Economic damage by invasive grey squirrels in Europe

Abstract

Invasive alien species (IAS) comprise a global threat to biodiversity and may also cause economic harm by damaging natural resources and property. Indirect costs associated with IAS control and protection of native species add further economic burdens. The eastern grey squirrel (Sciurus carolinensis) is a successful invader in Europe, and this chapter reviews the current knowledge on the types and scale of damage it has inflicted on the continent’s economies.

Full text

393
20. Economic Damage by Invasive Grey Squirrels in
Europe
JONATHAN J DERBRIDGE, HARRY W PEPPER and JOHN L KOPROWSKI
Summary
Invasive alien species (IAS) comprise a global threat to biodiversity and may
also cause economic harm by damaging natural resources and property.
Indirect costs associated with IAS control and protection of native
species add further economic burdens. The eastern grey squirrel (Sciurus
carolinensis) is a successful invader in Europe, and this chapter reviews the
current knowledge on the types and scale of damage it has inflicted on the
continent’s economies.
Introduction
Invasive alien species (IAS) comprise a globally pervasive threat to biodiversity
and are known to cause direct and indirect economic harm in Europe (Pimentel
et al. 2005; Kettunen et al. 2009). The non-native eastern grey squirrel (Sciurus
carolinensis) has established invasive populations in the United Kingdom (UK),
Republic of Ireland, and Italy following multiple introductions during the past
150 years (Middleton 1931; Teangana et al. 2000; Martinoli et al. 2010). In these
countries, grey squirrels have caused economic damage through direct eects
on timber (Kenward 1983), agriculture (Signorile & Evans 2006), property
(Williams et al. 2010) and through indirect costs, related to control eorts
to protect woodlands and the native Eurasian red squirrel (Sciurus vulgaris),
which it has widely replaced in Britain (Gurnell & Pepper 1993). Indirect eects
of biodiversity loss, altered ecosystem services, and aesthetics are dicult to
quantify, but are assumed to increase the overall cost to any economy aected
by IAS (Pimentel et al. 2005). Here, we review the available knowledge on
quantifiable economic damage by grey squirrels in Europe, and highlight some
areas of damage research that should be explored. Finally, we provide a detailed
description of the assessment of economic damage in timber production.
The primary direct economic eect is to timber value as a result of crop
bark stripping by grey squirrels (Shorten 1957; Kenward 1983). The primary
indirect monetary cost to European economies following arrival, comes
from the attempts to control grey squirrels to protect timber and/or aid in
conservation of the native red squirrel (Huxley 2003). Although other eects
have been reported, empirical studies of their economic impacts have not been
conducted.

The Grey Squirrel: Ecology & Management of an Invasive Species in Europe
394
Timber damage
Why grey squirrels strip bark is not fully understood, but the behaviour is
correlated with high juvenile density where agonistic gnawing and exploratory
feeding occur (Kenward & Parish 1986). Grey squirrels also strip and gnaw bark
associated with scent marking activities (Taylor 1977; Koprowski 1993), however,
marking points are typically found only on mature to over-mature timber and
areas are generally small (~300 cm2) unlike the large scale damage that appears
unrelated to marking at traditional sites (Koprowski 1991). Whatever drives
bark stripping behaviour, its targets and eects have been well documented,
including with respect to tree species, age, size, and timing of damage. Such
information has been critical in understanding and predicting the economic
consequences to the timber industry, in part because crop rotation times mean
the full extent of damage cannot be immediately assessed (Gurnell 1996).
In the UK and Republic of Ireland, broadleaved tree species are the most
vulnerable, with beech (Fagus sylvatica), sycamore (Acer pseudoplatanus), and
oak (Quercus spp.) the most commonly aected (Shorten 1957; Rowe & Gill
1985; Lawton 2003). Although grey squirrels are more common in, and cause
more damage to, broadleaved woodlands (Gurnell 1996), they also strip bark
from conifers; Scot’s pine (Pinus sylvestris) and Norway spruce (Picea abies)
are particularly vulnerable in Britain (Mayle & Broome 2013). Semi-mature trees
age 10 to 40 years old suer the most damage across species (Rowe 1984;
Mayle & Broome 2013) with larger trees in oak woodland the most likely targets
(Mayle et al. 2009). Regardless of species and age, bark stripping is seasonal,
occurring almost exclusively between March and August (Middleton 1931; Rowe
1984; Lawton 2003).
Vulnerable tree species are attacked in various ways by grey squirrels. In Britain,
bark stripping on beech tends to be on the lowest one metre of the main stem
(Mercer 1984). Oaks and conifers suer crown damage in the main canopy.
On sycamore, birch (Betula spp.), larch (Larix spp.), and lodgepole pine (Pinus
contorta), stem damage is most common between base and canopy. In each
case, severe damage can kill trees, and damage to surviving trees may not be
detectable until harvested (Breummer et al. 2000). Secondary damage from
bark stripping can also occur in the form of rotting and staining at wound sites
infected by fungi (Dagnall et al. 1998; Mayle et al. 2007) (Figure 1). Wounds to
the bark of a tree disrupt the flow of fluids from the foliage to other parts of the
tree causing the tree to become stressed. This in turn can result in dieback in
the crown and root system and colonization by wood decaying organisms (e.g.
fungi). The larger the wound, the greater is the severity of the decay (Pawsey
& Gladman 1965). In northern Italy, the tops of poplars (Populus spp.) girdled
by grey squirrels may die and be blown o in strong wind (Currado et al. 1987)

