The tens of thousands of tourists, scientists, and support personnel who visit Antarctica every year are carrying with them large numbers of seeds from alien plants that could one day pose a threat to Antarctica’s limited array of native plant species. A team of scientists vacuumed the clothes and luggage of 850 visitors to Antarctica during the 2007-2008 summer season, finding more than 2,600 stowaway seeds from other regions of the world, including the Arctic and the Alps. That season, 33,000 tourists and 7,000 scientists and support personnel visited Antarctica, carrying with them an estimated 70,000 seeds, according to a study published in the Proceedings of the National Academy of Sciences. The Antarctic continent only has two native species of vascular plants (a hair grass and a pearl wort), and so far scientists have discovered no alien plant species that have taken hold there. The study noted, however, that invasive species such as dandelions have taken root on some sub-Antarctic islands, and that as Antarctica continues to warm, alien species could eventually begin growing on the continent proper.