Eradication of African clawed frogs near the French-Belgian border

Eradication of African clawed frogs near the French-Belgian border

From the mountains in Italy to an internship at INBO

After getting my master’s degree in Natural Science in December 2022 in Italy, at University of Florence, I obtained a scholarship for continuing the work started for my thesis. I am carrying on the sampling of the Italian native crayfish in the Foreste Casentinesi National Park, a wonderful mountainous and wooded protected area straddling the regions of Tuscany and Emilia Romagna. It is a job that I really love both because I am trying to understand how to help this species that suffers from invasion of North American species, such as red swamp crayfish and raccoon, and for the type of work. Lots of fieldwork, boots on the ground and waders on the creeks.

But all this work will start in May 2023, with higher temperatures, and I figured that I could spend the time in between training in wildlife conservation and management. So, I packed my bags and headed for Belgium for a work experience at INBO (Research Institute for Nature and Forest). A government institute I had never heard of before, in a country I had never been to, but with good people and good references, ensured my (ex-)supervisor. See you in a month, Florence!

Cappuccino in Ghent & indoor skiing in Mesen

I ended up in the cozy town of Ghent which, among canals, castles, chocolate and “frieten”, I immediately liked. My supervisor’s work decided to meet me in the city’s most authentic coffee bar, perhaps to make me feel a little in Italy. I was very curious to know what he had in store for me, even though I already knew a little bit from the emails we sent each other. Just the time of a coffee and the four weeks I was supposed to spend in Flanders were already filled with a range of activities. We headed off for a government building by the station named after a famous Flemish poet and, after a quick tour up to the 21st floor (what a view!), we opened an agenda and started feeding it with fieldwork, meetings and data analysis. My most important activity was clear from the beginning: follow-up on a rapid eradication attempt for an unwelcome newcomer in the north of Belgium.

Straight away the next day, we headed to the very south of West Flanders, near Mesen, the smallest of Belgian towns, with an impressive cathedral and a "frietkot" on the market square (how Belgian is that!). During the journey I observed the landscape, so flat, and what I noticed was a mostly agricultural landscape with arable fields and villages along the way, and very few trees. For a moment I am reminded of the forested mountains I'm used to in Italy, when all of a sudden, a tall and extravagant structure looms in front of me. They tell me it's an indoor ski slope. At first, I thought it was a joke or maybe I misunderstood, but it's just like that. It must be a special experience to spend a summer Sunday skiing an artificial ramp in the middle of the Belgian countryside.

Reminiscence of World War I

Arriving at the site, I realize the very round shape of the ponds due, in all probability, to the explosion of a bomb during the First World War. It is not uncommon in these parts to see pools that have such origin. The whole area was affected by the war, and it is also for this reason that the landscape is bare and with poor environments. Despite my past experience with aquatic invaders, I have a bit of a hard time imagining how species, even the hardiest, could survive in this ecological wasteland. Yet, here, on a very rainy January 2023 morning, an important biodiversity conservation action was underway. A new alien, potentially invasive, species is threatening biodiversity here. Here is the African clawed frog, scientific name Xenopus laevis.


African clawed frogs on the lookout for new territory

African clawed frogs started to invade Wallonia and the south of the province of west Flanders. They are known to reproduce in three ponds. They probably got there after an aquarium dump across the border, in the quiet French village of Armentières, just before the pandemic. Good dispersers, an extensive hydrographic network (including the Lys river), a cobweb of administrative national and regional borders... The ideal place for an invader… But first, let's take a step back.

Alien species with three claws

How did this beast get its name? Easy, it has three short claws on each hind foot, which it uses to tear apart its food. The frog belongs to the family Pipidae, which are known to scavenge for food. They have no tongue, no teeth so they have to suck in their food for which they also use their front legs. The name also gives us another hint. Indeed this frog originates from Africa, from the sub-Saharan region to be precise. But then, how did it get to a place thousands of kilometers away from its home? Not by itself for sure. But with a little help from human beings. This is the reason it is called an alien species. Where it establishes and spreads in natural environments, it impoverishes native biodiversity and therefore is called an invasive alien species.

