Themat; ... Session: Conservation & Management

Proceedings of the I European Congress of Mammal }gy. Museu Bocage. Lisboa, 1996, pp. 67-81.

THE GREY SQUIRREL IN BRITAIN: PROBLEMS FOR MANAGEMENT

AND LESSONS FOR EUROPE

by

JOHN GURNELL

School of Biological Sciences, Queen Mary & Westfield College. London El 4NS

INTRODUCTION

Twenty-three species of mammals have been deliberately or accidentally introduced into Great Britain, many within the last century, and several of these species, including the grey squirrel, are now recognized as pests (Brown, 1985 in Usher, 1986). Grey squirrels were first deliberately introduced from eastern North America into Britain in 1876 and there followed at least 10 further in­troductions by 1910, eight from the U. S. A. and two from Canada (Shorten, 1954). In addition, grey squirrels were translocated within Britain many times between 1906 and 1937 (Shorten, 1954, Lloyd, 1983). Grey squirrels were re­leased as exotic animals to enrich many parks and gardens. For some people, but by no means all, that previously held view that grey squirrels are attractive and desirable animals has changed substantially over the ensuing years. Why?

68 John Gurnell

Virtually all of the original introductions and translocations were sucessful. By 1930, grey squirrels covered some 26000 km2 with established populations in many places, particularly in the north-east, cen­tral and south-east parts of England (Shorten, 1954). After this period of establishment, grey squirrels began to spread throughout the country, with significant progress being made during the· 1930'.s and 1940's (Lloyd, 1983). In fact, Williamson & Brown (1986) have described the spread of the grey squirrel as a generally steady advance comprising of random dispersal with occasional major advances. The changing distribu­tion of red and grey squirrels in Great Britain is well documented and further details will not be presented here (Lloyd, 1983, Gurnell, 1987, 199Ia,b).

To come to the present, the distribution of the red squirrel continues to decline and this is leading to several increasingly isolated populations in England and Wales (Gurnell & Pepper, 1993). Grey »<juirrels continue to extend their range with advances, for example, in Scotland and northern England. Furthermore, and somewhat worrying, is that translocations within Britain still appear to take place from time to time (Pepper pers. comm.). Thus, there are fears that the red squirrel will become extinct in England and most of Wales within the next 20 years. Moreover, once gone, it is unlikely that the red squirrel will ever again be seen in the deciduous woodlands within these areas. The grey squirrel is far too established in these habitats to allow room for the red squirrel without intensive management.

In Britain, the wildlife value of the red squirrel is high. They fill a major ecological niche, and, rather like the feelings that many people had and have for the grey squirrel, they are handsome and enchanting animals to the public. The decline of the red squirrel is closely linked with the appearance of the grey, and so the grey must take the blame for the loss of our native species. The grey squirrel also attracts the wrath of many people because it causes serious damage to trees, particularly young broadleaved trees such as beech, sycamore and oak which are 10-40 years old (see below). Consequently, grey squirrels are both liked and disliked in Great Britain.

The Grey Squirrel in Britain 69

Squirrel damage

Both red and grey squirrels strip bark from trees; descriptions of damage are provided by Gumell (1987) and Shorten (1954). Grey squirrels occur at much higher densities (two to eight animals per hectare) than red squirrels (less than one squirrel per hectare) in broadleaved woodland and can cause considerable amounts of economic damage in these habitats (tab. I). There is also increasing concern about grey squirrel damage to conifer trees in certain parts of the country. Sometimes red squirrels also damage conifers and this poses problems for forest managers who could be faced with the contrary problems of conserving red squirrels and control­ling them to minimise damage (Gurnell & Pepper. 1988).

