Genetic consequences of beaver reintroduction in a multi-lineage contact zone

The Eurasian beaver (Castor fiber Linnaeus, 1758) has experienced a remarkable recovery in Europe, spurred by reintroductions following near-extinction. In Belgium and the Netherlands, reintroduction programs launched since the late 20th century have resulted in rapid population growth between 2000 and 2020. These efforts involved individuals from genetically distinct source populations (Bavarian, Elbe, and Polish). To assess genetic consequences of these programs, particularly in the context of range expansion and conservation success, we investigated patterns of population structure, admixture, and genetic diversity. We analysed 129 individuals across northern Belgium and the Netherlands using 13 microsatellite markers. Our analysis reveals three major genetic clusters, reflecting the different reintroduction sources. Crucially, we show that the Meuse Basin now acts as a genetic convergence zone, facilitating secondary contact and admixture between individuals from the three main source clusters and showing higher levels of genetic diversity. Our findings underscore the permeability of landscape barriers and the dynamic nature of beaver range expansion.