Reinstating trophic cascades as an applied conservation tool to protect forest ecosystems from invasive grey squirrels (Sciurus carolinensis)

Red squirrel (Sciurus vulgaris) range within the United Kingdom (UK) has retracted significantly due to the spread of an Invasive Alien Species, the North American Eastern grey squirrel (Sciurus carolinensis). Where grey squirrels are sympatric, red squirrel populations decline through inter-specific competition and squirrelpox virus (SQPV) infection. Grey squirrel eradication from the island of Anglesey facilitated the complete restoration of native red squirrels. Although native species recovery delivered significant ecological and economic benefits, the eradication extended only to a narrow sea-channel boundary, across which grey squirrel dispersal continues to occur. Hence, the long-term sustainability of Anglesey's red squirrel population is vulnerable to grey squirrel re-establishment without continuous intervention. Recent research has demonstrated that as pine marten (Martes martes) landscape use intensity increases, so too does red squirrel occupancy, likely linked to parallel declines in grey squirrel occupancy. Restoration of this mustelid predator is a potential tool to deliver sustainable grey squirrel control by restoring a missing trophic component, depressing grey squirrel incursion rates onto Anglesey, reducing red squirrel exposure to SQPV. Recent UK pine marten translocations have sourced animals under licence from wild Scottish populations. We explore the alternative use of captive-bred founders, simultaneously introducing new genetic variability against a limited diversity within extant populations. A current conservation translocation is paired with an ongoing assessment of founder behaviour and ‘personality’ measured before release. We then highlight the multi-disciplinary approach to delivering applied red squirrel conservation programmes in the face of invasive species.