The reasons for damage are complex and will not be reviewed here (Gurnell. 1987. Kenward. 1989. Kenward & Parish. 1986. Kenward. 1983). Nevertheless. it is clear that certain properties of trees. including species. age. quality and annual growth patterns. affect their vulnerability to damage (tables I and 2). Similarly. demographic and spatial properties of squirrels affect the likelihood of damage occurring. Damage can be serious when the density of grey squirrels in or near damage-vulnerable habitats is high. especially when there are a large number of young animals in the popula­tion (Gurnell. 1989). The main damage period in Britain is April to July

TAB. I. GREY SQUIRREL DAMAGE TO TREES IN BRITAIN AND ITALY.

(a) Grey squirrel bark stripping damage to vulnerable tree species in slate and private forests in the U. K. in 1983 (from Rowe, 1984). Damage assesment defined as the presence of exposed wood due to bark removal by squirrels.

Number of Areas Damage Cat. (% trees attacked)

Species Assessed Damage 0 1- 21- 41- 61- 81-

20 40 60 80 100

Beech 143 87 13 39 17 17 13 <I Sycamore 118 86 14 39 20 13 9 5 Oak 139 41 59 31 6 2 <I Poplar 45 29 71 13 4 9 0 2

70 John Gumell

(b) Grey squirrel damage, feeding activity and nest sites in relation to tree species an area to the southwest of Turin in Italy (after Currado, Scaramozzino & Brussino, 1987).

Tree species

Wild Hornbeam Aspen/white poplar Pedunculate oak Red oak Wild cherry

Hazel Robinia Grey willow Elm Sycamore/maple Alders Hawthorn Ash

Cultivated Poplar hybr. Walnut Apple Cherry Willow

Damage

++ ++ +

++

Buds

+

Fruits

++ ++ +

+

++ + +

Nests

++ ++ +

+

+

(sometimes later) and therefore the focus of much research has been on predicting squirrel densities within different forest habitats, and whether there is an influx of animals into damage-vulnerable habitats, during this period. Considerable advances have been made in relation to the prediction of squir­rel numbers in recent years.

For example. food shortage and the severity of winter weather are believed to be the important limiting factors affecting numbers during the annual cycle and an index method of predicting future relative abundance based on squirrel trappability is currently being evaluated (Gurnell, 1989). The criteria for assessing damage-vulnerable sites and predicting high den­sity grey squirrel habitats in relation to damage-prediction assessment are outlined in table 2.

The Grey Squirrel in Britain 71

TAB. 2. CRITERIA FOR ASSESSING DAMAGE-VULNERABLE AND POTENTIALLY HIGH DENSITY SQUIRREL HABITATS (AFTER GURNELL & PEPPER, 1988).

Damage-vulnerable habitats

(i) Tree species

Most important species according to relative damage frequency (Rowe & Gill, 1985):

I. Beech 2. Sycamore 3. Oak 4. Maple 5. Sweet Chestnut 6. Ash 7. Birch 8. Hornbeam 9. Many other species occasionally damaged

(ii) Age

Pole-stage trees, 10·40 years old

(iii) Time of the year

Nearly all economic damage occurs between April and July.

(iv) Sile characteristics and forest management

Conditions which will give most vigorous growth and highest sap content during spring and summer (Kenward & Parish, 1986), Can be affected by local climate. soil type, and forest practices such as thinning.

PotentiaUy high squirrel density habitats

Broadleaves, especially those with large fruits or seeds. e. g. oak, beech, sweet chestnut hazel, cherry.

Mixtures of species for good year-to-year supply of seed food.

Plant and animal rutemative foods.

Mature - seed bearing age

Population levels in April to July. High numbers depend on overwinter food supply, especially tree seeds, immigration, early breeding and the recruitment ot springborn young.

Good understorey (e. g. hazel) and high diversity of plants and animals. Good supplies of alternative foods to tree seeds.

