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.     

Density-dependent variation in body mass of voles

We studied inter-annual, spatial and sexual variation in the body mass of bank volesMyodes glareolus Schreber, 1780 and grey-sided volesMyodes rufocanus Sundevall, 1846 using live trappings from two grids on the southand north-facing slopes of a mountain valley in Southern Norway. Variation in spring density of the four populations was consistent with cyclic dynamics (n=7,s-values >0.5). Individuals caught on the south-facing slope were larger than those caught on the north-facing slope. Reproductively mature bank vole males were smaller than females, whereas reproductively mature grey-sided vole males were larger than females. Body mass was related to density in both species. In bank voles, we found a direct positive density dependence caused by a higher rate of survival at higher densities resulting from individual allocation of resources from reproduction to survival and growth. In grey-sided voles, we found a negative delayed density dependence resulting from grazing on preferred plants that determined the resource available for individual vole growth the following year.     

Density-dependent reproduction causes winter crashes in a common vole population

Common voles in western France exhibit three-year population cycles with winter crashes after large outbreaks. During the winter of 2011–2012, we monitored survival, reproduction, recruitment and population growth rate of common voles at different densities (from low to outbreak densities) in natura to better understand density dependence of demographic parameters. Between October and April, the number of animals decreased irrespective of initial density. However, the decline was more pronounced when October density was higher (loss of ≈54 % of individuals at low density and 95 % at high density). Using capture-mark-recapture models with Pradel's temporal symmetry approach, we found a negative effect of density on recruitment and reproduction. In contrast, density had a slightly positive effect on survival indicating that mortality did not drive the steeper declines in animal numbers at high density. We discuss these results in a population cycle framework, and suggest that crashes after outbreaks could reflect negative effects of density dependence on reproduction rather than changes in mortality rates.     

Strong “bottom‐up” influences on small mammal populations: State‐space model analyses from long‐term studies

“Bottom‐up” influences, that is, masting, plus population density and climate, commonly influence woodland rodent demography. However, “top‐down” influences (predation) also intervene. Here, we assess the impacts of masting, climate, and density on rodent populations placed in the context of what is known about “top‐down” influences. To explain between‐year variations in bank vole Myodes glareolus and wood mouse Apodemus sylvaticus population demography, we applied a state‐space model to 33 years of catch‐mark‐release live‐trapping, winter temperature, and precise mast‐collection data. Experimental mast additions aided interpretation. Rodent numbers in European ash Fraxinus excelsior woodland were estimated (May/June, November/December). December–March mean minimum daily temperature represented winter severity. Total marked adult mice/voles (and juveniles in May/June) provided density indices validated against a model‐generated population estimate; this allowed estimation of the structure of a time‐series model and the demographic impacts of the climatic/biological variables. During two winters of insignificant fruit‐fall, 6.79 g/m² sterilized ash seed (as fruit) was distributed over an equivalent woodland similarly live‐trapped. September–March fruit‐fall strongly increased bank vole spring reproductive rate and winter and summer population growth rates; colder winters weakly reduced winter population growth. September–March fruit‐fall and warmer winters marginally increased wood mouse spring reproductive rate and September–December fruit‐fall weakly elevated summer population growth. Density dependence significantly reduced both species' population growth. Fruit‐fall impacts on demography still appeared after a year. Experimental ash fruit addition confirmed its positive influence on bank vole winter population growth with probable moderation by colder temperatures. The models show the strong impact of masting as a “bottom‐up” influence on rodent demography, emphasizing independent masting and weather influences; delayed effects of masting; and the importance of density dependence and its interaction with masting. We conclude that these rodents show strong “bottom‐up” and density‐dependent influences on demography moderated by winter temperature. “Top‐down” influences appear weak and need further investigation.     

Delayed density-dependent season length alone can lead to rodent population cycles

Studies of cyclic microtine populations (voles and lemmings) have suggested a relationship between the previous year's population density and the subsequent timing of the onset of reproduction by overwintered breeding females. No studies have explored the importance of this relationship in the generation of population cycles. Here we mathematically examine the implications of variation in reproductive season length caused by delayed density-dependent changes in its start date. We demonstrate that when reproductive season length is a function of past population densities, it is possible to get realistic population cycles without invoking any changes in birth rates or survival. When parameterized for field voles (Microtus agrestis) in Kielder Forest (northern England), our most realistic model predicts population cycles of similar periodicity to the Kielder populations. Our study highlights the potential importance of density-dependent reproductive timing in microtine population cycles and calls for investigations into the mechanism(s) underlying this phenomenon.     

