Grey squirrel (Sciurus carolinensis) bark-stripping damage in UK woodlands: motivation and mitigation

The introduction of eastern grey squirrels (Sciurus carolinensis) to Britain from the late 19th century has led to extensive and ongoing damage to trees, caused by their bark-stripping behaviour. This damage undermines the UK Government’s ambitious afforestation targets for expanding woodland area to sequester carbon. Previous observational studies have identified a spectrum of susceptibility to grey squirrel damage amongst tree species and tree stands. With important native broadleaves such as beech (Fagus sylvatica) and oak (Quercus spp.) amongst the most susceptible, advocating for planting of alternative species is not an effective management solution for afforestation that aims to address the coupled emergencies of climate change and biodiversity loss. As current correlative knowledge of tree susceptibility does not allow for a causal understanding of the behaviour, the motivations of grey squirrel bark-stripping are still poorly understood, hindering the development of non-lethal management options that target bark stripping. This thesis aims to determine the current knowledge gaps in drivers, impacts, and control of grey squirrel bark stripping, and to explore the chemistry that mediates interactions between grey squirrels and broadleaf trees that vary in their susceptibility to squirrel damage. Firstly, I examined previous literature on sciurid bark-stripping research using systematic mapping methods. I demonstrated a paucity of knowledge about the causes explaining bark-stripping behaviours, particularly for species responsible for significant economic damage (e.g. S. carolinensis, Callosciurus erythraeus). Future research should prioritise experimental designs and interdisciplinary collaborations that aim to establish causal understanding of the behaviour, with a goal to inform the development of successful mitigation strategies. To address this knowledge need, I described the composition of volatile organic compounds (VOCs) produced by the foliage of ten Populus cultivars varying in their susceptibility to squirrel damage. I identified compounds strongly associated with resistance and with susceptibility to squirrel damage, suggesting that tree chemistry could be mediating grey squirrel bark-stripping behaviour through either deterrent or attractant properties. I then demonstrated the effects of six VOCs (DMNT, β-eudesmol, camphene, acetophenone, ocimene and farnesene) on squirrel behaviour in three Hampshire woodlands. I detected evidence of repellent properties of DMNT, β-eudesmol and camphene, and attractant properties of acetophenone, ocimene and farnesene to squirrel feeding. These VOCs have the unique potential to inform development of ‘push-pull’ strategies to protect vulnerable trees and woodlands against squirrel damage. The novel insights into the bark-stripping behaviour of grey squirrels described in this thesis provide the first empirical evidence of natural plant VOCs influencing grey squirrel bark-stripping behaviour. I discuss the potential to harness these interactions into effective non-lethal management methods.