Density plot showing the distance between any two random points within a country and between two random points in different countries (t = 15.6, df = 64.4, P < 0.001). The distance between random points within a country (‘within-country’) represents the distance between native-alien populations and their native range, while the distance between random points in different countries (‘between countries’) represents the distance between alien populations introduced from other countries and their native range. See Suppl. material 1 for full methods.