DOI: 10.5061/dryad.pzgmsbch7
title: Declining invasive grey squirrel populations may persist in refugia as native predator recovery reverses squirrel species replacement

description: Invasive species pose one the most serious global threats to biodiversity.
 Investigations into the interactions of native and non-native species
 focus on the impacts of single species, despite being embedded in a
 network of direct and indirect interactions between multiple species and
 their environments. We developed 1 km2 resolution, single-species and
 multi-species, occupancy models using quantitative camera trap data
 collected by citizen scientists at 332 sites in a regional survey
 comprising the 14,130 km2 of Northern Ireland. Recent research suggests
 that native red and invasive grey squirrels in Britain and Ireland, are
 linked by resource and disease mediated competition, and by a shared
 enemy, the European pine marten. We demonstrate that the presence of the
 pine marten reverses red squirrel replacement by grey squirrels on a
 regional basis, with red squirrel occupancy positively affected by
 exposure to pine marten. In contrast, the grey squirrel has a strongly
 negative response to the presence of pine marten. Modelling habitat
 suitability of pine marten and both squirrel species
 suggests that, despite the potentially strong effect of a
 recovering population of pine martens in controlling grey squirrel
 distribution, the latter is likely to persist in urban refugia that are
 either inaccessible or avoided by the pine marten. Synthesis and
 applications. Sustainable recovery of both the red squirrel and the pine
 marten populations across the wider landscapes of Ireland, Scotland and
 Northern England seems probable, whilst in Southern England and Wales,
 although possible, chances are more remote. Increasing native woodland
 cover on a landscape scale may assist in realising this scenario. Despite
 the recovery of the pine marten, isolated populations of grey squirrels
 are likely to persist. Human-lead control of grey squirrel populations in
 urban refugia requires urgent funding and implementation to avoid the
 development of novel genotypes in grey squirrel source populations that
 enable the species to better avoid predation by the pine marten.
description: Data was collected by 70 citizen scientsts over 330 sites using a
 standardised camera trap and feeder method in Northern Ireland in
 2015.  Camera traps were returned to lead author and
 all images processed and detections of focal species recorded
 All detections from camera traps returned by volunteers are
 provided in the
 csv "Allsitesallspeciesoccurencedata2015citizenscience" An overview of sites sampled, XY coordinates, Irish Grif refs, and deployment details of camera traps are provided in excel document"2015citizensciencesquirrelsurveyoverview" Finally all habitat covariate values for all sites included in analysis, and all 1km squares of Northern Ireland are included in csv "Habitat info full all sites". These values were extracted from Land Cover Maps, 2007
description: In viewing the Overview dataset, please note the rows highlighted
 red show sites where volunteers did not provide the images from their
 camera traps (for example deleted them prior to providing photos to
 authors). These sites were therefore not suitable for inclusion in the
 work and creation of detection histories.