New views on how population-intrinsic and community-extrinsic processes interact during the vole population cycles

Habitat type modulates sharp body mass oscillations in cyclic common vole populations

Cyclic rodent populations exhibit pronounced changes in body mass associated with the population cycle phase, long-known as Chitty effect. Although Chitty effect is a common epiphenomenon in both America and Europe, there is still incomplete evidence about the generality of these patterns across the entire range of most species. Moreover, despite decades of research, the underlying factors driving Chitty effect remains poorly understood. Here, we examined the influence of intrinsic and extrinsic factors that may underlie observed patterns in vole size variation in the Iberian common vole Microtus arvalis asturianus. We weighed and measured 2816 adult voles that were captured during 6 trapping periods. Vole numbers and body mass showed strong period- and phase-related variation both in females and males, demonstrating marked Chitty effect in the studied population. Body mass of adult males correlated with body length, evidencing that heavier males are also structurally larger. Statistical models showed that probability of occurrence of large-sized vole (> 37 g) was significantly more likely in reproductive males, during increase and peak phases, and it was modulated by habitat, with crop fields and field margins between crops showing an increased likelihood. We suggest an effect of the habitat on vole body mass mediated by predation.

Shrinkage of body size in rodents as a strategy of populations under anthropogenic conditions (results of 50 years of study of rodent populations)

Based on results of 50 years of continuous observations of populations of the pine vole (Microtus subterraneus de Selys-Longchamps, 1836) and yellow-necked wood mouse (Sylvaemus flavicollis Melchior, 1834), the phenomenon of reducing body size of individuals was revealed, particularly a significant reduction in body dimensions and body mass. The research was conducted at the Kaniv Nature Reserve covering various stages of the local ecosystem's existence under anthropogenic impact. Four cycles of density dynamics of two populations are chosen for comparison of their characteristics. The first three cycles correspond to qualitatively different periods in the existence of the protected ecosystem and populations of the studied species, whereas the last cycle reflects the current situation. It has been established that the tendency to decrease body dimensions is also characteristic for other rodent species, its manifestations are observed for 35 years, and the scale gradually increases. By fatness indexes, individuals of the pine vole on average lose 23.3% and yellow-necked wood mice lose 16.7% of the former values. It was shown that the level of reduce in exterior parameters is always greater in females than in males, and in the group of breeding adults it can reach 32%. It has been found that the phenomenon also applies to juveniles, whose fatness decreases by an average of 21%. The phenomenon occurs against the background of violations of various aspects of population dynamics, which allows it to be associated with anthropogenic changes in the environment. It is suggested that shrinking can be realized by various mechanisms. First, as a result of mortality, the largest individuals and reproductive females with their greatest energy needs fall out of the population, and, second, the growth and weight gain of young animals is slower. As a result, the decrease in external parameters of individuals reduces their specific energy needs and allows them to better survive under adverse conditions. From these points of view, reducing body dimensions can be considered as a specific population strategy to maintain ecological balance.

Plant-herbivore interactions: Combined effect of groundwater level, root vole grazing, and sedge silicification

Accumulation of silica (Si) by plants can be driven by (1) herbivory pressure (and therefore plant-herbivore interactions), (2) geohydrological cycles, or (3) a combination of (1) and (2), with (1-3) possibly affecting Si concentration with a 1-year delay.To identify the relative significance of (1-3), we analyzed the concentration of Si in fibrous tussock sedge (Carex appropinquata), the population density of the root vole (Microtus oeconomus), and the groundwater level, over 11 years.The largest influence of autumn Si concentration in leaves (Sileaf) was on the level of the current-year groundwater table, which was positive and accounted for 13.3% of its variance. The previous year's vole population density was weakly positively correlated with Sileaf, and it alone explained 9.5% of its variance.The only variable found to have a positive, significant effect on autumn Si concentration in rhizomes (Sirhiz) was the current-year spring water level, explaining as much as 60.9% of its variance.We conclude that the changes in Si concentration in fibrous tussock sedge are predominantly driven by hydrology, with vole population dynamics being secondary.Our results provide only partial support for the existence of plant-herbivore interactions, as we did not detect the significant effects of Si tussock concentration on the vole density dynamics. This was mainly due to the low level of silicification of sedges, which was insufficient to impinge herbivores.Future studies on plant-herbivore interactions should therefore aim at disentangling whether anti-herbivore protection is dependent on threshold values of herbivore population dynamics. Furthermore, studies on Si accumulation should focus on the effect of water-mediated Si availability.

