From the Asian tiger mosquito in the American South, to the Eurasian zebra mussel in the Great Lakes, to European quackgrass
throughout the United States,
invasions of non-native species can disrupt ecosystems, cause havoc with local economies, and even threaten health. To test the relative weight of human versus other ecologic factors on freshwater fish invasions, Fabien Leprieur and colleagues collated species data on over 1,000 river basins throughout the world (collectively draining over 80% of Earth''s land surface), characterizing each species as native or non-native. In these areas, more than a quarter of species were non-native, and they had higher-than-average numbers of native species threatened by extinction. There was no correlation between the richness of native species and that of non-
natives, at once refuting biotic acceptance (for which high numbers of natives predict high numbers of non-natives) and biotic resistance (for which high numbers of natives predict low numbers of non-natives).