Experimental translocation of juvenile water voles in a Scottish lowland metapopulation

Population density affects dispersal success because residents can hinder or facilitate immigration into a new site, via a “social fence effect” or “social attraction” (or “conspecific attraction”), respectively. These mechanisms can affect the dynamics of fragmented populations and the success of translocations. However, information on the settlement behaviour of dispersers is rare. We conducted a manipulative field experiment using wild water voles, which exist in metapopulations along waterways in Scotland. We translocated 17 young of dispersal age into either an occupied site or a vacant site containing good habitat, which had recently become extinct due to a feral predator (American mink) moving through. We monitored the movements of translocated voles using radio telemetry. Translocated voles were less likely to settle in occupied sites with higher densities of residents, suggesting a possible social fence effect at high density. There was evidence of a social attraction mechanism, because voles never remained at new sites unless another individual arrived soon after translocation, and they were more likely to settle in occupied or colonised sites than vacant ones. Voles remained in the transient phase of dispersal for many days, and often followed a “stepping stone” trajectory, stopping for several days at successive sites. We suggest that trajectories followed by dispersing water voles, the time scale and long dispersal distances found in this species are conducive to locating conspecifics at low density and colonising vacant habitat. These results are encouraging for prospects of metapopulation persistence and future translocation success.     

Travelling wave dynamics and self-organization in a spatio-temporally structured population

We analyse spatial population dynamics showing that periodic or period-like chaotic dynamics produce self-organization structures, such as travelling waves. We suggest that self-organized patterns are associated with spatial synchrony patterns that often depend on geographical distance between subpopulations. The population dynamics also show statistical spatial autocorrelation patterns. We contrast our theoretical simulations with empirical data on annual damages in young sapling stands caused by voles. We conclude, on the basis of the periodicity, synchrony, and spatial autocorrelation patterns, and our simulation results, that vole dynamics represent travelling waves in population dynamics. We suggest that because such synchrony patterns are frequently observed in natural populations, spatial self-organization may be more common in population dynamics than reported in the literature.     

Multi-annual density fluctuations and habitat size enhance genetic variability in two northern voles

In order to gain a better understanding of the consequences of population density cycles and landscape structure for the genetic composition in time and space of vole populations, we analyzed the multiannual genetic structure of the two numerically dominant, sympatric small rodent species of northernmost Fennoscandia. Red voles Myodes rutilus and grey-sided voles M. rufocanus were trapped in the subarctic birch forest along three fjords over five years. Along each fjord, there were four or five altitudinal transects each with five trapping stations. Spring and fall population densities were estimated from mark–recapture data. Grey-sided voles exhibited higher amplitude density fluctuations than red voles. Polymorphism at eight or nine microsatellite loci, determined in 1228 voles, was used to estimate local genetic diversity and differentiation among samples. Genetic diversity was higher in grey-sided voles than in red voles. Spring densities had no effect on local genetic diversity or on differentiation. The amplitude of density fluctuations and the extent of favorable habitat (sub-arctic birch forest) surrounding each site had a positive effect on genetic diversity, and the amplitude of density fluctuations had a negative effect on differentiation in red voles, for which fluctuating populations were compared with more stable populations. The harmonic mean of densities, reflecting average population sizes, had a negative effect on genetic diversity in red voles, but a positive effect in grey-sided voles, for which only fluctuating populations were compared. No other effects were significant for grey-sided voles. A temporal assignment test showed that the spatial structure was more stable in time for populations with more stable population dynamics. Altogether our results suggest that high amplitude density fluctuations lead to more gene flow and higher genetic diversity in vole populations.     

围栏条件下母体密度应激对根田鼠 F1代性器官指数的影响

为探讨自然条件下母体密度应激对根田鼠F1代性器官指数的影响,通过在围栏内建立不同密度的母体种群获得相应的F1代个体,以此建立了不同密度的子代种群,并测定了母体种群建群者的血浆皮质酮含量,将建立子代种群后剩余的F1代个体带回实验室饲养.实验结束后,测定了全部F1代个体的性器官指数及皮质酮含量.结果表明,高密度母体种群建群者的皮质酮含量显著高于低密度母体种群建群者;出生于高密度母体种群的F1代个体处于高密度子代种群,其性器官指数显著低于出生于低密度母体种群且处于低密度子代种群的F1代个体,而皮质酮含量显著高于后者;出生于高密度母体种群的F1代个体处于低密度子代种群,其性器官指数及皮质酮含量与出生于低密度母体种群且处于低密度子代种群的F1代个体间无显著差异.此外,实验室饲养条件下,出生于高密度和低密度母体种群的F1代个体间的睾丸指数无显著差异.本研究结果说明,在根田鼠种群中,单一的母体密度应激对子代的性器官指数无影响,但生前应激子代在性成熟后,当再次遭遇密度应激时,其性器官指数显著降低,母体密度应激和当前环境对种群的未来繁殖能力具有耦合效应.     

