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The bank vole is found in a variety of habitats, including hedgerows, conifer plantations and road verges, but shows a strong preference for mature broad leaved and mixed woodland (Flower dew et al., 2004). The diet comprises fruits, seeds and leaves from broad leaved trees, although other food sources such as flowers, grasses and moss are taken opportunistically. Unlike populations in mainland Europe, the bank vole forms a high proportion of its winter diet in Britain from dead leaves (Hansson, 1985).
Limited recent research is available on the factors affecting the population density of bank voles, although abundance is positively associated with the quality and size of woodlands. The species requires dense ground vegetation (Fernando et al., 1994). It is found frequently in field margins and hedgerows, which can support large resident populations (Gelling et al., 2007), but only rarely in arable fields (Harris and Yalden, 2008). Arable habitats were therefore excluded from the population estimates. There is no evidence of multi-annual cycles for bank voles in Britain (Flowerdew et al., 2004).
IUCN Red List (GB: LC; England: [LC]; Scotland: [LC]; Wales: [LC]; Global: LC). • This species has not been assessed for Article 17 of the EU Habitats Directive
Gaps in the distribution in England and Wales are likely to represent areas lacking survey effort, rather than true absences. It is unclear whether the larger gaps in Scotland reflect a lack of recorder effort or true absences. Further survey effort is recommended in these areas to increase confidence in the current distribution.The literature search (using both Myodes glareolus and Clethrionomys glareolus as species names) identified 63 papers. Fourteen provided information on population size or distribution; of these, six gave pre-breeding estimates of population density, and one contained percentage occupancy for hedgerows (Gelling et al., 2007). The remaining papers reported post-breeding estimates, assessed the relative effects of environmental variables on population size, or gave distribution data online.
Percentage occupancy data were not available for most habitats; the population size is therefore overestimated. Most of the population estimate is derived from broadleaved woodland (39%). Yet broadleaved woodland forms a low proportion of the land cover within the species’ range (Table 7.6a), and its importance is therefore largely a consequence of high density estimates relative to other habitats. Many of the density estimates for these other habitats were derived from expert opinion, highlighting the need for detailed surveys in habitats lacking empirical data.
Just over half of the habitat within range is improved grassland. It is estimated that this large habitat area supports only 1% of total bank vole population because of low population density (0.1ha-1). This estimate is again the average of values provided by two expert opinions, and validation of the estimated values would improve future calculations of population size. Despite the low suitability of improved grassland and arable land for bank voles, a significant proportion of the population is found in hedgerows in these areas. It is possible that many individuals captured within improved grassland reside in hedgerows, and therefore the inclusion of both these habitats could have slightly overestimated the population. (Arable land was excluded.)
Harris et al. (1995) reported a total population size of 23,000,000, comprised of 17,750,000 in England, 3,500,000 in Scotland and 1,750,000 in Wales. Those values fall within the confidence limits of our estimates here, except in Scotland for which current estimates are somewhat higher. Population sizes were calculated by Harris et al. (1995) from density estimates for hedgerows, woodland, scrub and bracken only, where the density per habitat type was assigned using a combination of empirical data and expert opinion. The methods to estimate current population size are, therefore, similar, although the difference in selected habitats and use of expert opinion mean that comparisons should be made with caution.
Nationally, there are changes between the two reviews in the estimated availability of key habitats, generated by a combination of true change and methodological differences, irrespective of any range change. The adjusting of results to reflect more probable temporal changes in the composition of the British landscape — using differences between the 1990 and 2007 Countryside Surveys (Carey et al., 2008) — produces a population size only 5% different from the original (and within the original confidence limits). These differences in assumed habitat areas therefore do not materially affect the comparison of population sizes between the reviews.
Population range and habitat all deemed to be stable.
