Red vs Grey: The cause of decline in Sciurus vulgaris populations due to the introduction of Sciurus carolinensis in the UK
In Scotland, the Eurasian red squirrel (Sciurus vulgaris) suffered near extinction in the 1700s as a result of deforestation, but populations recovered following extensive reforestation during the 1800s. At the beginning of the 20th century the species was considered abundant across the whole of the mainland UK (Harris et al. 1995; Holm 2010). The red squirrel is now considered one of the most popular mammals in the UK, particularly in Scotland (Lurz 2010).
Currently, the red squirrel is listed as ‘Least Concern’ on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species™, due to its large global population which reaches across the Palearctic and as far as Japan (Shar et al. 2008). However, in mainland UK the Joint Nature Conservation Committee lists S. vulgaris as vulnerable in England and Wales; it is currently still common across Scotland as a whole (Harris et al. 1995). The plight of the red squirrel over the past half century now means that the UK Biodiversity Action Plan lists S. vulgaris as a priority species for conservation.
The reason for the decline in red squirrel populations is the deliberate human introduction of the North American grey squirrel (Sciurus carolinensis) across the UK, between 1876 and 1929. Invasive species are one of the greatest threats to native biodiversity (Bryce et al. 2002) and the introduction of the grey squirrel to the UK has led to significant declines of red squirrel populations, often resulting in extinction. Today, grey squirrel populations continue to thrive and expand their range while reds squirrel populations decline in their presence (Harris et al. 1995; Lurz et al. 2005; Rushton et al. 2006; Sandro 2008). While many discoveries have been made towards understanding the mechanics of this replacement, the exact reasons behind it are still unclear (Wauters et al. 2001).
Some of the first studies into the newly introduced grey squirrel were carried out, via survey, in the 1930s (Middleton, 1932; Middleton & Parsons, 1937). A clear trend of expansion was observed then and it is a trend which has continued to this day (Mayle & Broome, 2006). From only nineteen initial introduction sites – fourteen in England, two in Wales and three in Scotland – the grey squirrel has expanded its range to the extent that it now covers a significant proportion of the UK. Currently in England and Wales the red squirrel exists now only in isolated pockets, while the grey has spread to cover almost the entirety of both countries. In Scotland the red squirrel still continues to survive across the vast majority of its initial range, but the grey continues to expand into this range from the South and has driven several populations to extinction (Harris et al. 1995; Mayle & Broome 2006). At present, the woodlands north of central Scotland provide the greatest and safest refuge from grey invasion, thanks in part to their fragmented nature (Poulsom et al. 2005).
While it can be hard to accurately assess population numbers for both species, current estimates suggest there are approximately 161,000 red squirrels and 2.5 million grey squirrels in the UK (Harris et al. 1995).
Interspecific competition can be defined as the resource exploitation and/or interference by individuals of one species that affects the survivorship, growth and/or fecundity of the individuals of another species (Begon et al, 2006). The shared resources and habitats of S. vulgaris and S. carolinensis mean there are many areas where such competition and conflict can occur.
Interference competition is the defined by direct action of one individual against another; such action may involve threat behaviours and/or actual, physical attack (Keddy 2001). With this in mind, an intuitive approach would suggest that the larger, more robust grey squirrels have caused declines in red squirrel populations by physically out competing them – using attacks and/or the threat of physical confrontation – for space and food. However, several studies have shown that direct aggression or interference between red and grey squirrels is not a factor in red decline; indeed no overt interspecific aggression has ever been observed between the two species (Bryce et al., 2002; Wauters & Gurnell, 1999). While some studies have observed instances of reds moving off at the approach of greys, such observations were either not statistically significant or simple anecdotal observations.
The hypothesis that greys actively push reds from an area has further been disproved by Wauters and Gurnell (1999) who showed that range was defined by almost purely by intraspecific interactions between reds, as opposed to any kind of interspecific interactions with greys.
