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Britain’s Mammals: Introducing the Species
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Box G. Coypu (Myocastor coypus)

Box H. European beaver (Castor fiber)

As a group, the rodents contain some of the most imperilled and the most pestilential species. Red squirrels and water voles are among Britain’s most rapidly declining species, while house mice, brown rats and grey squirrels cause serious economic damage. The black rat encompasses both – it is perhaps our rarest naturalized mammal, but was once a serious pest, and is still controlled. The coypu (Box 3:G) was also once a member of the British fauna, but was successfully eradicated by 1989. Conversely, there are schemes to reintroduce red squirrels, water voles and dormice to areas where they are no longer present, and, excitingly, plans are afoot to reintroduce the European beaver (Box 3:H), extinct from Britain for at least 400 years.

Box G. Coypu (Myocastor coypus)

Coypu are native to South America but were imported to Britain for fur farming around 1929 (all farms were closed by 1945). A feral population was established in Norwich by 1944, and within 20 years coypu had extended their range throughout East Anglia, west to Lincolnshire, Cambridgeshire and Hertfordshire, numbering approximately 200,000 in 1962. In the 1950s coypu were responsible for serious damage to dykes and banks through burrowing. They also damaged agricultural crops and indigenous flora. An extensive control campaign initiated in 1962, coupled with the severe winter of 1962/63 reduced coypu to about 2000 individuals restricted to Norfolk and Suffolk by 1970. A second campaign was initiated in 1981 with the aim of eradicating coypu from Britain within 10 years. About 34,900 coypu were trapped during this campaign; only 12 animals were captured in 1987 and two found dead on the roads in 1988. The campaign was terminated in January 1989. One male was trapped by MAFF field staff in December 1989 – no further evidence of coypu has been seen. It now seems certain that coypu have been successfully eradicated from Britain.

Further reading

Gosling, L.M. & Baker, S.J. (1987). Planning and monitoring an attempt to eradicate coypus from Britain. In: Mammal Population Studies. (ed. S. Harris). Symp. Zool. Soc. Lond. 58, 99-113. Oxford University Press, Oxford. 382pp.


Box H. European beaver (Castor fiber)

Beavers were once widely distributed throughout England, Wales and Scotland, but were probably rare by the 10th century and extinct by the 16th century. Following a public consultation and scientific feasibility studies, a trial reintroduction to Scotland is planned in the near future (see section 4.4.1.iv).

Beavers are Europe's largest rodent, weighing around 20kg. They also have a distinctive hairless, flat tail and webbed hind feet, both adaptations to their unique aquatic life. They live in lakes, swamps, streams, rivers and ditches where they have year round access to water and food. They eat terrestrial and aquatic herbaceous and woody vegetation (alder, poplar and willow are favourites), usually within 10m of the water. Beavers fell trees to obtain their food, preferring those with a diameter less than 10cm, although they are capable of bringing down trees as large as 1m in diameter. In winter, branches and tree trunks are stored underwater, enabling beavers to feed when the ice covers the water and they cannot forage above ground.

Beavers are generally nocturnal, but can be seen at dusk where there is little human disturbance. Where possible they use a simple burrow in the river-bank, with an entrance below the water surface. Where this is not possible they construct ‘lodges’ (a large hollow mound of branches plastered with mud, also with underwater entrances). Home range sizes vary from 0.5-12.8km depending on habitat configuration and vegetation quality; one family will typically use about 3km of river bank or shore. Family groups (an adult pair, kits, yearlings and occasionally one or more sub-adults) may remain together during the summer if sufficient food is available, but otherwise the female and young of the year use one lodge, while the rest of the group reside in others. At the end of the summer, the group reassembles to repair the main lodge, build a food store, and over-winter together.

Mating occurs in February and births in June. Beavers are monogamous and produce one pair of young per year. Although they become sexually mature at 1.5-2 years old, beavers do not usually breed until their third summer, when they have left the family group. Beavers usually disperse at about two years of age, moving anywhere between 10-120km from their natal range, in order to establish new territories. They can live for up to 25 years, but seven or eight is more usual. The majority of deaths among reintroduced populations are from poaching, entanglement in nets and road accidents. Population densities vary from 3-9 individuals per km on river and 3-22 individuals per ha on ponds and lakes, determined largely by the availability of winter food.

Beavers are a key-stone species and have a major impact on their environment through their damming activities. Dams are carefully and deliberately made from mud and branches, the gaps plugged with twigs, and mud packed against the upstream side. Finally, long supporting pieces of wood are dragged over the crest and pinned against the bottom on the downstream side. Even quite a small dam will take a person’s weight. Dam repair is ongoing, stimulated by the sound of running water. Dams range in length from less than a metre to hundreds of metres, and in height from several centimetres above water-level to a couple of metres. Beavers build dams to maintain water-levels at sufficient depth to cover the entrance to the burrow and allow the animals to swim, dive and store food. Ponds resulting from damming also improve beavers' access to surrounding food supplies. A beaver group may have a series of dams, usually downstream from the lodge, which raises the water level by an average of 30-50cm (up to 1m). Building activity and felling trees may interfere with forestry and fishery interests but may also be enormously beneficial for biodiversity.

Further reading

Macdonald, D.W., Tattersall, F.H., Brown, E.D. & Balharry, D. (1995). Reintroducing the European beaver to Britain: nostalgic meddling or restoring biodiversity? Mammal Rev. 25,161-200.

Red squirrel (Sciurus vulgaris)

STATUS Origin Native
UK Population size 160,000 ??
UK Distribution East Anglia, Isle of Wight, Poole harbour islands; north England and parts of Wales and Ireland; widespread in Scotland
UK Status Endangered; declining
IUCN designation Lower risk
NATURAL HISTORY Habitat Broadleaved and coniferous woodland, though becoming confined to coniferous forest
Diet Conifer and other tree seeds, also nuts, fungi and buds
Behaviour Diurnal; arboreal; generally solitary
Home range 1-7ha in mixed broadleaved woodland; 9-30ha in conifer-dominated woodland
Breeding 1-2 litters of 1-8 (average 3, usually max. 5) young per year
ISSUES Habitat change 2.3.1.ii, 2.3.2.i, 2.3.2.ii, Box 4:CC
Impact of grey squirrel 2.1.3.ii, 2.1.4.i
Reintroduction 4.4.1.ii.a


Since the introduction of the grey squirrel (see below) into Britain, the native red squirrel has become increasingly rare. By the 1990s it was effectively extinct in southern England, with the exception of a few isolated remnant populations in Norfolk, the Isle of Wight, and Brownsea and Furzey islands in Poole harbour. Isolated populations still occur in Wales, Lancashire and County Durham and red squirrels are still abundant in large areas of Cumbria, Northumberland, outside the central belt of Scotland and parts of Ireland. They are widespread throughout continental Europe. They were introduced to Jersey in the 19th century and are currently thriving there.

With the exception of a modest range expansion in Scotland, associated with upland afforestation, the red squirrel in Britain has been declining, both in range and in numbers since the 1930s. In the early 1990s the population was estimated at 160,000 individuals, but has probably declined since then.

Originally, the British red squirrel was considered to be a separate subspecies, but this distinction may have been blurred by Continental introductions to Scotland during the 18th and 19th centuries.

Natural history

Since the arrival of the grey squirrel (a broadleaf specialist), red squirrels have become increasingly confined to large blocks of conifer forest – formerly they had been found in broadleaved and coniferous woodland in Britain, as they are in the rest of Europe. They feed mainly in the tree canopy on conifer and other tree seeds as well as fruits, nuts, berries, fungus, bark and sap tissue, tree buds and flowers, other green plant material, invertebrates (e.g. caterpillars), lichen and the occasional vertebrate material (including birds’ eggs).