<-----------Spatial relationships among damage vulnerable and potentially -----------> high squirrel density habitats

72 John Gurnell

Squirrel management

Within many forests in Britain, both private and public, grey squirrel numbers are controlled to reduce tree damage. At the same time, there is an increasing demand to manage forests in such a way as to conserve the re­maining red squirrel populations. (Licences to control red. squirrels to re­duce damage can be obtained from Government Agencies, but these are seldom applied for.) Squirrels in Britain are not managed for hunting, food or furs (Rowe, 1983). There are five necessary ingredients to a species management policy: legislation, education (both the public and the authori­ties), habitat management, population management and long-term monitor­ing (Gumell & Pepper, 1988). There are several Acts of Parliament govern­ing squirrel management in Great Britain; they include the Grey squirrels (Warfarin) Order 1973 in relation to controlling grey squirrels, and the Wildlife & Countryside Act 1981 which extends full protection to the red squirrel. Further details can be found in Pepper (1989a) and Cooper (1991). Habitat and popUlation management will be further considered below in terms of grey squirrel control and red squirrel conservation.

(a) Grey squirrel control

Control of grey squirrels is almost entirely achieved by poisoning them with 0.02% warfarin dispensed in hoppers; few squirrels are now shot or trapped (Gumell & Pepper, 1988). It is illegal to poison red squirrels or to poison grey squirrels where red squirrels are at risk. In recent years, con­siderable improvements to hopper design have been made and risks to non­target species have largely been removed (Pepper 1989b, 1990). The eco­nomics of grey squirrel control in state forests is outlined in table 3. With­out control, there is a danger that the trees will not reach a sufficient timber quality to be worth harvesting, in which clIse losses would approximate their standing value. Moreover, the problems of damaged timber are not fully realized in Britain because the timber being harvested at the present time was not subjected to damage when they were vulnerable. In addition, emerg­ing from ongoing research is that it is unnecessary to control grey squirrels

The Grey Squirrel in Britain

TAB. 3. OUTLINE COSTS OF GREY SQUIRREL DAMAGE PREVENTION IN FORESTRY COMMISSION BROADLEAVED WOODLAND IN 1991.

73

Figures are actual figures, not discounted (Forestry Commission, unpublished). Updated from Gumell & Pepper, 1988. Note that private finns contracted to control squirrels in small woods may charge up to £30ha-'.

Area of vulnerable woodland Area controlled Cost of protection (Warfarin) Actual cost of protection for ages 10 through to 40 years

e. g. Beech Yield class 6 at 100 years standing value Loss due to damage without control

e. g. Beech Yield class 10 at 100 years standing value Loss due to damage without control

22,000 ha 32,000 ha £3.08ha-' £92.40ha-'

£8500ha-\ £255ha-'

£19100ha-\ £573ha-\

outside the damage period (August to March), and that it may be possible in the future to estimate where and how much control will be necessary each year based on damage-prediction assessments made prior to the dam­age season (Gurnell, 1989, Gurnell & Pepper, 1988).

(b) Red squirrel conservation

Numbers of red squirrels have always exhibited marked fluctuations in numbers and these have been influenced by loss of habitat, food availabil­ity, and disease epidemics (e. g. coccidiosis, parapoxvirus), particularly at times when animals are already undernourished and in poor body condition (Gurnell, 1987). Red squirrels sometimes disappeared from areas before the arrival of grey squirrels (e. g. Reynolds, 1985) and in many cases it appears that red squirrels were unable to recover from very low numbers once the grey squirrel had replaced them. Thus, the replacement of the red by the grey squirrel is attributed to how efficiently the two species utilise food supplies in coniferous and broadleaved habitats. It is not accepted that it is due to disease brought into the country by grey squirrels, to the larger grey

74 John Gurnell

squirrel 'aggressively chasing out' the smaller red squirrel, or, as has been suggested, because grey squirrels interfere in the mating behaviour of red squirrels (Bertram & Moltu, 1987) .

