The conservation paradox of plants that are threatened at home but naturalized elsewhere

Human actions pose threats to numerous native plant species. At the same time, humans have introduced many plant species beyond their native ranges, where some of these plants have become naturalized or even invasive. Invasive plant species are listed as one of the main threats to many threatened native plants. Interestingly, however, there are also plant species that are threatened in their native range but have become naturalized or even invasive elsewhere. Such threatened-but-naturalized plants species constitute a conservation paradox. The number of threatened-but-naturalized plants species and what characteristics distinguish them from other threatened species remains unknown. Here, we combined databases of global threat status and naturalization success of the world’s vascular plants, and added information on growth forms, economic uses and their native and non-native ranges. We found that among the 26,036 species assessed as threatened by the IUCN, only 238 (ca. 1%) have become naturalized elsewhere, and 21 of those are considered invasive in at least one country. If we extrapolate from the number of IUCN-assessed species to the entire extant global vascular flora, we estimate that between 1000 to 2000 plant species might be threatened at home but naturalized elsewhere. Most of the threatened-but-naturalized species are trees or shrubs with economic uses such as provisioning of materials and use for landscaping. More threatened species than expected have naturalized in Europe from nearly all other continents. Furthermore, although Africa is the largest donor of threatened-but-naturalized species in absolute numbers, the numbers are nevertheless lower than expected due to the continent’s overall large number of threatened species. Our study shows that, although the number is relatively low, some threatened plant species have managed to become naturalized outside their native range, and deviate in some of their characteristics from the other threatened species. Future studies should assess the potential conservation value of the naturalized populations of threatened species.