Rate of population change in voles from different phases of the population cycle

Factors involved in causing cyclic vole populations to decline, and in preventing populations from recovering during the subsequent low density phase have long remained unidentified. The traditional view of self-regulation assumes that an increase in population density is prevented by a change in the quality of individuals within the population itself, but this is still inadequately tested in the field. We compared the population growth of wild field voles ( Microtus agrestis ) from the low phase (conducted in 1998) with that of voles from the increase phase (conducted in 1999) in predator-proof enclosures (each 0.5 ha) in western Finland. Within a few months, enclosed vole populations increased to high density, and the realised per capita rate of change over the breeding season did not differ between the populations from different cycle phases. This implies that the recovery of populations from the low phase was not hindered by an impoverishment in quality of individual voles. Accordingly, we suggest that population intrinsic factors (irrespective of the mechanisms they are based on) are unlikely to play a significant role in the generation of cyclic density fluctuations of voles. Instead, we discovered direct density-dependent regulation in the vole populations. Accurate estimates of population growth and the observed density dependence provide important information for empirically based models on population dynamics of rodents.     

Delayed density‐dependent onset of spring reproduction in a fluctuating population of field voles

Delayed density‐dependent demographic processes are thought to be the basis for multi‐annual cyclic fluctuations in small rodent populations, but evidence for delayed density dependence of a particular demographic trait is rare. Here, using capture–recapture data from 22 sites collected over nine years, we demonstrate a strong effect of population density with a one‐year lag on the timing of the onset of spring reproduction in a cyclically fluctuating population of field voles Microtus agrestis in northern England. The mean date for the onset of spring reproduction was delayed by about 24 days for every additional 100 voles ha^?1 in the previous spring. This delayed density dependence is sufficient to generate the type of cyclic population dynamics described in the study system.     

Population‐level manipulations of field vole densities induce subsequent changes in plant quality but no impacts on vole demography

Grazing‐induced changes in plant quality have been suggested to drive the negative delayed density dependence exhibited by many herbivore species, but little field evidence exists to support this hypothesis. We tested a key premise of the hypothesis that reciprocal feedback between vole grazing pressure and the induction of anti‐herbivore silicon defenses in grasses drives observed population cycles in a large‐scale field experiment in northern England. We repeatedly reduced population densities of field voles (Microtus agrestis) on replicated 1‐ha grassland plots at Kielder Forest, northern England, over a period of 1 year. Subsequently, we tested for the impact of past density on vole life history traits in spring, and whether these effects were driven by induced silicon defenses in the voles’ major over‐winter food, the grass Deschampsia caespitosa. After several months of density manipulation, leaf silicon concentrations diverged and averaged 22% lower on sites where vole density had been reduced, but this difference did not persist beyond the period of the density manipulations. There were no significant effects of our density manipulations on vole body mass, spring population growth rate, or mean date for the onset of spring reproduction the following year. These findings show that grazing by field voles does induce increased silicon defenses in grasses at a landscape scale. However, at the vole densities encountered, levels of plant damage appear to be below those needed to induce changes in silicon levels large and persistent enough to affect vole performance, confirming the threshold effects we have previously observed in laboratory‐based studies. Our findings do not support the plant quality hypothesis for observed vole population cycles in northern England, at least over the range of vole densities that now prevail here.     

Geographical differences in erythemally-weighted UV measured at mid-latitude USDA sites.