Differences between red and grey squirrels in body weight, build, three­dimensional activity and population densities in coniferous and broadleaved habitats have been described (table 4) . It appears that Ted squirrels are not so well adapted to broadleaved habitats as grey squirrels. They only reach the same densities as they do in coniferous habitats which is considerably less than grey squirrels in broadleaved habitats (table 4). One example of the differences between red and grey squirrels in broadleaved habitat utili­sation, is that red squirrels do not exploit acoms crops as efficiently as grey squirrels (Kenward & Holm, 1989). Thus, grey squirrels have the competi­tive advantage over red squirrels in broadleaved woodland. This is not to say that red squirrels do not live in broadleaved woodland, even though it may be less suitable for them (Wauters & Dhondt, 1989). Red squirrels were once found throughout all broadleaved woodland areas in Britain, and, of course, they are found in broadleaved woodland across Europe (Shorten, 1954, Tonkin, 1983, Wauters & Dhondt, 1988).

TAB. 4. A COMPARISON OF SOME CHARACTERISTICS OF RED AND GREY SQUIR­RELS. DIMENSIONS APPROXIMATE AND AVERAGES FROM MANY STUDIES (FROM GURNELL. 1987, 199IA.B, KENWARD & HOLM. 1989. KENWARD & TONKIN, 1986).

Characteristic Red Grey

Head and body length (mm) 220 260 Tail length (mm) 180 220 Body weight (g) 300 550 Longevity (yr) 6-8 7-9 Autumn weight increase (% body weight) !O 20 Arboreal activity (% time. average for year) 67 14

Preferred habitat Coniferous woodland Broadleaved woodland Acorns preferred No Yes Population density (oolha) Coniferous woodland < 1.2 < 1.0 Deciduous woodland < 1.0 Usually 2.0-8.0

The Grey Squirrel in Britain 75

In contrast to broadleaved woodland, red squirrels can survive in large blocks of pure conifer whereas there is evidence that grey squirrels may not be able to do so (Gurnell & Taylor unpubl.). It is believed that blocks or belts of seed producing broadleaved trees are necessary for the initial inva­sion of grey squirrels into coniferous forest and required to maintain their presence. These broadleaves act as survival habitats, from which-grey squirrel populations can expand and contract into and out of the conifer stands ac­cording to prevailing conifer seed supplies. The long-term future of the red squirrel on the British mainland, therefore, depends on maintaining large areas of continuous, mixed species coniferous forest with no broadleaved component. In the short-term the aims are to manage forests in such a way that the range and numbers of red squirrels are increased and, at the same time, the presence of grey squirrels discouraged. Once the structure of the habitat is right, intensive management should not be necessary.

Since there are many, conservation demands made on forest managers, they must plan where possible to combine the different management objec­tives (McIntosh, 1988). Red squirrels require the setting aside of large areas of coniferous forest (e. g. >2,OOOha) as squirrel protection areas or Reserves. Because grey squirrels can make foraging movements of I kilometre or more (e. g. Kenward, 1985), the Reserve should be round or square rather than long and thin. A border at the edge of the forest of at least 3km wide should be established as a buffer against the infiltration of grey squirrels. The border should consist of habitats which are unsuitable for grey squirrels and, there­fore, need not consist of coniferous forest, but could consist of open land (common, agricultural but not urban).

Management within the Reserve should concentrate on developing and maintaining a mixed species coniferous forest. Importantly, most large-seeded broadleaves (e. g. oak, beech, sweet chestnut) should be removed and new plantings of broadleaves should be confined to small seeded species such as birch. The introduction of broad leaved woodland corridors into coniferous forests underpins many plans to diversify these forests for wildlife in gen­eral (McIntosh, 1988), and this must be a cause for concern for the future of red squirrels in the large coniferous forest in the North of England and Scotland (e. g. Kielder Forest).