Red squirrels are arboreal and diurnal, with major peaks of activity two to three hours after dawn and two to three hours before dusk in summer, and a single morning peak in winter. Home range size is strongly dependent on the size and structure of woodlands in the present highly fragmented landscape, and varies from 1-7ha in mixed broadleaved woodland, to 9-30ha in conifer-dominated woodland. Males have larger home ranges than females. Red squirrels are generally solitary, although individuals of either sex sometimes share dreys.

Mating occurs in winter and in summer leading to spring (February-April) and summer (May-August) cohorts of 1-5 young (average three). Individuals are capable of breeding from 10-12 months of age and live to an average of three years (maximum 6-7 years). Juveniles disperse (up to 10km or more) in May and October following the two breeding peaks. Mortality in the first year is high (75-85%) but declines to an average of 40% (range 20-50%) in adults. Causes of death include starvation, road accidents, predation and diseases (coccidiosis and parapoxvirus).

Annual fluctuations in density are considerable, and are influenced mainly by seed supplies. Densities are highest in mature pine-dominated woodland as pine cones retain their seeds longer than broadleaved species. Typical values are 0.1-3.4 (usually <1) individuals per ha in conifer forest and 0.76/ha in oak-hazel wood.

The issues

Grey squirrels have decimated red squirrel populations, probably through competition for food, but possibly also by transmitting deadly parapox virus. Red squirrels are also threatened by fragmentation of forests, and the persistence of many isolated populations is questionable.

Increased conifer plantation creates additional habitat for red squirrels but the benefit depends on their management - too many sitka spruce, for example, may be disadvantageous. Supplemental feeding may attract red squirrels but there is little evidence that this leads to a long-term increase in density, and feeders can act as foci for disease transfer. Red squirrels may cause economic damage in conifer plantations at a local level by bark stripping, especially at high densities.

A captive-breeding and reintroduction program was established in the UK in 1995. The species action plan aims to maintain and enhance current populations of red squirrel through ‘good management’ (including attempting to create or maintain 2,000ha of conifer reserves in Wales), and to re-establish populations where appropriate.

Further reading

Bryce, J. (1997). Changes in the distribution of red and grey squirrels in Scotland. Mammal Rev. 27, 171-176.
Gurnell, J. (1987). The natural history of squirrels. Christopher Helm, London.
Gurnell, J. (1996). Conserving the red squirrel. In: Thetford Forest Park: the ecology of a pine forest (ed. by. P. Ratcliffe & J. Claridge), pp. 132-140. Forestry Commission, Edinburgh.
Gurnell, J. & Lurz, P.W.W. (1997). The conservation of red squirrels, Sciurus vulgaris L. PTES, London.
Gurnell, J. & Pepper, H. (1994). A critical look at conserving the British red squirrel Sciurus vulgaris. Mammal Rev. 23, 127-137.
JNCC. (1996). UK strategy for red squirrel conservation. JNCC, Peterborough. 16pp.
Kenward, R.E., Hodder, K.H., Rose, R.J., Walls, C.A., Parish, T., Holm, J.L., Morris, P.A., Walls, S.S. & Doyle, F.I.B. (1998). Comparative demography of red squirrels (Sciurus vulgaris) and grey squirrels (Sciurus carolinensis) in deciduous and conifer woodland. J. Zool., Lond. 244, 7-21.
Pepper, H. & Patterson, G. (1998). Red squirrel conservation. Forestry Commission Practice Note 5. Forestry Commission, Edinburgh. 8pp.
Red squirrel (Sciurus vulgaris) Action Plan: http://www.jncc.gov.uk/ukbg/default.htm

See also Grey squirrel.

Grey squirrel (Sciurus carolinensis)

STATUS Origin Introduced
UK Population size c.2.5 million ??
UK Distribution Most of England, Wales and parts of Ireland; patchy in central and east coast of Scotland
UK Status Common throughout England and Wales, widespread in central Scotland and northern Ireland; increasing
IUCN designation Not listed
NATURAL HISTORY Habitat Forests and woodlands, also urban parks and gardens
Diet Seeds, also tree shoots, flowers, and invertebrates
Behaviour Diurnal; usually solitary
Home range Good habitat: 0.5-4ha; poorer habitat: 5-12ha
Breeding Single litter (maybe two) of 1-8 (average 3, usually max. 5) young per year
ISSUES Impact on red squirrels 2.1.3.ii, 2.1.4.i
Damage and control 2.2.2.i, 2.2.2.ii.e, 2.4.1.i.c, 4.1.1.iii, 4.3.1.ii.a


The grey squirrel was introduced from the USA between 1876 and 1929, and continues to expand its range and abundance.

Grey squirrel populations are still increasing in the border counties of England and possibly also in Scotland. A British pre-breeding population of about 2.5 million was estimated in the early 1990s, but may have increased since then.

Natural history

Grey squirrels are most abundant in mature broadleaved forests, although they are also found in mixed and conifer woodlands, as well as in hedgerows, parks, gardens and urban areas with mature trees. They are primarily herbivorous with a preference for tree seeds, feeding mainly on acorns, beechmast, tree shoots, flowers and nuts, as well as fruits, roots and cereals, insects and occasionally birds’ eggs.

The grey squirrel is diurnal. Home ranges in optimal habitat with abundant food supply may be between 1-4ha, but possibly as small as 0.5ha. In less optimal mixed broadleaved woodlands, males’ home ranges may be 6-12ha and females’ 5-7ha. Range peripheries overlap extensively, but core areas in breeding females’ ranges are generally exclusive.

There are two peaks of mating, one in December to February and the other March to May; females usually only produce one litter per year, two when conditions are favourable. Litters of 1-8 young (average three) emerge in April to June and July to September. Sexual maturity is attained at 10-12 months. In Britain, dispersal (<1–12km) of spring-born young occurs in the summer, yearlings in the spring and adults and juveniles in the autumn. Grey squirrels may live up to 8-9 years. Adult mortality averages 40% (range 20-50%) although mortality during the first year is high (75-85%). Annual survival is affected by the availability of autumn tree seed. Neither predation nor disease are thought to significantly affect population dynamics in Britain, although road traffic is a contributing factor. In commercial forestry, there may be intensive trapping and poisoning of grey squirrels. Densities may range between 5-10 individuals per ha in mature oak woodland and 2-8/ha generally in broadleaved woodland, but may be <1/ha in conifer forests.

The issues

Grey squirrels are the primary cause of the loss of red squirrels throughout England and Wales, and much of Scotland and Ireland (see above). They also cause significant damage to market gardens, orchards and arable crops, and their bark stripping behaviour is a major threat to the management of commercial forests.

While conservationists might welcome the eradication of the grey squirrel in Britain, this is improbable. Firstly, because of the financial and logistical implications involved (for such a widespread species) and, secondly, because grey squirrels are popular with the public, particularly in urban areas where they are among the most visible wild mammals. Grey squirrels are controlled for tree protection by poisoning and trapping and sometimes for red squirrel conservation (where red squirrels are present only live trapping is used). They are also shot for sport.

Further reading

Gurnell, J. (1996). The grey squirrel in Britain: problems for management and lessons for Europe. In: European mammals (ed. by M.L. Mathias, M. Santos-Reis, G. Amori, R. Libois, A. Mitchell-Jones & M.C. Saint-Girons), pp. 67-81. Museum Bocage, Lisbon, Portugal.
Gurnell, J. (1999). Grey squirrels in woodlands: managing grey squirrels to prevent woodland damage. Enact 7, 10-14.
Pepper, H. & Currie, F. (1998). Controlling grey squirrel damage to woodlands. Forestry Commission Practice Note 4. Forestry Commission, Edinburgh.
Sheil, J. (1999). The grey squirrel (Sciurus carolinensis) – a UK perspective on a vertebrate pest species. J. Environ. Mgmt 55, 145-156.

