Fluctuating Vole Populations Research Articles

Body Condition in the Tawny Owl Strix aluco near the Northern Limit of Its Range: Effects of Individual Characteristics and Environmental Conditions.

Simple SummaryThe present study examines how the variations in food supply and winter weather are reflected in the body condition of female and male Tawny Owls Strix aluco of different colour morphs in a population near the southern coast of Finland. Winter weather conditions before breeding seemed to have effects on the food availability of Tawny Owls: the depth of the snow cover showed a positive relationship, and the frequency of temperature fluctuations around the freezing point had a negative relationship. In females, intrinsic factors such as colour morph and age, as well as the body condition of the mate and the stage of the season, governed body condition. In males, only age and the stage of the season suggested associations with body condition. Probably due to the efficient use of alternative prey, the effects of fluctuations of vole populations on the body condition of Tawny Owls are only moderate.The body condition of boreal species of vole-eaters seems to vary largely according to fluctuations in vole populations and weather conditions of the preceding winter. I studied females and males of the Tawny Owl Strix aluco of temperate origin near the northern limit of the species’ range in southern Finland to reveal if they show similar patterns to the boreal species. Winter weather conditions before breeding seemed to have pronounced effects on the food availability of Tawny Owls. In females, intrinsic factors such as colour morph and age, as well as the body condition of the mate and the stage of the season (Julian date), governed body condition. In males, only age and Julian date showed pronounced relationships with body condition. The results suggest that deep snow cover protects vole populations through winter until spring better than a minor amount of snow and that frequent temperature fluctuations around the freezing point in early spring make voles more available for owls that are preparing for breeding. This was also reflected positively in the body condition of female owls. Probably due to the efficient use of alternative prey, the effects of fluctuating vole populations on the body condition of Tawny Owls are, in general, only moderate.

Bilberry seed production explains spatiotemporal synchronicity in bank vole population fluctuations in Norway

AbstractPopulation fluctuations of small rodents are often synchronized over larger areas (>100 km) than what could be explained by dispersal, suggesting that the synchronizing factor is weather‐related and possibly mediated through changes in food quality. Because bank vole (Myodes glareolus) populations usually peak 1 year after peaks in reproduction of the staple winter food plant bilberry (Vaccinium myrtillus), we tested for a possible link between food and spatial synchrony by comparing the synchrony in bank vole population indices and bilberry seed production indices between three study areas across about 20,000 km2 in South Norway during a four decade period (1979–2019). There were subperiods of spatial synchrony and asynchrony between the study areas in the fluctuations of bank vole numbers and bilberry seed production, with the latter part of the study period displaying more pronounced synchrony than the first and middle part. However, with a few marked exceptions, when vole fluctuations were spatially out of phase across study areas so was bilberry seed production. Thus, we conclude that bilberry seed production to a large extent explained the spatiotemporal synchronicity in bank vole population fluctuations. Although bilberry seed production seems to be a causal driver of vole fluctuations, it remains to be seen to what extent the chemical composition of bilberry plants influences vole performance. Finally, certain weather factors may still influence voles directly, or indirectly by triggering bilberry seed production.

Density-Dependent Prevalence of Francisella tularensis in Fluctuating Vole Populations, Northwestern Spain.

Tularemia in humans in northwestern Spain is associated with increases in vole populations. Prevalence of infection with Francisella tularensis in common voles increased to 33% during a vole population fluctuation. This finding confirms that voles are spillover agents for zoonotic outbreaks. Ecologic interactions associated with tularemia prevention should be considered.

Strong territoriality and weak density‐dependent reproduction in Lesser Spotted Eagles Clanga pomarina

We assessed whether populations of the migratory, philopatric, territorial Lesser Spotted Eagle Clanga pomarina are regulated through territoriality and density‐dependent reproduction rates. In the north‐western part of the distribution to the east of the Baltic Sea, territories were spaced regularly and consistently between years. We did not find strong support for an improvement of reproductive output with an increase in distance to the nearest neighbour or with a decrease of the number of conspecifics within a 2‐km radius around nest‐sites. Eagles differed greatly in territory spacing across three studied geographical areas, but breeding performance did not follow the same pattern. Performance of birds at the northern limit of their distribution did not differ from that in an adjacent southern area. Moreover, breeding performance fluctuated synchronously across different geographical areas, perhaps indicating climatic spatial autocorrelation or trophic interactions with synchronous vole population fluctuations over large areas. Our data suggest population regulation through territoriality but do not identify density‐dependent breeding performance in this internationally protected raptor. Population density may be a good criterion for the selection of priority sites for conservation, but synchronous fluctuations in reproductive success over large geographical areas suggest that habitat conservation should similarly be focused at large scales.

