Vole Density Research Articles

Structure and Movements of a Community of Small Mammals Along a Powerline Right-Of-Way in Subalpine Coniferous Forest

Abstract We examined diversity, density, and movements of small mammals in a subalpine coniferous forest in Colorado by comparing three sites in a 40-m-wide powerline right-of-way to three control sites in undisturbed forest. We captured 181 individuals of 10 species at powerline sites and 128 individuals of nine species at control sites. Density of southern red-backed voles (Myodes gapperi) was higher at control sites, whereas least chipmunks (Tamias minimus) had a greater density at powerline sites. At powerline sites, southern red-backed voles occupied forested habitats >30 m away from the right-of-way, whereas least chipmunks occupied habitats within the right-of-way. Following translocation across the right-of-way, North American deermice (Peromyscus maniculatus) readily crossed the right-of-way, while the right-of-way formed a barrier to southern red-backed voles. During their natural movements, southern red-backed voles exhibited significant directional movements parallel to the right-of-way, where...

Spatiotemporal dynamics of Puumala hantavirus in suburban reservoir rodent populations

The transmission of pathogens to susceptible hosts is dependent on the vector population dynamics. In Europe, bank voles (Myodes glareolus) carry Puumala hantavirus, which causes nephropathia epidemica (NE) in humans. Fluctuations in bank vole populations and epidemics in humans are correlated but the main factors influencing this relationship remain unclear. In Belgium, more NE cases are reported in spring than in autumn. There is also a higher incidence of human infections during years of large vole populations. This study aimed to better understand the link between virus prevalence in the vector, vole demography, habitat quality, and human infections. Three rodent populations in different habitats bordering Brussels city, Belgium, were studied for two years. The seroprevalence in voles was influenced first by season (higher in spring), then by vole density, vole weight (a proxy for age), and capture site but not by year or sex. Moreover, voles with large maximal distance between two captures had a high probability for Puumala seropositivity. Additionally, the local vole density showed similar temporal variations as the number of NE cases in Belgium. These results showed that, while season was the main factor influencing vole seroprevalence, it was not sufficient to explain human risks. Indeed, vole density and weight, as well as the local habitat, were essential to understanding the interactions in these host-pathogen dynamics. This can, in turn, be of importance for assessing the human risks.