See also Red squirrel.

Bank vole (Clethrionomys glareolus)

STATUS Origin Native
UK Population size 23 million ??
UK Distribution Throughout mainland Britain, incl. some of the islands
UK Status Very common
IUCN designation Not listed
NATURAL HISTORY Habitat Woods, scrub, banks and hedges
Diet Fruits, seeds, leaves
Behaviour Active day and night; gregarious, females seasonally territorial
Home range 0.05-0.73ha
Breeding 4-5 litters of 3-5 (average 4.1) young per year
ISSUES Habitat change Box 2:W, 2.3.3.i, Box 2:AA


Bank voles are common throughout western and central Europe, as they are in mainland Britain (including some of the islands). A separate subspecies exists on the island of Skomer, Dyfed, Wales (C. g. skomerensis).

Based on density and distribution estimates, a total pre-breeding population estimate for British bank voles is 23 million, of which a third is thought to populate hedgerows in arable areas in England. Bank voles are believed to be spreading in Ireland. The Skomer vole is locally common on Skomer and thought to be relatively stable. The pre-breeding population is believed to number approximately 7,000 (based on extrapolations from summer live-trapping surveys).

Natural history

Bank voles are found in deciduous woodlands, scrub and areas with high herbaceous growth such as park lands, banks and hedges. They are almost entirely herbivorous, preferring fleshy fruits, soft seeds and the leaves of woody plants although they will also eat fungi, moss, roots, grass, buds and occasionally insects, worms and snails, as well as dead leaves in winter.

Bank voles are active day and night but tend to become more nocturnal in summer. They are active throughout the winter. Home range size varies between 0.05-0.73ha, with male home ranges being larger than females’, particularly in summer. They are gregarious but females hold exclusive territories during the breeding season.

The breeding season is usually April to September or October but may continue all year-round in favourable conditions. Females may produce 4-5 litters of 3-5 (average 4.1) young per year. Bank voles may reach sexual maturity at 4.5 weeks, although individuals born late in the year will not become sexually mature until the following spring; both sexes disperse at sexual maturity. Maximum life span in the wild is about 18 months. Predators include owls, kestrels, weasels and foxes.

Population densities are typically between 10-80/ha (and up to 150/ha following good mast years in deciduous woodlands), but may be up to 475/ha (e.g. on Skomer Island). There are considerable seasonal fluctuations in their numbers, typically being high in late summer and autumn then declining in the winter to a low in April or May. Densities also vary greatly between years, dependent on the seed crop and the severity of winter weather.

The issues

Bank voles may be vulnerable to lead exposure where they occur near roads, and to pesticide ‘drift’ into field margins, from molluscicides applied to arable land and rodenticides placed around farm buildings. Hedgerow removal and woodland fragmentation make it harder for bank voles to persist in agricultural landscapes, and woodland populations may suffer from overgrazing of ground cover by deer and other large mammals.

Further reading

Flowerdew, J. (1993). Mice and voles. Whittet Books, London.
Flowerdew, J.R., Gurnell, J. & Gipps, J.H.W. (Eds). (1985). The ecology of woodland rodents: bank voles and woodmice. Symp. Zool. Soc. Lond. 55. Oxford University Press, Oxford. 304pp.
Mallorie, H.C. & Flowerdew, J.R. (1994). Woodland small mammal population ecology in Britain: a preliminary review of the Mammal Society survey of Wood Mice Apodemus sylvaticus and Bank Voles Clethrionomys glareolus, 1982-1987. Mammal Rev. 24, 1-16.

Field vole (Microtus agrestis)

STATUS Origin Native
UK Population size c.75 million ??
UK Distribution Widespread but patchily distributed throughout British mainland
UK Status Locally common; poss. declining
IUCN designation Not listed
NATURAL HISTORY Habitat Rough, ungrazed grassland
Diet Grasses and herbaceous plants
Behaviour Active day and night but esp. crepuscular; males territorial
Home range 0.01-0.1ha
Breeding 2-7 litters of 4-6 young per year
ISSUES Habitat change 2.1.5.ii, Box 2:W, 2.3.3.i, 2.3.3.iv, 2.3.5.ii, Box 2:AA


Field voles, thought to be the most numerous of all British mammals, are widespread but patchily distributed throughout the British mainland. They are present on most of the Hebridean islands, but are absent from the Isles of Scilly, Lundy, Orkney and Shetland. They are widespread in central and northern Europe.

There are few data with which to gauge population size. Based on their relative abundance to other small mammals, the field vole population is estimated at approximately 75 million. However, there is a widespread suspicion that populations have been declining since the 1970s, although corroborating data are lacking.

Natural history

Field voles are found in rough, ungrazed grassland, where the vegetation provides sufficient cover. They are most abundant in meadows, field margins and forest plantations with lush grass, but are also found in lower numbers in hedgerows, blanket bog, dunes and open moorland. They feed mainly on the green leaves and stems of grasses.

Field voles may be active by day and night, but are particularly crepuscular. Home range size varies between 100-1000m² and is related to population density and food availability; male home ranges are twice the size of females’. Although adult ranges are fairly static, subordinates tend to be more nomadic. Males are territorial, while females are aggregated and largely non-territorial.

Most breeding occurs between April and September, although young may be found throughout the year. Males can breed at about 40 days of age, females at 28 days producing 2-7 litters of 4-6 young per year. Adults and subadults of both sexes may disperse. Maximum lifespan for a field vole is 2 years, but few survive to their second autumn. They are eaten by most predatory birds and mammals.

There are few data on population density from upland areas, although these areas probably harbour a considerable proportion of the population. In the lowlands, density is higher in grassland (100-300/ha) than in arable land (1-15/ha); field voles are usually absent from cropped areas. There is some evidence for fluctuations in density (with 3-5 year periodicity), particularly in Scotland and northern England. The situation is less clear in the south, where annual fluctuations occur only in some areas, particularly in patchy habitats.

The issues

At high densities, field voles can cause damage to grassland, young plantations and crops. This problem was prevalent in Britain in the 1920s and 1930s but is now largely confined to Scandinavia.

The field vole may be threatened by increased grazing pressure and a loss of rough grassland (resulting from natural and anthropogenic changes), removal of linear features, development and scrub encroachment. They may be subject to poisoning by rodenticides. Field voles are an important prey item for many raptors, and this makes a proper assessment of their status all the more vital.

Further reading

Flowerdew, J. (1993). Mice and voles. Whittet Books, London.

Orkney vole (Microtus arvalis orcadensis)

STATUS Origin Introduced
UK Population size c.4 million1
UK Distribution Orkney islands
UK Status Common
IUCN designation Not listed
NATURAL HISTORY Habitat Old peat cuttings, rough grassland
Diet Leaves, stems and roots of grasses and dicotyledons
Behaviour Active day and night (diurnal in winter)
Home range c.0.03-0.08ha ((up to 0.37ha)
Breeding 1-6 young (average 2.7-2.9) poss. every month through the breeding season
ISSUES Habitat loss 2.3.3.iii


The Orkney vole is an endemic subspecies of the common vole and is found only in the Orkney Islands; a different subspecies is present on Guernsey (M. a. sarnius). The Orkney subspecies is distinguished by its large size (up to 90g, compared with 35g for continental animals). The common vole itself is widespread in continental Europe. The Orkney subspecies arrived on Orkney around 5,000 years ago, probably on the boats of early Neolithic settlers from the Low Countries, and is present on the islands of Mainland, Rousay, Sanday, Westray and South Ronaldsay. Orkney vole numbers declined dramatically with changes in land use, and may still be decreasing. Simulation modelling suggests that the likely population (of perhaps four million1) is viable indefinitely. Orkney voles are important not only because they are an endemic subspecies, but also because they represent a vital source of food for the island’s short-eared owls, kestrels and hen harriers.