Habitat fragmentation, vole population fluctuations, and the ROMPA hypothesis: An experimental test using model landscapes.

Increased habitat fragmentation leads to smaller size of habitat patches and to greater distance between patches. The ROMPA hypothesis (ratio of optimal to marginal patch area) uniquely links vole population fluctuations to the composition of the landscape. It states that as ROMPA decreases (fragmentation increases), vole population fluctuations will increase (including the tendency to display multi-annual cycles in abundance) because decreased proportions of optimal habitat result in greater population declines and longer recovery time after a harsh season. To date, only comparative observations in the field have supported the hypothesis. This paper reports the results of the first experimental test. I used prairie voles, Microtus ochrogaster, and mowed grassland to create model landscapes with 3 levels of ROMPA (high with 25% mowed, medium with 50% mowed and low with 75% mowed). As ROMPA decreased, distances between patches of favorable habitat (high cover) increased owing to a greater proportion of unfavorable (mowed) habitat. Results from the first year with intensive live trapping indicated that the preconditions for operation of the hypothesis existed (inversely density dependent emigration and, as ROMPA decreased, increased per capita mortality and decreased per capita movement between optimal patches). Nevertheless, contrary to the prediction of the hypothesis that populations in landscapes with high ROMPA should have the lowest variability, 5 years of trapping indicated that variability was lowest with medium ROMPA. The design of field experiments may never be perfect, but these results indicate that the ROMPA hypothesis needs further rigorous testing.

Barn Owl Productivity Response to Variability of Vole Populations.

We studied the response of the barn owl annual productivity to the common vole population numbers and variability to test the effects of environmental stochasticity on their life histories. Current theory predicts that temporal environmental variability can affect long-term nonlinear responses (e.g., production of young) both positively and negatively, depending on the shape of the relationship between the response and environmental variables. At the level of the Czech Republic, we examined the shape of the relationship between the annual sum of fledglings (annual productivity) and vole numbers in both non-detrended and detrended data. At the districts’ level, we explored whether the degree of synchrony (measured by the correlation coefficient) and the strength of the productivity response increase (measured by the regression coefficient) in areas with higher vole population variability measured by the s-index. We found that the owls’ annual productivity increased linearly with vole numbers in the Czech Republic. Furthermore, based on district data, we also found that synchrony between dynamics in owls’ reproductive output and vole numbers increased with vole population variability. However, the strength of the response was not affected by the vole population variability. Additionally, we have shown that detrending remarkably increases the Taylor’s exponent b relating variance to mean in vole time series, thereby reversing the relationship between the coefficient of variation and the mean. This shift was not responsible for the increased synchrony with vole population variability. Instead, we suggest that higher synchrony could result from high food specialization of owls on the common vole in areas with highly fluctuating vole populations.

Effects of Variable Food Supply on the Body Condition of Breeding Tawny OwlsStrix alucoin Southern Finland

Body condition of boreal species of vole-eaters seems to vary in accordance with fluctuating vole populations. Tawny owlsStrix alucoof temperate origin were studied to reveal if they show a similar pattern during nestling period near the northern limit of the species' range. The body condition of tawny owls was not considerably affected by the annual fluctuations of small voles indicated by regional vole indices and local prey samples. There were some significant relationships with some alternative prey but the variation in the body condition of owls was largely explained by the bird's age, time of the season, and stage of the nestling period. Old birds that bred earlier in the season were in better condition than young ones that bred later. Due to alternative prey, the body condition seemed to be relatively resistant to the effects of variable foraging conditions as expected for a generalist species of southern origin.

Density-dependent vole damage in silviculture and associated economic losses at a nationwide scale