Changes in population structure and reproduction during a 3-yr population cycle of voles

Cyclic changes in population growth rate are caused by changes in survival and/or reproductive rate. To find out whether cyclic changes in reproduction are an important part of the mechanism causing cyclic fluctuations in small mammal populations, we studied changes in the population structure and reproduction of field voles ( Microtus agrestis ), sibling voles ( M. rossiaemeridionalis ), bank voles ( Clethrionomys glareolus ), and common shrews ( Sorex araneus ) in western Finland during 1984–1992, in an area with 3-yr vole cycles. We also modelled the population growth of voles using parameter values from this study. The animals studied were collected by snap trapping in April, May, June, August, September, and, during 1986–1990, also in October. We found several phase-related differences in the population structure (age structure, sex ratio, proportion of mature individuals) and reproduction (litter size, length of the breeding season) of voles. In non-cyclic common shrews, the only significant phase-related difference was a lower proportion of overwintered individuals in the increase phase. According to the analyses and the vole model, phase-related changes in litter size had only a minor impact on population growth rate. The same was true for winter breeding in the increase phase. The length and intensity of the summer breeding season had an effect on yearly population growth but this impact was relatively weak compared to the effect of cyclic changes in survival. The population increase rates of Microtus were delayed dependent on density (8–12-month time lag). Our results indicate that cyclic changes in reproduction are not an important part of the mechanism driving cyclic fluctuations in vole populations. Low survival of young individuals appeared to play an important role in the shift from the peak to the decline phase in late summer and early autumn.     

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.     

Cyclic voles and shrews and non-cyclic mice in a marginal grassland within European temperate forest

Cyclic population dynamics of small mammals are not restricted to the boreal and arctic zones of Eurasia and North America, but long-term data series from lower latitudes are still less common. We demonstrated here the presence of periodic oscillations in small mammal populations in eastern Poland using 22-year (1986–2007) trapping data from marginal meadow and river valley grasslands located in the extensive temperate woodland of Białowieża Primeval Forest. The two most common species inhabiting meadows and river valleys, root vole Microtus oeconomus and common shrew Sorex araneus, exhibited synchronous periodic changes, characterised by a 3-year time lag as indicated by an autocorrelation function. Moreover, the cycles of these two species were synchronous within both habitats. Population dynamics of the striped field mouse Apodemus agrarius was not cyclic. However, this species regularly reached maximum density 1 year before the synchronized peak of root voles and common shrews, which may suggest the existence of interspecific competition. Dynamics of all three species was dominated by direct density-dependent process, whereas delayed density dependent feedback was significant only in the root vole and common shrew. Climatic factors acting in winter and spring (affecting mainly survival and initial reproduction rates) were more important than those acting in summer and autumn and affected significantly only the common shrew. High temperatures in winter and spring had positive effects on autumn-to-autumn changes in abundance of this species, whereas deep snow in combination with high rainfall in spring negatively affected population increase rates in common shrew.     

Dampening of population cycles in voles affects small mammal community structure,decreases diversity,and increases prevalence of a zoonotic disease

Long‐term decline and depression of density in cyclic small rodents is a recent widespread phenomenon. These observed changes at the population level might have cascading effects at the ecosystem level. Here, we assessed relationships between changing boreal landscapes and biodiversity changes of small mammal communities. We also inferred potential effects of observed community changes for increased transmission risk of Puumala virus (PUUV) spread, causing the zoonotic disease nephropatica epidemica in humans. Analyses were based on long‐term (1971–2013) monitoring data of shrews and voles representing 58 time series in northern Sweden. We calculated richness, diversity, and evenness at alpha, beta, and gamma level, partitioned beta diversity into turnover (species replacement) and nestedness (species addition/removal), used similarity percentages (SIMPER) analysis to assess community structure, and calculated the cumulated number of PUUV‐infected bank voles and average PUUV prevalence (percentage of infected bank voles) per vole cycle. Alpha, beta, and gamma richness and diversity of voles, but not shrews, showed long‐term trends that varied spatially. The observed patterns were associated with an increase in community contribution of bank vole (Myodes glareolus), a decrease of gray‐sided vole (M. rufocanus) and field vole (Microtus agrestis) and a hump‐shaped variation in contribution of common shrew (Sorex araneus). Long‐term biodiversity changes were largely related to changes in forest landscape structure. Number of PUUV‐infected bank voles in spring was negatively related to beta and gamma diversity, and positively related to turnover of shrews (replaced by voles) and to community contribution of bank voles. The latter was also positively related to average PUUV prevalence in spring. We showed that long‐term changes in the boreal landscape contributed to explain the decrease in biodiversity and the change in structure of small mammal communities. In addition, our results suggest decrease in small mammal diversity to have knock‐on effects on dynamics of infectious diseases among small mammals with potential implications for disease transmission to humans.     