Natural history

Orkney voles may be found in old peat cuttings, rough grass habitats and in linear features such as grassy fence-lines, drainage ditches and roadside verges. Yet, Orkney voles may be absent from intensively farmed cereal and root crops and even re-seeded grassland that might, at first sight, appear to be appropriate habitat. They feed on the leaves, stems and roots of grasses and dicotyledons, favouring plants with high nitrogen levels.

Orkney voles are active on the surface both day and night, but are predominantly diurnal. Home ranges of 0.084ha for males and 0.062ha for females have been recorded at a low-density moorland site, whereas values at a lowland grassland site, where population density was high were 0.0394ha and 0.0296ha, respectively; ranges as large as 0.37ha have been known. Ranges are probably defended, although there can be much overlap between neighbours (especially at high densities). Local migrations are common.

On Orkney, females may be pregnant from March to November (mating has been recorded in February on Guernsey). Litters of 1-6 young (average 2.7-2.9) may be born every month and the young disperse shortly after leaving the nest. Juveniles breed the year following their birth and usually die before the winter. Predators include birds of prey and domestic cats.

Population densities vary from 31/ha in Calluna moorland or blanket bog, 100/ha in wet heath, 206/ha in rough grassland to around 500/ha in fence-line rough grass and old peat cuttings. There is no indication of population cycles.

The issues

The current Orkney vole population is highly fragmented between patches of natural and semi-natural habitat, many of which are small. On farmland, in rough-grass-tall-herb vegetation associated with fence-lines, ditches and road verges. These linear habitats support high densities of voles and also act as habitat corridors allowing the flow of voles between patches of natural habitat. Orkney voles rarely, if ever, leave these fence-lines, verges or ditches and damage to this corridor network might result in disproportionate damage to the population.

Further reading

1Gorman, M.L. & Reynolds, P. (in press). The impact of changes in land-use on the Orkney vole Microtus arvalis orcadensis. In: Conservation and Conflict: Mammals and Farming in Britain (ed. by F.H. Tattersall & W.J. Manley). Linnean Society Occasional Publication, Westbury Publishing, Yorkshire.

Water vole (Arvicola terrestris)

STATUS Origin Native
UK Population size 875,0001
UK Distribution Limited to aquatic habitats away from main rivers throughout England and Wales and southern and eastern Scotland
UK Status May be locally common but also locally extinct in many areas; rapidly declining
IUCN designation Not listed
NATURAL HISTORY Habitat Lowland freshwater habitats
Diet Bankside plants, also roots, rhizomes and bulbs
Behaviour Diurnal; social, may be seasonally territorial
Home range 90-500m
Breeding 2-5 litters of 4-6 young per year
ISSUES Pollution 2.2.3
Habitat change 2.1.1.i, 2.3.1, 2.3.4, 2.3.4.i, 2.3.4.ii, Box 2:Z, 4.4.1.v, Box 4:BB, 4.4.2
Impact of mink 2.1.1.i, 2.1.5, 2.1.5.iii
Reintroduction 4.3.3.i.a, 4.4.1.ii.b, 4.3.1.ii.b
Monitoring Box 4:O, 4.2.3.i
Climate change 2.2.5


Water voles typically are found in suitable habitat throughout England and Wales and southern and eastern Scotland, but only locally in north-west Scotland. They are present on some islands, including Anglesey and the Isle of Wight, but absent from the Isle of Man and Ireland. Local distribution may be limited to headwaters, side streams, ditches and brooks away from the main rivers and large tributaries. On the Continent they are widely distributed in the Pyrénées, north-eastern Europe, and the European Palaearctic.

Water voles may be Britain’s fastest declining mammal. They are undergoing a precipitous long-term decline, which can be separated into two distinct periods, both of which have seen a radical intensification of agriculture. During the 1940s and 1950s the first phase of their decline may have been associated with afforestation and the subsequent acidification of waterways. The second, most dramatic, period of decline, took place in the 1980s and correlates with the spread of American mink. Two consecutive VWT surveys estimate relative population sizes of 7,293,000 (England: 4,658,000; Scotland: 2,374,000; Wales: 261,000) in 1990 and 875,000 (England: 486,000; Scotland: 354,000; Wales: 35,000) in 1998, a decline of almost 90%. Furthermore, voles have entirely disappeared from many catchments in which they were previously abundant.

Natural history

Water voles in Britain are found in a variety of lowland fresh water habitats. On the Continent, however, especially in the southern parts of their range, they are more fossorial, burrowing in pastures like moles. In Britain, there are fossorial populations on Read Island in the Humber estuary and on Eilean Gamnha, Argyll, Scotland. Characteristically, the semi-aquatic water vole feeds on bankside plants, favouring grasses, common reed, sedges, rushes and dicotyledons, although they also eat roots, rhizomes and bulbs in winter.

Water voles are predominantly diurnal, but may be active day and night. There is no evidence of hibernation, although some individuals undergo short periods of torpor and activity outside the burrow is rare in winter. Home ranges are linear along the river banks and range in size from 90-500m depending on density and season (larger in summer and at low population densities) with male home ranges being larger than females’ (e.g. at low density, female home ranges may be 300m and those of males, 1,500m). Females are territorial in the spring but, at high densities, home ranges remain undefended and overlap. In contrast, breeding males are more territorial when densities are high. Water voles nest communally in the winter, with a female, her daughters and unrelated males sharing a nest.

The mating season can extend from March to October. Most births occur in April and May, with a second peak in July to September. Females produce 2-5 litters of 4-6 young per year. Water voles usually reach sexual maturity after their first winter, although they may occasionally breed in the year of their birth. Juvenile males disperse at four months, while some young females disperse in the spring. Average lifespan is less than six months, but up to five years in captivity. Resident females have the highest survival rate and dispersing individuals (particularly females) the lowest; monthly mortality rates for all individuals ranges from 10-40%. Mortality is especially high over long, hard winters (up to 70% in adults) and predation by a variety of mammalian and avian predators is common.

Densities are highest in the autumn and lowest in the spring. Locally, density may be extremely high (up to 200/ha), otherwise values vary between 4-16/100m and 6-9/ha.

The issues

In addition to predation by American mink, other threats to water voles include: habitat loss, particularly from waterway management; changes in agricultural land use adjacent to riparian habitats; fragmentation and isolation; water pollution; increased cattle grazing; human disturbance; and climatic changes (severe winters and dry summers). These factors probably all act synergistically. For example, vegetation loss through habitat destruction and cattle grazing may increase predation risk; mink predation, in turn, increases isolation of populations and thus, the risk of demographic threats. The water vole’s reproductive potential, however, means that this species has considerable potential for population increases. Upland and non-linear areas (such as extensive reed beds) are becoming important refuges as they are less affected by mink and agriculture, and they are potential sources for recolonisation or reintroduction in other areas. It remains to be seen whether a network of high-quality sites will enable coexistence of water voles and mink. Research on translocation of water voles is currently being carried out in the Thames region. The species action plan aims to maintain the current distribution and abundance of this species in the UK and to ensure they are present throughout their 1970s range by the year 2010, utilizing habitat management and translocation where necessary.

