Small rodent winter survival: snow conditions limit access to food resources

1. In Fennoscandia during winter small rodents spend most of their time in the subnivean space, between the snow cover and the ground. The subnivean space is probably not a uniform habitat, but broken into accessible and inaccessible patches by ice covering the vegetation. This might reduce access to otherwise available food resources. 2. To test whether ice formations reduce access to food and thus limit winter survival of small rodents, we conducted an experiment where we increased subnivean space by adding corrugated aluminium sheets on the ground before onset of winter. The sheets prevented ice formation, thus mimicking natural occurring subnivean space, and providing more room for animals living in the subnivean space to forage. 3. During the experiment 142 Microtus oeconomus were passive induced transponder (PIT)-tagged, and a system consisting of fixed tube-shaped antennas and PIT-tag readers were used to provide data to analyse winter survival and individual subnivean space use. The extent of winter grazing was measured after snow melt by examining percentage area grazed. 4. The treatment resulted in increased survival which corresponded well with significantly higher space use and more grazing under the sheets. 5. Females showed a positive correlation between probability of survival and body mass while no such effect was observed in males. 6. The results suggest that the snow cover reduces survival in winter by physically enclosing the vegetation in ice and thus reducing access to otherwise available food resources. The amount of ice and its configuration might vary between years due to changing weather patterns. Our results offer a mechanistic explanation for variations in winter survival and suggest incorporating climate variables in future small rodent models. 7. Directional and long-term changes in climate might result in increased ice formation in the subnivean system. Such deterioration may lead to reduced winter survival and act by stabilizing population dynamics and dampening vole cyclicity.     

Winter energy requirements of soricine shrews

The energy requirements of shrews under snow cover have not been determined. This is an expository paper attempting to ascertain the daily energy budget of a soricine in subnivean conditions. By means of extrapolation from past work (Morrison, Ryser & Dawe, 1959; Gebczynski, 1965, 1971), a winter individual of Sorex cinereus (mean weight of 2–6 g) in southern Manitoba has a rather high energy budget at 2^oC of about 38-6 kj or 9-2kcal per day. It is assumed that winter prey include invertebrates and small mammals, and their biomass and energy content are estimated. Compensatory factors reducing energy needs are considered, i.e. Dehnel's Phenomenon, extended periods in the nest, low population densities and adequate snow cover. We can probably reduce the above figure for a daily energy budget when considering all the above factors, so that about 30 kj are required daily at the mean subnivean temperature of – 4–5^oC.     

Immediate and short‐term responses of bird and mammal assemblages to a subalpine wildfire in the Snowy Mountains,Australia

Abstract Over 35 days in January–February 2003, wildfires burnt across much of the subalpine/alpine landscape of south‐eastern Australia, including about 70% of the land above 1500 m in the Snowy Mountains. At the time of the fire, studies of two subalpine faunal assemblages were being undertaken. The opportunity to resurvey the assemblages was taken in order to examine the immediate impact of fire in an environment where it is uncommon but predicted to occur increasingly with global warming. A study area in the Whites River Valley, where the number of bird species was counted monthly from 1996 to 2003, and weekly in late winter–spring from 2000 to 2003, was burnt in one fire. As well as the expected decrease in the number of individual birds, the fire resulted in an immediate decrease in the number of bird species, unlike in previously studied montane forest, with only the regularly wintering species plus the olive whistler and the ground‐feeding flame robin remaining. During the post‐winter avian immigration, few regular spring migrants appeared on burnt sites despite their nearby presence on the unburnt side of the valley. Five of six small mammal trapping grids were burnt. As with fires at lower altitudes, there was an immediate reduction in mammal numbers on burnt grids following the fire, but in addition, one species, Mastacomys fuscus, declined further in the ensuing 2 months both on burnt and unburnt sites. Numbers of Antechinus swainsonii and Rattus fuscipes stabilized until autumn/winter when there was a further decline due to the unavailability of subnivean space to allow winter foraging, allied with a concentration of fox predation on areas still carrying populations of small mammals.     