Reproduction and population density affect humoral immunity in bank voles under field experimental conditions

Density dependence is a common feature in the dynamics of animal populations. Availability of food resources critical to immunity is likely to be one of the mechanisms mediating the effect of population density on individual fitness. The ability to mount an immune response to an antigen is also affected by levels of immunosuppressive hormones associated with reproduction or mediating the response to ecological and social stress. We assessed variation in condition and intensity of humoral immune response to a T-cell-dependent antigen in bank voles (Clethrionomys glareolus) by experimentally altering population density before immunisation. Consistent with our prediction, males had lower humoral immunocompetence in the breeding than in the non-breeding season. Contrary to our expectation, males did not show enhanced immunocompetence and females showed depressed humoral immune response under experimentally lowered population density. Variation of immune response in relation to population density depended on sex, with females but not males showing lower immune response under experimentally reduced density. We conclude that humoral immunity of bank voles was affected by reproduction and social environment rather than by population density. Received: 2 November 1999 / Accepted: 22 March 2000     

Does maternal stress influence winter survival of offspring in root voles Microtus oeconomus? A field experiment

Maternal stress can have long‐term adverse consequences on immunocompetence and disease risk of offspring, and winter survival is a crucial demographic parameter in the life‐history of an individual that can substantially affect northern rodent population dynamics. An understanding of the effects of maternal stress on winter survival of offspring may help identify mechanisms driving population fluctuations of northern small mammals. Thus, we assessed the effects of maternal stress, resulting from high population densities, on winter survival of first generation (F₁) and second generation (F₂) in root voles Microtus oeconomus. Replicate high‐ and low‐density enclosed parental populations were established, from which we obtained F₁ generation that were used to establish new enclosed, equal‐density populations. The adults of the high‐density parental populations had higher corticosterone levels, an indication of physiological stress, than did those of the low‐density parental populations. Over‐winter survival of the F₁ generation voles from the low‐density parental populations was greater than that of those from the high‐density parental populations. Over‐winter survival of F₂ generation voles did not differ between the two treatments. Our results suggest that maternal stress affected over‐winter survival of first generations but not second generations. Reduced immunocompetence, resulting from high population density stresses, transferred to offspring may be a factor in annual (winter) population declines. Because the effect is transitory, i.e. immunocompetence of F₂ voles is not affected, reduced immunocompetence resulting from high density stresses would not contribute to lengthy periods of low population densities that are characteristic of multi‐annual population fluctuations.     

Optimal age of maturity in fluctuating environments under r‐ and K‐selection

Optimality models for evolution of life histories have shown that increased environmental stochasticity promotes early age of maturity. Here we argue that if r‐selection for early maturation implies a tradeoff making those phenotypes more sensitive to a change in population size than phenotypes maturing at older ages, K‐selection can favor delayed onset of maturation. We analyze a general stochastic Leslie‐matrix model with a simplified density regulation affecting all survivals equally through a function of the population vector, often called the ‘critical age class’. We show that the outcome of such an age‐dependent r‐ and K‐selection is that the expected value of the ‘critical age class’ is maximized by evolution, a strategy strongly influenced by the magnitude of the environmental stochasticity. We also demonstrate that evolution caused by such density‐dependent selection influences the population dynamics, showing a possible reciprocal effect between ecology and evolution in age‐structured populations. This modeling approach reveals that changes in population size affecting the fitness of phenotypes with different age of maturity may be an important selective agent for variation in onset of reproduction in fluctuating environments. This provides a testable hypothesis for how patterns in the population dynamics should affect life history variation.     