 So, why have people moved this species from its natural range? Well, they were not directly taken from a pond in Africa and released in Europe. In fact, this frog is kept as a pet or as an animal to do experiments in laboratories in many parts of the world, and it usually lives in a controlled environment. But sometimes things can go wrong. Some individuals escaped indeed, more or less accidentally, and they ended up in nature without all the benefits of living in an aquarium. In such a situation, a species often struggles to survive since it did not evolve there and so it is not adapted to that environment. In other words, individuals don’t have the right “weapons” to face the challenges of that place. But it is not always like this and sometimes it happens that an alien species survives and reproduces. This way, year by year, it increases in numbers and the animals need to move away in order to find other places to colonize.


Alien and invasive 

And that is where the problem arises. If a native species is not able to compete with the alien for resources such as food or space, or if it is preyed upon by the newcomer, the natives perish. This makes an alien species invasive and this is what researchers noticed for the African clawed frog in several European countries. In particular, it can prey on native amphibian species such as newts and their larvae, and the latter may also leave an area to escape predation pressure. This alien frog also reduces invertebrates and zooplankton, important food sources for native aquatic fauna such as fish and amphibians. African clawed frogs are also healthy carriers of amphibian disease such as chytrid fungus and Ranavirus. For these reasons, after extensive risk assessment, the species was added to the list of invasive alien species of European Union Concern. This European legislation imposes a ban on trade and keeping of the species and obliges countries to take rapid eradication action when these species pop up. The biggest populations of African clawed frog in Europe occur in France and Portugal, where it is under management, and in Sicily, and species distribution modeling shows the European climate will become more suitable for the species establishment.

Environmental DNA to trace African clawed frogs

Going back to the project I was following, after a discovery of a population in a pond in nearby France, in 2022, in West Flanders, scientists from INBO have found this species in three different ponds in an agricultural area close to the border with Wallonia and France. After the discovery in France, they promptly decided to run a survey using environmental DNA, which allows detection based on water samples. No positive detection at that time, however. Yet, a few years later, it became evident that they had been looking in the wrong neighborhood. The frog apparently spread via the river Lys and took a smaller tributary to swim upstream, the Douvebeek (see picture below). A new survey was undertaken in 2022 and the frogs were detected at several places, including three ponds where reproduction was ascertained.

Early detection & rapid response

Being the first time this species was found in Flanders, it was decided to quickly act in order to try and stop a potential invasion. This kind of approach matches the concept known as Early Detection and Rapid Response in the field of alien species management. This good practice is fundamental for trying to remove a species spending little money and energy, and also avoiding much more animal suffering. Postpone the decision and the populations will be much bigger so more money will be needed for removal or management with a lower probability of success. The rules are: immediate action, hit them hard, minimum effort, maximum yield.

New method

Now the big question: how to remove those animals? Well, the answer is not easy, and it has been decided to apply a quite challenging method. Moreover, the method was never applied for eradicating this species. It is however frequently used in aquaculture to sterilize ponds after the fish harvest so they are free from biota and disease. Also, it has been successfully used for eradicating invasive crayfish in France. But frogs are not (cray)fish.

On my first day of fieldwork, conditions were quite hard to face. It had been raining heavily for days. The area around the ponds was very muddy. I was wearing waders, not for going into the water as I am used to, but just for walking. In some spots, there was so much mud that I sank almost up to my knees. Also, it was really cold and was starting to be a bit challenging. The sandwich I brought for lunch fortunately brought some relief…

Draining the pond and putting up a frog-proof fence

The ponds are first drained using a hydraulic pump plugged to a diesel generator brought there with a tractor. As the water level drops, frogs feel the hydro regime is changing and try to get out of the water to find a more suitable place.