UV measurements from instruments maintained by USDA at 16 mid-latitude sites were analysed to investigate geographic differences. Fifteen of the sites are in North America, and one is in New Zealand. The instruments measure erythemally weighted UV radiation, and the results are presented in terms of UV Index (UVI). The focus of this work is on data from 2003, but the main results are also shown for years 2002 and 2004. In the North American sites, the peak UVI values increase by approximately 15% between latitudes 47 degrees N and 40 degrees N, and they show an increase with altitude of approximately 15% in the first kilometer, but much smaller rates of increase above that level. Peak UV intensities in the New Zealand site (45 degrees S, alt. 0.37 km) exceed those at comparable latitudes and altitudes in North America by 41 +/- 5%, and are more comparable with those over 1 km higher and 5 degrees closer to the equator. The number of observations on these days that exceeded various thresholds of UVI showed similar patterns. Furthermore, the number of days in which the peak values exceeded various thresholds also showed similar patterns, with the number of extreme values in New Zealand being anomalously high. For example, the only sites in North America where UVI exceeded 12 were at the high altitude sites in Colorado and Utah, for which there were 53 days, 6 days and 2 days respectively at the 3.2 km, 1.6 and 1.4 km sites. By contrast, the peak UVI at Lauder (0.37 km) exceeded 12 on 17 days. Lauder was the only site under 1 km altitude where the UVI exceeded 11 on a regular basis (48 days). The optical depths at Lauder were significantly lower than at all North American sites. These, together with the lower ozone amounts and the closer Earth-Sun separation in summer all contribute to the relatively high UV intensities at the New Zealand site. Other sites in New Zealand show similar increases compared with corresponding sites in North America, and the differences persist from year to year. The contrast in UV between New Zealand and North America is similar to that observed previously between New Zealand and Europe. During winter months, the UVI in New Zealand is not particularly high, giving a larger summer/winter contrast in UVI, which may be important from a health perspective.     

Effects of forage availability on growth and maturation rates in water voles

1. In populations of small mammals, food supplementation typically results in higher population densities, body weights, growth rates and reproductive rates. However, few studies have demonstrated a relationship between forage levels and demographic rates in wild populations in the absence of supplementation. 2. We examined the association of levels of available forage with individual growth rates and time to sexual maturity in eight re-introduced and three naturally occurring populations of water voles (Arvicola terrestris). 3. Range sizes were smaller at sites with higher population densities. Mean forage availability and individual growth rates covaried with range size at each site. 4. The weight at which water voles became sexually mature was 112 g for females and 115 g for males and did not vary between study sites. Differences in growth rates therefore translated into differences in the time taken to reach maturity between sites. 5. In the re-introduced populations, mean days to maturity varied inversely with mean range length. Females took 7 days (18%, range 40-47 days) longer and males 5 days (13%, range 40-45 days) longer to reach breeding condition at the sites with the shortest mean range lengths. 6. Evidence from this study suggests a possible mechanism by which increased population densities may reduce maturation rates in water voles through a reduction in mean range size, thereby limiting the availability of forage to each individual.     

Modeling spatiotemporal dynamics of vole populations in Europe and America

The mathematical models proposed and studied in the present paper provide a unified framework to understand complex dynamical patterns in vole populations in Europe and North America. We have extended the well-known model provided by Hanski and Turchin by incorporating the diffusion term and spatial heterogeneity and performed several mathematical and numerical analyses to explore the dynamics in space and time of the model. These models successfully predicted the observed rodent dynamics in these regions. An attempt has been made to bridge the gap between the field and theoretical studies carried out by Turchin and Hanski (1997) [23] and Turchin and Ellner (2000) [24]. Simulation experiments, mainly two-dimensional parameter scans, show the importance of spatial heterogeneity in order to understand the poorly understood fluctuations in population densities of voles in Fennoscandia and Northern America. This study shed new light upon the dynamics of voles in these regions. The nonlinear analysis of vole data suggests that the dynamical shift is from stability to chaos. Diffusion driven model systems predict a new type of dynamics not yet observed in the field studies of vole populations carried out so far. This has been termed as chaotic in time and regular in space (CTRS). We observed CTRS dynamics in several simulation experiments. This directs us to expect that dynamics of this animal would be de-correlated in time and simultaneously mass extinctions might be possible at many spatial locations.     