Age variation in a fluctuating population of the common vole

We analysed variation in age in a fluctuating population of the common vole (Microtus arvalis) in southern Moravia, Czech Republic, to test the assumption of the senescence hypothesis that the age of voles increases with increasing population density. Between 1996 and 1998, we monitored the demographic changes by snap-trapping and live-trapping in a field population passing through the increase, peak and decline phase of the population cycle. We used the eye lens mass method to determine the age of snap-trapped animals and those that died in live-traps. The average age of winter males was clearly higher after the peak phase breeding season than before it. No such phase-dependent shift in age, however, was observed in the female component. Male age continued to increase from autumn to spring over the pre-peak winter, and the highest age was in spring of the peak phase year. However, after the peak phase breeding season the highest age was achieved in winter, with the decline phase males during the next spring tending to be younger. The average age of females in spring populations was always lower than in winter populations. The average age of voles from live-traps was always higher than voles from snap-traps, particularly in winter and spring populations, suggesting the presence of senescent animals. Although the density-dependent changes in age are consistent with those observed for other voles, they provide only weak evidence that population cycles in the common vole are accompanied by pronounced shifts in individual age, particularly in female voles.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

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.     

Numerical response of small mustelids to vole abundance: delayed or not?

One of the most studied problems in population ecology has been to understand the relative roles of top–down and bottom–up forces in regulating animal populations. This has also been a key issue in studies of vole population dyna mics. Vole populations exhibit a wide variation of dynamics, from seasonal fluctuations to multiannual variations or cyclicity. One of the hypotheses to explain cyclic population dynamics is predation by the specialist predators. A common counterargument against the predation hypothesis has been the lack of conclusive observations of the time delay in the predators’ numerical response. We studied the interaction between voles and their specialist small mustelid predators, the stoat Mustela erminea and the least weasel Mustela n. nivalis, by modelling their interaction to data sets that cover large areas of Finland. Vole abundance was monitored with biannual trappings and their predators with snow‐tracking. Results show a high dependence of the predators on the voles, and this connection is generally tighter in weasels than in stoats. Weasel abundance is affected most strongly by the vole abundance in previous spring, 8.5– 10 months earlier, while in stoats the effect of autumn abundance of voles, 2.5–6 months earlier, was the strongest. These results, together with the observation that the weasels’ effects on voles are stronger after a time lag of 6–9.5 than 2–4.5 months, indicate the existence of a time lag in weasels’ numerical response. A time lag in the predators’ numerical response is a necessary condition for the predators to drive population cycles in its prey, and therefore our results support the specialist predation hypothesis.     

Contrasting consequences of climate change for migratory geese: Predation,density dependence and carryover effects offset benefits of high‐arctic warming

Climate change is most rapid in the Arctic, posing both benefits and challenges for migratory herbivores. However, population‐dynamic responses to climate change are generally difficult to predict, due to concurrent changes in other trophic levels. Migratory species are also exposed to contrasting climate trends and density regimes over the annual cycle. Thus, determining how climate change impacts their population dynamics requires an understanding of how weather directly or indirectly (through trophic interactions and carryover effects) affects reproduction and survival across migratory stages, while accounting for density dependence. Here, we analyse the overall implications of climate change for a local non‐hunted population of high‐arctic Svalbard barnacle geese, Branta leucopsis, using 28 years of individual‐based data. By identifying the main drivers of reproductive stages (egg production, hatching and fledging) and age‐specific survival rates, we quantify their impact on population growth. Recent climate change in Svalbard enhanced egg production and hatching success through positive effects of advanced spring onset (snow melt) and warmer summers (i.e. earlier vegetation green‐up) respectively. Contrastingly, there was a strong temporal decline in fledging probability due to increased local abundance of the Arctic fox, the main predator. While weather during the non‐breeding season influenced geese through a positive effect of temperature (UK wintering grounds) on adult survival and a positive carryover effect of rainfall (spring stopover site in Norway) on egg production, these covariates showed no temporal trends. However, density‐dependent effects occurred throughout the annual cycle, and the steadily increasing total flyway population size caused negative trends in overwinter survival and carryover effects on egg production. The combination of density‐dependent processes and direct and indirect climate change effects across life history stages appeared to stabilize local population size. Our study emphasizes the need for holistic approaches when studying population‐dynamic responses to global change in migratory species.     

Direct negative density‐dependence in a pond‐breeding frog population

Understanding population dynamics is critical for the management of animal populations. Comparatively little is known about the relative importance of endogenous (i.e. density‐dependent) and exogenous (i.e. density‐independent) factors on the population dynamics of amphibians with complex life cycles. We examined the potential effects of density‐dependent and ‐independent (i.e. climatic) factors on population dynamics by analyzing a 15‐yr time series data of the agile frog Rana dalmatina population from Târnava Mare Valley, Romania. We used two statistical models: 1) the partial rate correlation function to identify the feedback structure and the potential time lags in the time series data and 2) a Gompertz state‐space model to simultaneously investigate direct and delayed density dependence as well as climatic effects on population growth rate. We found evidence for direct negative density dependence, whereas delayed density dependence and climate did not show a strong influence on population growth rate. Here we demonstrated that direct density dependence rather than delayed density dependence or climate determined the dynamics of our study population. Our results confirm the findings of many experimental studies and suggest that density dependence may buffer amphibian populations against environmental stress. Consequently, it may not be easy to scale up from individual‐level effects to population‐level effects.     

