Information and Advsiory Note Number 70, January 1997
1.1 The red squirrel Sciurus vulgaris is indigenous to Britain. It has the largest geographic range of any of the tree squirrels and is found throughout the Palearctic south to the Mediterranean, the Caucasus Mountains, the southern Ural Mountains and through to north-east China. Approximately 42 subspecies have been described but the taxonomic definition of these is not clear. The British form Sciurus vulgaris Ieucuorus was the fourth species to be described (Lowe & Gardiner, 1983).
1.2 The current British red squirrel population is estimated to be 161,000 animals of which 121,000 are estimated to be in Scotland.
(Harris et al., 1995).
Records of squirrel numbers over the past few hundred years shows that the British red squirrel population has often fluctuated. Indeed, records show that numbers fell to very low levels in Scotland around the 18th and 19th centuries, to the point of extinction in some areas. Several translocations were carried out in Scotland from English stock, e.g. Dalkeith 1772; Minto (Roxburghshire) 1824; Dumfriesshire 1837; Mauchline (Ayrshire) 1866; Minard House (Argyllshire) 1847; Beaufort Castle (Inverness-shire) 1844 (Lowe & Gardiner, 1983). Introductions were also made using stock from other countries, which included different subspecies, e.g. S. v. vulgaris from Scandinavia to Dunkeld (Perthshire) in 1793 (Lowe & Gardiner, 1983).
2.1 Red squirrel re-stockings continued throughout Britain until around 1910 when squirrels again became abundant and were noted for the damage they caused to trees.
Re-stockings then ceased and the population again declined, but subsequently began to recover by the 1930s. By this time, however, the situation had become complicated by the spread of the grey squirrel (see below).
2.2 The red squirrel has been lost from large areas of the country, particularly during the past 50 years. It is now restricted primarily to Scotland, the north of England, and small pockets in Wales and southern England.
3.1 Although the precise reasons for the decline remain unclear, three main factors have been suggested. These are
Squirrels carry a variety of internal and external parasites. Although most of these cause little discomfort to the animals, there are two in particular which are known for their debilitating and often fatal effects.
Coccidiosis is caused by a parasitic protozoan of the genus Eimeria. This attacks the epithelial lining of the intestine, and serious infections may result in bloody diarrhoea as pieces of the gut wall become sloughed off. The disease is transmitted as an oocyst in the faeces which may remain infective in the environment for many months. The precise method of ingestion is not known but it appears that individuals may be prone to reinfection, causing a progressively debilitated state which may result in death (Gumell, 1987).
Red squirrels are also susceptible to the parapox virus. Affected animals exhibit similar symptoms to rabbits infected by the myxomatosis virus, with skin lesions, scabs, a swollen face and discharge from the eyes. Parapox was originally considered to be restricted to the East Anglian population but two red squirrels in Cumbria were confirmed with the virus in 1992. A grey squirrel also appears to have contracted a parapox-like virus in Surrey in 1994 (Gurnell, 1994). The disease has not yet been recorded in Scotland and it is unclear as yet whether the grey squirrel may act as a vector for its spread.
There is currently no evidence to identify the extent to which either of these diseases has contributed to the widespread loss of red squirrels across Britain.
3.1.2 Loss and fragmentation of habitat
Continuity of habitat is an important factor in red squirrel occupation. Home range sizes vary between 3 and 12 hectares (or greater) depending on the availability of food and shelter. However, felling of woods and hedgerows, or altering their species composition, may divide previously suitable areas and discourage occupation by red squirrels. It has been suggested that small woods which are 5 km or more from a resident population of red squirrels are, in fact, unlikely to support a population (Gurnell, 1994). Fragmentation of woodland habitat increases the vulnerability of a red squirrel population to a variety of stochastic event, including bad weather or disease. The population also becomes more susceptible to competition from grey squirrels (see below). The extent to which forest diversification policies have contributed to this process have only recently been recognised (see below).
