The role of the pine marten in Irish squirrel population dynamics

Sheehy, Emma
There are two squirrel species found in Ireland. The native red squirrel, Sciurus vulgaris, and the invasive North American grey squirrel, Sciurus carolinensis. In Ireland, the UK and Italy the grey squirrel threatens the survival of the red squirrel as the effects of competition and disease from the grey squirrel almost inevitably lead to total replacement of the red squirrel population. In 2007, the results of a national squirrel survey suggested that the normally invasive grey squirrel population in the midlands of Ireland had gone into decline. Whilst occasionally grey squirrel introductions have failed elsewhere, such a decline in range has not been recorded anywhere else for this species subsequent to having established itself as an invasive population. In the 2007 report, the decline in grey squirrel range was anecdotally attributed to an increase in European pine marten, Martes martes, range and numbers. The pine marten population in Ireland reached a nadir in the early 20th century as a result of habitat loss and persecution, but the core population in the west and midlands of Ireland have recovered in recent decades. The 2007 survey recommended the theory that the pine marten population may be involved in the decline of the invasive grey squirrel's range be investigated. This project serves to carry out that study by investigating the role, if any, of the European pine marten in red and grey squirrel population dynamics in Ireland. The distribution and status of all three species were examined in both the midlands of Ireland and a control region in the east, where all three species are also present but the grey squirrel range had not been reported as having gone into decline. Detailed distribution surveys of the study and control regions were carried out by means of sightings and field surveys which confirmed that the grey squirrel is now rare in approximately 9,000km(2) of its former invasive range. The distribution survey also established that the red squirrel has successfully recolonised much of this area and a live-trapping program confirmed the red squirrel population in the midlands is now in competitive release. The grey squirrel continues to thrive in other parts of the country however, including the control region in the east of the country. Results from a pine marten diet and density study suggest that the abundance of the native predator population may be a critical factor in the grey squirrel's success or failure as an invasive species. Non-invasive methods of studying this elusive carnivore were also investigated, and the genotyping of remotely plucked hair samples in the study and control regions revealed the pine marten to be considerably more abundant in the midlands than the east of Ireland. Real-time PCR techniques were utilised to determine the frequency of occurrence of squirrels and other small mammals in pine marten diet and the results were compared to a sub sample of scats which were subject to traditional macro analysis. Both techniques were found to yield similar results suggesting molecular techniques are a useful tool in studying pine marten diet. The first evidence of European pine marten predating upon the North American grey squirrel is reported, however a negative correlation between pine marten and grey squirrel presence at woodland level resulted in a small sample size of scats in areas that the grey squirrel was available as a prey item. This negative correlation in distribution also suggested that the non-lethal effects of an abundant predator population may be influencing the grey squirrel's ability to succeed as an invasive species in Ireland more so than direct predation itself. The possibility that pine marten scent marking may be influencing grey squirrel behaviour was also explored through a literature review and an experiment with grey squirrel sensitivity to predator odour. Red squirrel distribution at landscape level was positively correlated with that of its natural predator, and the area which the red squirrel has recolonised falls entirely within the midlands pine marten population's range. The dietary study confirmed that the red squirrel is a low frequency prey item in the abundant pine marten population's diet. The true relationship between the pine marten and the grey squirrel has yet to be established however, and recommendations for future studies are provided.
Publisher DOI
Attribution-NonCommercial-NoDerivs 3.0 Ireland