Natural strongholds for red squirrel conservation in Scotland

Abstract

The Eurasian Red Squirrel (Sciurus vulgaris) is under threat from the invasive North American eastern Grey Squirrel (Sciurus carolinensis) with 80% of the remaining red squirrel populations in the British Isles found in Scotland. In this study we develop a spatially explicit mathematical model of the red and grey squirrel system and use it to assess the population viability of red squirrels across Scotland. In particular, we aim to identify existing forests – natural strongholds for red squirrels – that can successfully support red squirrels under UK Forestry Standard management and protect them from potential disease-mediated competition from grey squirrels. Our model results indicate that if current levels of grey squirrel control, which restrict or reduce the distribution of grey squirrels, are continued then there will be large expanses of forests in northern Scotland that support viable red squirrel populations. Model results that represent (hypothetical) scenarios where grey squirrel control no longer occurred indicated that grey squirrel range expansion and the process of red squirrel replacement would be slow. Model results for an assumed worst-case scenario where grey squirrels have expanded to all regions in Scotland identified forest regions – denoted natural strongholds – that could currently support red squirrels under UK Forestry Standard management practice. The results will be used to inform forest management policy and support a strategic review of red squirrel management by land management agencies and other stakeholders.

Habitat mediates coevolved but not novel species interactions

“…The native prey is, however, suppressed in the presence of the native predator in the habitat where the invasive competitor does not occur, suggesting an alternate mechanism. In the absence of pine martens, red squirrels are thought to be able to persist in conifer plantations owing to a competitive advantage over grey squirrels [ 20 ]. Thus, in conifer plantations, the red squirrel does not benefit from competitive release from grey squirrels following pine marten recovery but is subject to predation by the shared predator [ 33 ].…”

“…Our multi-species approach provides key insights into factors driving the occurrence and interactions of a complex and conservationally important interaction network which were not otherwise evident. For example, previous research has suggested that interactions between red and grey squirrels depend on habitat alone, with predictions ranging from complete extirpation of the native inferior competitor in native broadleaf woodlands to the persistence of the inferior native competitor in large commercial conifer plantations [ 11 , 20 ]. This has led to recommendations that national conservation strategies for red squirrels should focus on the planting of commercial conifer plantations as opposed to native broadleaf forests, where grey squirrels have a competitive advantage [ 11 , 20 ].…”

“…For example, previous research has suggested that interactions between red and grey squirrels depend on habitat alone, with predictions ranging from complete extirpation of the native inferior competitor in native broadleaf woodlands to the persistence of the inferior native competitor in large commercial conifer plantations [ 11 , 20 ]. This has led to recommendations that national conservation strategies for red squirrels should focus on the planting of commercial conifer plantations as opposed to native broadleaf forests, where grey squirrels have a competitive advantage [ 11 , 20 ]. Our results suggest that such management strategies could undermine ongoing red squirrel recovery efforts, with consequences likely antithetical to their intention.…”

“…When accounting for additional actors, and the mediating role of habitat, we observed a reduction in the occurrence of red squirrels in large structurally simple conifer plantations where pine martens were present. Such commercial plantations constitute the majority of Ireland and Scotland's forest cover, where they continue to be planted under the guise of saving the red squirrel [ 20 ]. Our results suggest that landscape management strategies for red squirrel conservation would be best focused on planting native broadleaf woodlands alongside continued pine marten restoration efforts.…”

“…We use multi-species occupancy models applied to a dataset collected on three occasions over 5 years from 2015 to 2020 to examine whether species co-occurrences and interactions differ along environmental and spatial gradients. We expect: (i) the impact of the pine marten on grey squirrels to be consistent regardless of local habitat owing to the naivety of the invasive species to the native predator [ 19 ]; (ii) the interactions between the pine marten and the red squirrel to be dynamic and dependent on habitat, with more structurally complex and diverse habitats resulting in lower impacts on the native prey species; and (iii) the competitively linked native-invasive prey species interactions to be mediated by habitat [ 20 ].…”