Further reading

Macdonald, D.W. & Strachan, R. (1999). The mink and the water vole: analyses for conservation. EA/WildCRU, Oxford.
Strachan, C., Jefferies, D.J., Barreto, G.R., Macdonald, D.W. & Strachan, R. (1998). The rapid impact of resident American mink on water voles: case studies in lowland England. In: Behaviour and ecology of riparian mammals (ed. by N. Dunstone & M.L. Gorman). Symp. Zool. Soc. Lond. 71, 339-357. Cambridge University Press, Cambridge. 391pp.
Strachan, C., Strachan, R. & Jefferies, D.J. (2000). Preliminary report on the status and distribution of the water vole (Arvicola terrestris) in Britain 1996-1998. The Vincent Wildlife Trust, London.
Strachan, R. (1997). Water voles. Whittet books, London.
Strachan, R. (1998). Water vole conservation handbook. EA/EN/WildCRU, Oxford.
Strachan, R. & Jefferies, D. (1993). The water vole Arvicola terrestris in Britain 1989-1990: its distribution and changing status. The Vincent Wildlife Trust, London.
Woodroffe, G.L. (1994). The water vole – some aspects of its ecology. British Wildlife 5, 296-303.
Woodroffe, G.L. (2000). The water vole. The Mammal Society, London.
Water vole (Arvicola terrestris) Action Plan: http://www.jncc.gov.uk/ukbg/default.htm

Wood mouse (Apodemus sylvaticus)

STATUS Origin Native
UK Population size c.38 million ??
UK Distribution Widespread
UK Status Very common; stable?
IUCN designation Not listed
NATURAL HISTORY Habitat Most habitats
Diet Seeds, invertebrates, also seedlings, buds, fruit, nuts etc.
Behaviour Nocturnal; territorial in summer, nest communally in winter
Home range 0.2-3.7ha
Breeding 4-7 litters of 2-9 young per year
ISSUES Pesticides, poisoning and pollution 2.2.1, Box 3:H, 2.2.2.ii, 2.2.2.ii.a, 2.2.2.ii.b, 2.2.2.ii.e, Box 2:K, Box 2:L, 4.3.3.iii
Habitat change 2.3.1.ii, 2.3.3, 2.3.3.i, Box 2:W, Box 2:AA


Wood mice are widespread throughout Continental Europe, Scandinavia and Britain. They are absent from many of the smaller British islands and where they do occur on islands this is most likely the result of introductions.

The British population is believed to be stable and, based on distribution and density estimates, suggested to number in the region of 38 million individuals.

The wood mouse is an important prey to many mammalian and avian predators.

Natural history

The wood mouse is a highly adaptable species that occurs in most habitats that are not too wet. They eat predominantly seeds (including cereal grains) and invertebrates (such as snails, arthropods and earthworms), but will also take seedlings, buds, fruits and nuts, fungi, moss and galls, as well as larval moths and butterflies, and centipedes.

Wood mice are largely nocturnal, although they may be active up to two hours before dusk and after dawn in summer; pregnant and lactating females and some males may be active for short bursts during the day. Home ranges are larger in woodland (male 0.63ha, female 0.19ha) than in farmland (male 1.77ha, female 0.26ha), and even larger in sand dunes (male 3.65ha, female 1.58ha). In summer, home ranges of males overlap those of other males and females, the females defending exclusive breeding territories against other females; males, but not females, may share a burrow. In winter, all ranges overlap and males and females nest communally.

The breeding season occurs in March to October, with a peak in July and August, but may occasionally extend throughout the year. Usually 4-7 litters of 2-9 young are produced per year, although this varies geographically. Females disperse mainly in autumn and winter, males may disperse throughout the year. The maximum lifespan is 18-20 months and few adults survive from one summer to the next. Mortality of over-wintered adults is high in spring, and juvenile mortality tends to be higher in spring and summer. Predators include foxes, weasels, martens, cats, kestrels and owls.

Population densities vary between 0.25-100/ha, being stable in the summer, generally rising in the autumn, and declining over winter and spring. Winter densities fluctuate according to food supply.

The issues

Although occasionally considered a pest in arable crops, wood mice adapt well to modern farming and are a mainstay of many food chains in the agro-ecosystem. However, the loss of woodlands and hedgerows and intensification of farming practices impacts on wood mouse habitat. They are affected indirectly by the impact on their food of agrochemicals, and may be at risk directly from such toxins and pollution (partly molluscicides). In common with many other small rodents, wood mice carry various diseases transmissible to livestock and humans, such as Leptospirosis (Weil’s disease).

Further reading

Flowerdew, J. (1993). Mice and voles. Whittet Books, London.
Flowerdew, J.R., Gurnell, J. & Gipps, J.H.W. (Eds). (1985). The ecology of woodland rodents: bank voles and woodmice. Symp. Zool. Soc. Lond. 55. Oxford University Press, Oxford. 304pp.
Mallorie, H. & Flowerdew, J.R. (1994). Woodland small mammal population ecology in Britain. A preliminary review of the Mammal Society survey of woodmice (Apodemus sylvaticus) and bank voles (Clethrionomys glareolus), 1982-1987. Mammal Rev. 24, 1-15.
Tattersall, F.H. & Macdonald, D.W. (in press). Wood mice in the arable ecosystem. In: Conservation and Conflict: Mammals and Farming in Britain (ed. by F.H. Tattersall & W.J. Manley). Linnean Society Occasional Publication, Westbury Publishing, Yorkshire.

Yellow-necked mouse (Apodemus flavicollis)

STATUS Origin Native
UK Population size 750,000 ??
UK Distribution Found predominantly in south, south-east and west England and central and eastern Wales.
UK Status Locally common; poss. declining?
IUCN designation Not listed
NATURAL HISTORY Habitat Mature woodland, also wooded gardens, hedgerows etc.
Diet Seedlings, buds, fruit, invertebrates
Behaviour Nocturnal; social (structure unknown)
Home range <0.5ha
Breeding 3 litters of 2-11 (usually 5) young per year
ISSUES Habitat change 2.3.2.iv, 2.3.3.iv


The yellow-necked mouse is found only in the south and west of England (but absent from Cornwall and Cheshire) and in central and eastern Wales where their patchy distribution approximately conforms to the distribution of remaining areas of ancient woodland. There is also some evidence of association with the drier areas of south Britain. In Europe, however, their range extends further north than the wood mouse in Scandinavia although they generally have a more restricted distribution, occurring predominantly in montane areas in southern Europe.

Throughout their range, yellow-necked mice are generally scarce although they may be locally common. Because they are associated with mature and ancient semi-natural woodland, they may be declining, although there are no substantiating data. The estimated population size of 750,000 is doubtful.

Natural history

Yellow-necked mice occur mainly in ancient or mature, semi-natural, deciduous woodland, often close to arable farmland, as well as in orchards and wooded gardens and more marginal habitats (e.g. hedgerows, field margins and rural buildings). They feed largely on seedlings, buds and fruit as well as occasional invertebrates.

Yellow-necked mice are normally active for a single period during the night. Home ranges can be less than 0.5ha, although their size varies with habitat quality. Male home ranges are larger than females and ranges overlap, both within and between sexes. Individuals may nest in groups of three or four in winter, but other details of social structure are unknown.

Breeding takes place between March/April and October, but occasional births may occur throughout the year. Females have approximately three litters per year of 2-11 (usually five) young per litter. They are capable of breeding after their first winter, or (if they are born early) in the year of their birth. Lifespan may be up to two years, but normally is less than one. Juvenile survival is high in spring and summer, but mortality is steady over the winter (c.60% per month). Dispersal may occur throughout the year and both sexes are capable of up to 1200m journeys over open ground. Nearly a third of the population is transient.

Population densities can vary from 1-10/ha, but may be up to 50/ha, depending on habitat quality and masting of tree species (which may vary widely between years).

The issues

The close association of yellow-necked mice with ancient woodland means that they may be vulnerable to habitat fragmentation. However, their dispersal behaviour and high reproductive potential may enable them to colonise new areas rapidly, making them less vulnerable than other species to demographic problems associated with small populations. Numbers may be reduced by the loss of seed-producing trees, such as elms and yews, as a result of increased conifer planting. Coppicing in years 1-3 also adversely affects numbers, but appears to be beneficial in years 10-30.

