Quantifying Habitat Breadth.

Each EVA survey plot was assigned to one of 229 habitat types of the European Nature Information System (EUNIS) using the classification expert system EUNIS-ESy version 2020–06-08 (95). These habitat types were further merged into 47 broader habitat types used in this study (SI Appendix, Table S18). Habitat classification was possible for 66% of the nearly 390,000 EVA plots that included aliens and 60% of over 1.2 million plots that included native species; these were used to quantify the habitat breadth of species naturalized and native ranges, respectively. Because not all habitat types are equally distinct (e.g., wet and mesic grasslands are more similar to each other than either is to taiga), our measure of habitat breadth accounts for floral similarity among habitats by calculating the effective habitat number for each species following the method in ref. 96 (SI Appendix, Effective habitat number and Fig. S9). In addition to calculating habitat breadth, we determined the habitat where each species most frequently occurred by aggregating EVA plots in 1° × 1° grid cells and counting the number of grid cells where each species was observed in each habitat.