An international group of scientists and conservationists is calling for tougher action to stop the spread of alien species - the animals and plants which can cause massive damage when they are transported into habitats other than their own.

Invasive species are one of the main factors behind large-scale extinctions.

The group, known as the Global Invasive Species Programme (GISP), has been working for three years to come up with an effective and globally acceptable 10-point plan to tackle the problem.

The measures include tighter controls at borders to check that potentially harmful species are not being carried in freight, and fines for those who are found to have brought animals or plants into an area which is later damaged as a result.

Recent examples of the havoc caused by invasive species include the arrival on the Pacific island of Guam of the brown tree snake from south-east Asia. Its introduction led to 10 of the island's 13 bird species becoming extinct.

Hard fight

In Africa, the introduction of the South American water hyacinth to Lake Victoria led to large areas of water being covered and starved of oxygen, severely damaging marine life.

Professor Harold Mooney, from Stanford University in the United States, presented the GISP plan to the annual meeting of the American Association for the Advancement of Science.

He told the BBC that alien species were having a devastating effect all around the world.

"They're causing diseases, they're devastating our crops, they're destroying our forests, they're impeding navigation in water, they're even modifying the course of evolution and they're driving species to extinction," he said.

Despite the scale of the problem, Professor Mooney said his group still had a hard fight ahead to try to persuade governments to introduce legislation and to spend the money needed to tackle the issues.

'Fire truck' mechanism

Scientists say the human propensity to travel, carrying plants, animals and bacteria, is essentially taking our ecosystems back some 200 million years, when the Earth consisted of a supercontinent called Pangea.

During that time, plant seeds and animals could move freely across the land, since they were not yet separated by thousands of kilometres of ocean.

Currently there is no global network set up to deal with or to prevent future ecosystem invasions.

"We're looking at designing something like the CDC [Centers for Disease Control]," said Professor Mooney. "We need something comparable for invasive species."

He added that there was a requirement for a "rapid response mechanism" - a fire truck for invasive species.

If nations developed the resources to react immediately to an invasion, they would save money and time by controlling the invasive species before it established itself, he said.