Voles inflict damage to silviculture by debarking or severing tree seedlings. The large-scale impacts of vole damage to silviculture, both in terms of severity and financial losses are, however, poorly known. In autumn 2005, cyclically fluctuating vole populations were at their highest in Finland for over 15 years, which led to extensive damage to silviculture during the winter 2005/06. We carried out a nationwide assessment of the incidence, spatial extent and economic value of damage and its relation to vole abundance in privately owned forests during this winter. Damage data were obtained with a questionnaire addressed to the directors of all Forest Management Associations (FMAs) operating in Finland, and vole abundance data from 15 long-term monitoring projects across the country. Voles were confirmed to have destroyed ca. 4.7 million tree seedlings, covering a total effective damage area of ca. 2600 ha. The directors of the FMAs estimated that the actual level of damage was likely to exceed 8.5 million seedlings, or 5400 ha. Roughly 80% of all damage was inflicted on Norway spruce ( Picea abies), ca. 10% on Scots pine ( Pinus sylvestris) and ca. 10% on birch ( Betula sp.) and other species. Considering costs of replanting alone, a most likely very conservative estimate of the financial impact of vole damage during the winter 2005/06 lies between 2.2 and 4.0 million €. The occurrence of damage during the winter was positively related to vole abundance in the previous autumn. This validates vole population monitoring as an effective tool for forecasting near-future damage to silviculture. Our results suggest that if vole populations continue to fluctuate as they currently do, levels of damage to Finnish forests will be great also in the future, far exceeding damage levels recorded in earlier decades.

Demography of fluctuating vole populations: Phase homogeneity of demographic variables

Most research on fluctuations of arvicoline rodents has focused on changes in demographic variables across phases. Here, we examined whether demographic variables varied within a phase (trough, increase, and decline) across 30 fluctuations of Microtus ochrogaster and 14 fluctuations of Microtus pennsylvanicus over 25 years. For each species, we tested for phase homogeneity (i.e., values of demographic variables were invariant across all fluctuations within a given phase) for the following demographic variables: monthly survival (total, adult, young), proportion of reproductively active adult males and females, proportion of young, and proportion of immigrants. We found phase homogeneity for 96.7% of the 582 phase/fluctutation data sets for the seven variables, in the three phases of across all fluctuations in the three habitats for M. ochrogaster and 98.0% of 254 of those for M. pennsylvanicus in two habitats. We conclude that for each of these two species demographic variables are relatively invariant both within a phase and across fluctuations. Die Untersuchungen zu Fluktuationen von Wühlmäusen konzentrierten sich meist auf die Veränderungen von demographischen Variablen zwischen den Phasen. In dieser Studie untersuchten wir, ob die demographischen Variablen innerhalb von Phasen (Populationstief, Wachstums- und Abnahmephase) variierten. Dreißig Fluktuationen von Microtus ochrogaster und 14 von M. pennsylvanicus aus 25 Jahren wurden betrachtet. Für beide Arten testeten wir sieben demographische Variablen auf "Phasenhomogenität" (d.h., daß die Werte der betreffenden Variablen für eine gegebene Phase invariant waren): monatliche Überlebensrate (total, Adulte, Jungtiere), Anteil reproduktiv aktiver Männchen und Weibchen, Anteil der Jungtiere und Anteil der Immigranten. Wir fanden "Phasenhomogenität" in 96.7% der 582 Phase/Fluktuation-Datensätze für M. ochrogaster (3 Habitate) und in 98.0% der 254 Datensätze für M. pennsylvanicus (2 Habitate). Wir schließen, daß bei beiden Arten die demographischen Variablen sowohl innerhalb einer Phase als auch über Fluktuationen hinweg relativ unveränderlich sind.

Spatial dynamics of Microtus vole populations in continuous and fragmented agricultural landscapes

Small mammal populations often exhibit large-scale spatial synchrony, which is purportedly caused by stochastic weather-related environmental perturbations, predation or dispersal. To elucidate the relative synchronizing effects of environmental perturbations from those of dispersal movements of small mammalian prey or their predators, we investigated the spatial dynamics of Microtus vole populations in two differently structured landscapes which experience similar patterns of weather and climatic conditions. Vole and predator abundances were monitored for three years on 28 agricultural field sites arranged into two 120-km-long transect lines in western Finland. Sites on one transect were interconnected by continuous agricultural farmland (continuous landscape), while sites on the other were isolated from one another to a varying degree by mainly forests (fragmented landscape). Vole populations exhibited large-scale (>120 km) spatial synchrony in fluctuations, which did not differ in degree between the landscapes or decline with increasing distance between trapping sites. However, spatial variation in vole population growth rates was higher in the fragmented than in the continuous landscape. Although vole-eating predators were more numerous in the continuous agricultural landscape than in the fragmented, our results suggest that predators do not exert a great influence on the degree of spatial synchrony of vole population fluctuations, but they may contribute to bringing out-of-phase prey patches towards a regional density level. The spatial dynamics of vole populations were similar in both fragmented and continuous landscapes despite inter-landscape differences in both predator abundance and possibilities of vole dispersal. This implies that the primary source of synchronization lies in a common weather-related environment.