• IUCN Red List (GB: LC; England: [LC]; Scotland: [LC]; Wales: [LC]; Global: LC). • This species has not been assessed for Article 17 of the EU Habitats Directive
The field vole is most abundant in rough low-productivity grassland under low intensity management and untreated by artificial fertilisers. Long tussocky grass allows for the formation of runs and nests (Gelling et al., 2007), and provides protection from aerial predators. The species’ density is negatively associated with grazing pressure, but low intensity grazing, particularly by sheep, may be beneficial as it leads to more diverse vegetation structure (Schmidt et al., 2005). The field vole may also live in marginal habitats: it can occupy open grassy patches within fragmented woodlands, as well as moorlands, at low densities (Bellamy et al., 2000; Tattersall et al., 2000), and marginal rough grasslands may support high densities.
Linear habitats, such as hedgerows, are becoming an increasingly important because of habitat fragmentation and the loss of tussocky grasslands (Tattersall et al., 2002). Not only can hedgerows provide useful corridors, but the vegetation in hedgerow bottoms can also provide the sole habitat for the species, especially within pastoral landscapes with typical grazing intensities (Gelling et al., 2007).
Gaps in the species’ distribution throughout the mainland are likely to represent areas lacking survey effort, rather than true absences. Further survey effort is recommended in these areas to increase confidence in the current distribution.
Field voles use long tussocky grass as their primary habitat, so it was assumed that animals within both arable and improved grassland would be transient individuals that would be largely accounted for by the estimates for hedgerows. Arable and improved grassland habitats were therefore not included in the calculation of population size. Annual and multiannual cycles (three to four year periodicity) in population size are known in Britain, but noncyclic populations are also present in different areas (Lambin, 2008). These differences make it difficult to assess the status of the population as a whole, or even the stage of the cycle reached at any given time. Therefore, whenever temporal data were available, troughs in population size were used as the best estimate of pre-breeding density, even if these were not obtained in the spring (Lambin et al., 2000; Loughran, 2006).
Nine papers were identified by the literature search. Of these, four contained pre-breeding estimates of population size, four reported post-breeding estimates of population size, and one examined the effect of habitat variables on relative population size.
No percentage occupancy data were available; the population size is therefore overestimated. Most of the estimated population (88%) is derived from unimproved grassland (Figure 7.7b). The population density used for this habitat is based on 18 sites reported in three papers. Unimproved grassland accounts for 19% of the habitat found within the species’ distribution. Coniferous woodland, broadleaved woodland and urban areas form most of the remaining area (67%), although the densities within these habitats are thought to be low. Improved grassland was excluded from the analysis on the grounds that it offers poor habitat for field voles, and individuals using this environment would be accounted for by the hedgerow density estimates.
Grassland habitats have spectrally similar profiles in remotely sensed datasets, so areas of rough grassland may have been misclassified as improved grassland, and vice versa, in the LCM2007 dataset. The area given for rough grassland may therefore be inaccurate. We also grouped improved, neutral, acid and calcareous grasslands together for the analysis, combining those habitats most likely to be mistaken for each other but which are also functionally similar. Some of this ‘improved grassland’ grouping may support a low density of field voles, particularly if grazing intensity is low (Schmidt et al., 2005), but the habitat has been excluded from this analysis
The population size estimate in Harris et al. (1995) was 75,000,000. This estimate was based on the ratio of field voles to other small mammals in a range of samples, including live traps and owl pellets. Values of 1.9 field voles per wood mouse and 1.8 field voles per common shrew were derived across all samples. As this method is not based on the area of suitable habitat within the species’ distribution, a comparison between population size estimates from Harris et al. (1995) and the current estimate is not advised.
Population -stable, range-stable, habitat- stable.
Status. Non-native naturalized island endemic.
IUCN Red List (GB: VU; England: n/a; Scotland: [VU]; Wales: n/a; Global: LC). • This species has not been assessed for Article 17 of the EU Habitats Directive.
No estimates for pre-breeding population densities were available from the literature review, and no data were available from the previous population review (Harris et al., 1995). However, overall population estimates were made for the years 1998-1990: post-breeding estimates for Orkney were 3,000,000 on Mainland, 500,000 on Rousay, 300,000 on Westray, 200,000 on South Ronaldsay and 100,000 on Sanday (Reynolds, 1992). Prebreeding populations can therefore be inferred to be 1,000,000-2,000,000. The population size is not known to experience cycles (see(Gorman and Reynolds, 2003; Harris and Yalden, 2008; Fraser et al., 2015a).
