English abstract

As a result of its recent range expansion throughout Flanders, the European beaver has also started colonizing less favorable habitats, with associated risks of excavation damage to dikes. The question then arises of how great these risks are and to what extent they can be made manageable. The Vlaamse Waterweg nv therefore asked INBO to help map out the presence of beavers and the way in which they use a specific area along the Dyle and Demer river in Flemish Brabant. This knowledge is crucial to inform a balanced damage management that also maximizes opportunities for beaver presence. Next to traditional inventory methods, research via VHF-telemetry proved to be a suitable, but labor-intensive, addition.

A total of 22 beavers could be provided with a transmitter, of which 14 provided sufficient data for detailed analysis. The results showed little to no territory overlap, so that the number of established territories was equal to the number of families present. On the Dyle between Werchter and Muizen, a stretch of approximately 15 km, three distinct territories were identified, each covering 5 – 6 km of river length. Although it is clear that there are three families within this zone, the number of beavers present is more difficult to estimate and probably fluctuates between 12 and 20 individuals throughout the year. Rare intrusions from beavers from neighboring territories and dispersion of young beavers from more upstream territories may temporarily increase this number, but this effect is short-lived.

We found that, where possible, beavers built their dens in accessible, disconnected meanders close to the main river and that they also spent a significant amount of their time there. This implies that preserving these meanders, making them accessible or improving their quality as beaver habitat, can be good management measures. We also found that territories with continuously accessible meanders, as well as territories with meanders that temporarily dry up and territories without any meanders, all roughly occupied the same river length. This implies that the biological carrying capacity of the system investigated here is fairly constant and that food and habitat is sufficiently guaranteed throughout.

Although adjacent meanders can form a very important part of the beaver habitat, they are not a prerequisite for beaver presence. Beavers that have access to meanders which hold sufficient water, have little disturbance and provide sufficient shelter and food, will locate their den there and spend a large part of their time there, reducing the risk of burrows and excavated resting places in the dikes of the main watercourses. Due to the strong territorial behavior, the presence of a beaver family further guarantees the absence of additional settlement of other beaver families on the main watercourse in a wide zone. In function of the carrying capacity and possible dike damage, there are strong indications that systems with more hiding possibilities in the form of accessible adjacent meanders have less risk of damage.