Implications of nest-site limitation on density-dependent nest predation at variable spatial scales in a cavity-nesting bird

Nest‐site limitation may have different implications in the spatial distribution of breeding pairs depending on the availability of suitable habitat and the types of nest‐sites. Distribution of cavities suitable as nest sites may allow circumstantial aggregation or active choice of colonial nesting, which may have different implications on breeding performance through effects on breeding density, with variable costs and benefits depending on the consequences of intraspecific competition, social interactions and predation. We evaluated the effects of breeding density derived from nesting site limitation on breeding performance and predation at different spatial scales and considering multiple social, population and environmental limiting factors in the red‐billed chough Pyrrhocorax pyrrhocorax. The results indicate that variable breeding density may arise within the population depending on the availability and spatial distribution of nest‐sites. Nest‐site availability and distribution may also determine social breeding systems (isolated or aggregated) at variable densities, thus resembling differences found at different spatially distant populations under contrasting environmental conditions. Breeding performance was related to density‐dependent processes of population regulation, especially density‐dependent nest predation due to predator attraction to nest clusters. Results also indicate that predation pressure depend on density patterns at large scales. This suggest that predation may have important consequences on population dynamics of spatially structured populations depending on the strength of this kind of density dependence, which in turn may depend on habitat features affecting the prey but also the spatially variable guild of predators. Because habitat and nesting site availability may vary spatially depending on multiple human influences, understanding the strength and form in which breeding density and nest predation at different spatial scales may influence the size and persistence of populations can help to manage them more adequately.     

Effects of conspecific and heterospecific residents on patterns of immigration in two species of voles

Studies with birds have shown that presence and density of resident conspecifics and heterospecifics can influence patterns of habitat selection. There have been few studies on the effects of social cues on rates of immigration in mammals. We report results from a long-term live trapping study of immigration in two species of voles, Microtus ochrogaster Wagner, 1842 and Microtus pennsylvanicus Ord, 1815, in bluegrass habitat in east-central Illinois, USA. We compare immigration into control sites from which no individuals of either species were removed with immigration into experimental sites from which either conspecifics or heterospecifics were removed. We focus on characteristics of immigrants and rates of immigration in relation to density in destination habitats. Within each species, immigrants into control and removal sites were similar with respect to body mass and reproductive condition, indicating no major differences in the physical condition of immigrants into sites with established populations and sites without established populations. For both species, density of conspecifics at a site positively influenced rate of immigration at that site. Density of heterospecifics at destination sites did not significantly influence rate of immigration for either species. These results suggest that site selection by dispersing M. ochrogaster and M. pennsylvanicus is characterized by conspecific attraction.     

Spatio-temporal patterns of habitat use in voles and shrews modified by density, season and predators

1.?Although the intrinsic habitat preferences of a species can be considered to be fixed, the realized habitat use depends on the prevailing abiotic and biotic conditions. Often the core habitats are occupied by dense and stable populations, while marginal habitats become occupied only at times of high density. In a community of interacting species, habitat uses of different species become inter-related, for example an increased density of a strong competitor forcing a weaker competitor to use more marginal habitats. 2.?We studied the spatio-temporal distribution patterns of three common small mammal species, the bank vole Myodes glareolus; the field vole Microtus agrestis; and the common shrew Sorex araneus, in a 4-year trapping study carried out on six large islands, each containing a mixture of three main habitat types (forest, field and clear-cut). We experimentally released least weasels (Mustela n. nivalis) to some of the islands to see how the focal species respond to increased predation pressure. 3.?Both vole species were largely restricted to their core habitats (bank voles to forests and field voles to fields) at times of low population density. With increasing density, the relative habitat use of both species increased in the clear-cut areas. The common shrew was a generalist in its habitat use at all population densities. 4.?The release of the weasels changed the habitat use of all study species. 5.?The vole species showed a stronger aggregated pattern than the common shrew, especially at low population density. The vole aggregations remained in the same localities between seasons, except in the case of bank voles after the weasels were released. 6.?Bank voles and field voles avoided each other at high density. 7.?We conclude that intrinsically differential habitat requirements and flexibility to modify habitat use facilitate the coexistence of the two competing vole species in mosaic landscapes consisting of boreal forests and open habitats.     

Owl winter irruptions as an indicator of small mammal population cycles in the boreal forest of eastern North America

