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Exotic marsh frogs from distant lands are colonizing the south-east of the Netherlands. This has been demonstrated by biology students from Leiden University faculty. Although the amphibians thrive in the little country, they are harmful to native biodiversity.

The marsh frog is gaining ground in the Netherlands, which is bad news for native frogs. Biology students from the IBL and Naturalis, together with RAVON (Reptiles Amphibians Fish Research Netherlands), where the exotic marsh frogs originally came from. The research is published in the journal Amphibia-Reptilia.

Citizen scientists help out

Three species of green frogs occur naturally in the Netherlands: the pool frog, the edible frog, and the marsh frog. "It is difficult to between these three frog species. But distinguishing between our native tree frogs and their exotic cousins is even more complex," says Ph.D. candidate Anagnostis Theodoropoulos, who supervised the student projects.

To gain clarity on the distribution of green frogs, RAVON asked its network of volunteers for help. These people take a saliva sample from a frog using a swab. This sample contains DNA that can be used to determine the identity of the frog.

From distant lands to the Netherlands

"By determining the DNA profile of each frog, we were able to identify its species and origin," explains lead researcher Ben Wielstra.

The results are crystal clear: the exotic tree frogs do not originate from the Netherlands, but from Central/Eastern Europe, the Balkans, and Northern and Southern Turkey. The findings fit into the bigger picture: a hodgepodge of tree frog lines was introduced into Western Europe in the past—for frog legs on the menu or as exotic inhabitants for people's ponds.

Gluttony, disease and hybrid species

The invasive tree frog competes with the native species for food. Frogs eat pretty much anything that fits in their mouths—and the marsh frog has a pretty big mouth. They can spread disease and even hunt native species. Now that the marsh frog front has reached our native green frogs, native green frogs will mate with non-native marsh frogs. This also makes hybridization between the two species possible.