395
Economic damage by grey squirrels in Europe
Figure 1. Cross-section from beech stem showing callus around grey squirrel
damage (from Mayle. et al. 2007)
The cost of timber damage
The commercial value of a woodland can decline significantly if trees are
degraded through bark stripping by grey squirrels. High quality timber from
the main stem of trees that may, for example, be converted into planks or cut
into veneers for furniture can sell for around £15,000 per hectare (ha) at 2015
prices. If severely damaged by bark stripping (Figure 1), the same tree crop may
only be suitable for firewood worth 30% of that figure (i.e., £4,500).
An evaluation of the costs of grey squirrel bark stripping damage in British
woodlands (Broome & Johnson 2000) estimated 43,000 hectares of beech,
sycamore, and oak woodland were vulnerable to damage in Britain. The
estimated undamaged value of this woodland was around £40 million.
Historical data from surveys assessing presence of squirrel damage indicated
28% of the beech, 24% of the sycamore, and 7% of the oak were aected,
resulting in an estimated loss of £10 million on the current crop rotation (Mayle
& Broome 2013); a similar estimate for Britain was also provided in another
study (reviewed in Bertolino 2008). As damaged trees were assumed to have
no value, this was believed to be the upper limit of the loss in Britain.

The Grey Squirrel: Ecology & Management of an Invasive Species in Europe
396
Costs of control
Eradication is the most eective method for limiting impacts from mammalian
IAS, but success becomes increasingly costly with expanded geographical area
and lengthened time since invasion (Robertson et al. 2016). Complete eradication
of grey squirrels is considered practically or economically unfeasible in the UK
and Republic of Ireland (Gurnell & Pepper 1993; Lawton 2003) and unlikely in
Italy (Bertolino & Genovesi 2003; Signorile et al. 2014). However, control of grey
squirrels will need to continue in order to maintain economically viable timber
plantations and conserve legally protected red squirrels. Historically, attempts
at control have been sporadic and unsuccessful, but improved knowledge
of grey squirrel population dynamics, behavior, and vulnerability to control
methods have helped develop reliable estimates of cost eective control (see
Gurnell & Pepper this volume, Chapter 21).
Timber damage control
The first concerted eort to reduce invasive grey squirrel populations was a
bounty system implemented in Britain from 30th September 1953 to 31st March
1958 to protect state- and privately-owned woodland (see Box 3, in Gurnell &
Pepper this volume, Chapter 21). Initially set at one shilling per tail (five pence)
for two years, increasing to two shillings in 1956, this ineective scheme was
eventually abandoned after over a million bounties had been paid out at a total
cost of £100,000 (Sheail 1999), about £2.2 million adjusted for inflation in 2015.
Subsequent control for which economic data are available in Britain has
been less centrally coordinated. Evaluation of four damage surveys in British
woodlands from 1949 to 2000 indicated no correlation between the amount of
damage control and reduction in timber damage (Mayle & Broome 2013). Until
recently, damage control methods included shooting, trapping, and where red
squirrels did not occur, poisoning with warfarin. Warfarin was the most cost-
eective method (Mayle et al. 2007). With warfarin as an available option, the
minimum cost of control to keep grey squirrel damage below a moderately
serious level was around £20 per ha (Mayle & Broome 2013), giving an annual
cost of control in 43,000 ha of vulnerable broadleaved woodland of £860,000.
However, European Union approval for use of warfarin was revoked in 2014
(Willoughby 2014), meaning annual costs of control will likely increase.
Few data are available on costs of control in the Republic of Ireland and Northern
Ireland, but one report estimated the worst case scenario for potential costs of
damage prevention in all forested areas at about £19.2 million (Kelly et al. 2013).
At least in the Republic of Ireland, however, the worst case scenario seems
unlikely as broadleaved forest accounts only for 26% of stocked areas, and only
2,811 ha of stocked forest are impacted by squirrel damage (Anon. 2013).