Contrary to what is observed in Fennoscandia, it seems to be widely accepted that small mammals do not exhibit multi-annual population cycles in the boreal forest of North America. However, in the last thirty years, irruptions of vole predators such as owls have been reported by ornithologists south of the North American boreal forest. While such southerly irruptions have been associated in Fennoscandia with periods of low abundance of small mammals within their usual distribution range, their possible cyclic nature and their relationships to fluctuations in vole densities at northern latitudes has not yet been demonstrated in North America. With information collected from existing data-bases, we examined the presence of cycles in small mammals and their main avian predators by using temporal autocorrelation analyses. Winter invasions of boreal owls ( Aegolius funereus ) were periodic, with a 4-yr cycle in Québec. Populations of one species of small mammal, the red-backed vole ( Clethrionomys gapperi ), fluctuated periodically in boreal forests of Québec (north to 48°N). Boreal owls show invasion cycles which correspond to years of low density of red-backed voles, the main food item for this owl species. In addition, winter observations of northern hawk owls ( Surnia ulula ) and great gray owls ( Strix nebulosa ) south of their usual range increased in years of low density of red-backed voles. Our results suggest that a 4-yr population cycle exists in the eastern boreal forest of North America for voles and owls, which is very similar to the one observed in Fennoscandia.     

Trading populations—can biodiversity offsets effectively compensate for population losses?

Biodiversity offsetting promotes the protection or restoration of biodiversity at one site to compensate for the loss of biodiversity due to development at another site. Thus populations of species at a development site may be extirpated in the belief that offsetting elsewhere will compensate for the loss of biodiversity. In this study we tested the replaceability of roadside populations of the orchid Diuris platichila threatened by development (populations 1–5, n = 50–541 plants) with a potential offset population occurring in nearby natural vegetation (population 6, n = 143 plants). We measured differences in habitat among the populations and associated differences in flowering and fruiting. We also measured genotypic diversity within and among the populations, and the capacity of soil from each population to promote the symbiotic germination of outcrossed seed from the two largest populations (populations 1–2). An evaluation of the performance of the relevant offset policy was also undertaken which was informed by these studies. Compared to the roadside populations, the potential offset site had limited flowering (except after fire) and was genotypically less diverse. Soil from the potential offset site, and populations 2, 3 and 5, supported significantly less seed germination than soil from population 1. Translocating individuals from the most genotypically diverse populations into the offset population could help to buffer against the loss of genetic diversity if offsetting was required; however, the limited reproduction and recruitment opportunities at the offset site could result in the eventual erosion of any initial increase in diversity. The offset policy failed to secure a suitable offset for a hypothesized loss of plants. More generally we conclude that offsetting approaches which do not assess genotypic diversity and recruitment capacity may fail in their objective of protecting species and that maintaining populations in safe sites may be required.     

Effects of competition and season on survival and maturation of young bank vole females

In territorial microtines intra-specific density dependent processes can limit the maturation of individuals during the summer of their birth. This may have demographic consequences by affecting the number and the age distribution of breeding individuals in the population. Little is known about this process on a community level, though populations of many northern microtine species fluctuate in synchrony and are known to interfere socially with each other. We experimentally studied the influence of the field vole Microtus agrestis on maturation, breeding, space use and survival of weanling bank voles, Clethrionomys glareolus. Two additive competition experiments on bank vole populations were conducted in large outdoor enclosures, half of them additionally housing a field vole population. In a mid-summer experiment low population density and absence of older breeding females minimised intra-specific competition. Survival was not affected by the presence of field voles. Season had a significant effect on both the probability of maturation and breeding of the weanlings. Competition with field voles significantly delayed breeding, and coupled with seasonal effects decreased the probability of breeding. In a late-summer experiment breeding and survival of bank vole weanlings were studied for three weeks as part of a high density breeding bank vole population. Weanlings did not mature at all nor were their space use and survival affected by the presence of field voles. Our results show that competition with other species can also have an impact on breeding of immatures. In an extreme seasonal environment, even a short delay of breeding may decrease survival chances of offspring. Seasonal and competition effects together may thus limit the contribution of year born females to reproductive output of the population. Other studies have shown that adult breeding bank voles suffer lower survival in the presence of field voles, but this study showed no survival effects on the weanlings. Thus it might be beneficial for weanlings to stay immature especially in the end of the breeding season and postpone reproduction to the next breeding season if densities of competing species are high.     

Landscape geometry and travelling waves in the larch budmoth

Travelling waves in cyclic populations refer to temporal shifts in peak densities moving across space in a wave‐like fashion. The epicentre hypothesis states that peak densities begin in specific geographic foci and then spread into adjoining areas. Travelling waves have been confirmed in a number of population systems, begging questions about their causes. Herein we apply a newly developed statistical technique, wavelet phase analysis, to historical data to document that the travelling waves in larch budmoth (LBM) outbreaks arise from two epicentres, both located in areas with high concentrations of favourable habitat. We propose that the spatial arrangement of the landscape mosaic is responsible for initiating the travelling waves. We use a tri‐trophic model of LBM dynamics to demonstrate that landscape heterogeneity (specifically gradients in density of favourable habitat) alone, is capable of inducing waves from epicentres. Our study provides unique evidence of how landscape features can mould travelling waves.     