Further reading

Marsh, A.C.W. (1999). The national yellow-necked mouse survey report. The Mammal Society, London.
Marsh, A.C.W. & Harris, S. (2000). Partitioning of woodland habitat resources by two sympatric species of Apodemus: lessons for the conservation of the yellow-necked mouse in Britain. Biol. Cons. 92, 275-283

See also Wood mouse.

Harvest mouse (Micromys minutus)

STATUS Origin Probably a post-glacial introduction
UK Population size Unknown
UK Distribution Southern England and Welsh coastal belt
UK Status Locally common; poss. declining
IUCN designation Lower risk
NATURAL HISTORY Habitat Long, dense grass in reedbeds, bramble patches etc.; also linear features such as hedgerows
Diet Grass seeds, cereals, fruits, berries, insects
Behaviour Intermittently active throughout 24 hours; largely solitary
Home range Female: 0.035ha; male: 0.04ha
Breeding 3-7 litters of 1-8 young per year
ISSUES Habitat change 2.3.3.iv, 2.3.3.i, Box 2:AA


The harvest mouse in Britain is limited to southern England and parts of the coastal belt of Wales; scattered colonies outside this area are believed to be the result of introductions (possibly from the 19th century). In general, their distribution is patchy, both spatially and temporally. Given the species’ preference elsewhere for dry conditions, its distribution in Britain may be limited by summer rainfall.

Harvest mice numbers are thought to be declining (in a recent national survey they were found at only 30% of 800 sites known to have been occupied in the 1970s) and litter size also appears to have decreased (litter sizes of 4-12 being recorded pre-1917 compared with a maximum of eight in the 1970s). There is no evidence of a decline in range, but there is little information about the species in general to estimate its population size.

Natural history

Harvest mice are generally found in long, dense grass in dry reedbeds, rushes, bramble patches, ungrazed hay meadows, and occasionally in crops (where field headlands and rough grass banks provide refuge during the winter). They are also recorded in linear habitats (e.g. hedges, ditches, field and wood edges and roadside verges) and sometimes even in rough grassland and waste land in urban areas. Harvest mice eat mainly grass seeds, cereals, fruits, berries, and insects and green shoots in the spring.

Harvest mice are intermittently active throughout a 24-hour period, but are largely nocturnal and often crepuscular, becoming more nocturnal in summer and more diurnal in winter. Activity in the summer is within the seedhead or stalk zone of the vegetation, while in winter they are active at ground level. On the South Downs, home ranges were 0.04ha for males and 0.035ha for females. Although harvest mice are largely solitary, there is considerable overlap of adjacent ranges.

Harvest mice breed between May and October, and occasionally until December when the weather is suitable. Females produce 3-7 litters of 1-8 young per year. Harvest mice may breed at about six weeks old. The young are driven away from the nest by the female (at about 18 days old) before the next litter is produced. Few harvest mice survive six months (maximum lifespan recorded is 18 months). Cold, wet weather, particularly persistent rain, sudden temperature drops, and frost, may be major causes of mortality, especially for the young (entire litters may freeze to death in the nest in cold wet conditions). Juvenile mortality is variable and over-winter mortalities are high (95% in adults), with the greatest number of mortalities occurring in February.

Population density varies with habitat, being lowest in cereal fields (0.05-0.4/ha) and highest in reedbeds (20->50/ha); in very localized areas, densities can be extremely high (>200/ha), often followed by several years of very low numbers. Although over-winter losses may be high, they may be compensated for by an equally rapid population increase in the summer. Thus, throughout the year, the population may peak during November, and crash during February to March. Population cycles occur in some European populations.

The issues

Agricultural practices and land use changes, including combine harvesting, stubble burning, pesticide use and hedge management, are probably having a negative impact on harvest mice numbers. Harvest mice are vulnerable to habitat loss, particularly of linear features, while the early removal of crops from fields (e.g. modern winter cereals) and ploughing can be particularly devastating. Harvest mice may also be particularly sensitive to climate change in Britain where they are at the extreme of their range. The importance of flooding in reedbeds is unknown, but almost certainly detrimental.

Harvest mice have been the subject of numerous informal small-scale releases which may have confused distribution. A national, co-ordinated breeding programme has since been established and the first large-scale reintroduction (300-400 animals) will take place in 2001.

Further reading

Harris, S. (1980). The harvest mouse. Blandford Press, Poole. 43pp.
Perrow, M. & Jowitt, A. (1995). What future for the harvest mouse? British Wildlife 6, 356-365.

House mouse (Mus domesticus)

STATUS Origin Introduced
UK Population size Unknown
UK Distribution Widespread, especially in urban areas
UK Status Locally common
IUCN designation Not listed
NATURAL HISTORY Habitat Urban areas; farm buildings and hedgerows in rural areas
Diet Cereals; also insects, roots, arthropods
Behaviour Nocturnal; social and territorial
Home range 0.0004– >0.01ha
Breeding 5-10 litters of 4-8 (average of 5.4 in Britain) per year
ISSUES Habitat change 2.3.3
Damage and control 2.4.1.d, 4.1.1.iii


The house mouse was introduced to Britain in the Iron Age. It is currently widespread in the British Isles and Ireland wherever there is human habitation, including most small islands. The house mouse has a cosmopolitan distribution greater than any other mammal, with the exception of man.

House mice underwent a decline in urban areas in the late 1970s probably due to more effective control measures. Numbers may currently be stable, but estimating population sizes is difficult because local infestations are erratic in occurrence but potentially numerous. Despite its obvious importance as a pest, very little is known of its ecology in Britain, especially in urban or suburban areas.

Natural history

House mice are common and widespread in a range of urban habitats; in rural areas they are largely restricted to farm buildings and very occasionally hedgerows or fields. They live independently of humans on islands, such as the Isles of May or Stockholm, where there are no competitors. House mice eat almost anything edible, although they favour cereals.

Home range size may vary from as little as a few square metres to over 100m², although in buildings mice rarely move more than 3-10m and in fields they are often semi-nomadic. Social organisation is very flexible, but generally involves territorial, mixed-sex groups.

Breeding can occur throughout the year when food is sufficient, although in rural populations there is a peak in May and June and there is little breeding over the winter months. Females are capable of producing 5-10 litters of 4-8 young (average in Britain 5.4) per year, which are sexually mature at 8-12 weeks. Maximum lifespan is rarely greater than 18 months in the wild. Juveniles have a high mortality rate due to desertion and infanticide. In adults, causes of death include exposure to cold, wet weather coupled with poor food supply. Mice are frequently poisoned in commensal habitats, where they may have few predators except man, cats and the occasional brown rat.

Extremely high densities of house mice have been reported in farm situations where they have easy access to abundant food (e.g. densities equivalent to 476/ha in a piggery and up to 70,000/ha in a battery chicken barn). However, densities are generally much lower: free-living feral island populations are usually less than 60/ha and, in hedgerows and arable fields, are typically <1/ha. In late autumn and winter, there is a general tendency for movements of house mice away from fields and hedgerows into farm buildings. In spring minor movements occur in the reverse direction.

The issues

Although there is no concern for their conservation, this species does raise important management issues in terms of food contamination and disease. In Britain, the house mouse is economically important as a pest of stored products. Although the amount of product that they eat is relatively small, removing contaminants such as hairs, urine and droppings can be very expensive. House mice also cause physical damage to building materials and wiring. Some urban populations (e.g. Birmingham) have developed unusual food preferences that appear to be heritable. In such cases, control using cereal-based rodenticide formulations often fails and it is necessary to determine beforehand what these mice will eat. When cereals were stored in ricks in open fields over winter, house mice were much more common in the countryside.

