The beaver’s menu: species and spatial selection of a European beaver population and implications for riverbank bioengineering

Abstract

Riparian ecosystems are of utmost importance for many key ecosystem services, such as nutrient filtering or flood and erosion control. Restoring degraded riverbanks using soil bioengineering techniques initiates secondary succession improving both protection from erosion and riparian ecological functions. These techniques often rely on the settlement of Salicaceae and sometimes Tamaricaceae species that can be strongly impacted by beaver browsing. During the last decade European beaver (Castor fiber) populations have largely grown in France and can thus impact restored riverbanks, leading practitioners to adapt their management strategy. We conducted a study to assess the feeding preference of C. fiber and their browsing spatial pattern in semi natural riverbanks in the French Alps (Isere, France) by measuring browsing impacts along a transect. Our results indicate selective browsing occurrences at two nested spatial scales: firstly, beavers create large preferentially browsed patches (~ 8 m diameter) secondly, at a finer scale within these patches, they demonstrate a selective browsing between plant species. Among the species used in soil bioengineering, Myricaria germanica showed the lower browsing selectivity with only 2.5% of browsing probability while Salix eleagnos and Salix purpurea (10%) and Populus nigra, Salix daphnoides and Salix alba (20%) showed significant higher selectivity by beaver. Our results suggest that within the beaver presence area soil bioengineering techniques should consider adjusting both the composition and the spatial pattern of plant species settlement to mitigate the impacts of beaver on restored riverbanks.