The Southeast of the Netherlands is being colonized by exotic marsh frogs, according to research by Ravon, Leiden University, and Naturalis. Although these non-native marsh frogs thrive in our country, they pose a threat to native biodiversity.
Invasive species are among the leading causes of biodiversity loss. In the Netherlands, exotic marsh frogs are on the rise. Biology students from Leiden investigated where these frogs originally came from.
'Nicelegs!'
Why are these marsh frogs being moved around in the first place? The frog leg industry is considered the main source of introductions. Every year, hundreds of cubic meters of frogs are shipped to France.
However, the trade in frog legs isn’t the only introduction route. Until recently, you could even buy exotic tadpoles at garden centers to decorate your pond. “No wonder marsh frogs are being spread over large distances,” says Tariq Stark from RAVON, the species organization focused on reptiles, fish, and amphibians.
Rapidexpansion
Because the marsh frog naturally occurs in the Netherlands, its introduction here is less noticeable, but it still plays a significant role. “Exotic marsh frogs are moving up from the south and have now colonized a considerable part of our country where they were never present before,” says Richard Struijk from RAVON. The first sightings date back to around 1990 in South Limburg. As of 2025, they have spread further across the Netherlands. It’s high time to take stock of the situation.
Cryptic speciesand citizen science
In the Netherlands, three species of green frogs occur naturally. Besides the marsh frog, there are the pool frog and the edible frog. These species are already difficult to tell apart, and the new marsh frogs make it even trickier. To track the distribution of this latest group, volunteers from RAVON collected oral mucus samples from frogs in Limburg.
Frogcocktail
Using DNA barcoding, the exact sequence of a small but variable piece of DNA was determined for all the frogs. “By establishing the DNA profile for each frog, we were able to determine which species each individual belongs to and where in its natural range it originally came from,” explains lead researcher Ben Wielstra (Naturalis/Leiden University).
The results are crystal clear: the exotic marsh frogs do not originate from the Netherlands, but from various regions: Central/Eastern Europe, the Balkans, and Turkey. Wielstra explains, “Here, a mix of different marsh frog variants has been introduced, some of which are so distinct that they could be considered separate species.” Such highly genetically mixed populations do not occur in their natural ranges. The marsh frogs clearly aren’t bothered by this: the introduced populations are expanding rapidly.
Gluttony, diseaseand hybridization
The marsh frog is considered one of the most significant invasive amphibians. Frogs will eat almost anything that fits in their mouths - and the marsh frog has a rather large one. Introduced marsh frogs not only compete with native amphibians but can also prey on them. In part of the invasion front in Limburg, highly endangered amphibians live, such as the yellow-bellied toad and the midwife toad. Introduced marsh frogs can also spread amphibian diseases like chytridiomycosis. Wielstra adds, “Now that the marsh frog front has reached our native green frogs, we also have to consider hybridization, because native green frogs will mate with the non-native marsh frogs.”
Scientists and policymakers have long advocated for stricter regulations on the import of live frogs. Now that the scale of the marsh frog invasion in the Netherlands has been mapped, it is crucial to study its impact and develop management plans. How can we get exotic marsh frogs under control?
Moreinformation
The article titled The invasive marsh frogs advancing into the Netherlands carry a diverse mix of mtDNA has been published in Amphibia Reptilia.
Photos: Tariq Stark (frog from which oral mucus is being sampled); Maarten Gilbert (marsh frogs from the Balkans); Benny Trapp (lead photo: marsh frog from the Balkans).