House mice are relatively unimportant as vectors of human diseases, but they can carry Salmonella and Cryptosporidium in their droppings and Leptospirosis (Weil’s disease) in their urine.

Further reading

Berry, R.J. (1981a). Town mouse, country mouse: adaption and adaptability in Mus domesticus (Mus musculus domesticus). Mammal Rev. 11, 91-136
Humphries, R.E., Sibley, R.M. & Meehan, A.P. (2000). Cereal aversion in behaviourally-resistant house mice in Birmingham, UK. Appl. Anim. Behav. Sci. 66, 323-333.
Tattersall, F.H. (1999). House mice and woodmice in and around an agricultural building. J. Zool., Lond. 249, 469-472. 

Brown rat (Rattus norvegicus)

STATUS Origin Introduced
UK Population size More than 7 million ??
UK Distribution Widespread
UK Status Common; pest
IUCN designation Not listed
NATURAL HISTORY Habitat Urban areas (esp. in sewers), agricultural land and unpopulated islands
Diet Invertebrates, small mammals, birds’ eggs, amphibians, cereals, seeds
Behaviour Nocturnal (diurnal if predation risk high at night); social
Home range Linear home ranges: female 340m, male 660m
Breeding c.5 litters of 1-15 (usually 7-9) young per year
ISSUES Impact on black rat 2.1.3.i
Damage and control 2.2.2.ii.e, Box 2:J, 2.4.1.i.a, 2.4.1.i.d, 4.1.1.iii, 4.3.1.ii.a, Box 4:T


The brown rat is thought to have originated in Asia, possibly China. It arrived in Europe in the early part of the 18th century and was first recorded in England around 1720. Although also known as the Norway rat, it was not introduced to Norway until much later, in 1762. It was introduced to North America by 1740. It is now widespread in urban areas throughout the world except in some tropical and subtropical areas. In Britain, it is widely distributed in urban and rural areas except in the most exposed mountain regions and on some of the smaller offshore islands.

The total brown rat population in Britain is estimated at an absolute minimum of 7 million. Results from the 1993 MAFF national rodent survey (for house mice and brown rats) found that 4.8% of urban premises were infested with rats, a significant increase since a previous survey 14 years earlier. In agricultural areas, results revealed that 42% of properties were infested with rats, although the trend over time is unknown.

Natural history

The brown rat is widely distributed in both rural and urban areas and is commonly associated with farm buildings. Substantial populations can occur on refuse tips and in poorly maintained sewer systems. Occasionally, rats can survive all year round in hedges and field margins. They are not totally dependent on human activities and survive on unpopulated islands (e.g. in the Hebrides). They are omnivorous, favouring protein and starch-rich foods. Animal prey includes invertebrates (e.g. slugs, snails and larvae) and vertebrates (e.g. frogs, birds’ eggs and young small mammals) as well as meat and bones that may be scavenged. Vegetable matter, such as agricultural cereals and seeds, is also commonly taken.

Brown rats are mainly nocturnal but may become diurnal in the absence of human disturbance. Home ranges are typically linear rather than circular; in one study, mean home range length on arable land was recorded as 340m for females and 660m for males. Individuals can move over 3km in one night. In farm buildings, where resources are plentiful, rats tend to be sedentary. Rat colonies develop from a pair or a single pregnant female, communities probably comprising an aggregation of smaller social units (‘clans’) consisting of a dominant (and territorial) male with a harem of females and associated subordinate males.

Brown rats can breed all year if there is abundant food and mild weather, theoretically producing up to 13 litters per year at 4-5 week intervals. In reality, up to five litters are generally produced, although reproduction may be increased as populations recover from partial control campaigns. Litter size is usually 7-9 (range 1-15), varying with female body size. Females have been known to produce up to 56 young in a single year. Females are generally sexually mature at 8-12 weeks. Brown rats may live to three years in captivity although longevity in the wild is probably less than 18 months. It is likely that farm rat populations have a relatively rapid turn over, with few individuals (<5%) surviving longer than a year. While a range of predators (cats, dogs, foxes, mink, stoats and owls) may take rats, particularly young ones, the greatest cause of mortality is often likely to be the use of rodenticides.

An average population around farm buildings might number 110 individuals although populations in excess of 1,000 can occur. Urban domestic colonies tend to be much smaller. 

The issues

Brown rats are vectors of a suite of dangerous pathogens, including toxoplasmosis, Q-fever, Hantaan fever and Cryptosporidiosis, although their reputation as a vector of Leptospirosis (Weil’s disease), and as a host for Salmonella has been based on surprisingly scant evidence. Control of rats was greatly improved with the introduction of anticoagulant formulations around 1950, but within 10 years resistance to these compounds developed. In the mid- to late-1970s more potent variants (second-generation anticoagulants) were developed to which rats also developed some degree of resistance. Unfortunately, these newer compounds proved to be particularly toxic to a wide range of other vertebrates, although the impact of their widespread use on wildlife populations is currently unknown.

Further reading

Buckle, A.P. & Smith, R.H. (1994). Rodent pests and their control. CAB International, Wallingford.
Macdonald, D.W., Mathews, F. & Berdoy, M. (1999). The behaviour and ecology of Rattus norvegicus: from opportunism to kamikaze tendencies. In: Ecologically-based management of rodent pests. (Ed. G.R. Singleton, L.A. Hinds, H. Leirs & Z. Zhang), pp. 49-80. ACIAR Monograph No. 59. ACIAR, Canberra.
Meehan, A.P. (1984). Rats and mice: their biology and control. Rentokil, East Grinstead.
Meyer, A.N. et al. (1995). National commensal rodent survey 1993. Environmental Health 103, 127-135.

Black rat (Rattus rattus)

STATUS Origin Introduced
UK Population size 1,300
UK Distribution Lundy, Garbh Eilean, Shiants, Inner Hebrides; restricted to port areas on mainland
UK Status Endangered; former pest
IUCN designation Not listed
NATURAL HISTORY Habitat Buildings; also rocky shores and cliffs on islands
Diet Fruit and cereals, crabs on islands
Behaviour Nocturnal; social
Home range 80-90m
Breeding 3-5 litters of 1-16 (mean=7) young per year
ISSUES Impact of brown rat 2.1.3.i
Damage and control 4.1.1.iii


Black rats came to Britain with the Romans. Until the introduction of the brown rat, they were widespread throughout Britain, as they currently are throughout the world, particularly in the tropics.

In Britain, their range has contracted substantially since the 1950s. Since 1984, black rats on the mainland have been more or less restricted to Southwark, London and Avonmouth and these are probably transient. They are also present on the Isle of Lundy in the Bristol Channel, and the Shaint Islands, Outer Hebrides. The total British population is thought to be fewer than 1,300 individuals.

Natural history

Throughout the world black rats are invariably associated with buildings. However, in Britain, they live mainly on rocky shores and cliffs on islands, although they are sometimes also found around buildings and rubbish dumps on the islands. Black rats are omnivorous but have a greater tendency to eat vegetable food (e.g. fruit and cereals) than does the brown rat. On Lundy they forage along the shore line where they may feed on crabs.

Black rats are nocturnal, with peaks of activity two to three hours after sunset, although they may become more diurnal in undisturbed habitats or when there is a shortage of food. Home range size on Lundy appears to be small (80-90m). Black rats live in groups of several males and two or more dominant females.

Breeding is from mid-March to mid-November. Average litter size is seven (range1-16) and is correlated with female body size. Three to five litters are possible in a year and thus the number of young produced per year by a single female may be considerable (56 has been documented on London ships). Females are capable of breeding at 12-16 weeks or when they have achieved a body weight of c.90g. Little is known concerning dispersal. Maximum lifespan in the wild is probably less than 18 months and populations may have a high mortality rate (71-97%) largely due to poisoning.

