Immigration can be a real problem in the frog world. Native species in western Europe are finding it difficult to compete with hostile invaders. With many frogs already suffering population declines, a recent study offers another explanation for disappearing anuran species.

Water frogs indigenous to France and Spain, including Graf’s hybrid frog (Rana grafi) and the Iberian water frog (Rana perezi) are being marginalized by the robust Central Asian marsh frog (Rana ridibunda). The marsh frog is a formidable neighbour. It is the largest European frog, develops quickly, and lives longer than the native frogs. Female marsh frogs have more offspring. In freshwater, the species is particularly hardy. So the newcomers are able to outcompete the native residents, flooding the habitat with ravenous young marsh frogs. The story resembles many other scenarios where alien introductions have swamped the indigenous species.

The twist is how the marsh frogs have spread. Frogs by their very nature are forced to inhabit moist areas and breed in water. Typically, this does not lend itself to a mass invasion across continents. However, the marsh frog is imported and bred for culinary purposes - frogs’ legs. Haute cuisine in France and Spain has led to the accidental introduction of these frogs along several large river areas. Ironically, in Romania, marsh frogs are subject to a fishing quota due to their edible limbs. This species is also used extensively for physiological and biochemical research.

Another quirky aspect is the marsh frog’s reproductive habit, known as hybridogenesis. If marsh frogs mate with native water frog species, the resultant offspring are marsh frogs - rather than intermediate types. Only marsh frog chromosomes go into any gametes (eggs or sperm) produced by offspring. Essentially, the other species’ genetic contribution is never available, and offspring tend to mate with other marsh frogs.

About 60 to 80 million frogs are used in French kitchens each year. Local species tend to be protected, with many edible frogs imported from Russia. In the past, Indonesia was also a major supplier, until the practice led to frog population declines. Frog farms, which allow the production of specialty frogs for consumption, reduce the impact on wild and local species. Unfortunately, escaped frogs then cause their own problems.