Red and Grey Squirrels
The most crucial reason that red and grey squirrels cannot coexist is that grey squirrels carry a virus known as Squirrelpox (SQPV). Grey squirrels who carry the virus show no symptoms, when grey and red squirrels come into contact with each other the virus is easily spread. Although this is just one of several factors, it is estimated that the loss of red squirrels in Britain has happened 20 times faster than it would have done had squirrel pox not been a factor.
Grey squirrels are approximately twice the size of the native red squirrel, and can live in much higher population densities. As a result the grey squirrel has proven to outcompete the red squirrel for habitat and resources resulting in a steady decrease in red squirrel numbers across Britain.
The grey squirrel also has behavioural advantages. The red squirrel, is primarily arboreal and spends only a third of its time on the ground, the grey squirrel on the other hand spends more than three quarters of it’s time on the woodland floor foraging for food. This adaptation means that ahead of the winter, grey squirrels can increase their body weight by as much as 20%, while reds which feed far less efficiently in broadleaved woodland, rarely manage to gain 10%. This inability to gain weight results in many red squirrels not meeting critical mass for survival and breeding.
Grey Squirrels as an invasive
As a result of the grey squirrel’s behavioural advantages over the native red squirrel which has resulted in such a dramatic decline in red squirrel population numbers across Britain, the grey squirrel has now been listed within an IUCN list of the 100 worst invasive species globally.
In addition the grey squirrel has been included as a key species for action in an EU regulation on Invasive Alien Species. The regulation focuses on the effective and coordinated management of alien invasive species, and entered force on the 1st January 2015.
In order for the re-introduction to take place it is essential we not only remove grey squirrels from the target areas but demonstrate that future reinvasion by grey squirrels can be precluded. The primary method for this is live trapping, meaning that any non-target animal caught by mistake can be released unharmed. Traps are checked at least every 24 hours and humane grey squirrel dispatch is carried out following legislative and best practice guidance, included within the Forestry Commission paper on grey squirrel control. This approach is the same as that undertaken successfully on Anglesey, which led to the re-colonisation of the island by native red squirrels, and is employed in the north of England and Scotland as a grey control method.