There are no reliable population density estimates for black rats Britain; elsewhere, densities may range between 1–52/ha.

The issues

On the few islands where black rats remain, there is concern that they may be harmful to nesting seabirds, although it is debatable whether this warrants their control. Where black rats are closely associated with human activities they are particularly vulnerable to control measures. Furthermore, due to better hygiene and control methods, future immigration from ships is also unlikely, making the future of the black rat in Britain limited. Whether, as a once-serious pest, black rats should now receive some protection is contentious.

Further reading

Key, G., Fielding, A.H., Goulding, M.J., Holm, R.S. & Stevens-Wood, B. (1998). Ship rats Rattus rattus on the Shiant Islands, Hebrides, Scotland. J. Zool., Lond. 245, 228-233.
Mcdonald, R.A, Hutchings, M.R., & Keeling, J.G.M. (1997). The status of ship rats Rattus rattus on the Shaint Islands, Outer Hebrides, Scotland. Biol. Cons. 82, 113-117
Smith, P.A., Smith, J.A., Tattersall, F.H., Lancaster, V., Natynczuk, S.E. & Seymour, R.E. (1994). The ship rat (Rattus rattus) on Lundy, 1991. J. Zool., Lond. 231, 689-695
Twigg, G.I. (1992). The black rat Rattus rattus in the United Kingdom in 1989. Mammal Rev. 22, 33-42.

Hazel dormouse (Muscardinus avellanarius)

STATUS Origin Native
UK Population size 500,000 ??
UK Distribution Patchily distributed in England and Wales
UK Status Vulnerable and locally endangered; declining
IUCN designation Lower risk
NATURAL HISTORY Habitat Mature, mixed deciduous woodland, scrub, hedges
Diet Flowers, pollen, nuts, berries, fruit, insects
Behaviour Nocturnal; hibernate; sedentary
Home range 0.5ha
Breeding 1-2 litters of 2-7 young per year
ISSUES Habitat change Box 2:S, 2.3.1.i, 2.3.1.ii, 2.3.2.iii, 2.3.3.iv,
Reintroduction 4.4.1.ii.c


Hazel dormice in England and Wales are patchily distributed south of an imaginary line stretching from the River Thames to the River Severn, extending into the Welsh borders. In northern Britain, they occur only in three isolated populations in Cumbria and Northumberland. They seem better adapted to the drier, more predictable climate of continental Europe and this may be an explanation for their limited distribution in Britain where they are at the edge of their range. However, in the 19th century dormice occurred as far north as Carlisle and Tyne. Dormice are often absent from apparently suitable areas, possibly as a consequence of past management.

Dormice have declined in both abundance and in range this century due to woodland loss and fragmentation, and their population in Britain is estimated at 500,000. However, the isolated nature of many populations is probably of more concern than the number of individuals nationally.

Natural history

Hazel dormice are found in ancient deciduous woodland, dense shrubbery and coppices, particularly in areas where there is plenty of secondary growth and trees with edible seeds such as hazel, sweet chestnut and beech. The physical structure of dormouse habitat is important, particularly the availability of arboreal pathways formed by sprawling coppice and climbing plants, such as honeysuckle or bramble. A diversity of trees is also needed to provide a succession of foodstuffs during the active part of the year. Tree holes may be limiting in coppice and in such areas nest boxes may be important. Hazel dormice are specialist arboreal foragers, feeding on flowers and pollen in spring and early summer, and nuts, berries and fruits later in the year, as well as insects.

Dormice are nocturnal, and hibernate over winter. Their small home ranges average 0.5ha. Several individuals may share a nest, but their social organisation is poorly known. They probably have limited ability for dispersal, due to their reluctance to move through open areas.

In Britain, young may be found from May to September and occasionally as late as October. Dormice of one year and older breed but the breeding rate is slow (1-2 litters of 2-7 young per year) and reproductive potential may be constrained by hibernation and climate. Dormice can live up to five years; the adult mortality rate is low (30%) that of juveniles, higher (50%). The principal cause of mortality is probably starvation during hibernation due to insufficient fat reserves. Consequently, young born late in the season have a lower chance of survival.

Population densities vary with habitat quality (1-10/ha) and are much lower than wood mice and bank voles (even in prime habitat).

The issues

Hazel dormice live in an unsuitable climate in Britain, being better suited to warm, dry summers and cold, dry winters (continental climate). Our unpredictable, wet and mild winters, and wet and cloudy summers create special problems for this species and may reduce breeding success; climate change may exacerbate these problems. In addition, low population densities and low reproductive potential mean that dormouse populations are slow to recover from population lows. Dormice are also threatened by habitat fragmentation and loss, especially of ancient woodland and hedgerow.

The conservation of dormice raises interesting questions in terms of habitat restoration, captive breeding and reintroduction. The species action plan aims to maintain and enhance dormouse populations in all counties where they still occur, and to re-establish populations in at least five counties where they have been lost. Since 1992, dormice have been reintroduced in North Yorkshire, Cambridgeshire, Cheshire, Suffolk, Warwickshire, Buckinghamshire and Nottinghamshire; it is not yet clear how successful these reintroductions will be in the long-term.

Further reading

Bright, P.W. & Morris, P.A. (1996). Dormouse conservation handbook. English Nature, Peterborough.
Bright, P.W. & Morris, P.A. (1996). Why are dormice rare – a case study in conservation biology. Mammal Rev., 26, 189-195.
Bright, P.W., Morris, P.A. & Mitchell-Jones, A.J. (1996). A new survey of the dormouse Muscardinus avellanarius in Britain, 1993-94. Mammal Rev. 26, 189-195.

Edible dormouse (Glis glis)2

STATUS Origin Introduced
UK Population size 10,000-30,000 ??
UK Distribution Restricted to the Chilterns
UK Status Increasing in range
IUCN designation Not listed
NATURAL HISTORY Habitat Mature, deciduous woodland, also gardens and orchards
Diet Nuts, fruit, fungi, insects, occ. eggs and nestlings
Behaviour Nocturnal; social
Home range 100m diameter
Breeding Single litter of 2-9 per year
ISSUES Control 4.1.1.iii


The edible dormouse is distributed throughout central Europe. It was introduced to Britain in 1902, where it has a very restricted distribution in the Chiltern Hills. However, its range has been increasing slowly since its introduction and they are locally numerous. True to their name, they are often eaten – a practice which was common during Roman times. They are also known as the fat dormouse due to their corpulence before hibernation. Population estimates range from 10,000-30,000.

Natural history

Edible dormice are found in mature deciduous woodland, predominantly in the canopy of trees, as well as in gardens, orchards and houses. They are omnivorous, feeding on vegetable food including nuts, fruit, fungi and bark, as well as animal foods such as insects, and occasionally eggs and nestlings.

Edible dormice are nocturnal and sociable, living in loose groups of close relatives. Home ranges are about 100m in diameter. They hibernate between October and April in hollow trees or underground.

Breeding occurs from June-August. From the age of two, females can produce one litter per year of 2-9 young. They live to about seven years. Population densities vary between 0.2-4/ha.

The issues

In the Chilterns, edible dormice can damage fruit crops and forestry plantations (‘bark-stripping’ after emergence from hibernation). They also chew through wiring and woodwork in houses. Consequently, they are controlled by woodland owners, fruit-growers and Environmental Health Departments of District Councils.

Further reading

Morris, P. (1997). The edible dormouse. The Mammal Society, London.

2Note: Some authorities include the edible dormouse in the genus Myoxus, on the taxonomic grounds that Glis is invalid (Holden, 1993; McKenna & Bell, 1997). However, for the purposes of this report (which is not taxonomic in scope), we retain usage of the more widely-used Glis.

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