397
Economic damage by grey squirrels in Europe
Red squirrel conservation control
Exclusion or containment of grey squirrels at low numbers is considered critical
for red squirrel conservation (Gurnell & Pepper 1993). In Britain, 37 mainland
and 10 island red squirrel strongholds have been identified and grey squirrel
control has been conducted to maintain their conservation value (Gurnell et al.
2015).
In the only complete eradication, invasive grey squirrels were removed from
Anglesey through trapping control between 1998 and 2015 (Shuttleworth et
al. 2015). This involved the trapping and euthanasia of 6,371 grey squirrels by
2010 (Schuchert et al. 2014), and the final 26 individuals by 2013. The project
also involved trapping of over 3,200 from the adjacent mainland population.
Including some costs for red squirrel reintroductions, the total project cost was
£1,019,000 (CM Shuttleworth, personal communication).
In a five year control project, the conservation group, Saving Scotland’s Red
Squirrels (SSRS), targeted areas of native red squirrel populations in northeast
Scotland and the Central Lowlands with the goal of reducing and containing
invasive grey squirrel populations. A three-pronged approach using SSRS sta,
contracts with private landowners and a trap-loan scheme with local volunteers
was implemented in 2009. Grey squirrel capture rates were reported as near
zero in some areas by 2011, indicating project goals were being attained. After
the first two years, at least 4,323 grey squirrels had been removed at a cost of
£436,213 (Tonkin & Mackenzie 2011).
In northern Italy, eradication costs for a trial programme of live capture and
euthanasia with halothane were estimated at 50 Euros per animal removed.
A legal challenge to removal by animal rights groups halted the programme
and led to discussions of surgical sterilization as an alternative that would
increase the cost by an estimated 50 to 80 Euros per animal. The cost for an
eective sterilization campaign in 1999 was estimated at up to 900,000 Euros
(Perry 2004). However, by the time this case was resolved in the courts the
invasive grey squirrel population had expanded to an unmanageable level and
eradication plans were abandoned (Bertolino & Genovesi 2003).
Few other squirrel eradication eorts have attempted to estimate costs. A
notable exception was the removal of Mexican red-bellied squirrels (Sciurus
aureogaster) from islands in the Florida Keys, USA where cost of eradication of
fewer than 50 animals during 2007 to 2011 was about $80,000 (Pernas & Clark
2011).

The Grey Squirrel: Ecology & Management of an Invasive Species in Europe
398
Other economic damage
Data on other forms of economic damage are very limited; few peer-
reviewed studies have been conducted on non-timber industry costs, and
available government reports lack verifiable sources. However, there are some
indications that grey squirrels have the potential to cause considerable damage
to agriculture, property, and urban economies.
Agriculture
A study that surveyed farmers on the Turin plain of northern Italy reported
damage to newly generated seedlings of maize by grey squirrels. Damage was
scattered and limited to field edges with broadleaved trees, and in the worst case
would cost 70,000 Euros in replanting across the 40,000 ha study area (Signorile
& Evans 2006). The authors also noted damage to maize as potential cause of
concern for agriculture in Britain, where invasive grey squirrels are more extensive
and maize is increasingly planted as a commercial crop (Signorile & Evans 2006).
Some damage of economic importance is caused to market gardens, orchards and
arable crops if they are located near favourable grey squirrel habitat, particularly
when other food sources are in short supply (Gurnell & Hare 2008). Beyond the
above, a recent report has concluded that there is little evidence for significant
wider agricultural damage (Williams et al. 2010).
Property
The frequency of grey squirrel damage to buildings has not been well studied,
but problems include gnawing of electrical cables and damage to thatched
or shingle roofs (Gurnell 1987). Reports to local councils and pest control
contractors from private households in Britain were used to calculate a potential
annual economic cost of £1.9 million based on an average service charge of
£56 (Williams et al. 2010). The same report indicated that insurance companies
rarely cover rodent damage.
Although not reported as a serious problem in Europe, in urban centres of their
native range, grey squirrels interfering with electrical infrastructure cause power
outages that may cost millions of dollars annually across the USA (Mooalem
2013). In the state of New York, USA, grey squirrels caused 55% of all power
faults during 1977 to 1988 at a mean cost per fault of $12,550 and a total of $5.5
million (Enck 1989).
Quantifying effects on timber value
For managers to calculate financial loss from bark stripping damage to a
woodland, they must assess the quantity of timber in the trees. This is usually
expressed in terms of solid volume, which can be assessed when trees are
standing or felled. The assessment methodology (Hamilton 1975) and forest

