The fringes of Antarctica are being invaded by alien plants and tiny animals, scientists have found.
Researchers scoured the clothes and boots of tourists and scientists visiting the continent and found that most were carrying plant seeds.
Alien plants already grow on the fast-warming Antarctic Peninsula.
Writing inProceedings of the National Academy of Sciences (PNAS), the team says the plants are likely to spread as the climate warms.
"People in the past have been sceptical, saying, 'It's largely ice-covered so it's unlikely that plants will establish themselves'," said lead researcher Steven Chown from Stellenbosch University in South Africa.
"[They're] forgetting that probably less than 1%, but still a significant area, is ice-free - some of that's in the peninsula region, and it's been warming very quickly."
The Antarctic Peninsula, which runs up towards the southern tip of South America, has warmed by about 3C over half a century, much faster than the global average.
As a result, ice cover is dwindling.
Many islands in the sub-Antarctic region have seen significant ecological changes due to invasive species that have either arrived accidentally or deliberately.
The research team believes that the Antarctic Peninsula and some other areas around the continent's coast could see similar changes in decades to come.
"Antarctica has a native ecology - a very well-established microbial ecology, and on the peninsula it has two species of indigenous plants," Prof Chown told BBC News.
"And it will be changed by species coming in."
The marine environment is changing too, with giant crabs establishing themselves in waters that were previously too cold.
During International Polar Year in 2007-08, the research team took samples from tourists and tourism operators, and scientists and their support staff.
On average, each visitor carried 9.5 seeds into the White Continent, though scientists carried far more each than tourists.
"What we found was that people's boots and bags were the things that had most material attached," said Kevin Hughes from the British Antarctic Survey (BAS).
"I guess the tongue of the boot is an ideal place for seeds to be caught when you're tying up your laces. But we did find them in various bits of clothing as well."
Extrapolating from their figures, this means that about 70,000 seeds arrive on Antarctica each year.
The places that tourists visit tend to be the warmest bits of the continent - which are also the places where seeds are most likely to survive.
The researchers found that although many of the seeds originated in South America, a large number came from the Northern Hemisphere.
About half of them came from cold regions and would probably be viable in the warmer bits of Antarctica.
The researchers also collated evidence from other scientists on organisms that have already established themselves.
Deception Island, 100km north-west of the peninsula, has already been colonised by two grass species and two springtails - tiny animals that live in topsoil and leaf litter.
On the western slopes of the peninsula itself, the grass speciesPoa annuahas established itself close to four research stations - implying that it has probably been brought, inadvertently, by visiting scientists.
Poa annuahas already taken over several sub-Antarctic islands where it dominates vegetation.
The researchers suggest that measures be taken as soon as possible to tackle invasive species that are already there, and to prevent the arrival of new ones, as far as possible.
Dr Hughes has already "eradicated" a South American member of the aster family from Deception Island, where tourists regularly stop to visit an old whaling base, by the simple measure of pulling up the single specimen he found.
But with some of the more widespread species, they fear it could already be too late.
The International Association of Antarctic Tour Operators (IAATO), which covers most companies in the area, already takes pains to make sure tourists arrive seed-free; and some countries' science organisations have similar requirements.
"We can use guidelines for vehicles, make sure cargo hasn't got seeds and invertebrates on it, make sure clothing is clean and that we bring fresh boots," said Dr Hughes.
"[However,] I think it's safe to say that wherever people go, it's inevitable that they bring other species with them; and no matter what we do, our best efforts will only reduce the rate at which species are introduced, we'll never prevent it altogether."
There is no legal obligation to clean up accidentally introduced alien species under the Antarctic Treaty.
But this team of scientists believes there is a moral obligation to do so, and to block new arrivals as far as possible.
One complicating factor for the Antarctic Peninsula and its islands is that some seeds are known to arrive carried on the wind from South America.
But, argues Prof Chown, there is still an element of human agency about these wind-blown cases in that the plants can only establish themselves under climatic conditions created largely through humanity's production of greenhouse gases.
If nothing is done, he says, small pockets of the unsullied continent may, in 100 years, look very like sub-Antarctic islands such as South Georgia where alien plants and animals, particularly rats, have dramatically changed the local ecology.
"South Georgia is a great sentinel of what could happen in the area in the next few hundred years," he said.
"My suspicion is that if you didn't take any biosecurity measures you'd end up with a system that would look like a weedy environment with rats, sparrows andPoa annua."
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