Seasonal patterns in habitat use by the harvest mouse (Micromys minutus) and other small mammals

The ecology of the harvest mouse (Micromys minutus) is poorly understood, partly because it is a difficult species to monitor. It is commonly associated with reedbeds, where evidence suggests that it experiences strong seasonal fluctuations in abundance. However, it is unknown whether these fluctuations are caused by real changes in population size, or by movement between habitats. This study investigated seasonal changes in population size and habitat use by harvest mice, and other small mammal species, by trapping the reedbed and three associated habitat types: woodland, pasture and arable land. A sampling effort of 9887 trap bouts across nine months, resulted in 70 captures of harvest mice, as well as wood mice (N = 1022), bank voles (N = 252), field voles (N = 9), common shrews (N = 86) and pygmy shrews (N = 7). The reedbed was the habitat with the most captures and highest diversity. Harvest mice were caught exclusively in the reedbed at the beginning of autumn. Wood mice and bank voles experienced fluctuations in population numbers and wood mice also showed seasonal variation in habitat use. Our study supports the idea that harvest mice undergo extreme seasonal fluctuations in abundance in reedbeds, but these do not appear to be related to changes in habitat use.

Behavioural plasticity of motor personality traits in the common vole under three-day continual observation in a test box

In animals, behavioural personality traits have been well-documented in a wide array of species. However, these traits, different between individuals, are not completely stable in individuals. They show behavioural plasticity like many other phenotypic traits. This plasticity is able to overcome some weak aspects of personality trait behavioural strategy. In the present study, we examined the relationship between motor personality traits and behavioural plasticity in the common vole (Microtus arvalis) using a PhenoTyper (PT) box (Noldus). During a three-day test, four behavioural motor activity parameters were monitored in 47 voles: distance moved, (loco)motion duration, motion change frequency, sprint duration. Consistency repeatability (R_C) of the parameters from the PT test was very high, with all values ≥ 0.91. To select the best linear mixed-effect models (LMMs), several predictors (test day, sex, body weight) were tested. Only test day had a significant effect on the dependent variables and other predictors did not improve the LMMs. Further, we found significant effects of random intercepts (motor personality traits) and slopes (behavioural plasticity), as well as significant negative correlations between them for all behavioural parameters. Our results indicate that motor personality traits were connected with behavioural plasticity. Moreover, we revealed a significant positive correlation between the random slopes of (loco)motion duration and motion change frequency. This relationship could indicate some central plasticity of motor personality traits. In conclusion, negative correlations between the motor personality traits and the behavioural plasticity demonstrate expression of convergent tendency from both opposite trait values. This corresponds with different ideas on ability to compensate personality effects or to prepare for potential future conditions. In the laboratory, plasticity of personality traits take place whenever an animal is placed e. g. in a breeding box for the first time or is left for a long time in an experimental apparatus.

Population cycles and outbreaks of small rodents: ten essential questions we still need to solve

Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

The hidden effect of inadvertent social information use on fluctuating predator–prey dynamics

Understanding biotic interactions and abiotic forces that govern population regulation is crucial for predicting stability from both theoretical and applied perspectives. In recent years, social information has been proposed to profoundly affect the dynamics of populations and facilitate the coexistence of interacting species. However, we have limited knowledge about how social information use influences cyclic and non-cyclic fluctuations of populations and if any population-level effects can be expected in species where individuals do not form social groups. In this study, I built individual-based models in a factorial design to investigate how predator avoidance behaviour and associated inadvertent social information (ISI) use alters the predictions of classical predator–prey population models in non-grouping (e.g., randomly moving) animals. Simulation results showed that ISI use in prey stabilized population dynamics by disrupting high-amplitude cyclic fluctuations in both predator and prey populations. Moreover, it also decreased the strength of the negative feedback of second-order dependence between predator and prey. I propose that if social cues are commonly used sources of information in animals regardless of the level of social organization, then similar social information-mediated effects on trophic interactions and population dynamics may be prevalent in natural communities.

Population cycles: generalities, exceptions and remaining mysteries

Population cycles are one of nature's great mysteries. For almost a hundred years, innumerable studies have probed the causes of cyclic dynamics in snowshoe hares, voles and lemmings, forest Lepidoptera and grouse. Even though cyclic species have very different life histories, similarities in mechanisms related to their dynamics are apparent. In addition to high reproductive rates and density-related mortality from predators, pathogens or parasitoids, other characteristics include transgenerational reduced reproduction and dispersal with increasing-peak densities, and genetic similarity among populations. Experiments to stop cyclic dynamics and comparisons of cyclic and noncyclic populations provide some understanding but both reproduction and mortality must be considered. What determines variation in amplitude and periodicity of population outbreaks remains a mystery.

