The countings take place every year, on exactly the same spots. This allows researchers to get an idea of how the species is doing. The stretches cover almost 118 square kilometres, with countings taking place four times a year.
In 2009, these countings resulted in an average of 174 roe deer being spotted. Last year this number had plummeted to reach 82, less than 50 percent. Jim Casaer, a biologist working for the National Institute for Research into Nature and Woodlands (INBO), explains that the downward trend started in 2013.
Explanations are now being sought, but nobody is really sure what the cause acually is. In fact, the sharp decline is a kind of a mystery. "It's possible that the deer have become less visible", says Casaer. But this is being contradicted by others, who claim low vegetation has dimished.
"The animals are perfectly healthy"
Another possibility is that the animals are perishing due to a new type of disease. "We examined roe deer that had been run over by traffic.
But they turned out to be completely healthy. And if traffic would be the cause, then their number should rise again in the years to come, because large stretches of woodlands are now being joined together again, to avoid that the deer have to cross the road."
Could it be wild boars causing the dramatic fall? "There are not so many wild boars in the Sonian woods. But it's possible that recreation and dogs on the loose could have an impact. Some are talking about poachers, but I have no figures about this."
Forest ranger Dirk: "I can confirm you: the deer have gone"
Dirk Raes has been a forest ranger in the area for 34 years now. He dismisses the possibility of poaching: "I have known one case throughout my career. And wild boars are not involved either. Recently, six were run over by cars. Their population is very small."
Raes is quite clear about the cause. He thinks it's human intervention: "It's the human pressure as recreation is increasing. People are crossing large parts of the woods, often without keeping their dog on a leash. The dogs frighten the roe deer and chases them away. Recently, a pregnant deer was overrun by a car. This is almost certainly the consequence of a dog chasing it."
"You see, roe deer are intelligent animals. They keep well away from the walking trails. But the dogs are scaring them. I can say one thing: I used to know where the deer were hiding. Now I can tell you: they have gone."