Data from: A cryptic invasion within an invasion and widespread introgression in the European water frog complex: consequences of uncontrolled commercial trade and weak international legislation

In western Europe many pond owners introduce amphibians for ornamental
purposes. Although indigenous amphibians are legally protected in most
European countries, retailers are circumventing national and international
legislation by selling exotic non-protected sibling species. We
investigated to what extent non-native species of the European water frog
complex (genus Pelophylax) have become established in Belgium, using
morphological, mitochondrial and nuclear genetic markers. A survey of 87
sampling sites showed the presence of non-native water frogs at 47
locations, mostly Marsh frogs (P. ridibundus). Surprisingly, at least 19%
of all these locations also harboured individuals with mitochondrial
haplotypes characteristic of Anatolian water frogs (P. cf. bedriagae).
Nuclear genotyping indicated widespread hybridisation and introgression
between P. ridibundus and P. cf. bedriagae. In addition, water frogs of
Turkish origin obtained through a licensed retailer, also contained P.
ridibundus and P. cf. bedriagae, with identical haplotypes to the wild
Belgian populations. Although P. ridibundus might have invaded Belgium by
natural range expansion from neighbouring countries, our results suggest
that its invasion was at least partly enhanced by commercial trade, with
origins as far as the Middle East. Also the invasion and rapid spread of
Anatolian lineages, masked by their high morphological similarity to P.
ridibundus, is likely the result of unregulated commercial trade. We
expect that Anatolian frogs will further invade the exotic as well as the
native range of P. ridibundus and other Pelophylax species elsewhere in
western and central Europe, with risks of large-scale hybridisation and
introgression.