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As a group, the rodents contain some of the most imperilled and the most pestilential species. Red squirrels and water voles are among Britains 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.
Red squirrel (Sciurus vulgaris)
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.
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.
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.
Bryce, J. (1997). Changes in the distribution of red and
grey squirrels in Scotland. Mammal Rev. 27, 171-176.
See also Grey squirrel.
Grey squirrel (Sciurus carolinensis)
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.
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 (<112km) 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.
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.
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.
See also Red squirrel.
Bank vole (Clethrionomys glareolus)
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).
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.
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.
Flowerdew, J. (1993). Mice and voles. Whittet Books,
Field vole (Microtus agrestis)
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.
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.
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.
Flowerdew, J. (1993). Mice and voles. Whittet Books, London.
Orkney vole (Microtus arvalis orcadensis)
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 islands short-eared owls, kestrels and hen harriers.
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 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.
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)
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 Britains 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.
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.
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 voles 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.
Macdonald, D.W. & Strachan, R. (1999). The mink and
the water vole: analyses for conservation. EA/WildCRU, Oxford.
Wood mouse (Apodemus sylvaticus)
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.
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.
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 (Weils disease).
Flowerdew, J. (1993). Mice and voles. Whittet Books,
Yellow-necked mouse (Apodemus flavicollis)
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.
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 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.
Marsh, A.C.W. (1999). The national yellow-necked mouse
survey report. The Mammal Society, London.
See also Wood mouse.
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.
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.
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.
Harris, S. (1980). The harvest mouse. Blandford Press,
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.
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.
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 (Weils disease) in their urine.
Berry, R.J. (1981a).
Town mouse, country mouse: adaption and adaptability in Mus domesticus (Mus musculus
domesticus). Mammal Rev. 11, 91-136
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.
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.
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 (Weils 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.
Buckle, A.P. & Smith, R.H. (1994). Rodent pests and
their control. CAB International, Wallingford.
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.
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 152/ha.
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.
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,
Hazel dormouse (Muscardinus avellanarius)
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.
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).
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.
Bright, P.W. & Morris, P.A. (1996). Dormouse
conservation handbook. English Nature, Peterborough.
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.
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.
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.
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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