Linking demography and host dispersal to Trichuris arvicolae distribution in a cyclic vole species

Spatial structure in the distribution of pathogen infection can influence both epidemiology and host–parasite coevolutionary processes. It may result from the spatial heterogeneity of intrinsic and extrinsic factors, or from the local population dynamics of hosts and parasites. In this study, we investigated the effects of landscape, host dispersal and demography (population abundance and phase of the fluctuation) on the distribution of a gastro-intestinal nematode Trichuris arvicolae in the fossorial water vole Arvicola terrestris sherman. This rodent exhibits outbreaks occurring regularly in Franche-Comté (France). Thirteen out-of-phase populations were studied in autumn 2003. They exhibited highly different T. arvicolae prevalences. The heterogeneity in prevalences was not explained by population structure, landscape or vole abundance, but by the phase of the vole population fluctuations. Populations at the end of the high density phase showed null prevalence whereas populations in increase or outbreak phases exhibited higher prevalences. Population genetic analyses based on microsatellites revealed significant differentiation between vole populations, and higher dispersal rates of young voles compared with old ones. These younger individuals were also infected more frequently than older voles. This suggested a role of host dispersal in the distribution of T. arvicolae. However, there was a strong discrepancy between the spatial patterns of prevalence and of host genetics or demographic phase. Genetic differentiation and differences in demographic phase exhibited significant spatial autocorrelations whereas prevalence did not. We concluded that the distribution of T. arvicolae is influenced by vole dispersal, although this effect might be overwhelmed by local adaptation processes or environmental conditions.

Large‐scale spatial and temporal patterns in population dynamics of the stoat, Mustela erminea, and the least weasel, M. nivalis, in Finland

The stoat and the least weasel are specialist predators of small rodents, and therefore their numbers are likely to depend on the availability of voles. These small predators are ecologically alike, but they differ somewhat in their diet. The stoat is larger in size than the least weasel and therefore capable of using a wider variety of prey species, while the least weasel is more restricted to small mammals. Voles in northern Fennoscandia exhibit cyclic dynamics of 3–5 years with large‐scale spatial synchrony and geographical trends in cycle length and amplitude. We predicted that the cyclic dynamics of voles are reflected in the dynamics of their predators with slight differences between the stoat and the least weasel. In this study we use snow‐tracking data to characterize the dynamics of small mustelids. The data were collected from different parts of Finland using permanent triangle‐shaped census routes of 12 km in 1989 to 2003. Population fluctuations of small mustelids were generally multiannually periodic and in synchrony over large areas, but we did not find any clear geographical gradient in the attribute of small mustelid dynamics comparable to those observed in vole population fluctuations. Instead, we found a similar decreasing temporal trend in the abundances of both species as has been recently reported for voles.

Experimental tests of the role of predation in the population dynamics of voles and lemmings

ABSTRACT Reasons for fluctuating populations of small mammals have been intensively investigated since the early days of modern ecology. Particular interest has been taken in vole populations exhibiting multiannual oscillations. Much empirical and theoretical work has been accomplished to find out the key factor(s) driving these population cycles and many reviews have been written about the results. One of the most plausible processes for explaining regular fluctuations in small mammals is predation. Here I review the existing literature on the experimental studies of the role of predation in vole population dynamics in the hope that a critical examination of these studies will help researchers improve the design of future experiments. Most predation manipulations have been done in exclosures, but there are also studies that have attempted to reduce or increase predator numbers in non‐fenced areas, islands and enclosures. As the number of experimental studies has increased, their quality in terms of replication, use of controls and realistic spatial and temporal scales has also improved. Most studies have found population‐level effects of predator manipulations on prey populations. The effects have varied from very weak to very strong, reflecting dissimilar experimental designs and the great variety of predator–prey interactions among different kinds of species in different landscapes. Most of these studies show that predation limits population growth of voles, and in some circumstances even regulate vole population fluctuations, but none of them clearly demonstrates that predation consistently changes fluctuation patterns of voles. To be able to assess more reliably the true role of predation on (cyclic) population fluctuations of voles, more competent experiments are still needed not only over the geographical range of cyclic population dynamics, but also in areas of weakly or non‐cyclic populations of voles.

