Figure 2. Variable importance of the predictors of regional alien species richness of plants (A, C, and E; n = 347 regions) and birds (B, D, and F; n = 423 regions) using three statistical techniques: hierarchical partitioning (A and B), random forests (C and D), and mixed linear models (E and F) (results for the other taxonomic groups are shown in Figs. S4–S7). For hierarchical partitioning, the individual (green) and shared (yellow) contributions are shown. The conditional (threshold = 0.95) variable importance of all predictors of random forests is shown in light blue bars. Mixed models refer to the beta weights (brown circles) and their 95% confidence interval. For the HP and LMM, region area, and the relative and absolute measures of economy and population were log-transformed, and precipitation was square-root transformed. GDPc: Gross Domestic Product per capita, and HPD: Human Population Density.