Without fallow deer, the dune will bloom again

In recent years, an exploding fallow deer population has turned the Amsterdamse Waterleidingduinen into a bare hilly landscape. But in a few grid pieces, nature shows its resilience. “With fewer deer, the rest of the area can also recover.”

Cow Moons

Undaunted, two resting fallow deer stare at the approaching employees of Waternet, the organization that, in addition to water supply and wastewater treatment for Amsterdam and the surrounding area, also manages the Amsterdam Water Supply Dunes. The animals are right in front of the fences of the so-called exceptionwhere the fallow deer cannot reach.

As Mark van Til, ecologist at Waternet, gets closer, the deer still choose the hare trail. The impressive antlers disappear into the distance. “In other areas you really can’t get that close, but here there are so many fallow deer that the meeting between humans and fallow deer has become completely normal.”

The fact that the population is so large has serious consequences for the area. “Because there were so many of them, they just ate everything, it looked like a golf course,” says Van Til. “You can see here, grasses have been eaten down to the ground, and you only see two flowering plants: Broomwort, an exotic, and burdock. Both are poisonous, the only reason these two haven’t been eaten away yet. As a visitor, you could therefore still think that it is quite blooming here, but in the spring almost nothing blooms here, and there is therefore not much to collect for insects.”

To the left of the fence, the fallow deer cannot come, there are many flowers and plants growing. That is why Waternet is closing off areas in the Amsterdam Waterleidingsduinen.Picture Patrick Post

Shooting of fallow deer

Even the thickets, important to many species of butterflies and birds, had to suffer. Reason why Waternet wanted to intervene in 2013: a large part of the fallow deer had to be shot. But it wasn’t without a fight. Opponents of the hunt rebelled and brought legal proceedings to the Council of State to prevent the shooting of ‘their’ beloved fallow deer.

In 2017, the judge ruled that a permit to significantly reduce the population by means of a cut was properly issued. From around 3,900 animals that were there in 2016, to gradually 600 to 800. However, it will happen step by step and will take a few more years. Last spring, 1,800 fallow deer were counted. In the meantime, in order to preserve the resource stocks in the area, sixteen enclosures have been set up, areas of several hectares surrounded by fences where the deer cannot reach. They have been there for over two years now.

And it works, as can be seen as we progress towards the exclusion. Trimmed grass and bare earth on one side of the fence, on the other a colorful lush patchwork of green, yellow and purple. “You’re already seeing so many more species,” says Daan Kinsbergen, a PhD student at the University of Amsterdam who is researching the exclusions. “From here I can already see wet straw, suckling pig, thyme, mouse’s ear, clover, burnet rose. The deer also just eat the last, which is surprising when you see how many thorns such a plant has.”

Left Mark van Til, ecologist at Waternet, and squatting Daan Kinsbergen, PhD student at the University of Amsterdam, who researches the dune areas where fallow deer cannot reach.  Picture Patrick Post

Left Mark van Til, ecologist at Waternet, and squatting Daan Kinsbergen, PhD student at the University of Amsterdam, who researches the dune areas where fallow deer cannot reach.Picture Patrick Post

Sand lizards, pearl butterflies

After we enter the cordon, a lizard darts into a clump of grass in front of us. “A sand lizard”, says Kinsbergen. “There are also students looking specifically at the species, others are researching, for example, the small pearl butterfly, and one group is working on DNA from microbes in the soil.”

Many parties and people participate in the research in the dunes: PWN, University of Amsterdam, Butterfly Foundation, Bargerveen Foundation, volunteers and students from intermediate vocational training to university level, they all participate partially. Kinsbergen: “This is how I see the entire ecosystem, along with many others. It gives us a really good idea of ​​the effect the exclusions have on the whole system.”

Although the biologists can regularly be found inside the fences, their enthusiasm for the development is no less. “Look, you see all kinds of bumble bees on the purple thyme. And I see a caterpillar killer and a wasp. They are really huge sources of nectar. Inside the enclosures they can really grow into beautiful purple bulbs, outside they are immediately trampled by the deer.”


Further on, Van Til sees another species that is suddenly well again. “The dewberry, which is normally a fairly common species in the dunes, has been greatly degraded here due to overgrazing. And with the strawberry moth, whose larvae depend on dewberries.” Van Til believes that many species were still present as plants or as remnants underground.

“The species are often still there, but the vegetation no longer functioned because the plants did not get a chance to flower and form fruits and seeds. The research must show whether the recovery really ensures a well-functioning ecosystem, where species can also enter into the necessary relationships with each other.”

In the field there are four pyramids of black carrot cloth with a plastic pot at the top. “These are the insect traps, emergence traps to be exact,” explains Kinsbergen. “There are many insects that live part of their lives in the ground and at some point crawl out. Anything that creeps out within a square meter will eventually end up at the top of the pot. The Bargerveen Foundation will identify all these creatures in the laboratory.”


Ahead is a separate research setup, some poles with a funnel on them. “My showpiece, which I developed together with a postdoc, is nitrogen traps. Because overgrazing is not the only threat to dune nature, nitrogen still causes grass here. The nitrogen traps do not give a precise picture of the total nitrogen deposition at this site, but they provide enough information to be able to compare the different exclusions. In this way, we can see whether nitrogen can play a role in how nature recovers well or less well here.”

Ultimately, the results must also give a picture of the desired deer density. “We keep talking about overgrazing, but we also need grazing. Otherwise, the entire dune will be wider,” emphasizes Kinsbergen. “But it must be done with far fewer animals than now. It is also better to have a wider variety of grazers because each animal grazes differently and has a different impact on the landscape. PWN also has a few exclusions in the Kennemerduinen, which not only give an idea of ​​the influence of the low fallow deer population there, but also of the effects of cattle and horses running there.”

Waternet’s electric van drives the group to another barrier, which is located in a lower part of the dune area. Here we find plant species from the moist dune valleys, which had also largely disappeared due to grazing. Parnassia, round evergreen, but also orchids. “In terms of varieties, it’s really a party here,” says Van Til.

The poisonous ragwort is a wild, usually biennial plant with yellow flowers.  Picture Patrick Post

The poisonous ragwort is a wild, usually biennial plant with yellow flowers.Picture Patrick Post

Back after 20 years

“It is also full of insects. Further on you can still see the faded reed orchid and here is the swamp wasp orchid. Thanks to the exclusion, it has returned and is also expanding. And here we see heather, which has really been gone for twenty years”, continues Van Til.

“Oh look, there’s heather too!” notes Kinsbergen. “I hadn’t seen that before. I always call it ‘the exclusion that keeps on giving’. Every time I walk around here, I see something new. That is the beauty of my research. You often hear in the news how terrible things are for biodiversity, and that’s true, but I’m going to focus on the restoration of biodiversity, a place where species come back. It’s nice to work on such a bright spot.”

But for now it will be limited to the places inside the fences. “We will further reduce the population of fallow deer in the coming years,” says Van Til. “We hope to reach the target by 2024. Hopefully, the restoration of biodiversity throughout Amsterdam’s water supply dunes will really kick off.”

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