Density,home range,seasonal movements and habitat use of the mountain pygmy‐possum Burramys parvus (Marsupialia: Burramyidae) at Mount Blue Cow,Kosciuszko National Park

A population of mountain pygmy‐possums Burramys parvus was studied at the Mount Blue Cow ski resort in Kosciuszko National Park between 1986 and 1989. Forty‐eight individuals were radiotracked during the snow‐free months and 21 individuals were tracked during winter over the 3 years of study. Trapping and radiotracking showed that the density, population structure, movements and home range sizes of B. parvus on Mount Blue Cow were strongly correlated with elevation and changed with the season. Female densities were greatest in habitats characterized by deep boulderfields, at high elevations with an abundance of Bogong moths. Males visited the areas where females were located to breed in November–December and then by February, the majority migrated to lower elevations or north and westerly aspects. Females that nested at lower elevations also visited high‐elevation habitats to access the high concentrations of Bogong moths, which were the main food source in summer. A high proportion of the juvenile males and some juvenile females dispersed to lower elevations in March and April. The resulting sexual segregation during autumn and winter may be a result of female aggression or scramble competition, but is also explainable by differences in energy requirements, seed availability and hibernation strategies between the sexes. The extraordinarily large nightly and seasonal movements between habitat patches of up to 2 km for females and 3 km for males, sexual segregation and the use of different hibernation sites have important implications for the management of this species. These include the need for movement and dispersal corridors and the conservation of boulder‐heath habitats outside the main boulderfields.     

A technique for using hair tubes beneath the snowpack to detect winter-active small mammals in the subnivean space

The study of winter-active small mammals beneath the snowpack has proved challenging for researchers because of the relative inaccessibility. We present a technique using hair tubes that permits the detection of small mammals active in the subnivean space. Hair tubes are cylindrical or funnel-shaped structures containing suitable bait and an adhesive surface that harvests hairs from small mammals as they attempt to reach the bait. Hair tubes eliminate many of the difficulties often associated with live trapping and permit the expansion of systematic sampling to larger scales than allowed by conventional live-trapping methods. The technique was used successfully to detect five small mammal species in the subnivean space in Kosciuszko National Park (KNP) in southeastern Australia. These included the common bush-rat, Rattus fuscipes; the dusky and agile antechinus, Antechinus swainsonii and A. agilis; the broad-toothed rat, Mastacomys fuscus; and the mountain pygmy possum, Burramys parvus. Although hair tubes have a number of limitations, such as not providing a measure of abundance or allowing the identification of individual animals, we believe that these limitations are balanced by the fact that the technique can be used at any spatial scale. Hair tubes are particularly suited to studies of animal distribution at the landscape-scale, because many hair tubes can be deployed and dispersed over large areas, and monitored on a regular basis by a small team of researchers. The technique also makes use of readily available, low-cost materials and could be easily adapted to a range of conditions and different target species.     

The contribution of grazing to plant diversity in alpine grassland and heath

Abstract Grazing on transplants of a grass, a forb and a tree was examined in low-diversity grassland and more diverse heath in Australia's Snowy Mountains. Transplants were surrounded by 2 mm mesh netting. In one grassland plot, grazers (probably soil invertebrates) attacked 40–90% of tree and forb seedlings but no grass seedlings. In heath, which had about half the grass cover of grassland, grazers consumed grasses but not trees or forbs. The results suggest that grazers can depress diversity in grassland by attacking species other than grass. In heath, they may promote diversity by attacking only grass and releasing other species from competition.     

Habitat selection,reproduction and predation of wintering lemmings in the Arctic

Snow cover has dramatic effects on the structure and functioning of Arctic ecosystems in winter. In the tundra, the subnivean space is the primary habitat of wintering small mammals and may be critical for their survival and reproduction. We have investigated the effects of snow cover and habitat features on the distributions of collared lemming (Dicrostonyx groenlandicus) and brown lemming (Lemmus trimucronatus) winter nests, as well as on their probabilities of reproduction and predation by stoats (Mustela erminea) and arctic foxes (Vulpes lagopus). We sampled 193 lemming winter nests and measured habitat features at all of these nests and at random sites at two spatial scales. We also monitored overwinter ground temperature at a subsample of nest and random sites. Our results demonstrate that nests were primarily located in areas with high micro-topography heterogeneity, steep slopes, deep snow cover providing thermal protection (reduced daily temperature fluctuations) and a high abundance of mosses. The probability of reproduction increased in collared lemming nests at low elevation and in brown lemming nests with high availability of some graminoid species. The probability of predation by stoats was density dependent and was higher in nests used by collared lemmings. Snow cover did not affect the probability of predation of lemming nests by stoats, but deep snow cover limited predation attempts by arctic foxes. We conclude that snow cover plays a key role in the spatial structure of wintering lemming populations and potentially in their population dynamics in the Arctic.     