Factors Influencing Emergence of Tularemia, Hungary, 1984–2010

To the Editor: Francisella tularensis, the etiologic agent of tularemia, is a highly infectious zoonotic agent. F. tularensis subsp. holarctica (type B) is found throughout the Northern Hemisphere and is the only subspecies found in Europe (1). Lagomorphs and rodents probably serve as the primary mammalian reservoir hosts, and hematophagous arthropods, such as ticks, play a role as vectors and hosts (2,3). Although F. tularensis is a potential agent of biological warfare and several emergences and reemergences of tularemia have been reported around the world (1,4), the epizootiology of the disease is only partially understood. The aim of our study was to analyze factors that influence the emergence of tularemia in Hungary. The study area (15,475 km2) included 3 counties in eastern Hungary. The analyzed data represented a period of 25 years, March 1984–February 2010. Annual F. tularensis–specific seroprevalence data for the European brown hare (Lepus europaeus) population were obtained by slide agglutination testing during the winter (December and January) screening of 2,500–25,000 animals (Technical Appendix). Population density data (animals/km2) for hares were based on February line transect counts and were obtained from the Hungarian Game Management database (www.vvt.gau.hu/vadgazdalkodasi_statisztikak.htm). Common vole (Microtus arvalis) densities (calculated from the number of active burrows/hectare during November) for 1996–2010 were obtained from the Central Agriculture Office, Budapest, Hungary. Vole density was scaled from 0 (absent) to 10 (peak population). The annual number of tularemia cases in humans (based on clinical history and tube agglutination test results) was obtained from the National Center for Epidemiology, Budapest. The data were regrouped according to the yearly biologic cycle (March–February) for hares and voles (Figure), and relationships between these yearly data were quantified by using the Spearman rank correlation coefficient (5) at county and regional levels. A 2–3 year cycle was characteristic for the analyzed data. A significant positive correlation was found among the number of tularemia cases in humans and the seroprevalence of F. tularensis among European brown hares (Spearman ρ = 0.73; p<0.0001) and the population density of common voles (Spearman ρ = 0.77; p = 0.0081). A significant negative correlation was found between the population density of hares and the seroprevalence of F. tularensis in hares (Spearman ρ = −0.41; p = 0.0365). Figure Correlation between the seroprevalence of Francisella tularensis in the European brown hare (Lepus europaeus) population, the population density of European brown hares and common voles (Microtus arvalis), and the number of tularemia cases in humans eastern ... The comprehensive and long-term annual data used in this study provide clues regarding the factors shaping the intraannual epizootiology and emergence or reemergence of tularemia. The European brown hare is moderately sensitive to F. tularensis subsp. holarctica. The hares produce a heterogeneous response to infection, which means that some die of overwhelming bacteremia and others survive with a protracted course of infection, thereby contributing to the maintenance of tularemia over longer periods and serving as useful sentinels of disease activity. Other studies have concluded that hares, together with infected ticks, may serve as disease reservoirs between epizootics (2,3,6,7). However, we instead hypothesize that the 2–3 year cycling feature of the population dynamics for the common vole (2) determines the ecology of F. tularensis subsp. holarctica in eastern Hungary. The common vole is highly susceptible to F. tularensis subsp. holarctica (3,8). When population densities among voles are high, F. tularensis disease transmission and spillover to hares may be facilitated by stress-related aggression, cannibalism, and F. tularensis contamination of the environment by infectious body discharges (2). Enhanced transmission and spillover can expand local outbreaks to epizootic proportions, and infected hares may, in turn, further enhance the spread of disease through bacterial shedding in urine (6,7). The disease in hares often results in septicemia and death (7), thus decreasing the population density of these animals. Hares and especially voles are also hosts for different stages of several tick species (2,6), so it can be expected, that higher numbers of infected rodents and lagomorphs result in an increased proportion of infected ticks and, thus, increased transmission of F. tularensis subsp. holarctica. It can be concluded that a higher number of infection sources in the environment results in elevated numbers of cases in humans, mainly through the handling and skinning of hares, but also through tick bites and, potentially, the inhalation of infectious aerosols originating from, for example, hay or grain. Technical Appendix: Detailed data of the annual Francisella tularensis–specific seroprevalence in the European brown hare (Lepus europaeus) population, the annual population density of European brown hares and common voles (Microtus arvalis), and the annual number of tularemia cases in humans, Hungary, 1984–2010. Data are for 3 counties in eastern Hungary: Bekes, Csongrad, and Jasz-Nagykun-Szolnok)*. Click here to view.(47K, xls)

To breed, or not to breed? Predation risk induces breeding suppression in common voles

Breeding suppression hypothesis (BSH) predicts that, in several vole species, females will suppress breeding in response to high risk of mustelid predation; compared to breeding females, suppressing females would gain higher chances of survival. Seminal evidence for BSH was obtained in the laboratory, but attempts to replicate breeding suppression under field conditions were less conclusive. We tested whether breeding suppression occurs in common voles (Microtus arvalis), and how population density and predation risk combined affect voles' reproductive activity. We found that, in contrast to males, female common voles suppress reproductive activity when faced with high predation risk. Population size was not reduced despite breeding suppression. A model of the interaction between predation risk and population density revealed that predator-induced breeding suppression depends on the density of conspecifics. We concluded that breeding suppression is a viable adaptation only at low vole densities, when per capita predation risk is high. Finally, we identified the key issues of experimental design required for the consistency of future studies on breeding suppression.