Why spacing behavior does not stabilize density in cyclic populations of microtine rodents

Summary There are several published hypotheses that consider spacing behavior to be a significant factor causing the multiannual density fluctuations characteristic of some microtine rodent populations. Recent modeling efforts have concluded, however, that spacing behavior should have a stabilizing rather than a destabilizing effect on population dynamics. Why doesn't spacing behavior stabilize these cyclic populations? We argue that while spacing behavior does have a stabilizing influence on population dynamics by limiting the number of breeding individuals, reproduction continues and population size is not limited in an asymptotic manner. Rather, microtine social organization produces demographic changes within a population that allow density cycles to occur under certain conditions. Using a simulation model, we demonstrate that in a strongly seasonal environment populations with low density dependence in reproduction will cycle whereas populations with high density dependence in reproduction will have relatively stable densities. Given such complicating factors as the annual species nature of microtine rodents, occasionally intense predation, and the tendency for territoriality to break down during the non-breeding season, individuals with low density dependence in reproduction will always be able to invade and eventually dominate populations with high density dependence in reproduction, regardless of the resulting destabilization of population dynamics.     

Habitat protection,cattle grazing and density‐dependent reproduction in a desert tree

Anthropogenic activities usually trigger changes in the population density of plants. Thus, land management practices can influence density‐dependent demographic parameters and species interactions. We investigated plant‐pollinator interactions and reproduction in Prosopis flexuosa, the largest tree species in the Central Monte desert of Argentina, an important economic and cultural resource for humans and a functionally prominent species. We hypothesized that reproductive output of P. flexuosa would be limited at low densities, and that exclusion of catle grazing would enhance population density and consequently interaction frequency with pollinators and reproductive success. The study was conducted in and around Ñacuñán Biosphere Reserve (Mendoza, Argentina), where cattle grazing has been excluded for over 35 years. Working in five pairs of protected and cattle grazed 1‐ha plots, we recorded density of adult trees, pollinator visitation frequency to inflorescences and seeds per inflorescence in focal trees. Adult tree density was higher in protected plots than in cattle grazed plots. Density of reproductive trees was positively correlated with seed production, suggesting positive density dependence for reproduction (Allee effect). Pollinator visitation to inflorescences and seed production was higher in protected plots compared with plots under cattle grazing. Suppression of anthropogenic degradation has resulted in higher adult tree density in protected plots, indirectly higher pollinator visitation to inflorescences and higher reproductive success of trees. Increased frequency of plant‐pollinator interactions and tree reproduction suggest success of management practices aimed at protecting P. flexuosa woodlands.     

Is there direct and delayed density dependent variation in population structure in a temperate European cyclic vole population?

Population structure, in terms of the body mass, condition, sex and reproductive status of individuals, has been found to vary in phase with population density in cyclic populations of microtine rodents. Because sustained population cycles involve delayed density dependent changes in the population growth rate, we would expect at least some life history traits also to depend on past densities. Detailed, long-term studies of changes in vole life history traits are however few, and are largely restricted to northern Europe. In view of the uncertainty as to whether the cyclic microtine populations of western Europe represent the same phenomenon as those of northern Europe, we studied temporal variation in the structure of a clearly cyclic population of the common vole Microtus arvalis Pallas, in the cereal plains of mid-western France. Our data set contains seasonal, individual-level data from long-term, large-scale trapping covering four entire population cycles. We found considerable cyclic variation in population structure in spring (April), but less so in summer (June). In spring of post-peak years, animals were of low body weight and body condition (particularly females), litter sizes were smaller and there was a reduction in the proportion of breeders. All of these could be proximal drivers of increased mortality rates, or decreased birth rates, contributing to the population declines. Few life history traits, however, showed direct density dependent variation, and none of the traits studied here showed delayed density dependence. We have shown declines in the fecundity and body condition of voles from a western European population that coincides with, and may be a proximal cause of, cyclic declines in population density. Closer attention to proximal causes, by which ecological processes drive cycles, could clarify the extent to which microtine cycles across Europe represent a single phenomenon.