Foreign plants such as annual bluegrass are establishing themselves on Antarctica, whose status as the coldest and driest continent had long made it one of the most pristine environments on Earth.

But a boom in tourism and research activities to the Antarctic Peninsula may be threatening the continent's unique ecosystems, scientists say. (See a high-res Antarctica map.)

For the study, ecologist Steven Chown at the University of Stellenbosch in South Africa and colleagues vacuumed the clothes, footwear, bags, and gear of approximately 2 percent of people who visited during the Antarctic summer from late 2007 to early 2008. That amounted to 853 scientists, tourists, and accompanying support workers and ships' crew members. (