Predation risk and food: opposite effects on overwintering survival and onset of breeding in a boreal rodent

1. In seasonal environments, optimal onset of breeding and survival plays major roles in individual fitness. Many physiological and behavioural factors related to breeding increase the risk of predation; thus, breeding decisions should be based on current risks and benefits. According to life-history theory, if current predation risk is high and breeding itself increases the risk, it may be beneficial to postpone breeding. 2. During winter in northern hemispheres, food availability is limited and is at its lowest just prior to the onset of breeding in spring. Food constraint may lead to poor condition and reduced ability to start breeding. 3. We studied the effects of food and predation risk on winter survival and onset of breeding in a common boreal rodent, the bank vole (Myodes glareolus). In a 2 × 2 factorial experiment, we manipulated food availability (food supplemented or not) and predation risk (presence/absence of predator odour) in 20 large outdoor enclosures in central Finland. 4. Survival probabilities were highest in no predation risk treatments, whereas they were lowest in the predator risk treatment. The same trend was observed in vole densities and the weight change in individuals. Voles with food addition bred earlier than in the other treatments. 5. We conclude that during energy constrained harsh conditions in winter, predation risk causes behavioural changes throughout the winter and has significant negative survival and fitness effects for small mammals, reflected as delay in the start of breeding.

Vole and lemming activity observed from space

In northern ecosystems, vole and lemming densities vary between years in a regular pattern known as vole and lemming cycles. This study shows that the rodents drive corresponding cycles in vegetation that can be detected from space. The findings should help understand how climate warming will affect tundra ecosystems.

Avian predators as a biological control system of common vole (Microtus arvalis) populations in north‐western Spain: experimental set‐up and preliminary results

Ecologically based rodent pest management using biological control has never been evaluated for vole plagues in Europe, although it has been successfully tested in other systems. The authors report on the first large-scale replicated experiment to study the usefulness of nest-box installation for increasing the breeding density of common kestrels (Falco tinnunculus) and barn owls (Tyto alba) as a potential biological control of common vole (Microtus arvalis) abundance in agricultural habitats in north-western Spain. The results show that: (1) population density of both predator species increased in response to both nest-site availability and vole density; (2) voles are a major prey for the common kestrels during the breeding period; (3) vole density during the increase phase of a population cycle may be reduced in crop fields near nest boxes. The installation of nest boxes provides nesting sites for barn owls and kestrels. Kestrel populations increased faster than in areas without artificial nests, and the common vole was one of their main prey during the breeding season. The results suggest that local (field) effects could be found in terms of reduced vole density. If so, this could be an environmentally friendly and cheap vole control technique to be considered on a larger scale.

Integration of vole management in boreal silvicultural practices

Voles of the genera Microtus and Myodes are widespread and among the most abundant of small mammal species in the boreal zone of the Northern Hemisphere. They are keystone herbivore species in northern ecosystems, and they have profound impacts on both higher and lower trophic levels. Voles are also major silvicultural pests, damaging millions of tree seedlings in years of peak abundance. Prevention of vole damage to silviculture has proven to be very difficult owing to the ubiquity of both suitable vole habitat and potential damage sites across landscapes. The degree of damage inflicted by voles on seedling stands is largely, but not solely, determined by prevailing vole densities, which often fluctuate in 3-4 year population cycles. Silvicultural practices related to site habitat manipulation and/or choice and rearing of seedling material may also greatly influence the severity of vole damage to seedlings. The manipulation of these practices is currently at the forefront of methods potentially applicable to control vole damage in boreal forests. This paper reviews current evidence for the efficacy and present recommendations for further development and application of these methods to mitigate vole damage to seedling stands in boreal silviculture.

Rodent damage to natural and replanted mountain forest regeneration.

Impact of small rodents on mountain forest regeneration was studied in National Nature Reserve in the Beskydy Mountains (Czech Republic). A considerable amount of bark damage was found on young trees (20%) in spring after the peak abundance of field voles (Microtus agrestis) in combination with long winter with heavy snowfall. In contrast, little damage to young trees was noted under high densities of bank voles (Myodes glareolus) with a lower snow cover the following winter. The bark of deciduous trees was more attractive to voles (22% damaged) than conifers (8%). Young trees growing in open and grassy localities suffered more damage from voles than those under canopy of forest stands (χ 2 = 44.04, P < 0.001). Natural regeneration in Nature Reserve was less damaged compared to planted trees (χ 2 = 55.89, P < 0.001). The main factors influencing the impact of rodent species on tree regeneration were open, grassy habitat conditions, higher abundance of vole species, tree species preferences- and snow-cover condition. Under these conditions, the impact of rodents on forest regeneration can be predicted. Foresters should prefer natural regeneration to the artificial plantings.