Phase- and season-dependent changes in social behaviour in cyclic vole populations

Background

Social behaviour has been linked to hypotheses explaining multiannual population cycles of small rodents. In this paper we aimed to test empirically that the degree of space sharing among adult breeding female voles is higher during the increase phase than in the crash phase, and that the degree of sociality is positively related to population growth rate as suggested by Lambin and Krebs (Oikos 61:126–132, 1991) and Andreassen et al. (Oikos 122:507–515, 2013). We followed 24 natural bank vole Myodes glareolus populations over an area of 113 km² by monthly live trapping throughout a complete population cycle of three summers and two winters.

Results

Using spatially explicit capture-recapture models, we modelled the overlap in adult female home ranges and total population growth rate per season. We identified an increase phase before and during the peak density observation and a crash phase following the peak. Female home range overlap were seasonal- and phase-dependent, while population growth rate was associated with season and female home range overlap. High female home range overlap in the increase phase corresponded to a high population growth rate.

Conclusions

We suggest that intrinsic social behaviour plays a key role in the increase phase of vole population cycles, as social behaviour leads to an increased growth rate, whereas extrinsic factors (predation and/or food) initiate the crash phase. Our results are consistent with those of other studies in a variety of small rodent species.

Population dynamics, synchrony, and environmental quality of Hokkaido voles lead to temporal and spatial Taylor's laws

Taylor's law (TL) asserts that the variance in a species' population density is a power-law function of its mean population density: log(variance) = a + b × log(mean). TL is widely verified. We show here that empirical time series of density of the Hokkaido gray-sided vole, Myodes rufocanus, sampled 1962-1992 at 85 locations, satisfied temporal and spatial forms of TL. The slopes (b ± standard error) of the temporal and spatial TL were estimated to be 1.613 ± 0.141 and 1.430 ± 0.132, respectively. A previously verified autoregressive Gompertz model of the dynamics of these populations generated time series of density which reproduced the form of temporal and spatial TLs, but with slopes that were significantly steeper than the slopes estimated from data. The density-dependent components of the Gompertz model were essential for the temporal TL. Adding to the Gompertz model assumptions that populations with higher mean density have reduced variance of density-independent perturbations and that density-independent perturbations are spatially correlated among populations yielded simulated time series that satisfactorily reproduced the slopes from data. The slopes (b ± standard error) of the enhanced simulations were 1.619 ± 0.199 for temporal TL and 1.575 ± 0.204 for spatial TL.

Surviving winter: Food, but not habitat structure, prevents crashes in cyclic vole populations

Vole population cycles are a major force driving boreal ecosystem dynamics in northwestern Eurasia. However, our understanding of the impact of winter on these cycles is increasingly uncertain, especially because climate change is affecting snow predictability, quality, and abundance. We examined the role of winter weather and snow conditions, the lack of suitable habitat structure during freeze‐thaw periods, and the lack of sufficient food as potential causes for winter population crashes. We live‐trapped bank voles Myodes glareolus on 26 plots (0.36 ha each) at two different elevations (representing different winter conditions) in southeast Norway in the winters 2013/2014 and 2014/2015. We carried out two manipulations: supplementing six plots with food to eliminate food limitation and six plots with straw to improve habitat structure and limit the effect of icing in the subnivean space. In the first winter, all bank voles survived well on all plots, whereas in the second winter voles on almost all plots went extinct except for those receiving supplemental food. Survival was highest on the feeding treatment in both winters, whereas improving habitat structure had no effect. We conclude that food limitation was a key factor in causing winter population crashes.

Let's stay together? Intrinsic and extrinsic factors involved in pair bond dissolution in a recolonizing wolf population

For socially monogamous species, breeder bond dissolution has important consequences for population dynamics, but the extent to which extrinsic or intrinsic population factors causes pair dissolution remain poorly understood, especially among carnivores.

Using an extensive life‐history data set, a survival analysis and competing risks framework, we examined the fate of 153 different wolf (Canis lupus) pairs in the recolonizing Scandinavian wolf population, during 14 winters of snow tracking and DNA monitoring.

