Return and recolonisation by pine marten, either naturally or by reintroduction, is seen as a silver bullet to stop the grey squirrel in its tracks and save the red, although not all are convinced, writes Dr Terry Mabbett.

MORE pine martens means fewer alien eastern grey squirrels and returning native European red squirrels, according to research carried out in Scotland (Sheehy et al. 2018). In places where pine martens were more common, red squirrels were much less likely to visit nut feeders, but greys were not.

“It suggests greys are totally naive to the risks of the pine marten as a predator,” said lead author Emma Sheehy. “In their native range (eastern North America), they don’t have similar predators and that leaves them much more susceptible here,” she said. These potentially exciting findings are now backed up by research carried out in Northern Ireland by scientists at Queen’s University, Belfast.

Pine marten (Martes martes) and European red squirrel (Sciurus vulgaris) have several things in common. Both are British natives, and both have previously faced real threats of extinction in mainland England at least. Pine martens, with a long-standing reputation for preying on game birds and raiding poultry runs, were persecuted by farmers and gamekeepers. Pine martens are especially susceptible to persecution because they are slow breeders, with females not usually breeding until their third year. By 1920, pine martens were facing extinction in Britain.

European red squirrels’ problems started towards the end of the 19th century when the North American eastern grey squirrel (Sciurus carolinensis) was introduced into Britain. Grey squirrels were imported from New Jersey, released in Bedfordshire and also sent to other estates around the country. Grey squirrels were also released at Regents Park in London, an act credited with creating contemporary London’s huge grey squirrel population (Shorten, 1954; Signorile, 2014; Knapton, 2016).

Consequent demise of the native red squirrel was quick and complete. In 1945, red squirrels were still present in southern England, albeit together with grey squirrels, but they still had a free run across East Anglia. Kevin Ross, a woodsman who grew up in a forested area of Suffolk, speaks fondly of red squirrels frequent there as late as the 1960s (Ross, 2018). However, by 2000 red squirrels were virtually excluded from the whole of mainland England and Wales, apart from Cumbria and across into Northumberland (Signorile, 2014). Greys had outcompeted reds right across the board in feeding and breeding, but most of all in the capacity to carry and transmit the squirrel pox virus, which is lethal to red squirrels.

Northern Ireland research confirming earlier findings in Scotland and indicating an antagonistic effect of pine martens on grey squirrels, but apparently not reds, is conducted by Josh Twinings and colleagues at Queen’s University, Belfast. Interest was initially triggered by landowners describing how the appearance of pine martens was accompanied by a disappearance of grey squirrels and reappearance of red squirrels. Josh and his colleagues confirmed the relationship by sampling 330 sites across Northern Ireland.

The speed with which this apparently happens suggests pine martens are driving grey squirrels away rather than killing and eating them in large numbers. Red squirrels’ ability to exist quite comfortably alongside a fellow native but nominally predatory species (the pine marten) would appear to be down to innateness and instinct rather than learning. The last time pine martens and red squirrels existed alongside each other in numbers across large tracts of the UK was the first half of the 19th century, if not earlier.

This fits in with what Josh Twining told the national press. He said: “Red squirrels have evolved alongside pine martens and adapted to find ways not to be eaten by them. The grey squirrel is completely naive and doesn’t know how to react, so it becomes lunch.”

Return and recolonisation by pine marten, either naturally or by reintroduction, is seen as a silver bullet to stop the grey in its tracks and save the red, although not all are convinced and claim it is not a black-and-white – or red-and-grey – situation. They point to areas in Europe where red squirrels make up a significant slice of prey taken by pine martens and how these weasel-like mammals will happily slit the throats of red squirrels if they are the most frequent potential prey.

However, the last word goes to the researchers at Queen’s University and how recolonisation of Britain by pine martens could have the knock-on effect of pushing more grey squirrels into urban areas. “Unless the issue of control within (human-) populated areas is addressed, we risk creating a situation where marten-savvy grey squirrels could recolonise the wider landscape in the future,” says Josh Twining (Bawden, 2019).

And here’s where the superior ability of the grey squirrel in solving problems, especially more complex ones, as demonstrated by behavioural research carried out at the University of Exeter, could come into play (Yee Chow et al., 2018).

Bawden, T. (2019) ‘Red squirrel numbers are rebounding in UK forests as pine martens eat grey squirrels’.
Knapton, S.  (2016) ‘11th Duke of Bedford blamed for unstoppable grey squirrel invasion’. Daily Telegraph 26th January 2016.
Ross, Kevin (2018) Personal communication.
Sheehy, E., Sutherland, C., O’Reilly, C. and Lambin, X. (2018) ‘The enemy of my enemy is my friend: native pine marten recovery reverses the decline of the red squirrel by suppressing grey squirrel populations’. Proceedings of the Royal Society B. Volume 285 Issue 1874. 
Shorten, M. (1954) Squirrels. Collins New Naturalist Monographs Series No.12. 212 pages.
Signorile, A. L. (2014) ‘Genetic determinants of the expansion of eastern grey squirrel populations across Europe’. PhD Thesis, Imperial College London. 
Yee Chow, P., K. Lurz, P. and Lea, S. (2018) ‘A battle of wits? Problem-solving abilities in invasive eastern grey squirrels and native Eurasian red squirrels’. Animal Behaviour, 2018; 137: 11.

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