Guild composition and habitat use of voles in 2 forest landscapes in south‐eastern Norway

It is widely believed that intensive forestry has influenced small mammal population dynamics, and thereby the entire mammalian community in Fennoscandian boreal forests. The nature of these impacts on the different species is subject to debate. We live-trapped voles between 2006 and 2009 in 2 commercially harvested forests in south-eastern Norway. We investigated the variation in vole abundance among habitat types (e.g. mature forest and clear-cut) and the hypothesis that graminivorous species such as field voles (Microtus agrestis L.) benefit from clear-cuts at the expense of forest dwellers (i.e. the bank vole, Myodes glareolus Schreb.), using fine-scale descriptors of the ground vegetation. We could not find support for the hypothesis that field voles show a preference for clear-cuts, and their overall abundance was low, while bank voles were the dominant species in all habitat types, including clear-cuts in the peak and pre-peak years. We found a positive association between bank vole abundance and bilberry (Vaccinium myrtillus L.) availability rather than a specific habitat type. Low field vole density in clear-cuts might be due to variation in local productivity and ground vegetation as well as to large variation in the species temporal dynamics. The latter is particularly associated with the widespread decline of field voles in Scandinavia. Logging has the potential to negatively affect bank vole population dynamics because of the negative effect on bilberry development.

Synchronous population fluctuations of forest and field voles: implications for population management

High population densities of field and forest voles cause economic losses by consuming crop or slowing down forest regeneration by damaging the bark of young trees. Consequently, programs to monitor abundances are often implemented as part of population management. Whereas abundances of the common vole (Microtus arvalis) in the Czech Republic has regularly been checked by the State Phytosanitary Administration for decades, no monitoring program has so far been invented to monitor forest voles. Because population numbers of different vole taxa are often observed to fluctuate in phase, we explored the possibility whether the monitoring data for the common vole can be used to forecast numbers of forest voles. Since 2002, we have monitored by snap-trapping bank vole (Myodes glareolus) populations in three forests situated in southern Moravia, Czech Republic. Correlation analysis of time series of yearly population changes for the common and bank vole revealed that populations of field and forest voles in southern Moravia fluctuate in a close synchrony, the correlations being consistently higher than 0.8. This result provides the reasonable possibility of exploiting the data from the extensive common vole population monitoring program for prediction in population management of forest voles.

Synergistic influences of phase, density, and climatic variation on the dynamics of fluctuating populations

Although ecologists have long recognized that certain mammalian species exhibit high-amplitude, often multiannual, fluctuations in abundance, their causes have remained poorly understood and the subject of intense debate. A key contention has been the relative role of density-dependent and density-independent processes in governing population dynamics. We applied capture-mark-recapture analysis to 25 years of monthly trapping data from a fluctuating prairie vole Microtus ochrogaster population in Illinois, USA, to estimate realized population growth rates and associated vital rates (survival and recruitment) and modeled them as a function of vole density and density-independent climatic variation. We also tested for phase dependence and seasonality in the effects of the above processes. Variation in the realized population growth rate was best explained by phase-specific changes in vole density lagged by one month and mean monthly temperatures with no time lags. The underlying vital rates, survival and recruitment, were influenced by the additive and interactive effects of phase, vole density, and mean monthly temperatures. Our results are consistent with the observation that large-scale population fluctuations are characterized by phase-specific changes in demographic and physiological characteristics. Our findings also support the growing realization that the interaction between climatic variables and density-dependent factors may be a widespread phenomenon, and they suggest that the direction and magnitude of such interactive effects may be phase specific. We conclude that density-dependent and density-independent climatic variables work in tandem during each phase of density fluctuations to drive the dynamics of fluctuating populations.

How predation and landscape fragmentation affect vole population dynamics.

