Figure 3. Box-plot of variable importance (conditional, threshold = 0.95) of the population and economic predictors in explaining regional alien species richness for all taxonomic groups. The variable importance for the random forests of the seven taxonomic groups is shown, separately for absolute (Gross Domestic Product, GDP; and Human Population, HP) and relative (Gross Domestic Product per capita, GDPc; and Human Population Density, HPD) predictors.