The tale of how the Eastern grey squirrel Sciurus carolinensis spread like wildfire through the British Isles from just a few introductions, out-competing and spreading lethal squirrel-pox to the native red squirrel Sciurus vulgaris, is one of our country’s most well-known species declines, and a conservation issue notoriously difficult and controversial to manage.
An estimated 2.5 million greys are now believed to be living in Britain, and have become a familiar part of ‘British’ wildlife. With these American cousins looking set to stay, it would appear any future for red squirrels in Britain would be on offshore island populations like the Isle of Wight.
However, the population recovery of another British mammal species – the pine marten Martes martes – may prove to be the red’s indirect saviour.
Pine martens used to be one of Britain and Ireland’s most numerous carnivores. Yet, around the same time that grey squirrels were introduced to England, forest clearance and intense persecution from gamekeepers during the 19th century left the pine martens restricted to remote upland areas, such as the Scottish Highlands.
After being granted full protection since 1981 however, pine martens are beginning to increase in number from these refuges, and may be significantly predating the greys.
A study by Emma Sheehy and Colin Lawton of the National University of Ireland, published in Biodiversity & Conservation, focused their research on pine marten, red squirrel and grey squirrel populations within the Irish midlands, where the former has been recovering in similar patterns to populations on mainland Britain.
Dividing the area into three study zones – one where a potential decline in greys and increase in reds had been reported, another where all three species had been recorded but with no reported grey squirrel decline, and finally a buffer zone surrounding the latter area.
Using a combination of hair tubes, live trapping of squirrels and direct sightings from both professionals and locals over a 2-year period to survey populations of the three animals, the researchers were able to gain a reliable model of the density of each species.
A high density of pine martens was found to be strongly associated with a low density of grey squirrels. Subsequently, with reduced pressure from greys, red squirrel density was much higher where pine martens were common.
This study has revealed the first documentation of grey squirrel population retraction that was not instigated by human control, creating intriguing implications for the grey squirrel’s future across the rest of Ireland and Britain as a whole.
Within the midlands zone, where pine marten and red squirrel densities were high and grey squirrels low, pine marten scats had no grey squirrel remains within them.
However they were discovered in the scats of the low density marten population in the east, where populations of grey squirrels are high. Pine martens are opportunistic hunters, so when squirrels present themselves in high densities they make an easy meal.
No red squirrel remains were found in any scats, perhaps because reds spend more time in trees, whilst the greys are less arboreal, making them easier prey to the pine marten.
Pine martens are becoming more widespread within their Scottish stronghold, and the species was confirmed in Wales back in 2012. Meanwhile, the Vincent Wildlife Trust is currently investigating the possibility of wider reintroductions in a two-year feasibility study.
So could the recovery of this charismatic carnivore within our forest be the salvation for the red squirrels at the cost of it’s invasive relative? Competition from greys may be reduced and reds will have the opportunity to recover in greater numbers in mainland populations. But if greys persist even in low numbers, squirrel pox will still be an obstacle for the reds to overcome. So only time will tell, but the Irish study provides exciting insight into the natural dramas that may be played out across British woodlands in the years to come.