BackgroundMicrotine species in Fennoscandia display a distinct north-south gradient from regular cycles to stable populations. The gradient has often been attributed to changes in the interactions between microtines and their predators. Although the spatial structure of the environment is known to influence predator-prey dynamics of a wide range of species, it has scarcely been considered in relation to the Fennoscandian gradient. Furthermore, the length of microtine breeding season also displays a north-south gradient. However, little consideration has been given to its role in shaping or generating population cycles. Because these factors covary along the gradient it is difficult to distinguish their effects experimentally in the field. The distinction is here attempted using realistic agent-based modelling.Methodology/Principal FindingsBy using a spatially explicit computer simulation model based on behavioural and ecological data from the field vole (Microtus agrestis), we generated a number of repeated time series of vole densities whose mean population size and amplitude were measured. Subsequently, these time series were subjected to statistical autoregressive modelling, to investigate the effects on vole population dynamics of making predators more specialised, of altering the breeding season, and increasing the level of habitat fragmentation. We found that fragmentation as well as the presence of specialist predators are necessary for the occurrence of population cycles. Habitat fragmentation and predator assembly jointly determined cycle length and amplitude. Length of vole breeding season had little impact on the oscillations.SignificanceThere is good agreement between our results and the experimental work from Fennoscandia, but our results allow distinction of causation that is hard to unravel in field experiments. We hope our results will help understand the reasons for cycle gradients observed in other areas. Our results clearly demonstrate the importance of landscape fragmentation for population cycling and we recommend that the degree of fragmentation be more fully considered in future analyses of vole dynamics.

Breeding state and season affect interspecific interaction types: indirect resource competition and direct interference

Indirect resource competition and interference are widely occurring mechanisms of interspecific interactions. We have studied the seasonal expression of these two interaction types within a two-species, boreal small mammal system. Seasons differ by resource availability, individual breeding state and intraspecific social system. Live-trapping methods were used to monitor space use and reproduction in 14 experimental populations of bank voles Myodes glareolus in large outdoor enclosures with and without a dominant competitor, the field vole Microtus agrestis. We further compared vole behaviour using staged dyadic encounters in neutral arenas in both seasons. Survival of the non-breeding overwintering bank voles was not affected by competition. In the spring, the numbers of male bank voles, but not of females, were reduced significantly in the competition populations. Bank vole home ranges expanded with vole density in the presence of competitors, indicating food limitation. A comparison of behaviour between seasons based on an analysis of similarity revealed an avoidance of costly aggression against opponents, independent of species. Interactions were more aggressive during the summer than during the winter, and heterospecific encounters were more aggressive than conspecific encounters. Based on these results, we suggest that interaction types and their respective mechanisms are not either–or categories and may change over the seasons. During the winter, energy constraints and thermoregulatory needs decrease direct aggression, but food constraints increase indirect resource competition. Direct interference appears in the summer, probably triggered by each individual’s reproductive and hormonal state and the defence of offspring against conspecific and heterospecific intruders. Both interaction forms overlap in the spring, possibly contributing to spring declines in the numbers of subordinate species.

Natal conditions alter age-specific reproduction but not survival or senescence in a long-lived bird of prey

1. Natal conditions and senescence are two major factors shaping life-history traits of wild animals. However, such factors have rarely been investigated together, and it remains largely unknown whether they interact to affect age-specific performance. 2. We used 27 years of longitudinal data collected on tawny owls with estimates of prey density (field voles) from Kielder Forest (UK) to investigate how prey density at birth affects ageing patterns in reproduction and survival. 3. Natal conditions experienced by tawny owls, measured in terms of vole density, dramatically varied among cohorts and explained 87% of the deviance in first-year apparent survival (annual estimates ranging from 0·07 to 0·33). 4. We found evidence for senescence in survival for females as well as for males. Model-averaged estimates showed that adult survival probability declined linearly with age for females from age 1. In contrast, male survival probability, lower on average than for female, declined after a plateau at age 1-3. 5. We also found evidence for reproductive senescence (number of offspring). For females, reproductive performance increased until age 9 then declined. Males showed an earlier decline in reproductive performance with an onset of senescence at age 3. 6. Long-lasting effects of natal environmental conditions were sex specific. Female reproductive performance was substantially related to natal conditions (difference of 0·24 fledgling per breeding event between females born in the first or third quartile of vole density), whereas male performance was not. We found no evidence for tawny owls born in years with low prey density having accelerated rates of senescence. 7. Our results, combined with previous findings, suggest the way natal environmental conditions affect senescence varies not only across species but also within species according to gender and the demographic trait considered.

