Five losers and one winner in our struggling freshwater nature | News

After years of steady recovery, the improvement of Dutch freshwater nature is stalling. Important populations of fish and dragonflies are declining, while invaders such as the American crayfish and gobies are hindering native species.

What do the bullhead, the northern vole and the green glazier have to do with each other? All three species symbolize the, in some ways sudden, decline of our freshwater nature. Although there are also species that fortunately do well.

More about those specific species – five losers and one winner – later, but first a little context.

It is thanks to some ten thousand volunteers who count species that we can sketch a long-term picture of how nature is developing for the whole of the Netherlands. Since 1990, organizations including the World Wildlife Fund (WWF), BedrijvenNL and the Mammal Society have been compiling the Living Planet Report based on this data.

Since 1990, many land-dwelling species have been struggling. Remarkably, the picture for freshwater nature was much rosier for a long time afterwards, and contrary to the global trend. In recent decades, populations of important species in and around rivers, streams and swamps have grown by an average of 60 percent.

“We made great strides in the 1990s,” says Judy Koppenjan, an expert in Dutch nature at WWF. “Discharges of heavy chemicals into our rivers were restricted. This resulted in better water quality and the successful reintroduction of the otter and beaver.”

Yet ecologists are now signaling a change. Koppenjan: “We are moving from improvement to a faltering recovery and decline.”

There are several threats that are piling up. For example, rivers and streams still contain an enormous number of obstacles for migratory fish and other species. Furthermore, climate extremes such as prolonged drought are disastrous for the development of larvae, causing dragonflies to disappear.

And then there is the direct influence of humans. Pesticides that leach into ditches are harmful, as is acidification by nitrogen. “In some ways we are now even further away from home than when we started nature restoration in the 1990s,” says Sander Turnhout of TelefoonNL.

To give a face to the struggling freshwater nature, below are five species that are having a hard time in the Netherlands. And we end with a winner.

The situation of the bullhead is ‘downright worrying’, the Living Planet Report outlines. Of all the fish in the large rivers, this bottom fish has declined the most in numbers. Initially the species benefited from the improved water quality, but after 2010 the species was virtually decimated.

And that is, strangely enough, due to the introduction of an exotic groundfish: the blackmouth goby. Turnhout: “The Main-Danube Canal opened in 1992. Since then, various goby species have reached our major rivers. Previously, our delta was not connected to that of the Danube, but now it is.”

The black-billed goby reproduces very quickly, eats small prey and displaces the bullhead. It happens in rivers, but also in the Marker and IJsselmeer.

The little loach, like the bitterling, first saw an increase in the population between 1990 and 2010. But since then the species has been under pressure again. Our non-nature-friendly design of streams and ditches in particular stands in the way of the small loach.

Turnhout: “Our water management has too little to offer this species. There are too few places that occasionally dry out. And it does need that, for example small ditches around the Biesbosch.”

What is a mouse doing among freshwater nature? Marshes and marshy grasslands are the ideal habitat for the Northern Vole. The species is special because it only occurs in the Netherlands. Yet this population has been under severe pressure since the 1990s: wherever there is no longer a natural water level (read: the water level is lower for agriculture), the species loses competition from other mouse species.

Turnhout: “The Northern vole is very special. Thanks to its special tail, it can move very quickly between the reeds.” Yet there are fewer and fewer in, for example, the Weerribben-Wieden (Ov). An additional problem is that existing populations are small and live isolated from each other.

Dragonflies show a diverse picture. Southern species are doing better thanks to climate change. Northern ones less so. But something special is going on with the green glazier: an invasive exotic species is destroying an indispensable part of its habitat.

“The species is dependent on the crab clam, an aquatic plant with serrated, sword-shaped leaves,” says Turnhout. “The larvae of the green glazier grow under the leaves and climb up the leaves. But the American crayfish destroys the plants, causing the dragonfly to disappear.

By the way: mowing farm ditches unnaturally tightly does not help either. As a result, other species disappear and the crayfish is given complete free rein.

The white-beaked dragonfly was briefly known as a success story. In the last century, the species, which only lives in really clean water bodies, was as good as extinct in the Netherlands. Yet he returned thanks to improved nature management and some coincidental wanderers from Germany or Belgium.

The population has been declining again since 2020. As soon as the water in fens and ponds becomes too rich in nutrients or the water level fluctuates too much, the species disappears. In addition, the larvae cannot withstand extreme drought, heat waves and heavy downpours.

Fortunately, small miracles still take place in Dutch freshwater nature. The last crane turned its back on our country about 250 years ago. And yet he is back again. Since the beginning of this century. There are now 81 breeding pairs living in our country.

According to ecologist Turnhout, the crane is proof that nature around raised bog vegetation can also recover well, such as in Korenburgerveen (Gld). “It has been ensured that the dirty drainage water no longer reaches the peat. As a result, all kinds of species have returned. And we were even surprised by the arrival of the crane.”

The species is very sensitive to disturbance. But the raised bog area is not accessible to anyone, not even dogs. “Or you have to want to end up as a bog corpse.” And so the crane gives our country a new chance.

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