Along with a lack of observed aggression, extensive range overlap is observed between the two species (Wauters et al. 2002a). Examination of hairs and species specific parasites from squirrel dreys demonstrates that individuals of both species will share their use (Kenward and Hodder 1998), though habitation by individuals from both species at the same time is unlikely and has not been observed.
In their test of, and ultimate rejection of, the interference competition hypothesis (ICH), Wauters and Gurnell (1999) observed that any increase in the interactions between red and grey squirrels was likely due to increases in population densities. They observed no evidence of aggressive action of greys towards reds. On the contrary, grey squirrels mostly seem to ignore the presence of red squirrels.
Discounting direct aggression, the presence of competitor may have an indirect detrimental effect by limiting or interrupting foraging behaviours, thereby reducing fitness. However, while reds have been observed to interrupt their foraging in the presence of greys, such interruptions were short (less than one minute in duration) and ultimately deemed insignificant.
Kenward and Hodder (1998) did observe behaviours indicative of interference competition between the two species, with greys displaying dominance over food acquisition and home range, however these observations involved a transplanted group of red squirrels into an area already occupied by greys and, in the face of the other studies mentioned, cannot be held as conclusive proof that direct competition is commonplace.
Red squirrels, with their lighter build, are more adapted to foraging in the thin branches of a conifer canopy. Heavier greys are more adapted for gathering food from the ground, and their added strength aids in digging when the ground may be harder in winter months. As expected from this, grey squirrels spend the majority of time the forest floor, whereas reds spend the majority of their time within the branches of the trees (Kenward 1986; Kenward and Hodder 1998; Koprowski 1994; Lurz et al. 2005). Given these niche differences between the two species, habitat – specifically woodland type – does not initially seem to present a source of competition between the two.
Despite the overall trend of red squirrel decline, successful co-habitation of S. vulgaris and S. carolinensis has and does occur in the UK. Bryce et al. (2002) observed an apparently stable situation of co-existence between both species at Craigvinean forest in Scotland – a situation that has existed for approximately thirty years. Craigvinean forest consists almost entirely of coniferous trees and within it the greys are seemingly confined to small mixed coniferous-broadleaf areas.
However, the division of squirrel species by woodland makeup is not as clear cut as this observation might initially suggest and certainly isn’t the case across the UK. At Craigvinean, both species of squirrel utilised both types of forests, with reds actually favouring mixed areas in autumn and winter. Additionally, greys are by no means incapable of utilising conifer woodlands and have successfully outcompeted reds in other conifer only areas (Bryce et al. 2002).
It is important to note that the increase in human planted woodland cover across the UK is a particularly well documented contributor to the spread of the grey squirrel in the past fifty years with the expansion of grey squirrels mirroring the expansion of human planted woodlands; the timber industry’s monitoring of the damage that grey squirrels cause to trees has demonstrated that, just as reds are, greys are perfectly capable of colonising both deciduous and coniferous woodland (Mayle & Broome, 2006).
Rather than interspecific competition, habitat selection and home range definition has been shown to rely more upon intraspecific interactions and species food preference (Bryce et al. 2002; Wauters et al. 2002b).
Differing food preference has been shown to exist between redsand greys (Keward and Holm 1993; Wauters et al. 2002b). Some overlap does exist though, particularly with hazelnuts. Red squirrel populations show preference for hazelnuts, when they are available, as their main food crop but also extensively utilise pine cones. Greys will also readily utilise hazelnuts, but are also extremely capable of exploiting acorns as a food source and to a much greater extent than reds are. Because of this, the presence of hazel trees in an otherwise coniferous forest is a distinct benefit to red squirrels, but only in the absence of greys.