Determinants of lemming outbreaks

Population outbreaks in tundra rodents have intrigued scientists for a century as a result of their spectacular appearances and their general lessons in ecology. One outstanding question that has led to competing hypotheses is why sympatric lemmings and voles differ in regularity and shape of their outbreaks. Lemming outbreaks may be lost for decades while vole populations maintain regular population cycles. Moreover, when lemming populations eventually irrupt, they do so more steeply than the vole populations. Norwegian lemmings exhibited a large-scale outbreak synchronously with gray-sided voles in Finnmark, northern Fennoscandia, during 2006 to 2007 for the first time in two decades. Analyses of spatial variability of this outbreak across altitudinal gradients allowed us to identify determinants of the contrasting lemming and vole dynamics. The steeper lemming outbreak trajectories were caused by breeding and population growth during winter, when nonbreeding vole populations consistently declined. The differently shaped lemming and vole outbreaks appear to result from a particular demographic tactic of lemmings that evolved as an adaptation to the long and cold Arctic-Alpine winters. The lemming outbreak amplitude increased with altitude and vole density, indicating that lemming outbreaks are jointly facilitated by low temperatures and apparent mutualism with voles mediated by shared predators. High sensitivity to variation in climate and predation is likely to be the reasons why lemmings have more erratic population dynamics than sympatric voles. The combination of continued climatic warming and dampened vole cycles is expected to further decrease the frequency, amplitude, and geographic range of lemming outbreaks in tundra ecosystems.

Climate change in Norway: warm summers limit grouse reproduction

AbstractGrouse and vole numbers may peak after peaks in the seed crop of bilberry (Vaccinium myrtillus) because of reduced levels of feeding deterrents in bilberry plants. We predicted that grouse reproduction depends also on summer (June–September) temperatures in the 2 previous years, because bilberry plants will be less exhausted after a high seed crop in or after warm summers, and thus rebuild their chemical defence more quickly. After berry peak years, population indices of capercaillie (Tetrao urogallus) and bank vole (Myodes glareolus) in southern Norway were negatively related to summer temperatures in the previous year or previous 2 years. Willow grouse (Lagopus lagopus) chick production in five areas in Norway was negatively related to summer temperatures in the 2 previous years when controlling for vole density. A similar pattern was found for the bilberry‐feeding moth (Eulithis populata), an important prey for grouse chicks. In eastern Norway, autumn densities of capercaillie and black grouse (Tetrao tetrix) were more likely to peak in vole peak years at high altitudes, where summer temperatures are low. We conclude that high summer temperatures may limit grouse reproduction through the effect on bilberry plants and that a warm climate thus adversely affects population levels of grouse.

An appraisal of the fitness consequences of forest disturbance for wildlife using habitat selection theory

Isodar theory can help to unveil the fitness consequences of habitat disturbance for wildlife through an evaluation of adaptive habitat selection using patterns of animal abundance in adjacent habitats. By incorporating measures of disturbance intensity or variations in resource availability into fitness-density functions, we can evaluate the functional form of isodars expected under different disturbance-fitness relationships. Using this framework, we investigated how a gradient of forest harvesting disturbance and differences in resource availability influenced habitat quality for snowshoe hares (Lepus americanus) and red-backed voles (Myodes gapperi) using pairs of logged and uncut boreal forest. Isodars for both species had positive intercepts, indicating reductions to maximum potential fitness in logged stands. Habitat selection by hares depended on both conspecific density and differences in canopy cover between harvested and uncut stands. Fitness-density curves for hares in logged stands were predicted to shift from diverging to converging with those in uncut forest across a gradient of high to low disturbance intensity. Selection for uncut forests thus became less pronounced with increasing population size at low levels of logging disturbance. Voles responded to differences in moss cover between habitats which reflected moisture availability. Lower moss cover in harvested stands either reduced maximum potential fitness or increased the relative rate of decline in fitness with density. Differences in vole densities between harvested and uncut stands were predicted, however, to diminish as populations increased. Our findings underscore the importance of accounting for density-dependent behaviors when evaluating how changing habitat conditions influence animal distribution.

