Tree frogs exposed

December 1st, 2022
Europese Boomkikker (Hyla arborea) - Jelger Herder

Biology students from Leiden University have discovered two tree frog species in the Dutch coastal dunes that do not occur there naturally. A special DNA technique revealed these potentially harmful tree frogs. Remarkable, but these are not the only exotic amphibians that have taken residency in the Dutch coastal dunes...

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In collaboration with RAVON (Reptile, Amphibian & Fish Conservation Netherlands) and Naturalis Biodiversity Center, eight biology students investigated the origin of several amphibian populations in the Dutch dune landscape. It was suspected that these dune populations were of foreign origin because they occur in an unusual location - far away from the natural populations - and have only been discovered relatively recently.

Dip nets
and waders

Under the supervision of researchers Ben Wielstra and Richard Struijk, the team ventured into the dunes armed with dip nets and waders. The team took a bit of saliva or skin mucus from each animal with a cotton swab. From this, the team was able to isolate DNA and precisely reconstruct the order of the DNA letters. The technique they used is called DNA barcoding.

DNA barcoding is a fast and convenient technique to identify DNA profiles. The DNA profile holds information about the identity of a species and shows from where in its natural range the ancestors of an introduced population were originally derived. You can compare it with languages and accents. By the language someone speaks, you can usually tell from which country that person is and by the accent, you can tell from which region exactly.

Wielstra team

Fieldwork images by Nienke Prins and Manon de Visser.


Veldwerk Wielstra team - Nienke Prins
Veldwerk Wielstra team - Manon de Visser


One of the amphibians the biology students studied, was the tree frog. This frog belongs to a group of 'cryptic species'. These are species that look alike, but turn out to be very different according to their DNA. The students compared DNA from the dune populations with other tree frog populations from the Netherlands and the rest of Europe. The result? The Eastern tree frog, originating from Greece, was found in the National Park Hollandse Duinen. In addition, they also found the Italian tree frog in the Westduinpark near The Hague. These species are genetically very different from the European tree frog, which is native to the Netherlands.

The odd one

These are not the only exotic amphibians the team identified. A population of spadefoot toads was discovered in the dunes near Callantsoog, which does not belong there either. “Spadefoot toads naturally only occur in the south and east of the Netherlands. The DNA profile shows that the population in Callantsoog is the same species as in the Netherlands, but is genetically dissimilar from the Dutch populations,” says Nienke Prins, one of the students who studied this population. “The DNA profile points to an origin in Eastern Europe.”

The midwife toad has also certainly been released in the dune area. In the Netherlands, this species lives exclusively in South Limburg. Furthermore, the presence of great crested newts is suspicious, but debatable: it could be a remnant population from a distant past. "The DNA profile of these two species appears to occur naturally in many European countries, including the Netherlands," explains Ph.D. candidate Manon de Visser, who helped supervise the students. "It is therefore difficult to determine their origin. We will have to conduct more DNA research to crack those codes.”

Tree frogs

The native European tree frog and the exotic Eastern tree frog. Images by Jelger Herder / RAVON.

Europese Boomkikker (Hyla arborea) - Jelger Herder
Oostelijke boomkikker (Hyla orientalis) - Jelger Herder


Does it matter where these species originally come from? “Yes,” says Struijk firmly. “Exotic species are one of the greatest threats to nature and can cause a lot of damage. If this is the case, the species can be seen as invasive. Exotic species can displace native species, for example, because they compete with or prey on native species," he mentions as one of the problems. Wielstra adds: “Exotic species can also carry nasty pathogens, such as fungi that cause the infectious disease chytridiomycosis. This is a disease that has killed many amphibians already.” The research team's advice is therefore clear: "Keep an eye on these suspicious populations, do everything we can to identify new exotic populations as quickly as possible, and recognize the danger of introduced populations."


The four student projects were carried out by the Institute of Biology Leiden (IBL) and Naturalis Biodiversity Center, in close collaboration with RAVON (Reptile, Amphibian & Fish Conservation Netherlands). ARTIS, Dunea, Natuurmonumenten, Staatsbosbeheer and the municipality of The Hague facilitated sampling. The articles are published in the journal Amphibia-Reptilia.

  • Tree frog: Marit Kuijt & Liam Oskam 
  • Spadefood toad: Stephanie Koster & Nienke Prins 
  • Midwife toad: Chris Vliegenthart & Maurits van de Vrede 
  • Crested newt: Jurian de Brouwer & Bas Helder


Links to the articles:

  1. Kuijt, M., Oskam, L., Den Boer, I., Dufresnes, C., France, J., Gilbert, M. J., De Visser, M.C., Struijk, R.P.J.H, Wielstra, B. (2022) The introduction of three cryptic tree frog species in the Dutch coastal dunes challenges conservation paradigms. Amphibia-Reptilia (published online ahead of print 2022)
  2. Koster, S., Prins, N., Dufresnes, C., France, J., De Visser, M.C., Sruijk, R.P.J.H., Wielstra, B. The conservation paradox of an introduced population of a threatened species: spadefoot toads in the coastal dunes of the Netherlands. Amphibia-Reptilia (published online ahead of print 2022)
  3. De Brouwer, J., Helder, B., France, J., De Visser, M.C., Struijk, R.P.J.H., Wielstra, B. An isolated crested newt population in Dutch coastal dunes: distribution relict or introduction? Amphibia-Reptilia (published online ahead of print 2022)
  4. Vliegenthart, C., Van de Vrede, M., Den Boer, I., Gilbert, M., Lemmers, P., France, J., De Visser, M.C., Struijk, R.P.J.H., Wielstra, B. The limits of mtDNA barcoding for determining the provenance of invasive species: a midwife toad example. Amphibia-Reptilia (published online ahead of print 2022)

Manon's DNA Talk (YouTube videos):

For additional questions, the media can contact Manon de Visser. For general inquiries, Naturalis' communications department is available. The images accompanying this press release can be found in higher resolution in this folder.