A reassessment of the biogeographic range of northern clade pool frogs (Pelophylax lessonae)

Distinguishing between native and introduced species can be difficult, particularly at range borders, where patchily distributed populations may occur away from a species' natural core range. The case of native pool frog (Pelophylax lessonae) populations at their northern range limit in Europe is particularly interesting. These are morphologically and genetically distinct populations which are patchily distributed and have been reported from the UK, Sweden and Norway, but up until 2013 were thought to be absent from Finland. When pool frog populations were discovered in southwestern Finland they were morphologically classified as belonging to this northern clade. However, the origin of these populations has been unclear and it is possible that the Finnish populations originated through human aided introductions, established themselves recently through natural migration, or are indeed previously undiscovered relic populations. To establish the origin and relationship of these frogs to other populations across Europe we used phylogeographical analysis based on microsatellite and mitochondrial DNA markers. Our results indicate that the Finnish, Norwegian, Swedish, UK, as well as Estonian populations belong to the northern clade. The Finnish frogs are most closely related to Swedish northern pool frogs, but are genetically more diverse. This suggests that the Finnish pool frogs are most likely a relic from postglacial migration, though we could not entirely rule out the possibility of a recent natural or human aided colonization from Sweden. This has implications for the conservation status of the pool frog in Finland, where it thus far has been considered an invasive alien species.