3.1.3 Grey squirrels
Grey squirrels are native to North America but are now well-established in Britain as a consequence of introductions carried out approximately 100 years ago. The first well-documented releases were in Woburn park in 1890 and at Loch Finnart on Loch Long in 1892 (Kenward & Holm, 1989). However, they were also known to be released in Cheshire in 1876 and there is some speculation that they may have existed in other areas as far back as 1830 (Kenward & Holm, 1989).
By 1920, the grey squirrel population was established in at least 12 different areas around Britain. Since then, the population has undergone a dramatic expansion - particularly rapid in the 1940s and 1950s - and has replaced the native red squirrel over much of its range. The grey squirrel population continues to expand, although in a more patchy manner. The current population estimate for grey squirrels in Britain is 2,252,000, of which 200,000 are estimated to be in Scotland.
Although the precise method of competition between the red and grey squirrels is, as yet, unclear, the decline in red squirrel range is mirrored by the spread of grey squirrels, indicating a gradual replacement of red squirrels by the greys. The primary theory suggests that the grey squirrel exerts competitive diet exclusion against the red squirrel, although a number of other factors, have also been considered.
4.1 Red squirrels are found in a variety of habitat types, from boreal coniferous forests to broadleaf woodlands in the south and west of Europe. They were formerly found in broadleaf woodland in Britain but have never been found to return to these once these have been occupied by grey squirrels. They now reside primarily in conifer strongholds in the north and west of the country, although they remain in broadleaf habitats in areas where grey squirrels do not occur.
4.2 The age of the woodland is particularly important to red squirrels and, although they will forage in young forests, they will not take up residence until the canopy is closed and the trees are of seed-bearing age (typically at 15 to 25 years of age). Conifers of between 20 and 40 years old, particularly Scots Pine and Norway Spruce, are favoured because they provide suitable amounts of both food and shelter.
4.3 Grey squirrels are seldom found in pure conifer plantations. In general, they occupy broadleaf forests, in keeping with their native North American habitat, but they are capable of taking advantage of conifer areas where these are within foraging distance of broadleaf woodland. Consequently, although they are rarely found co-existing with red squirrels in pure conifer stands, they are able to exploit the results of current forestry diversification policies, using deciduous trees along forest rides and riparian habitats to gain access to otherwise unsuitable areas.
5.1 Squirrels are opportunists and eat a wide variety of food depending on availability and tree seeds form the most important and consistent source of food. As would be expected of the ecological differences between the species, conifer seeds form the greatest component of red squirrel diet, while grey squirrels survive well on the seeds and nuts of deciduous trees and shrubs. Mast-producing species such as beech, oak, hazel, sycamore and chestnut are important to greys. When seeds are less readily available, both squirrels will seek other sources of food, such as shoots, buds, fungi and flowers.
In a number of conifer species, seed production varies significantly between years and this has been related to the local abundance of red squirrel populations. For example, good cone years in Scots Pine occurs approximately every 3-4 years in comparison with a 3-11 year cycle in Norway Spruce (Lurz et al., 1995). Populations show greatest reproduction and recruitment levels in years following good food crops.
5.2 Foraging behaviour in the two species also differs markedly. Red squirrels are primarily arboreal feeders, spending as little as 14% of their time on the woodland floor. In contrast, grey squirrels often spend up to 70% of their active time foraging on the ground. This provides the grey squirrel with the advantage in deciduous woodland where fallen seeds form the main winter food. In addition, grey squirrels are capable of taking some seeds earlier than the red squirrel, and thus reduce the food available to the red squirrel later in autumn (Kenward & Holm, 1989).
5.3 Weather conditions play an important role in the survival of squirrels. Bad weather can delay the onset of breeding, or can affect the survival of the young. It can also have a detrimental effect on foraging as squirrels store little fat and must forage constantly to retain good body conditions.
