English abstract

The European beaver has recolonized Flanders since 2000 and, as a European Habitats Directive species, is protected throughout the region. To warrant this, a regional species protection program has been drawn up, which has set a threshold of 100 reproductive units as a favorable conservation status for the species in Flanders. Since the presence of beavers can also result in potential conflict, the program also offers possibilities for management, provided this not jeopardize the favorable conservation status. To enable verification of this condition, long-term and high-quality monitoring data are required.

The evolution of the beaver population size in Flanders is currently monitored using the number of occupied territories as a proxy, assuming that 167 territories correspond to 100 reproductive units. This study evaluates the correctness of the current monitoring data and its repercussions for the evaluation of the favorable conservation status.

A comparison between the assumed and observed occupancies in 100 of 128 territories known on January 1st 2017, showed an accuracy of 82 %. Additional analyses further show that the occupancy chance of a territory is determined by its distance to the nearest occupied neighboring territory. More important than accuracy, which evaluates both presence and absence data, was the evaluation of precision (the accuracy of presence only data). This value was 91 % correct, which limits potential overestimation of occupancy to 9 %. Taking into account the level of uncertainty of this precision, the actual number of territories occupied has a 95 % probability of surpassing 85 % of the number of occupied territories assumed.

This results in a rule of thumb that, when the number of assumed occupied territories is multiplied by 0.85, there is a negligible chance of overestimation. In addition, the underlying assumptions that define the relation between the number of territories, the population size and the number of reproductive units are sufficiently conservative so that the likelihood of overestimation is also minimal. In order to increase the precision of the monitoring even further, the choice can be made to consider new, isolated territories as occupied only when occupation can be established for at least two years in a row. This reduces the chance that a too high proportion of territories occupied by loners will be included in the data. Not only do such territories have an increased chance of becoming vacant again, a too high share of such territories in the dataset could also lead to an excessive discrepancy between the assumed and the effective proportion of reproductive units per territory.