Genetic variability of New Zealand seagrass (Zostera muelleri) assessed at multiple spatial scales

We conducted RAPD analyses at multiple spatial scales to contribute to the conservation and future restoration of New Zealand's seagrass, Zostera muelleri Irmisch ex Asch. (Zosteraceae). Initially we focused on fine-scale genetic variation within two estuaries on the North Island, one on the East coast the other on the West coast. Within each estuary individuals were genetically similar, however, there was clear genetic separation between the two sites (genetic distance D = 0.2965). Genetic variation within a sampling location (m scale) was similar to that observed among sampling locations (km scale) within a site (21% and 28%, respectively) and smaller than that observed between sites (51%). We then expanded our sampling to include a further six populations distributed across almost the entire latitudinal (ca. 15°) gradient of the North and South Islands. At this scale genetic differences were closely correlated with coastal currents. There was a clear separation between North Island and South Island populations and further separation between the East and West coast populations of each Island. Sites located along the same section of coastline were more genetically similar than those from the opposite coast and other Island. Genetic similarity was highest within each of the sites, indicating a low degree of gene flow between populations. We recommend that any future restoration and conservation projects use only locally eco-sourced materials for population augmentations.     

Metapopulation processes and persistence in remnant water vole populations

We examined the spatial distribution of water vole populations in four consecutive years and investigated whether the regional population processes of extinction, recolonisation and migration influence distribution and persistence. We examined how such regional processes are influenced by spatial variation in habitat quality. In addition, we assessed the relevance of metapopulation concepts for understanding the dynamics of species that deviate from classical metapopulation assumptions and developing conservation measures for them. Populations were patchy and discrete, and the patchy distribution was not static between years. Population turnover occurred even in the absence of predatory mink, which only influenced the network of populations at the end of the study. Most populations were clustered close together in the upper tributaries. Local population persistence was predominantly influenced by population size: large populations were more persistent. Recolonisation rates were influenced by isolation and habitat quality. The isolation estimates which best explained the distribution of water vole populations incorporated straight‐line distances, suggesting water voles disperse overland. The distribution of recolonised sites indicated that dispersing voles actively selected habitat on the basis of its quality. Water voles depart from some of the assumptions made by frequently used metapopulation models. In particular there is no clear binary distinction between suitable and non‐suitable habitat. Accounting for variation in habitat quality before investigating temporal changes in population distribution allowed us to demonstrate that the key metapopulation processes were important. The significance of regional population processes relative to local population processes may have increased in declining, fragmented populations compared to pristine regional populations. We hypothesise that although mink predation is likely to eventually cause regional extinction in many areas, metapopulation processes have delayed this decline. Consequently, conservation measures should take into account mink predation rates and regional population processes, before considering aspects of habitat quality.     

Voles on small islands: effects of food limitation and alien predation

Ecosystems of three trophic levels may be bottom-up (by food-plant availability) and/or top-down (by predators) limited. Top-down control might be of greater consequence when the predation impact comes from an alien predator. We conducted a replicated two-factor experiment with field voles (Microtus agrestis) during 2004-2005 on small islands of the outer archipelago of the Baltic Sea, south-west Finland, manipulating both predation impact by introduced American mink (Mustela vison) and winter food supply. In autumn 2004, we live-trapped voles on five islands from which mink had been consistently removed, and on four islands where mink were present, and provided half of these islands with 1.8 kg oats per vole. Body mass of female voles increased as a response to supplementary food, whereas both food supplementation and mink removal increased the body mass of male voles in subsequent spring. During winter, there was a positive effect of supplementary food, but in the subsequent summer, possible positive long-term impacts of food supplementation on field voles were not detected. Mink removal appeared not to affect density estimates of field voles during the winter and summer immediately after food addition. Trapping data from 2004 to 2005 and 2007 suggested, however, that in two out of three summers densities of voles were significantly higher in the absence than in the presence of mink. We conclude that vole populations on small islands in the archipelago of the Baltic Sea are mainly bottom-up limited during winter (outside the growing season of food plants), when food availability is low, and limited by mink predation during summer which slows population growth during the reproductive season of voles.