Grey Squirrel Density and Damage in Relation to the Presence of Predators

The grey squirrel (Sciurus carolinensis) is an invasive species in Great Britain that causes substantial economic and ecological damage through stripping bark from broadleaf trees. The relationship between grey squirrel abundance and tree damage remains poorly understood, as does the potential role of recovering native predators in mitigating these impacts. We conducted an observational survey of 22 woodland sites across England and Wales with differing occurrences of pine martens (Martes martes) and northern goshawks (Accipiter gentilis), estimating grey squirrel density and quantifying bark-stripping damage. Grey squirrel density varied across sites. No significant relationship was detected between squirrel density and observed tree damage when analyses were restricted to sites where squirrels were present. However, when sites with no detected squirrels were included, squirrel density was positively associated with tree damage. Predator occurrence was associated with significantly lower grey squirrel densities. In contrast, predator presence was not significantly associated with the proportion of trees exhibiting bark-stripping damage. These findings suggest that predators may contribute to grey squirrel population suppression but that bark stripping is influenced by factors beyond contemporary squirrel abundance. Forest management practices that facilitate predator presence should be encouraged to support reduction in grey squirrel density and therefore improve future tree health. Further research should focus on fresh damage assessments and predator-mediated behavioural responses to better understand the mechanisms linking predator recovery, squirrel abundance and tree damage.