399
Economic damage by grey squirrels in Europe
management tables (Hamilton & Christie 1971) are both well tried and tested
and used by professional foresters. The value of any assessed solid volume (i.e.,
timber value) is a simple calculation of the volume (m3 per ha or tree) multiplied
by the price paid per m3.
There are alternative assessment procedures available such as the Helliwell
System and the Capital Asset Value for Amenity Trees (CAVAT) devised by
Tim Moya Associates, but no single method is universally accepted (M. Kelly
2010). Both protocols evaluate the quality of the visual amenity of trees either
as individuals, in groups, or whole woodlands, within the context of the wider
woodland landscape, but are not an alternative to solid volume measurement.
The Helliwell system calculates a monetary value following an assessment.
Various criteria including location, tree species, age, condition and area are
assessed and each given a value. Values are multiplied according to a formula
to give a monetary value that can be index linked to ensure it remains current.
The amount of bark stripping damage and its impact on the calculated timber
value of a solid volume assessment is assessed independently. The greater the
degree of precision required the greater the cost of assessment, and this has
to be set against the timber value. To obtain the exact extent of damage in an
area, it is necessary to examine every tree, which apart from small blocks is not
practical. Grey squirrel damage is typically patchy in distribution. Therefore,
the assessment method needs to be a representative sample that is ecient,
reliable and repeatable. Also, dierent assessors should be able to employ
the same method on a given area and obtain results that are not significantly
dierent from each other.
The UK Forestry Commission has developed an assessment protocol (The
Nearest Neighbour method) for estimating wildlife damage within a woodland
or plantation (Pepper 1998). This method is used to assess grey squirrel
bark stripping damage. It involves the systematic selection of points (cluster
points) spread throughout the woodland area and around each cluster point
a predetermined number of trees is assessed for damage. Trees are chosen
independently of damage observed and the presence or absence of damage is
recorded. The severity of damage can be determined by measuring the area of
bark removed. Dierent parts of the tree may be sold for dierent purposes and
will attract dierent values; thus the tree can be divided into dierent zones for
analysis. For example, the first three metres of the main stem of a broadleaved
tree are likely to be the most valuable followed by the next three metres. If both
three metre lengths are of high quality they may be sold for conversion into
slabs or veneers. The timber in the live crown is generally the least valuable and
may be sold for conversion into wood chips, pulp or firewood. The accuracy
of the results of an assessment depends on the number of trees assessed and
the assessor decides the degree of accuracy required. The method of any
assessment follows a nine-step sequence (Table 1).