76 John Gurnell

Attention must also be paid to the mix of conifer species within the Reserve and to the forest age structure. In Britain, commercial plantings now usually consist of the introduced Corsican pine or Sitka spruce instead of our native Scots pine and Norway spruce. Corsican pine and Sitka spruce produce less seed, less often than our native species. Planting some Scots pine, Norway spruce other conifer species such as larches and Douglas fir in groups or strips along ride edges and in small patches within the forest will provide a more diverse and balanced food supply for red squirrels (Gumell, 1987). Further, young plantations (e. g. 15 to 30 years old) which provide food and cover are preferred by red squirrels and it is believed that at least half the forest should be of seed-producing age. Therefore, a forest age structure should be developed such that about one third of the trees are 0-15 years old, one third 16-30 years old, and the remainder >30 years old. Other management practices are included in table 5.

TAB. 5. PRESENT AND FUTURE TACTICS FOR MANAGING RED AND GREY SQUIRREL IN COMMERCIAL FORESTS IN GREAT BRITAIN (MODIFIED FROM

GURNELL & PEPPER, 1988).

Control of red and grey squirrels.

]. Damage prediction assessment (a) Habitat damage vulnerability (b) Squirrel densities during damage period. April to July

2. Time of control (a) Not belwee~ August and March (b) Reduced control in years of low squirrel densities

3. Place of control (a) In potentially high squirrel density habitats (b) If possible, not in damage vulnerable habitats because squirrels can be attracted

to these areas if control is not started early on, in April. (e) Not in relation to low yield crops, unless of high conservation value.

4. Type of control (a) Minimise use of warfarin by damage prediction assessment (b) Use squirrel-only hoppers to avoid risk to non-target species areas (c) Trap for small scale control or in conservation-sensitive areas

5. Habitat management (a) Protect individual trees when valuable, e. g. with repellents (b) Minimising the planting of trees with large seeds or fruits, especially near dam­

age-vulnerable crops

The Grey Squirrel in Britain 77

6. Other (a) Maintain a programme of monitoring and damage assessment - this is essen­

tial (b) Carry out more research into damage economics (c) Research into alternative control methods to the use of poison

B. Conservation of red squirrels

1. Conservation Plan Integrate the conservation of red squirrels into the other management objectives of the forest manager. The strategy .should be to manage forest habitats so as to encourage red and discour­age greys squirrels.

2. Habitat management (a) Area ---;- set aside a large area of forest (e. g. > 2000 ha) and designate as a red

squirrel reserve. (b) Shape - round rather than long and thin to minimise incursions of grey squir­

rels. Cc) Border-at least 3km of coniferous forest or open land must be established as

a buffer against the infiltration of grey squirrels. (d) Broadleaves - the removal alliarge~seeded broadleaves (e. g. beech, oak, sweet

chestnut). (e) Conifers - In addition to the main commercial crop, mixtures of conifers

(e. g. pines, spruces, larches) should be planted in groups or ribbons along ride edges and in small patches within forest. This will improve cover and seed availability.

(f) Forest structure - young plantations (16 to 30 years) which provide food and cover are preferred by red squirrels. At least 50%-60% of the forest should be of seed~producing age. A forest age structure should be developed such that c. 30 % of trees are 0-15 years old, 30 % are 16-30 years old, and 40% are > 30 years old.

(g) When clear-felling, leave some good seed and nest trees. Seed~producing areas should be connected by continuous bands of trees to prevent isolation and facili ­tate movement between them.

(h) Thinning - the first thinnings to crops should, if possible, be deferred to about 30 years of age. 1 in 4, 2 in 8 or 2 in 10 row line thinning may benefit red squirrels by maintaining closed canopy cover in the unthinned strips.

3. Short-term Management Tactics. (a) Remove grey squirrels by trapping or shooting. (b) Providing a supplementary food source to red but not grey squirrels.

