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Better the Devil You Know
The battle for survival between native red and invasive grey squirrels has been ongoing in the UK ever since our ancestors introduced the grey species from North America in 1870s.
With the species competing for resources and the greys carrying squirrel-pox virus, which is harmless to them but lethal to reds, the contest seemed to be tipped in the greys’ favour.
But in nature such rivalry is likely to be influenced by more complex interactive networks than just two species. Now scientists from the University of Aberdeen have added a third species to the mix – the European pine marten. These native squirrel predators are recolonising Britain after years of persecution, and so the scientists set out to uncover the impact of this on red and grey squirrel populations.
Three study sites in Scotland were in the Highlands (marten recolonisation for over 45 years, no grey squirrels), Central Scotland (marten recolonisation for 8-14 years, grey squirrels since 1945) and the Borders (early stages of marten recolonization, grey squirrels since 1980).
Populations were monitored for five months using cameras and multi-species feeders – the latter with glue strips on the underside of the lid to obtain visitor hair samples. This material provided DNA to determine visitor species and identify individual martens.
The results showed that while red squirrels avoided feeders used by martens, greys were equally likely to use feeders whether frequented by martens or not. So grey squirrel numbers were reduced where exposure to pine martens was high, whereas numbers of red squirrels increased – unequivocal evidence that recovering marten numbers affect competition between red and grey squirrels.
This finding is likely explained by the fact that reds, having co-evolved with martens, instinctively avoid areas used by martens but invasive greys, having no such instincts, showed no avoidance behaviour at all.
Since pine martens’ range is increasing and extending southwards, scientists are optimistic that this will control the invasive grey squirrel species.
As they say in the title to their report, in this case ‘the enemy of my enemy is my friend’.
The native grey squirrel may have an unlikely ally. Photo: Darin Smith. T24_Red-Squirrel_01_no_Photo-Darin-Smith01
The pipe marten. Photo: Alastair Rae. no_a24PineMarten01