Population trend -declining. Range-stable. Habitat-declining.
The water vole in Great Britain is primarily riparian, usually occurring within 2m of water. Although fossorial ecotypes not associated with water are common in continental Europe (Berthier et al., 2014), and populations have been identified in localised areas of Glasgow and on some Scottish islands (Telfer et al., 2003; Stewart et al., 2017), these are currently considered to be a small proportion of the total population. The riparian water vole prefers
slow-flowing rivers, streams and marshes with tall dense vegetation (Strachan and Jefferies, 1993) that provides cover from avian predators (Lawton and Woodroffe, 1991). Reeds and grasses are used for food, cover and nesting material, while steep sandy banks allow it to construct extensive burrows above and below the waterline (Barreto et al., 1998b).
Unlike the larger colonies found in the lowlands, the upland water vole forms small scattered colonies, occupying dispersed patches of suitable habitat (Aars et al., 2001). These fragmented populations are vulnerable to stochastic variation and other threats (Capreolus Wildlife Consultancy, 2005). Nevertheless, upland areas and headwater streams are now the most important remaining sites for the water vole in some areas, despite low population densities (Walsh and Hall, 2005).Over the last century, intensification of agriculture has had a number of adverse consequences for water vole habitat. Factors detrimental to water voles have included wetland drainage, the encroachment of cultivated land into riparian and wetland habitats, overgrazing, and the degradation of the structural and vegetative suitability of banks for water vole burrows because of cattle poaching. River bank reinforcement programmes, and increased frequency of spate events because of altered drainage patterns and weather changes, have also negatively affected the suitability of riparian habitat. Together with predation by the non-native American mink (Neovison vison), these changes have resulted in a drastic decline in water vole populations (Jefferies et al., 2003; Gow, 2008; MacPherson and Bright, 2011). This decline has led to the establishment of the UK Water Vole Steering Group and the development of mink control strategies, such as the Scottish Mink Initiative, part-funded by the SNH Species Action Framework.
• IUCN Red List (GB: EN; England: [EN]; Scotland: [NT]; Wales: [CR]; Global: LC). • This species has not been assessed for Article 17 of the EU Habitats Directive.
24 papers and 10 government reports were identified by the literature search. Of these, 3 contained pre-breeding population density estimates, with the remainder containing postbreeding density estimates, habitat requirements, occupancy values or presence survey
Water vole population density depends on a number of factors that were not accounted for in our estimate. For example, density will be higher in areas with high vegetative cover and fewer mink. Non-linear wetland areas such as reed beds and grazing marsh can also support high population densities and potentially offer refuges from mink predation (Strachan and Moorhouse, 2006; MacPherson and Bright, 2010): these habitats were not included in our assessment because of a lack of i) data on occupancy, and ii) sufficiently fine resolution habitat data to permit identification of potentially suitable areas. Wider water channels may also contain higher densities of water voles than assumed here because separate populations can form on each bank (Harris et al., 1995), although recent evidence suggests that most surviving populations inhabit upper tributaries rather than main river channels owing to the presence there of mink (Telfer et al., 2001). The distribution of water voles can also change rapidly over time as local populations are lost to mink predation, or to a lesser extent because of habitat change: the occupancy data in this review may therefore be outdated despite being relatively recent (Strachan et al., 2000; Capreolus Wildlife Consultancy, 2005). Continuous monitoring of this species is therefore vital. Population densities vary between upland and lowland areas, with headwaters offering potential refuges for water voles (Walsh and Hall, 2005). Stratification into lowland and upland areas may provide a more robust population estimate, although more measures of population density would be required to ensure that variation between these areas is represented in the dataset.Harris et al. (1995) reported a total population of 1,169,000 water voles in Britain, comprising 752,000 in England, 376,000 in Scotland and 41,000 in Wales. The approaches taken by Harris et al. (1995) and the current review are comparable, except that the former obtained pre-breeding population