A Southward Invasion.

South America and tropical Asia have the most diverse plant life on earth, so it seems intuitive that these regions would supply much of the world’s invasive species. But that’s not the case, according to a new analysis of global plant databases led by Mark van Kleunen of the University of Konstanz in Germany. The data show more nonnative plants originating from temperate Asia and Europe than anywhere else. The numbers also indicate that more species move from the Northern Hemisphere south than vice versa, in part because of international trade patterns. Moreover, “it is very likely that the rate of invasion is increasing,” van Kleunen says, because expansion of that trade, as well as general travel, “makes it easier for species to move between continents.” — Mark Fischetti