To avoid this, a “frog-proof” fence is first installed around the water with buckets so that individuals trying to find an exit or entrance along the fence, can be recuperated. Buckets were also placed on the outside to intercept any native toads, salamanders or frogs on their way to a treated pond, or, who knows, new African clawed frogs climbing out of the brook.


Rain kept pouring constantly and, at a certain point, also Mr. Wind showed up which wasn’t helpful with the cold. At that moment I was freezing and one of the guys from Natuurwerk vzw, the social economy company who is in charge of the work shouted to me “Welcome to Belgium!”. I come from a country climatically quite different from this one and I am not used to it. Then I tried to take some photos with my smartphone because I wanted to report all the steps of the action. Usually, it is a very easy task, we do it several times a day every day, but with those conditions it wasn’t at all. I couldn’t properly move my fingers on the touchscreen. At the same time, I saw another guy from the company rolling a cigarette and I was amazed about how easily he could do that with heavy rain and wind. Clearly I needed a bit of time to settle in.

Applying calcium oxide

Two days later, the water level was lowered enough to move to the next step: application of calcium oxide. Also known as quicklime, it is a chemical compound that looks like a white powder and that, once added to the water, increases the pH considerably making the water unsuitable for African clawed frogs. It’s a dangerous product that requires the use of safety equipment such as overalls, gloves and goggles. After maintaining such a high pH level, also applying the quicklime on the lakeshore, all the frogs and all the living matter should disappear. Then, after a while, rainwater should fill the ponds again, the pH level will go back to normal values, the calcium oxide basically reacts to form carbon dioxide and water, and the environment returns to its original state so that native animals and plants can colonize them again.


There were several reasons why this drastic method was chosen. First, it is known to be effective. Adult, newly metamorphosed and juvenile frogs are known to be in the mud, which makes them almost impossible to get to with classic methods such as electrofishing or netting. Second, the action is swift, it requires for the pH to go above 12 for just 48 hours. This was a big advantage, as the ponds are used by farmers who needed to give permission to do this on their land. They let their cows drink here, so the ponds needed to be in healthy condition fast to be used again. Third, the ecosystem goes back to normal in just a few weeks. Remember we are January, and most native frogs, toads and salamanders start the reproductive season in just a few weeks from now.

There’s no fun in killing animals

Nonetheless, it became clear to me that what might appear a simple, rapid eradication, in reality represents a big challenge that requires dedication, time, people, money and a bit of luck for instance with the weather. Yet it is something that native biodiversity needs and deserves. I notice that with the people executing this action, there is total awareness about the fact that those alien frogs, who are not guilty of anything, are going to be killed. The people who do the job don’t have fun killing the animals but they are motivated by the prospects of preventing an alien species invasion that could eventually be dangerous to many more native species and would lead to many more frogs being killed, and with consequences that extend far beyond any collateral damage encountered through this action.


The Friday after, we are there again with the INBO communication team. Apart from the follow-up which includes weighing and measuring any frogs caught, as well as taking toe clips for DNA work and swabs for checking on pathogens, one important aspect is the communication. It is fundamental to let everyone know about the conservation efforts carried out here and the reasons why it has to be done. This is why I am writing this blog (and I am very glad you read up to here). I was also interviewed by a colleague of the INBO communication team, and I was a bit embarrassed. Actually, not to speak in front of a camera, even though it is not simple, but for speaking in another language. Anyway, I did it and she told me it was really fine. What a relief.

Delicate equilibrium

To conclude, there is the need to remind that animals, plants and all the living beings that live in the wild are linked one to the other. There are so many relationships among them: a species eats another, or can be helpful for creating new habitat, or can be a parasite, and so on. All these linkages build an imaginary dense ecological network, maintained by an incredibly fascinating but, at same time, very delicate equilibrium. This  equilibrium is fundamental for maintaining healthy  nature and biodiversity so that it can go on with all its processes that, in turn, are essential for OUR LIFE on this planet.

More information concerning the management of invasive alien species and INBO can be found here.


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