Spatial synchrony in vole population fluctuations – a field experiment

Three mechanisms have been proposed to induce spatial synchrony in fluctuations of small mammal populations: climate‐related environmental effects, predation and dispersal. We conducted a field experiment in western Finland to evaluate the relative roles of these mechanisms in inducing spatial synchrony among cyclic populations of field voles Microtus agrestis. The study was conducted during the increase and peak phases of a vole population cycle on four agricultural field sites situated 1.5–7.0 km apart. Each field contained two 0.5‐ha fenced enclosures and one 1‐ha unfenced control area. One enclosure per field allowed access by small mustelid predators and the other by avian predators; all enclosures prevented the dispersal of voles. The unfenced control areas allowed access by all predators as well as dispersal by voles. Enclosed vole populations were in a treatment‐wise asynchronous phase before the predator access treatments were applied. The growth rates of all enclosed populations were tightly synchronized during the course of the experiment. Conversely, synchrony both among the unfenced populations and between the fenced and unfenced populations was practically non‐existent. During winter, in the increase phase of the cycle, vole populations in all treatments declined to low densities due to a seasonal effect of winter food depletion. During summer, in the peak year of the vole cycle, all populations fluctuated in synchrony. At this time, both small mustelids and birds of prey appeared to be abundant enough to induce synchrony. Dispersal was identified as a potential contributor to synchronization, but the magnitude of its effects could not be reliably discerned. Our results indicate that no single mechanism can account for the observed patterns of spatial synchrony among cyclic northern vole populations. Rather, spatial synchronization is induced by different mechanisms, namely seasonality and predation, acting successively during different seasons and phases of the vole cycle.

Mechanisms of density dependence in fluctuating vole populations: deducing annual density dependence from seasonal processes

AbstractBased on recent advances in time‐series analyses of ecological dynamics using statistical and mathematical models, we summarise our recent results on the seasonal processes in the annual population dynamics of the grey‐sided vole Clethrionomys rufocanus (Sundevall, 1846) in Hokkaido, Japan, and report additional analyses on annual and seasonal density dependence. Annual direct density dependence was strong in almost all populations. In contrast, delayed density dependence was generally weak, although clear delayed density dependence was detected in some of the studied populations. Although seasonal density dependence was observed both in winter and summer, direct density dependence was much more profound during winter; thus, winter density dependence contributed most to the overall annual direct density dependence. We found no correlation between the seasonal components of annual direct density dependence; however, the corresponding seasonal components for annual delayed density dependence were positively correlated. We conclude that winter conditions influence the strength of annual direct density dependence most profoundly. Moreover, we conclude that direct density dependence during summer and winter may be generated by different mechanisms, whereas delayed density dependence seems to be generated by a common mechanism. Candidate mechanisms are discussed in relation to general knowledge of northern rodent populations and to specific insights provided by earlier studies of grey‐sided voles in Hokkaido.

Landscape effects on temporal and spatial properties of vole population fluctuations

Populations of northern small rodents have previously been observed to fluctuate in spatial synchrony over distances ranging from tens to hundreds of kilometers between sites. It has been suggested that this phenomenon is caused by common environmental perturbations, mobile predators or dispersal movements. However, very little focus has been given to how the physical properties of the geographic area over which synchrony occurs, such as landscape composition and climate, affect spatial population dynamics. This study reports on the spatial and temporal properties of vole population fluctuations in two areas of western Finland: one composed of large interconnected areas of agricultural farmland interspersed by forests and the other highly dominated by forest areas, containing more isolated patches of agricultural land. Furthermore, the more agricultural area exhibits somewhat milder winters with less snow than the forested area. We found the amplitude of vole cycles to be essentially the same in the two areas, suggesting that the relative amount of predation on small rodents by generalist versus specialist predators is similar in both areas. No seasonal differences in the timing of synchronization were observable for Microtus voles, whereas bank vole populations in field habitats appeared to become synchronized primarily during winter. Microtus populations in field habitats exhibited smaller spatial variation and a higher degree of synchrony in the more continuous agricultural landscape than in the forest-dominated landscape. We suggest that this inter-areal difference is due to differences in the degree of inter-patch connectivity, with predators and dispersal acting as the primary synchronizing agents. Bank vole populations in field habitats were more synchronized within the forest-dominated landscape, most likely reflecting the suitability of the inter-patch matrix and the possibility of dispersal. Our study clearly indicates that landscape composition needs to be taken into account when describing the spatial properties of small rodent population dynamics.