The impact of snow accumulation on a heath spider community in a sub-Arctic landscape

Patterns of snow cover across the Arctic are expected to change as a result of shrub encroachment and climate change. As snow cover impacts both the subnivean environment and the date of spring melt, these changes could impact Arctic food webs by altering the phenology and survival of overwintering arthropods, such as spiders (Araneae). In this field study, we used snow fences to increase snow cover across a series of large (375 m²) heath tundra plots and examined the effects on the local spider community during the following growing season. Fences increased snow cover and delayed melt on the treatment plots, paralleling the conditions of nearby shrub sites. Frequent sampling over the season revealed that increased snow cover did not affect spider abundance across different genera nor did it affect overall community composition. Further, our snow treatment did not affect the dates when plots achieved seasonal catch milestones (25, 50, 75 % of total seasonal catch). Increased winter snow cover did, however, produce higher body masses in adults and juveniles of the dominant species Pardosa lapponica (Lycosidae), beginning immediately after snow melt until midway through the growing season. In addition, ovary/oocyte mass of mature P. lapponica females was significantly higher on treatment plots during the peak reproductive period. This is the first experimental manipulation study to report a significant effect of landscape-level changes to winter snow cover on the biomass of an Arctic macroarthropod.     

Measuring the effects of reduced snow cover on Australia's alpine arthropods

Snow is one of the most important factors in the ecology of alpine ecosystems. In Australia, both the depth and duration of snow cover have declined significantly in recent decades and this trend is projected to continue with global warming. Many small arthropods remain active throughout the winter, within a space beneath the snowpack (subnivean) where the snow's insulation creates a thermally stable environment. Using field surveys and experimental manipulation of snow depth at two locations in the Australian alpine region, we explored the diversity of winter‐active arthropods and their response to reduced snow. Individuals from 18 arthropod Orders were detected beneath the snow during winter, with Collembola, Araneae, Acari and Coleoptera accounting for 95–98% of the individuals collected. The subnivean taxa represented a distinct subset of those active outside the winter months. Removal of the snow layer increased daily temperature fluctuations, increased the number of days below freezing and raised the mean surface temperatures. Community composition was altered by snow removal, driven by changes in the numbers of two abundant springtail taxa at each location. We found a strong reduction in the abundances of both taxa at one study site, and contrasting responses (one strong positive and one strong negative) to snow removal at the second study site. Subnivean arthropod communities in Australia thus appear sensitive to snow conditions at small spatial scales.     

Effect of snow cover on seasonal changes in diet, habitat, and regional distribution of raptors that prey on small mammals in boreal zones of Fennoscandia

Seasonal changes in spatial distribution of search effort of birds that prey on small mammals were studied in two structurally different coniferous forest habitats in the northern boreal zone in SE Norway. During the season with snow cover both the proportion of Microtus relative to that of Clethrionomys in the predators' diet, and their use of a clear-cut relative to that of older forest were lower than during the snow-free season. This was related to a lower relative availability of prey ( Microtus agrestis and M. oeconomus ) in the clear-cut when the ground was snow-covered than when it was snow-free. Based on this local pattern I suggest the following explanation for differences in migratory strategy between raptors that prey on small mammals in Fennoscandian boreal zones: species that migrate to snow-free areas in winter are either adapted to hunt by the energetically expensive method of quartering in open grassland habitats, where prey ( Microtus ) availability is relatively low during periods with snow cover (hen harrier Circus cyaneus , short-eared owl Asio flammeus , and longeared owl A. otus ), or by sit-and-wait in open grassland and forest habitats, the latter with relatively high prey availability during periods with snow cover, but unable to locate concealed prey (kestrel Falco tinnuculus , common buzzard Buteo huteo , and roughlegged buzzard B. lagopus ). In contrast, species that remain in areas with permanent snow cover during winter use the energetically cheap sit-and-wait tactic, and are able to hunt in closed forest habitat and localize concealed prey (the remaining owl species). Interspecific differences in prey availability as determined by hunting habitat and hunting mode is probably more important in shaping the migration patterns of Fennoscandian owls than is nest site availability.     

