Clipperton, Cocos/Christmas, Gilbert, Midway, Society, Socotra, Trinidad and Wake. He can be contacted by e-mail at dkab@bas.ac.uk.

Regulations forbidding the dumping of waste from ships has begun to make a difference but Barnes said more needs to be done because once an invading organism gets into an area it is impossible to remove it.