Cohort variation in the life-history parameters of stoatsMustela erminea in relation to fluctuating food resources: a challenge to boreal ecologists

This paper reviews field evidence suggesting that periodic temporary population irruptions of feral house miceMus musculus in New Zealand have a substantial effect on the reproductive success of stoatsMustela erminea Linnaeus, 1758. Stoats born during the summer of a peak in numbers of mice are more numerous and have higher fecundity (ovulation rate) but lower productivity (independent offspring per female) and shorter longevity than those born when mice are not abundant. This reversed silver-spoon effect is apparently correlated with intense competition for food within a much larger than usual cohort of young stoats. However, both stoats and mice are introduced in New Zealand, so it is possible that these effects are not natural. The question could be resolved by data demonstrating similar cohort effects in stoats in the northern hemisphere, living in areas with fluctuating vole populations and limited alternative prey.

Dynamic effects of predators on cyclic voles: field experimentation and model extrapolation.

Mechanisms generating the well-known 3-5 year cyclic fluctuations in densities of northern small rodents (voles and lemmings) have remained an ecological puzzle for decades. The hypothesis that these fluctuations are caused by delayed density-dependent impacts of predators was tested by replicated field experimentation in western Finland. We reduced densities of all main mammalian and avian predators through a 3 year vole cycle and compared vole abundances between four reduction and four control areas (each 2.5-3 km(2)). The reduction of predator densities increased the autumn density of voles fourfold in the low phase, accelerated the increase twofold, increased the autumn density of voles twofold in the peak phase, and retarded the initiation of decline of the vole cycle. Extrapolating these experimental results to their expected long-term dynamic effects through a demographic model produces changes from regular multiannual cycles to annual fluctuations with declining densities of specialist predators. This supports the findings of the field experiment and is in agreement with the predation hypothesis. We conclude that predators may indeed generate the cyclic population fluctuations of voles observed in northern Europe.

Population Fluctuations of Voles in North Fennoscandian Tundra: Contrasting Dynamics in Adjacent Areas with Different Habitat Composition

During 1991-1996. we studied population fluctuations of microtine rodents (primarily Clethrionomys rufocanus). of their winter food plants, and of their predators in a low arctic habitat complex, dominated by unproductive lichen dwarf-birch tundra. More productive habitats occurred patchwise throughout the landscape. On a south-facing slope, productive scrubland habitats prevailed, and luxuriant habitats were locally abundant. Our main method was live-trapping on 14 grids, representing typical lowland tundra (5 replicates), the productive slope (4 replicates) and barren high-altitude tundra (5 replicates). Within the slope vole populations were cyclic. In the lowland tundra. vole fluctuations were primarily seasonal, but the vole erash on the productive slope coincided with a phase of relatively low vole densities in the lowland. The highland was characterised by low vole densities. During the phase of rapid population growth, long-range dispersal occurred within the slope and from the slope to surrounding areas. Moreover, small mustelids which had initially been present only on the slope, started to move elsewhere, along natural dispersal corridors. Shoot mortalities of the main winter food plant. Vaccinium myrtillus. remained low. The observed scenario is consistent with the hypothesis that vole cycles represent a mustelid-microtine limit cycle, because cycles created by this mechanism should disappear when the productive habitats, capable of supporting resident predators, become fragmented and embedded in a vast unsuitable area.

Reproductive effort of female bank voles in a risky environment

In cyclically fluctuating vole populations, strong intraspecific competition and intense predation simultaneously or separately increase the costs of reproduction and so may set the framework for the optimal breeding tactic of voles. In a factorial experiment, we manipulated two factors in the breeding environment of bank vole (Clethrionomys glareolus) pairs, each with two treatment levels: no predation risk or high risk of specialist predators and low or high density of voles. In the manipulation, we used odours of conspecific voles and/or mustelid predators. Both over-wintered and young, summer-born, wild-caught bank vole females and males in breeding condition were used. Each female-male pair of voles was placed in a cage and the cages were distributed in large outdoor enclosures. All animals were fed ad libitum. Under predation risk, both old and young females suppressed breeding significantly. The density of conspecific voles did not affect overall breeding. However, there was some evidence that population density stimulated breeding of old females but suppressed breeding of young ones. Both risk factors appeared to increase litter sizes of those individuals who ‘decided’ to breed. Our results indicate that the risk of predation may be an important factor determining reproductive tactics of bank vole females. In risky environments, females seemed to choose between two totally opposite tactics: they suppressed breeding, which may increase their own survival to the next breeding event, or they continued to breed in spite of expected high survival costs. Females seemed to compensate the latter costs with a higher effort to the current and probably the last reproduction.