The use of habitat components by small mammals in eastern Australia

The effect of habitat components (vegetation density at two levels, litter, logs and roads) on the distribution of small mammals was assessed in adjacent areas of native forest and Pinus taeda plantation in north-eastern New South Wales. Rattus fuscipes was associated with structural complexity in native forest but not in pine plantation where it was found on downslope areas. R. rattus was associated with windrows in the pine plantation, R. lutreolus with areas devoid of a shrub layer in the pine plantation, Antechinus stuartii with logs and Melomys cervinipes with habitat components associated with rainforest areas. Road crossing by small mammals was inversely related to road width; roads severely restricted or stopped the movement of small mammals even when the road consisted of a long-unused and partly overgrown track.     

Influence of habitat characteristics on small mammals in a Mexican high-altitude grassland

Small mammals in a high-altitude grassland area close to Mexico City were studied. Populations of 10 species were censused using live traps in 48 sample quadrats. Within each quadrat, vegetation characterization, including complete floristic listings, cover values for species and layers and values of habitat modification, were assessed. Habitats were described according to plant communities identified using ordination and classification methods. Nine different plant communities were obtained. Densities and abundance of all small mammal species were calculated for each of the habitats classified. Peromyscus alsloni was the most abundant species in all habitats, reaching maximum densities of 55 ha⁻¹ in pine forest with dense ground and herb layer. Peromyscus melanotis also occurred in all habitats but at lower densities (maximum 29 ha⁻¹). Reithrodontomys megalotis was found in all habitats except in tall dense grassland. Densities for this species were generally low (1-9 ha⁻¹) but reached 19 ha⁻¹ in short dense grassland. All other species were largely absent from 4–8 habitats and showed very low densities (0.75–4 ha⁻¹). The densities of the more abundant species were largely correlated with more open habitats and higher indices of habitat modification. Lower altitude grassland habitats have a greater abundance of small mammals and a higher species richness than the medium and higher altitude, physiognomically more complex habitats. Species richness was highest in tall pine-alder forest with a species-rich, dense herb layer and lowest in pine forest with dense ground and herb layers. Species richness was positively correlated with overall small mammal density.     

Winter space use of coyotes in high-elevation environments: behavioral adaptations to deep-snow landscapes

In the last century, coyotes (Canis latrans) have expanded their range geographically, but have also expanded their use of habitats within currently occupied regions. Because coyotes are not morphologically adapted for travel in deep snow, we studied coyote space use patterns in a deep-snow landscape to examine behavioral adaptations enabling them to use high elevations during winter. We examined the influence of snow depth, snow penetrability, canopy cover, and habitat type, as well as the rates of prey and predator track encounters, on coyote travel distance in high-elevation terrain in northwestern Wyoming, USA. We backtracked 13 radio-collared coyotes for 265.41 km during the winters of 2006–2007 and 2007–2008, and compared habitat use and movement patterns of the actual coyotes with 259.11 km of random travel paths. Coyotes used specific habitats differently than were available on the landscape. Open woodlands were used for the majority of coyote travel distance, followed by mixed conifer, and closed-stand spruce–fir. Prey track encounters peaked in closed-stand, mature Douglas fir, followed by 50- to 150-year-old lodgepole pine stands, and 0- to 40-year-old regeneration lodgepole pine stands. Snowmobile trails had the most variation between use and availability on the landscape (12.0 % use vs. 0.6 % available). Coyotes increased use of habitats with dense canopy cover as snow penetration increased and rates of rodent and red squirrel track encounters increased. Additionally, coyotes spent more time in habitats containing more tracks of ungulates. Conversely, use of habitats with less canopy cover decreased as snow depth increased, and coyotes traveled more directly in habitats with less canopy cover and lower snow penetration, suggesting coyotes used these habitats to travel. Coyotes persisted throughout the winter and effectively used resources despite deep snow conditions in a high-elevation environment.     

Population regulation and dispersion of the smoky mouse,Pseudomys fumeus I. Dietary determinants of microhabitat preference

Habitat preferences of four species of small mammals were studied on a 7.5-hectare trapping grid in a subalpine heathland/woodland complex on Mt William, western Victoria. Animals did not show strong spatial separation and differences in dispersion appeared to represent response to some feature of habitat. Floristic cues were good predictors of preference for Rattus lutreolus, Antechinus swainsonii and Pseudomys fumeus. Structural factors might also be important for R. lutreolus and A. swainsonii. The entire grid was suboptimal for Antechinus stuartii. Pseudomys fumeus was mycophagous during winter and, in the summer breeding season, principally ate seeds and bogong moths. Habitat preferences appear to reflect selection for those areas providing a year round source of high quality food rich in nitrogen. The transition period between the end of production of fungal sporocarps and prolific subalpine flower and seed production during summer may be particularly important as preferred areas provide fungi and seeds over longer periods, and food for bogong moths during their annual migration.     

