The regional demise of a non-native invasive species: the decline of grey squirrels in Ireland.
Following the introduction of the grey squirrel (Sciurus carolinensis) to Ireland in 1911, the species quickly established and spread to cover the eastern half of Ireland. Historically, the River Shannon has delineated the western boundary of its distribution in Ireland, however the factors limiting the spread of the species westwards were unclear. The aims of this study were to assess the current squirrel distribution in the area directly bordering the River Shannon, and to identify habitat types and landscape characteristics that could be facilitating or impeding the spread of grey squirrels in Ireland. The current distribution was established through hair tube and live trapping surveys and through sightings from a citizen science survey. Grey squirrels are absent or in very low numbers in much of the study area. In some areas, red squirrels have reappeared where they previously had been displaced by the grey squirrel. Discriminant function analysis was used to identify significant differences in habitat types and landscape characteristics between a region with high grey squirrel occurrence records and a region where they are now rare. Several landscape attributes were found to be significantly different, including the presence of pine marten, water bodies, peatland and coniferous forests. The area in which grey squirrels have disappeared overlaps with the core pine marten population range, and in a landscape that is more fragmented than the areas in which grey squirrel are continuing to be invasive. The demise of the grey squirrel in Ireland is more widespread than previously believed.