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Declining invasive grey squirrel populations may persist in refugia as native predator recovery reverses squirrel species replacement

Cite this dataset

Twining, Joshua; Montgomery, Ian; Tosh, David (2020). Declining invasive grey squirrel populations may persist in refugia as native predator recovery reverses squirrel species replacement [Dataset]. Dryad.


  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.


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

Usage notes

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. 


Challenge Fund

Challenge Fund