Wintering space use and site fidelity in a nomadic species,the snowy owl

Migratory species can exploit many habitats over vast geographic areas and adopt various patterns of space and habitat use throughout their annual cycle. In nomadic species, determinants of habitat use during the non‐breeding season are poorly known due to the unpredictability of their movement patterns. Here, we analysed variability in wintering space and habitat use by a highly nomadic species, the snowy owl, in eastern North America. Using 21 females tracked by satellite telemetry between 2007 and 2016, we 1) assessed how space use patterns in winter varied according to the type of environment (marine vs terrestrial), latitudinal zone (Arctic vs temperate), local snow conditions and lemming densities and 2) investigated winter habitat and site fidelity. Our results confirmed a high inter‐individual variation in patterns of habitat use by wintering snowy owls. Highly‐used areas were concentrated in the Arctic and in the marine and coastal environments. Owls wintering in the marine environment travelled over longer distances during the winter, had larger home ranges and these were divided in more smaller zones than individuals in terrestrial environments. Wintering home range sizes decreased with high winter lemming densities, use of the marine environment increased following high summer lemming densities, and a thick snow cover in autumn led to later settlement on the wintering ground. Contrary to expectations, snowy owls tended to make greater use of the marine environment when snow cover was thin. Snowy owls were highly consistent in their use of a given wintering environment and a specific latitudinal zone between years, but demonstrated flexibility in their space use and a modest site fidelity. The snowy owls’ consistency in wintering habitat use may provide them with advantages in terms of experience but their mobility and flexibility may help them to cope with changing environmental conditions at fine spatial scale.     

Resource partitioning by small mammals in lowland heath communities of south-eastern Australia

Habitat selection by seven species of small mammals was investigated in three areas of heathland in Victoria. A total of 12 120 trapnights over 22 months was used to assess preference for different vegetation groups formed by clustering trap sites on (a) floristic and (b) structural criteria. Rattus lutreolus proved to have both floristic and structural requirements. Seasonal changes in distribution in relation to rainfall appeared to be dependent on soil type. Greatest movement occurred on sandy podzol soils. Pseudomys shortridgei selected the most diverse vegetation types. Rattus fuscipes preferred wet, structurally complex vegetation. Mus musculus appeared to fill the niche of Antechinus minimus and partially replace A. stuartii and A. flavipes when Antechinus spp. were absent. M. musculus seems to commonly fill three niches in these heathlands: fossorial insectivore, scansorial insectivore and immediate post-fire omnivore. A small mammal community structure of five major food niches may be common to both heathland and forest communities of south-eastern Australia.     

Small mammal responses to pyric successional changes in eucalypt forest

Sixteen 1-ha study plots covering five regeneration stages were simultaneously trapped five times over a 20-month period to provide data on small mammal response to vegetation changes following fire. Areas regenerating after fires from 9 years to 1 month before the investigation were sampled in a uniform open forest on a coastal sand plain. Two types of understorey were recognized: one dominated by true forest shrubs with which Rattus fuscipes, Antechinus stuartii and Sminthopsis murina were associated, and another dominated by heath elements where the addition of Pseudomys novaehollandiae and Mus musculus produced a significantly more diverse small mammal community. The two communities exhibited different responses to post-fire vegetation changes. Rattus fuscipes was the most abundant species and showed a logistic growth in biomass. No resident populations were established in the first 3 years, but a rapid increase in biomass occurred from 3 to 5 years to plateau after 8 years. Regeneration age had the greatest effect on R. fuscipes biomass mediated through the amount of accumulated leaf litter with additional variation being attributed to several vegetation structure variables and plant species diversity. A replacement sequence in time was observed for species reaching their maximum abundance (P. novaehollandiae and/or M. musculus → S. murina → A. stuartii → R. fuscipes) and was interpreted as species occupying stages in the succession when their optimal habitat requirements were fulfilled. These results have important implications for the design of management policies using fire or fire regimes as tools for habitat maintenance or alteration. A mosaic of forest patches of adequate size covering the entire range of seral stages is necessary to meet the optimum requirements of all the above species.     

