A reintroduced ecosystem engineer species may exacerbate ongoing biological invasion: selective foraging of the Eurasian beaver in floodplains.
Species reintroductions aim to re-establish populations of locally extinct, often keystone species. As a consequence of the reintroduction and protection of the Eurasian beaver (Castor fiber), the species has recolonized the suitable habitats (including floodplains) across Europe in the past few decades. Floodplain habitats have already been significantly altered during the period when the beaver was listed as a locally extinct species. Nowadays, alluvial forests are being highly impacted by rapidly spreading invasive woody species. We examined the supply of woody species and their utilization by beaver in this changed environment, with a fine-scale method, at 20 sites, along 39 transects (Danube catchment area, Hungary). The invasive species (Acer negundo, Fraxinus pennsylvanica and Amorpha fruticosa) were utilized by beaver, but the utilization ratio of softwood genera (Salix and Populus) dominated by native species was usually higher. Based on the preference analysis, performed for the thin branches category, at least one of the softwood genera was significantly preferred along 17 transects, and they were never avoided. In parallel, at least one of the invasive woody species was significantly avoided along 17 transects, and preferred along only one transect. We conclude that beaver's forage selection is capable of indirectly accelerating the spread of already present invasive species, a negative process from a nature conservation perspective. This adverse impact points to the need for increased attention on changes that can occur within the habitat of a reintroduced species, and on the possibility of unexpected conflicts among conservation targets.