Nature
Volume 525 Number 7567 pp5-152 3 September 2015
http://www.nature.com/nature/current_issue.html

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Global exchange and accumulation of non-native plants
Mark van Kleunen, Wayne Dawson, Franz Essl, Jan Pergl, Marten Winter+ et al.
A global database of alien plants, showing that over 13,000 species, nearly 4% of the global flora, have become naturalized in a new location.