Pulsed resources affect the timing of first breeding and lifetime reproductive success of tawny owls

1. According to life-history theory, environmental variability and costs of reproduction account for the prevalence of delayed reproduction in many taxa. Empirical estimates of the fitness consequences of different ages at first breeding in a variable environment are few however such that the contributions of environmental and individual variability remains poorly known. 2. Our objectives were to elucidate processes that underpin variation in delayed reproduction and to assess lifetime consequences of the age of first breeding in a site-faithful predator, the tawny owl Strix aluco L. subjected to fluctuating selection linked to cyclical variation in vole density (typically 3-year cycles with low, increasing and decreasing vole densities in successive years). 3. A multistate capture-recapture model revealed that owl cohorts had strikingly different juvenile survival prospects, with estimates ranging from 0.08 to 0.33 respectively for birds born in Decrease and Increase phases of the vole cycle. This resulted in a highly skewed population structure with >75% of local recruits being reared during Increase years. In contrast, adult survival remained constant throughout a vole cycle. The probability of commencing reproduction was lower at age 1 than at older ages, and especially so for females. From age 2 onwards, pre-breeders had high probabilities of entering the breeding population. 4. Variation in lifetime reproductive success was driven by the phase of the vole cycle in which female owls started their breeding career (26-47% of variance explained, whether based on the number of local recruits or fledglings), more than by age at first breeding or by conditions experienced at birth. Females who postponed reproduction to breed for the first time at age 3 during an Increase phase, produced more recruits, even when accounting for birds that may have died before reproduction. No such effects were detected for males. 5. Sex-specific costs of early reproduction may have accounted for females being more prone to delay reproduction. Contrary to expectations from a best-of-a-bad job strategy, early-hatched, hence potentially higher-quality females were more likely to breed at age 1, but then experienced rapidly declining food resources and so seemed caught in a life-history trap set by the multiannual vole cycle.

Barriers, Traps and Predators – An Integrated Approach to Avoid Vole Damage

Author(s): Fuelling, Olaf; Walther, Bernd; Nentwig, Wolfgang; Airoldi, Jean-Pierre | Abstract: Voles (Microtus spp. and Arvicola spp.) are the most abundant rodent species in open European landscapes. Due to their preference for agricultural habitats and their enormous reproductive potential, they are often regarded as pests. Several attempts have been made to reduce vole densities using rodenticide baits, gasses, or traps. No matter what method is used, they all bear the problem of voles reinvading from adjacent habitats. We tested a combination of fence and a new kind of trap that enabled terrestrial vole predators to take the captured prey. The combination of fence and traps was tested in a field trial and compared with mesh wire fences and an obstacle-free control line. This three-line setup was installed at each of three locations in Switzerland. Movement patterns of terrestrial predators were recorded by video observation during a 2-year period. Within this period, we made 1,224 observations of mammals; about ¾ of them, 951, were terrestrial predators and 157 were others like hares or hedgehogs. Due to their smaller size, only 116 voles were observed during the same time. Fences with traps were clearly preferred by predators, and their activity was significantly higher along fences with traps, as compared to the two other types. Predators seemed to check traps actively as they moved significantly more often close and parallel to fences with traps. Voles were not only removed from the traps but also caught while moving in the vicinity of the fences. The observed vole movements were ended by a predator in 11% of all instances. We found that barriers with additional trapping devices attracted predators, which started to patrol these fences regularly. Such a physical barrier in combination with its natural guard is a suitable device to protect high-value crops like orchards. As an integrated tool, it will reduce efforts, costs, and environmental impacts of vole control measures. Subsequent to this study, a new Hshaped double wall fence was constructed. This new type gives additional benefits, as it is easier to maintain and allows both terrestrial and avian predators to take captured voles.