6.1 The necessity for co-ordinated action was recognised earlier this year in the publication of a UK Strategy for Red Squirrel Conservation (JNCC, 1996). This sets out a range of conservation actions which aim to:
6.2 Work currently being carried out in Scotland focuses on two areas: habitat management and grey squirrel competition. The SNH contribution to this work is being coordinated as part of the SNH Species Action Programme.
6.2.1 Habitat management
As yet it is unclear what specific characteristics of a forest favour red squirrels and prevent occupation by grey squirrels. Consequently, it is difficult to provide prescriptive actions for long-term management of woodland for red squirrels. There are some habitat management recommendations, based on a study in Wales, which are cited in the UK Strategy (JNCC, 1996). However these recommendations need to be considered in the context of local geography and topography.
To contribute to the provision of management advice, SNH and the FC are supporting a three-year study to investigate habitat use in sympatric populations of red and grey squirrels. This is being conducted in an area where the two species have existed together for over 30 years - a relatively unusual occurrence in Britain. By relating time-budget analysis to forest composition and structure, the study aims to identify more clearly the subtle nature of inter-specific competition between red and grey squirrels. It is being carried out with a view to providing recommendations for habitat management to maximise the competitive advantage of the red squirrels over the greys. The study began in summer 1996 and will run for a period of three years.
6.2.2 Grey squirrels
Grey squirrel control is practised widely throughout Britain. Although there is little evidence to quantify the benefits this has to red squirrel conservation, it is generally accepted that some form of control may often be necessary for red squirrel conservation. Grey squirrel control is carried out most commonly through cage-trapping and shooting. However, these are expensive and the use of warlarin is currently being investigated as an alternative.
Under the Warfarin Order (1973), it was illegal to use warfarin to control grey squirrels anywhere in Scotland. However, the Pesticide Safety Directorate (PSD), MAFF approved an off-label agreement earlier in 1996 permitting amendments to be made to the conditions governing its use.
Warfarin can now be used to control grey squirrels in areas where it was formerly prohibited. This includes the whole of Scotland. However, it is still subject to label conditions and excludes areas which currently support a red squirrel population, plus woodland in adjacent 10 × 10km squares which is capable of supporting red squirrels (as defined in the user-guidance available at local FA offices). However, squirrel distribution maps used to identify areas for warfarin use must be up-dated annually.
Warfarin must only be used in the new-style hoppers (see below) and the onus is on the user to ensure that they comply with the agreement conditions. Detailed guidance on the conditions are held in local Forestry Authority offices and the onus is on the user to obtain these prior to using the poison.
Research is currently being concluded in Wales to investigate the efficacy of a new-style feeding hopper to target grey squirrels only. This is based on behavioural differences between the species in which the more timid red squirrels will be deterred by obstacles placed in the tunnel leading to the food. The hopper is also set on the ground to minimise contact with red squirrels.
The research aims to demonstrate that incidental effects on red squirrels are less than those currently occurring through cagetrapping techniques (red squirrels are often caught in grey squirrel traps). Provisional results appear to confirm this to be the case but approval must be sought from PSD before the hoppers can be used more widely. Further information will be distributed when the process has been concluded.
Gurnell, J. 1987. The Natural History of Squirrels. 201pp. Christopher Helm, Kent.
Gurnell, J. 1994. The red squirrel. The Mammal Society
Harris S., Morris P., Wray S. and Yalden D. 1995. A review of British Mammals. Joint Nature Conservation Committee, Peterborough.
Joint Nature Conservation Committee 1996. UK Strategy for red squirrel conservation. JNCC, Peterborough.
Kenward, R.E. & Holm, J.L. 1989. What future for British red squirrels? Biol. Journ. Linnean Soc. 38: 83-89
Lowe, V.P.M. & Gardiner, A.S. 1983. Is the British Squirrel (Sciurus vulgaris leucourus Kerr) British? Mammal Review 13: 57-67
Lurz, P.W.W., Garson, P.J. & Rushton, S.P. 1995. The ecology of squirrels in spruce dominated plantations: implications for forest management. Forest Ecology and Management 79: 79-90.