Beavers (Castor fiber) increase habitat availability, heterogeneity and connectivity for common frogs (Rana temporaria)

Abstract

Beavers (Castor fiber), as typical ecosystem engineers, alter living conditions especially for amphibians through the building of dams and felling of trees, thereby changing the hydroperiod and substantially affecting forest succession stages. In this study we quantify the effects of beavers on the availability of amphibian breeding waters in the Hürtgenwald, a woodland area in the Central European Rhenish Massif, its colonisation by common frogs (Rana temporaria) and the effects of age and succession stage of beaver ponds on ovipositional site selection. In 2013, beaver ponds comprised about half (49%) of all lentic water bodies but contained 82.5% of all common frog egg masses. Mature beaver ponds (>6 years old) harboured approximately half of the egg masses (), but new beaver ponds (1-3 years old) can also be home to large breeding aggregations. Abandoned beaver ponds are of minor importance as ovipositional sites for common frogs. High egg mass counts were also found in artificially-dammed ponds (). We believe that common frogs prefer occupied beaver ponds as ovipositional sites because of high insolation and a permanent hydroperiod, which lead to rapid tadpole emergence. Beaver ponds are generally located in close proximity to each other, facilitating movement and rapid colonisation by common frogs. Our research provides additional evidence to show that beavers enhance habitat availability, heterogeneity and connectivity, thereby fostering amphibian populations at a landscape level. As natural elements of small streams, beaver ponds must be taken into account in the context of the EU Water Framework Directive.