The red squirrel (Sciurus vulgaris) is native to the UK and could historically be found in both coniferous and deciduous woodlands, though it is now almost exclusively found in the former. It is primarily an arboreal species, feeding on a range of seeds and nuts during the warmer months before spending much of the winter in a state of torpor. Populations of S. vulgaris have declined markedly across most the UK since the 1930’s with current population estimates hovering around 160,000. Around 75% of these occur in Scotland with the other 25% being found in isolated patches throughout England and Wales. The reasons for this decline are threefold: i) extensive fragmentation and loss of suitable habitat, ii) inter-specific competition with and iii) transmission of the parapox virus from the introduced North American grey squirrel (S. caroliniensis) which now numbers in the region of 2.5 million.A survey carried out in Northern Ireland on red and grey squirrel habitat associations showed a specific distinction in preference between species for coniferous or deciduous forests. Red squirrels occupied predominantly large coniferous forests at higher latitudes than the lower deciduous forests favoured by greys which are also more tolerant of smaller forest fragments. More specifically, reds were found to require a mature canopy and a large number of trees of cone-bearing age. Such optimum conditions may not be met in managed forests where the age of trees and harvesting methods employed result in a relative scarcity of mature stands. Where there is sufficient food, red squirrels may also be found in predominantly deciduous woodland though such instances are in the extreme minority.
It is well documented that squirrel poxvirus (SQPV), a pustular dermatitis, is a major threat to red squirrel populations. Though the origin of the disease is unknown (indeed, it may have been present in the population prior to the introduction of grey squirrels), it is widely accepted that grey squirrels are the main vector for its transmission. While SQPV is not carried by all grey squirrels and is only pathogenic to a small minority, it has a very high mortality in reds. SQPV is particularly virulent, resulting in the death of the majority of red squirrel hosts within two weeks. This results in a swift community shift where the disease is introduced, with greys rapidly replacing reds. SQPV presents in a similar way to mixomatosis in rabbits: lethargy, poor coordination and facial lesions all manifest within two weeks of infection. It is possible to treat SQPV if caught at early enough, though if a squirrel is identified as having SQPV at any stage, it must be immediately removed from the population.
Control of grey squirrel populations is problematic at best. They are such an established species within the British countryside that many regard them as perhaps more integral and identifiable than the native red. Such widespread acceptance both highlights the speed with which the grey squirrel has spread across the UK and is indicative of the probable response to extreme and widespread actions such as culls. The best and most obvious course of action would be to arrest the spread of grey squirrels into areas inhabited by red squirrels. Supplementary feeding of red squirrels to the exclusion of greys could possibly bolster the red population and provide stability.
The long-term control and exclusion of grey squirrels may not be entirely feasible; the adaptability and fecundity of the species means that any form of control is likely to be costly and the benefits debatable. Despite this, the preservation of mainland red squirrel populations depends on minimising the impact of greys on prime red squirrel habitat. While control measures should be implemented, of greater importance is the management of forest habitat to favour red squirrels and the identification and utilisation of a vaccine against SQPV.