Both species are scatter-hoarders – they hoard food by either burying in the ground or caching in hollow trees during periods when food is plentiful, for use in the winter and spring months. Wauters et al. (2002a) showed that where an overlapping of red and grey home-ranges occurs, reds will exhibit a reduced energy intake from cached food sources. While food competition still exists on an intraspecific level, no such similar impact was observed in red only habitats. Clearly, the presence of grey squirrels has a negative impact on the cached food sources of red squirrels; the most likely explanation for this is pilfering of cached food by grey squirrels. Importantly, this impact is positively correlated with an increase in grey population density.
Red squirrels have two breeding seasons – one in winter and one in spring (Lurz et al. 2005). A study into whether the presence of grey squirrels interfered with the mating behaviours of red squirrels was carried out by Wauters and Gurnell (1999) in their test of the ICH. While they did observe greys occasionally interacting with the mating chase behaviour of reds, such interactions were short and never resulted in attempted copulation between red and grey squirrels. Further, these interactions were not seen to disrupt eventual breeding of the females involved; the ICH predicts that any reduction in red squirrel fecundity would be accompanied by a similar rise in grey squirrel fecundity, but this was not observed.
While the origins of Squirrel Poxvirus (SQPV) are not clear (Rushton et al. 2006), the effects of this disease upon red squirrels are well documented. The virus causes lesions and scabs in the skin around the eyes and nose initially, but spreading to cover the rest of the body. These lesions become severe after 2 weeks of infection and are accompanied by lethargy and appetite loss. The disease is usually fatal within 14-20 days from infection (Thomas et al. 2003, Rushton et al. 2006).
In Southern England, S. carolinensis populations can test seropositive for SQPV, sometimes with up to 100% of the grey population carrying the virus. However, rather than suffering from the disease, S. carolinensis appears to act as a reservoir for the infection (Thomas et al. 2003, Rushton et al. 2006). The virus is present in England, but so far has not been reported in Scotland (Rushton et al. 2006).
Mathematical models show that competition alone cannot account for the reductions and extinctions of red squirrel populations witnessed in the UK so far, while the inclusion of SQPV into the equation produces results similar to what is seen in the wild (Tompkins et al. 2003).
Where some populations of S. vulgaris have become extinct within only two years following the arrival of S. carolinensis, others have lasted up to twenty years in their presence; in other areas, both species continue to co-exist having seemingly reached some kind of ecological equilibrium, though this is a rare occurrence. The variation observed clearly indicates that a number of factors are in play with regards to red squirrel survival when placed in competition with greys.
Habitat seems to be a key factor in the survival of red squirrels when greys are involved. Red squirrels evolved in the largely coniferous woodlands of Europe and Asia, while grey squirrels evolved in the mixed deciduous woodlands of North America (Bryce et al. 2002). It is important to note that in many other parts of the world grey squirrel species are found co-habiting alongside other squirrel species; in North America, the Pine Squirrel (Tumiusciurus hudsonicus)(a.k.a. the North American Red) lives alongside S. carolinensis without significant detriment to either species. T. hudsonicus occupies a very similar niche to S. vulgaris i.e. while grey squirrel species are more adapted for ground foraging, red squirrel species are smaller and more adapted for life in the trees (Gurnell and Pepper 1993).
The co-habitation witnessed between red and grey squirrels in North America is possible because those native red squirrels species evolved alongside their grey counterpart and so are adapted to live together with greys as part of a community.
S. vulgaris had no such co-evolutionary step alongside S. carolinensis. The native UK squirrel has been denied the generations of competition, and trials of natural selection, against the North American grey that resulted in the subtle, yet key differences between the species; differences that have led to a level of divergence significant enough to allow successful co-habitation (Bryce et al. 2002).
The case of co-existence observed in Craigvinean forest is rare and, importantly, SQPV was not a factor in this situation. Given that the presence of SQPV can increase that decline in S. vulgaris populations by a factor of twenty (Rushton et al. 2006), it would be unwise to assume that Craigvinean forest represents a possible final solution to the predicament that grey squirrels pose.
