Interactive effects of beaver and deer on Scotland’s riparian woodlands

Abstract

Restoring lost interspecific interactions through reintroductions is a key aim in some habitat restoration projects, but few focus on the interspecific interactions specifically between herbivores. As a result, herbivore interactions are rarely considered in species management policies or woodland expansion targets, despite their potentially important role in shaping ecosystem function. The Eurasian beaver (Castor fiber) is an ecosystem engineer that is expanding across Scotland, following a long absence, into riparian woodlands already being structurally altered by deer (Cervidae). Riparian woodlands are high value habitats in terms of biodiversity and mitigation of climate change impacts, but their quality and extent have dramatically declined due to anthropogenic pressures. Beavers and deer both exert their own unique influence on riparian woodlands through browsing, yet no published research in Europe has so far investigated the relationship between beaver and deer browsing and how it could impact woodland structure, composition, and regeneration processes. This thesis examined beaver-deer interactions in riparian woodlands through a combination of field-based surveys and experiments in Scotland. The foundation of beaver-deer interactions was firstly examined by studying beaver tree foraging preferences and their temporal effects on woodland composition and structure (Chapter 2). While beavers can alter their habitat drastically at a small patch-scale over a short period through highly selective foraging, it was found that woodland composition or structure did not significantly change over an 11-year period. The second field-based survey (Chapter 3) confirmed that beaver herbivory can promote riparian woodland regeneration and habitat complexity by creating a mosaic of mature and multi-stemmed, coppiced trees. In addition, the secondary shoots from resprouted beaver-felled trees were found to be readily available, nutritious, and morphologically appealing in terms of their distribution and density, which could enhance resources for browsing deer and influence deer distribution. Finally, the mechanistic elements of beaver-deer interactions were explored by tracking the growth of experimental riparian willow (Salix cinerea) stands (Chapter 4). This demonstrated that deer browsing on resprouted beaver-felled trees is likely to alter tree structure and resource allocation over time, but effects will depend on deer density. It also revealed that changes in soil moisture (caused by beaver damming) may play a role in tree responses to beaver-deer interactions. As beaver and deer continue to expand into intensely-modified, populated landscapes throughout Europe, their interactions in riparian woodlands are likely to become increasingly commonplace. The research presented in this thesis highlights that nationwide riparian planting/enhancement, coupled with standardised monitoring of the impacts of herbivore interactions, should be considered a preventative priority in future beaver/deer management plans and woodland expansion targets.