Foreign seeds are hitching rides on scientists and tourists to the Antarctic and could someday upset the ecology of the warmest patches of the remote land, international researchers said.

Invasive species are "thought to be among the most significant conservation threats to Antarctica, especially as climate change proceeds in the region," said the study in the Proceedings of the National Academy of Sciences.

The research is the first attempt to quantify the threat of invasive plants and their potential effect on Antarctica.

People tend to unwittingly carry 9.5 seeds on average into Antarctica, with the estimated 7,000 scientists working in the region likely to have more seeds, or propagules, on them than the 33,000 tourists who visit annually.

Marc LeBouvier, a French researcher on the team, told AFP that the areas at risk include just one to two percent of the land mass which is not covered by ice.

"Invasive species are a global problem but the issue is particularly sensitive on islands where ecosystems are more fragile," he said.

"The risk in the Antarctic is an upset to the balance of the ecosystem which may result in the gradual replacement of native species with imported ones, which have the capacity to eliminate local plants."

The risks are greatest along the Antarctic Peninsula, in the coastal ice-free areas near the Ross Sea and in parts of East Antarctica, the study said.

An invasive grass species called Poa annua, two wind-dispersed vascular plant species of South American origin and two alien springtail species are believed to be among the new plants taking up residence in Antarctica.

The research was conducted during the first summer season of the 2007-2008 International Polar Year.

Scientists asked 5,600 people arriving aboard ships or airplanes to answer a questionnaire to determine where they had visited prior to arriving in Antarctica and where they were from.

They then proceeded to check 853 of the visitors' bags, purses, luggage, clothes and shoes for signs of seeds or propagules capable of becoming plants, and found more than 2,600 of them.

About 43 percent of the grains were identified to the species-level, which told scientists that several were "known invaders... from the sub-Antarctic or Arctic regions, similar in climate to parts of the Antarctic."

More than half of visitors said they had traveled to cold-climate areas in the year prior to their Antarctica visit, suggesting that they were toting hardy species capable of thriving in the region.

The research team was led by Steven Chowna of Stellenbosch University in South Africa and included scientists from the British Antarctic Survey, Japan Society for the Promotion of Science, Netherlands Institute of Ecology and the French Polar Institute.