In a study to investigate the niche differences between reds and greys (Wauters et al. 2002b), it was found that red squirrels, if given the opportunity, will occupy any woodland type they are presented with – not limiting themselves to the coniferous or broadleaved areas. Grey squirrels were shown to preferably establish a home range in broadleaf woodlands, but would utilise conifer only areas when their own population density pushed them to do so. Only in the presence of grey squirrels were reds found confined to conifer only areas. Naturally, woodland makeup is a defining factor of food availability.
The presence of greys does not seem to have an effect on the food choice of reds (Wauters et al. 2001). In the summer and autumn months, when food is plentiful, both species will utilise whichever food crop is most abundant. The niche differentiation (arboreal vs. ground foraging) ensures little competition between the species so long as there is no critical food shortage of any particular crop. Both species show a preference for hazelnuts where present, while pine cones are fed on almost exclusively by red squirrels and acorns are fed on almost exclusively by grey squirrels (though both species ultimately eat a mix of whatever is available).
Red squirrels survive the early winter months by feeding on the cones that continue to remain available in the canopy, while greys conserve energy during this period with reduced activity (Kenward and Holm 1993). As food grows scarcer, both species begin to utilising cached seed crops from the autumn. In later winter months, reds become more dependent upon their own cached food stores. However, greys, where present, are shown to utilise both their own food stores and those of red squirrels. This reduction in available food for the red squirrels results in a decrease in body mass for the red squirrels (Gurnell et al. 2004).
Female red squirrels must achieve or maintain a body mass greater than 300g in order to trigger oestrus (Kenward 1986), therefore the pilfering of red food caches by grey squirrels can have a direct effect on red squirrel ability to reproduce. Specifically, as spring breeding approaches, some reds may not have acquired/maintained the body mass necessary to enter oestrus. Thus, we see a reduction in the number of red squirrels breeding twice a year resulting in the ultimate decline of the red population (Gurnell et al. 2004).
It is important to note that without competition from greys, red squirrel populations are capable of recovering from extremely low numbers within only a few years (Lurz et al. 2005).
Red squirrel population density has been shown to correlate positively with hazel density; they show a strong food preference for hazels and pine cones. Greys will also readily consume hazels, but in the presence of oak trees greys will eat acorns whereas red squirrels, if given the choice, will not. The avoidance of this food source by red squirrels is due to the polyphenol (tannin) content of acorns which leads to problems with nutrient and water absorption in most mammals – an effect that grey squirrels have evolved a strategy to overcome (Kenward and Holm 1993). Therefore, the presence of oak trees in woodlands with both red and grey squirrel species provides a distinct advantage – in the form of a largely uncontested food source – for the greys.
Finally, the devastating effect of SQPV on red squirrels is of particular importance. When present, it can push a potentially robust situation for reds into a near guarantee of accelerated extinction.
The cause for the decline of the UKs Eurasian red squirrel populations in the presence of the alien North American grey squirrelappears to be the ability of the grey squirrel to better utilise the food resources available in a manner that ultimately impacts upon the year-round fecundity of the red squirrel. Essentially, while interference competition can largely, if not completely, be discounted, exploitation competition seems to be the area where greys win out over reds; greys are more capable of utilising the resources, food resources in particular, that are available to both species.
Beyond any doubt, the introduction of S. carolinensis creates a situation in which the S. vulgaris population is likely to decline to the point of the extinction, but importantly it does not guarantee this outcome. Human driven habitat alteration and destruction have been integral in the spread of grey squirrels, but human interaction is now also an absolute requirement of any efforts to halt the spread of S. carolinensis and safeguard S. vulgaris in the UK.
Complete, countrywide eradication of S. carolinensis is unrealistic and the complete halt of its advance is dependant greatly on the effort expended to stop it. Knowing this, learning more of exactly where the conflicts between red and grey squirrels occur, and using this knowledge to guide more efficient and productive conservation efforts is the key to successful conservation of the red squirrel in the UK.
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