Using a resource selection approach to predict the suitability of alpine habitats for a common herbivore

Climate change models predict that Australia's alpine areas will experience major declines in snow cover, which, in turn, may provide suitable habitat for species presently restricted to lower altitudes. As a result, there are concerns among land managers that many species will invade alpine areas and have a detrimental impact on fragile alpine ecosystems. However, species survival in such areas, irrespective of snow cover, is greatly dependent on the availability of suitable resources. This study investigated the selection of resources by common wombats Vombatus ursinus, which are currently restricted to, but widespread throughout, the subalpine zone of the Snowy Mountains. Our objectives were to identify habitat choices, and build a model of habitat suitability over the broader landscape, to investigate the likelihood of this common herbivore inhabiting the alpine zone. Global positioning system data were obtained from collared wombats, which were tracked for up to a year, to examine resource selection. Resource selection within the home range of individual wombats revealed that topographic position, vegetation cover, drainage, past fire disturbance, and roads were important predictors of locations. A global model showed that wombats selected locations with mid‐elevations, moderate slopes, closer to water courses and roads, and with a lower proportion of grassland, which are discussed in relation to foraging and burrowing requirements. Mapping of the global model illustrated that alpine areas had a low relative probability of use by this species. Consequently, wombats are unlikely to inhabit alpine areas under given climate change scenarios of less snow cover, because the area (presently) does not contain suitable resources necessary for a wombat to maintain a home range. Researchers and managers need to be mindful of how the spatial distribution of resources, in addition to species climatic tolerances, will influence potential range shifts.     

Demographic response of tundra small mammals to a snow fencing experiment

Snow cover is a key environmental component for tundra wildlife that will be affected by climate change. Change to the snow cover may affect the population dynamics of high‐latitude small mammals, which are active throughout the winter and reproduce under the snow. We experimentally tested the hypotheses that a deeper snow cover would enhance the densities and winter reproductive rates of small mammals, but that predation by mustelids could be higher in areas of increased small mammal density. We enhanced snow cover by setting out snow fences at three sites in the Canadian Arctic (Bylot Island, Nunavut, and Herschel Island and Komakuk Beach, Yukon) over periods ranging from one to four years. Densities of winter nests were higher where snow depth was increased but spring lemming densities did not increase on the experimental areas. Lemmings probably moved from areas of deep snow, their preferred winter habitat, to summer habitat during snow melt once the advantages associated with deep snow were gone. Our treatment had no effect on signs of reproduction in winter nests, proportion of lactating females in spring, or the proportion of juveniles caught in spring, which suggests that deep snow did not enhance reproduction. Results on predation were inconsistent across sites as predation by weasels was higher on the experimental area at one site but lower at two others and was not higher in areas of winter nest aggregations. Although this experiment provided us with several new insights about the impact of snow cover on the population dynamics of tundra small mammals, it also illustrates the challenges and difficulties associated with large‐scale experiments aimed at manipulating a critical climatic factor.     

Snow evens fragmentation effects and food determines overwintering success in ground-dwelling voles

Climate instability strongly affects overwintering conditions in organisms living in a strongly seasonal environment and consequently their survival and population dynamics. Food, predation and density effects are also strong during winter, but the effect of fragmentation of ground vegetation on ground-dwelling small mammals is unknown. Here, we report the results of a winter experiment on the effects of habitat fragmentation and food on experimental overwintering populations of bank voles Myodes glareolus. The study was conducted in large outdoor enclosures containing one large, two medium-sized or four small habitat patches or the total enclosure area covered with protective tall-grass habitat. During the stable snow cover of midwinter, only food affected the overwintering success, body condition, trappability and earlier onset of breeding in bank voles. However, after the snow thaw in spring, habitat fragmentation gained importance again, and breeding activities increased the movements of voles in the most fragmented habitat. The use of an open, risky matrix area increased along the habitat fragmentation. Our experiment showed that long-lasting stable snow cover protects overwintering individuals in otherwise exposed and risky ground habitats. We conclude that a stable winter climate and snow cover should even out habitat fragmentation effects on small mammals. However, along prolonged snow-free early winter and in an earlier spring thaw, this means loss of protection by snow cover both in terms of thermoregulation and predation. Thus, habitat cover is important for the survival of small ground-dwelling boreal mammals also during the non-breeding season.