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Red vs. Grey: The cause of decline in Sciurus vulgaris populations due to the introduction of Sciurus carolinensis in the UK

Author

Derek Elsby

Date

November 2013

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Contents

Introduction......................................................................................................... 3

Grey Squirrel Expansion ................................................................................. 3

Areas of Conflict ................................................................................................. 4

Physical Competition ...................................................................................... 4

Habitat and Home Range Competition ........................................................... 6

Food Competition ........................................................................................... 7

Reproductive Competition ............................................................................... 7

Squirrel Poxvirus ............................................................................................. 8

Discussion .......................................................................................................... 8

Natural History ................................................................................................ 9

Cause of Red Population Decline ................................................................. 10

Compounding Factors .................................................................................. 11

Conclusion........................................................................................................ 11

References ....................................................................................................... 13

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Introduction 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).

Grey Squirrel Expansion 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

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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).

Areas of Conflict 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.

Physical Competition 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

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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.

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Habitat and Home Range Competition 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).

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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).

Food Competition Differing food preference has been shown to exist between reds and 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.

Reproductive Competition 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

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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.

Squirrel Poxvirus 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).

Discussion 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

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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.

Natural History 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.

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Cause of Red Population Decline 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

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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).

Compounding Factors 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.

Conclusion The cause for the decline of the UKs Eurasian red squirrel populations in the

presence of the alien North American grey squirrel appears 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;

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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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