Tuesday, 7 February 2012

UK Invasive Species – War of the Grey and Black Squirrels

In the UK there are several very common and familiar animal, insect and plant species that are not actually native to our country.  They are species that have been introduced into our countryside in one way or another, and have often driven our native species from their habitats by out-competing them for food, passing on disease and taking over territory.  Invasive species can be very destructive to often fragile habitats, and can cost the economy millions of pounds a year. In fact, you may well have one of these alien animals in your garden right now, and you may have spent many happy hours watching their antics and admiring their aerial acrobatics in the trees.  This cute little invader is the grey squirrel, and although they seem so ubiquitous they have only been scampering around our gardens and woodlands for the last hundred years or so.

Our native squirrel species is the smaller red squirrel, and before the last quarter of the 19th century they numbered in the millions and ranged across the whole country. Red squirrels are easily recognisable by their striking red coats, bushy tails and tufts of red fur on their ears. Their preferred habitat is conifer forest, where they live off pine cones, seeds, shoots and fruit.  The red squirrel tends to be a solitary animal  except during the mating season, when they build large nests called dreys in the forks of trees producing a litter of between 2-3 kittens in the spring.  However, it is now estimated that there are as few as 120,000 red squirrels left in the wild, and the major cause of their decline was the introduction of grey squirrels into the UK.

The grey squirrel is a North American species, which arrived in the UK between 1876 and 1929 when they were introduced into many parks and private animal collections.  Inevitably some of the animals escaped or were released into the wild, where they thrived and bred successfully.  Because they were so much bigger, stronger and ate a wider variety of food than the native reds, they started to drive them out of their territory, so that now the red squirrel is confined to parts of Scotland, northern England, Wales and the Isle of Wight.  The grey squirrels also passed on disease to the reds, which they had no natural immunity to.

But although the grey squirrels have been the victorious conquerors of our gardens and parks for decades now, they do have a new challenger that is beginning to drive them out of their territory and ironically this new invasive species is a member of their own family.  So don’t go and get your eyes tested if the squirrel running down your fence looks black and not grey, as the black squirrel is slowly but surely increasing its numbers in some parts of Britain.

Like its grey cousin, the black squirrel also arrived from the US in the late 19th century, where they were kept as exotic pets in a private zoo in Bedfordshire.  Some of these animals escaped from captivity, and in 1912 the first wild black squirrel was spotted in the environs of Letchworth, Hertfordshire.  It is now estimated that there are more than 25,000 of them living in the UK, most of which are in the East Anglia region, and some scientists think that they could eventually become the dominant squirrel species in this country as there are more sightings of black squirrels being reported from other parts of the UK.

Grey and black squirrels are actually the same species, but a genetic mutation caused an excess of melanin to be produced in some squirrels that caused the black pigmentation of the fur.  It is thought that having black fur could help the squirrels to survive in colder climates, as it absorbs heat more readily.  Being so closely related, the grey and black squirrels can breed with each other, but when they do the black gene is dominant, which is another reason why they may eventually overtake the grey squirrels in numbers.  There has been a major research project undertaken on the spread of black squirrels by Anglia Ruskin University since 2004, where they have set up a website asking the public to report any black squirrel sightings they have made.  So a seemingly successful invasive species, can in its turn be overtaken by a more recently introduced species, so that in a few years time we could all be watching black squirrels in our parks, and the grey squirrels, just like the reds, might well have been pushed out and be only surviving in small colonies in the remoter, more inhospitable parts of the UK.

Black squirrel image Sujit kumar Wikimedia Creative Commons Attribution Share Alike 2.0 Generic

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