The growth of Sitka spruce plantations in Ireland over the past two decades may help explain why there has been a resurgence in the number of native red squirrel populations. The increase in the native pine marten population may also be a contributing factor as the invasive American grey squirrels appear to be a more popular choice for pine martens dining.
In Ireland, the invasive American grey squirrel, Sciurus carolinensis, threatens the survival of the native red squirrel, Sciurus vulgaris, as the effects of competition and disease almost inevitably lead to total replacement of red squirrel populations. However, the results of a national squirrel survey suggests that the normally invasive grey squirrel had gone into decline in the Irish midlands, which was anecdotally attributed to an increase in European pine marten, Martes martes, range and numbers*.
The American grey squirrel is also larger and heavier than its native counterpart and consequently cannot escape to the end of Sitka spruce branches to avoid becoming the main course for the increasing Pine marten population, although further research will be needed to confirm this hypothesis. Pine martens may also find the greys easier to catch given their greater activity time spent on the ground.
In other research conducted in 2014 it was noted that red squirrels did not select the habitat with the best tree seed crop (Scots pine) but preferred dense Sitka spruce plantations; they appeared to avoid the Scots pine area with its high grey squirrel density. Research data suggests that adult red squirrels suffered little from interspecific competition with grey squirrels and that the key factor is decreased juvenile recruitment in red squirrels.**
In a study aimed to quantify changes in squirrel distribution in Ireland and to investigate the role, if any, of the pine marten in red and grey squirrel population dynamics, a distribution survey of the midlands was carried out which confirmed the grey squirrel population has crashed in approximately 9,000 km2 of its former range and the red squirrel is common after an absence of up to 30 years. At landscape level, pine marten and red squirrel abundance were positively correlated, whereas a strong negative correlation between pine marten and grey squirrel presence at woodland level was found to exist. Squirrel demographics were determined by means of live trapping programs which confirmed that the red squirrel in the midlands is now in competitive release and the grey squirrel is present at unusually low density. This study provides the first evidence of a regional grey squirrel population crash and suggests that European pine marten abundance may be a critical factor in the American grey squirrel’s success or failure as an invasive species*.
*Population crash in an invasive species following the recovery of a native predator: the case of the American grey squirrel and the European pine marten in Ireland
Emma Sheehy, Colin Lawton 2014
** Interspecific effects of grey squirrels (Sciurus carolinensis) on the space use and population demography of red squirrels (Sciurus vulgaris) in conifer plantations
Luc A. Wauters, Peter W. W. Lurz, John Gurnell 2001