The alien marsh frog cocktail: Distribution, causes and pathways of a global amphibian invasion

While some biological invasions are well documented globally, others, more cryptic, are often underestimated despite multiple local warnings. This is the case of the marsh frog (Pelophylax ridibundus sensu lato), a Palearctic anuran amphibian distribured from Western Europe to Central Asia. Marsh frogs have been introduced into many European countries, where they pose threats to biodiversity, yet an integrative understanding of their invasion is lacking. Therefore, we combined diverse bibliographic sources with recent DNA barcoding to determine the extent and diversity of invasions in Europe and substantiate the causes and pathways of introductions. We document alien frog populations across 167 sub-regional administrative areas in 19 European countries, corresponding to nine phylogeographic lineages originating from three continents. Introduction pathways from the Balkans and the Pannonian Plain, Anatolia and the Levant coincide with the history of live frog imports into Europe, which involved the trade of hundreds of millions of individuals. Introductions were mainly associated with the consumption of frogs' legs, but also with ornamental, educational and research purposes. While some introductions date to the 18th century, most took place in the second half of the 20th century and new cases continue to emerge. Altogether these results show that the marsh frog is a widespread and complex invasive amphibian species in Europe, and ranks among the most worrying amphibian invaders in the world. Our assessment calls for the immediate ban on the commercial import of live water frogs, especially to prevent new lineage combinations in invaded areas, a Pandora's box that, if opened, could further boost the species' invasive potential.