Half of all widely naturalized species in the world are native to Europe, and more than 20 per cent of the continent's native vascular plants have naturalized outside the continent. This vast collection of introductions is three times more than would be expected by chance, making European flora an obvious candidate for studying naturalization patterns.
In this study, researchers used GBIF-mediated vascular plant occurrences to model the potential distribution of 1,485 European endemics outside Europe. Comparing these results with the realized ranges of the same species present on the Global Naturalized Alien Flora (GloNAF) database, they calculated a "naturalization debt"—that is, a proportion of species that do not yet occupy an otherwise suitable region.
The authors found that European endemic plants were only naturalized in less than five per cent of their suitable regions outside Europe, meaning that in terms of area, more than 95 per cent of expected naturalizations have not yet been observed.
Exploring socio-economic drivers, the study also found that naturalization debt decreased with the level of anthropogenic disturbances associated with human development, meaning that more naturalized species are observed in more developed regions.
Counter-intuitively, the average proportion of unsuccessful naturalizations in unsuitable environments was lower in developed regions with more international treaties relevant to invasive alien species. This, however, is more likely a result of regions with many invasive species joining treaties—rather than treaties promoting invasions.