The Grey Squirrel: Ecology & Management of an Invasive Species in Europe
400
Table 1. Nine-step sequence for timber damage assessment using the nearest neighbour method.
Steps Example
1. Set and record specic and clearly dened objectives for the assessment. Specify the
form of damage to be assessed and how it is to be recorded.
Presence or absence of bark stripping dam-
age on sampled trees to determine propor-
tion of trees with damage.
2. Decide the ± accuracy required (usually between 5% and 10%). The greater the accuracy
required the greater the intensity of the assessment therefore the greater the cost.
± 5%
3. Calculate the number of trees (N) to be assessed to provide the required accuracy. (100) = N
5
4. Choose the cluster size (c) (4 to 7 trees). c = 5
5. Determine the number of clusters (n) required; at least 20 clusters must be visited and
assessed. Adjust the cluster size if necessary to achieve the minimum 20 clusters. N = n
c
6. Calculate the distance between clusters (D) required to cover the area (A) evenly. A x 10000 = D
c
7. Complete the assessment eld work by visiting the cluster points systematically by
walking in a straight line, usually parallel to the edge of the area or, if a plantation, along
the rows of trees. Assess each cluster tree and record. Measure and record the stocking
density and stand top height if required.
(see Figure 2 for example of a eld assess-
ment record form)
8. Calculate the percentage of trees damaged (d = total number of damaged trees
counted, and a = total number of trees assessed).
d X 100 = percentage damage
α
9. Check the accuracy of the assessment. If the accuracy achieved is below the level
required more trees must be assessed. (n’ = actual number of clusters assessed, and a =
total number of trees assessed). = ± 200 sum of squares —
α
d2
η’
()
A single assessment is just a snapshot in time and it will not necessarily
reflect previous damage; thus it is necessary to decide upon a frequency of
assessments. Annual assessments in a Forestry Commission owned beech
woodland in southern England showed, after annual squirrel control over 12
years from 1972 to 1983, an annual decrease in the number of trees with visible
squirrel damage wounds with exposed wood. The percentage of damaged
trees decreased from 50% in 1972 to 8% in 1982 (H. Pepper, unpublished data).
A single assessment in 1982 would have given the erroneous impression that
damage to the trees was light and any loss of timber value would be negligible.
Therefore, once trees have reached the damage vulnerable stage (circa 15 to 20
years) it may be necessary to consider completing annual damage assessments
of fresh damage to evaluate the accumulated level of timber degradation that
has occurred. The interpretation of damage into a financial loss will inevitably
require a level of expert subjectivity, but accurate damage assessment data will
enable a more realistic costing.

401
Economic damage by grey squirrels in Europe
Figure 2. Suggested example of a field assessment record form.
Revenue forgone on sales as a result of timber degradation is not the only
financial penalty incurred due to grey squirrel bark stripping damage. There
may be additional related costs such as the price of damage assessments. If
the damage is found to be so severe that the trees cannot retain or increase in
value and should be felled prematurely the area will subsequently need to be
replanted. Restocking a woodland area may cost up to £6,000 per ha at 2015
prices.
Callus growth over bark stripping wounds results in abnormal growth rings
of wood creating possible weaknesses in a tree’s structure. Branch drop and
wind throw may result from wounding presenting a potential safety hazard to
people and property. This is a particular problem in areas where there is public
access. In these situations the employment of expensive arboricultural work is
unavoidable.

The Grey Squirrel: Ecology & Management of an Invasive Species in Europe
402
Ascribing a financial value to woodland amenities such as landscape, wildlife
conservation and recreation is currently being considered. The risk of grey
squirrel damage will have an impact on those considerations and likely incur
additional costs both subjective and real. This is likely to influence future
woodland management plans. Planting the most damage vulnerable tree
species may have to be avoided and the alternatives may not blend in with
the surrounding landscape. How do you evaluate such potential change in
economic terms?
Discussion
The grey squirrel is a prolific invader in wooded areas of the UK, Ireland, and
northern Italy, but its economic impacts appear greatest in Britain where it
is more widespread and bark stripping causes monetary losses to the timber
industry. Sporadic and largely unsuccessful control or eradication eorts
have not substantially altered economic damage to timber, but increasing
understanding of where and when damage occurs, and how to quantify it may
assist forestry decisions in future. The heaviest loss for wildlife conservation is the
decline of the native red squirrel throughout the invaded areas. Although total
ecological impact will be a challenge to quantify in monetary terms, evidence
presented in this chapter indicates annual budgets in the millions of Euros will
be needed to halt the march of grey squirrel invasions where red squirrels lie in
their path. Recent regional attempts at containment and eradication in the UK
provide encouragement that coordinated eorts can succeed, and suggest the
level of funding that may be required. Finally, despite this wealth of knowledge,
a recent article in an influential economics newspaper, which featured non-
native grey squirrels, suggested consensus on economic threats from IAS has
not been reached (Anon. 2015).
Acknowledgments
Sincere thanks to Emily A. Goldstein, Craig Shuttleworth, and two anonymous
reviewers for providing advice and ideas that helped improve this manuscript.
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