4. Other (a) Assess suitability of non~native conifers (b) Assess potential and feasibility of reintroductions

78 John Gumell

In the short-term, grey squirrels should be trapped and removed during the establishment phase of the Reserve. Two other management tactics are currently being studied. The first concerns the provision of supplemental feed in special hoppers which allow red but not grey squirrels access to the food (Bertram & Moltu, 1987), and the second concerns translocating and reintroducing red squirrels to strengthen declining populations. or to replace populations which have become extinct (Gumell & Pepper, 1988).

Grey squirrels in Europe

The invasion of grey squirrels into British forests has been particularly successful and they have had a considerable impact on forest ecosystems. Unlike other introduced mammals, such as coypu and muskrat (Gosling & Baker, 1989), the possibility for eradicating grey squirrels no longer exists and they must be considered a permanent member of the British fauna. Historically, grey squirrels have been introduced into other countries such as Australia and South Africa (Gumell, 1987), but it is the presence of grey squirrels in Europe which must be a cause for concern. There have been unofficial reports of grey squirrels in Belgium, but confirmed evidence of their presence comes from Italy.

Professor Currado and his colleagues have described the introduction and ecology of grey squirrels in Piemonte in Northern Italy (Currado & Scarramozzino, 1989, Currado el aI. , 1987). It appears that two pairs of grey squirrels were introduced into Piemonte in 1948 from the U.S.A. Another population at Genov Nervi is of uncertain origin but it appears to be more recent, about 20 years ago. In the ensuing years grey squirrels have spread over an area of about 200km2 to the South-west of Turin, between the Po River, the Pellice tributary and the Cotian Alps. Thus, as yet they are not widespread. However, it must be remembered that there were many intro­ductions and translocations throughout Britain, and although grey squirrels were first introduced in 1876, there was a considerable period of establish­ment before they really began to extend their range between 1930 and 1945. The spread of the Italian populations may also be slow because of the lim­ited amount of tree cover, but if they make their way along the rivers, they

The Grey Squirrel in Britain 79

could eventually reach the subalpine areas to the east and north. The Alps may be a more formidable geographic barrier, but with Man's obsession for translocating animals, grey squirrels could tum up over the other side of the Alps as well as elsewhere in Italy. There is evidence that grey squirrels are still being moved to new areas within Britain.

Equally worrying is that the grey squirrels in Italy are having exactly the same impact as in Britain. Red squirrels are no longer found in some areas (e. g. Stupinigi Woods), and forest trees (especially poplar and hornbeam plantations), fruit trees and field crops are damaged (see table I). There is a good chance that the grey squirrels can be eliminated while they stlll have a localized distribution (Currado et ai., 1987). It is widely realized that efforts to eliminate invaders are most effective at an early stage (e. g. Williamson & Brown, 1986). Such an opportunity probably never occurred in Britain because of the many and widespread introductions, although at that time tne full impact of the presence of grey squirrels was not known.

Although, as I understand it, there are laws against introductions in Italy, they are not implemented and it appears that other laws regard animals which exist naturally to be an integral part of the native fauna and as such gain protection. Moreover, as in Britain, many people in Italy wish the grey squirrels to be left alone. Thus, public opinion is against their eradication.

In Italy, therefore, it emerges that there are formidable obstacles against the instigation of an eradication campaign against the grey squirrel (Currado et ai., 1987). In particular, the local authorities and the general public in Piemonte need to understand why it is necessary to remove these animals. A concerted action is required by policy makers, funding organizations, and applied biologists (Gosling & Baker, 1989) to implement an eradication campaign (also see Currado et ai., 1987). If grey squirrels become firmly established in Italy, or elsewhere in temperate Europe, there are immense risks that the animal will spread over very large areas, cause serious dam­age to broadleaved trees and possibly field crops, and bring about the de­mise of the native red squirrel. This is one invasion we can anticipate, and we should leam from our past mistakes. The potential dangers are' there for all to see from the British experience. Grey squirrels must be kept out of Europe.

80 John Gurnell

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The Grey Squirrel in Britain 81

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