estimates by adjusting summer population sizes, whereas the current review computed the pre-breeding population size directly from spring density estimates. Both reviews adjusted for the percentage of habitat occupied based on the findings of the Vincent Wildlife Trust’s national water vole surveys. Harris et al. (1995) used the 1989-1990 surveys (Strachan and Jefferies, 1993), and the current review used the 1996-1998 surveys (Strachan et al., 2000), supplemented with additional data. Applying the same method as Harris et al. (1995), Strachan et al. (2000) estimated the overwintering population in 1996-1998 to be 262,500. These figures suggest a 78% decline in water vole population size between 1989-1990 and 1996-1998. The current review suggests a further decrease by 50% for the period 1998-2016. The occupancy values used in our estimate were measured in 1996-1998 (Britain; Strachan et al., 2000), supplemented by data collected in 2005 for upland Scotland (Capreolus Wildlife Consultancy, 2005), and the density estimates were derived in 2000-2005 (see Table 7.9b). Although trends in density are unclear, occupancy had decreased by 80% in most areas between 1989-1990 and 1996-1998 and, despite conservation efforts, the pressures of mink predation and habitat loss mean that this trend is highly likely to have continued. A notable exception may be parts of Scotland where systematic landscape-scale mink control has been conducted (Bryce et al., 2011; Gaywood et al., 2016; Robertson et al., 2017). For example, both upland and lowland regions of Aberdeenshire and the Cairngorms National Park have seen marked recoveries of water voles. Although recolonization is a slow process, particularly where starting population densities are low, water voles are now ubiquitous over large areas (Xavier Lambin, pers. comm.).
Applying the same method as Harris et al. (1995), Strachan et al. (2000) estimated the overwintering population in 1996-1998 to be 262,500. These figures suggest a 78% decline in water vole population size between 1989-1990 and 1996-1998. The current review suggests a further decrease by 50% for the period 1998-2016. The occupancy values used in our estimate were measured in 1996-1998 (Britain; Strachan et al., 2000), supplemented by data collected in 2005 for upland Scotland (Capreolus Wildlife Consultancy, 2005), and the density estimates were derived in 2000-2005 (see Table 7.9b). Although trends in density are unclear, occupancy had decreased by 80% in most areas between 1989-1990 and 1996-1998 and, despite conservation efforts, the pressures of mink predation and habitat loss mean that this trend is highly likely to have continued. A notable exception may be parts of Scotland where systematic landscape-scale mink control has been conducted (Bryce et al., 2011; Gaywood et al., 2016; Robertson et al., 2017). For example, both upland and lowland regions of Aberdeenshire and the Cairngorms National Park have seen marked recoveries of water voles. Although recolonization is a slow process, particularly where starting population densities are low, water voles are now ubiquitous over large areas (Xavier Lambin, pers. comm.).
Range-stable. Habitat-stable Population -declining.
Accurate maps are difficult to produce for this species; populations can disappear quickly where mink are present and may not recolonise, and variation in recording effort makes it difficult to determine areas of true absence. The National Water Vole Database and Mapping Project has collected presence records for water voles since 2008, and so map coverage is likely to be considerably more thorough than for other species of small rodents. The project does not, however, include systematic survey coverage for water voles, and so it is unclear whether gaps in the species’ distribution are caused by low recorder effort. In Wales, neither a National Key Site for Water Voles (Llanelli), nor several other populations, are shown on the smoothed distribution map
The lengths of riparian habitat for Scotland, Wales and each English region, were derived from Table 4 of Harris et al. (1995). These lengths were multiplied by the percentage of each region/country included in the species’ distribution, and then by the occupancy values per country or region (Table 7.9a). For Scotland, mean occupancy was taken from values in Strachan et al. (2000), Capreolus Wildlife Consultancy (2005), and Reynolds and Telfer (2000). The regional lengths for England were totalled to give an estimate for the whole country. The occupied lengths of riparian habitat were then multiplied by the abundance value (voles per 100m
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Bank vole. Field Vole and Water Vole. all feature.
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