Wolf pair dissolution was generally linked to a mortality event and was strongly affected by extrinsic (i.e. anthropogenic) causes. No divorce was observed, and among the pair dissolution where causes have been identified, death of one or both wolves was always involved. Median time from pair formation to pair dissolution was three consecutive winters (i.e. approximately 2 years). Pair dissolution was mostly human‐related, primarily caused by legal control actions (36·7%), verified poaching (9·2%) and traffic‐related causes (2·1%). Intrinsic factors, such as disease and age, accounted for only 7·7% of pair dissolutions. The remaining 44·3% of dissolution events were from unknown causes, but we argue that a large portion could be explained by an additional source of human‐caused mortality, cryptic poaching.

Extrinsic population factors, such as variables describing the geographical location of the pair, had a stronger effect on risk of pair dissolution compared to anthropogenic landscape characteristics. Population intrinsic factors, such as the inbreeding coefficient of the male pair member, had a negative effect on pair bond duration. The mechanism behind this result remains unknown, but might be explained by lower survival of inbred males or more complex inbreeding effects mediated by behaviour.

Our study provides quantitative estimates of breeder bond duration in a social carnivore and highlights the effect of extrinsic (i.e. anthropogenic) and intrinsic factors (i.e. inbreeding) involved in wolf pair bond duration. Unlike the effects of intrinsic and extrinsic factors that are commonly reported on individual survival or population growth, here we provide quantitative estimates of their potential effect on the social unit of the population, the wolf pair.

Generalist predator, cyclic voles and cavity nests: testing the alternative prey hypothesis

The alternative prey hypothesis (APH) states that when the density of the main prey declines, generalist predators switch to alternative prey and vice versa, meaning that predation pressure on the alternative prey should be negatively correlated with the density of the main prey. We tested the APH in a system comprising one generalist predator (pine marten, Martes martes), cyclic main prey (microtine voles, Microtus agrestis and Myodes glareolus) and alternative prey (cavity nests of common goldeneye, Bucephala clangula); pine marten is an important predator of both voles and common goldeneye nests. Specifically, we studied whether annual predation rate of real common goldeneye nests and experimental nests is negatively associated with fluctuation in the density of voles in four study areas in southern Finland in 2000-2011. Both vole density and nest predation rate varied considerably between years in all study areas. However, we did not find support for the hypothesis that vole dynamics indirectly affects predation rate of cavity nests in the way predicted by the APH. On the contrary, the probability of predation increased with vole spring abundance for both real and experimental nests. Furthermore, a crash in vole abundance from previous autumn to spring did not increase the probability of predation of real nests, although it increased that of experimental nests. We suggest that learned predation by pine marten individuals, coupled with efficient search image for cavities, overrides possible indirect positive effects of high vole density on the alternative prey in our study system.

Evidence for selection maintaining MHC diversity in a rodent species despite strong density fluctuations

Strong spatiotemporal variation in population size often leads to reduced genetic diversity limiting the adaptive potential of individual populations. Key genes of adaptive variation are encoded by the immune genes of the major histocompatibility complex (MHC) playing an essential role in parasite resistance. How MHC variation persists in rodent populations that regularly experience population bottlenecks remains an important topic in evolutionary genetics. We analysed the consequences of strong population fluctuations on MHC class II DRB exon 2 diversity in two distant common vole (Microtus arvalis) populations in three consecutive years using a high-throughput sequencing approach. In 143 individuals, we detected 25 nucleotide alleles translating into 14 unique amino acid MHC alleles belonging to at least three loci. Thus, the overall allelic diversity and amino acid distance among the remaining MHC alleles, used as a surrogate for the range of pathogenic antigens that can be presented to T-cells, are still remarkably high. Both study populations did not show significant population differentiation between years, but significant differences were found between sites. We concluded that selection processes seem to be strong enough to maintain moderate levels of MHC diversity in our study populations outcompeting genetic drift, as the same MHC alleles were conserved between years. Differences in allele frequencies between populations might be the outcome of different local parasite pressures and/or genetic drift. Further understanding of how pathogens vary across space and time will be crucial to further elucidate the mechanisms maintaining MHC diversity in cyclic populations.

Recovery of small rodent populations after population collapse

In this review we summarise published knowledge regarding small mammal population recovery following sudden population collapse, regardless as to whether the collapse is caused by natural or man-made events. We determine recovery mechanisms, recovery time and recovery rate, and suggest how to adapt and optimise current methods to regulate small mammal population size, for pest management and/or conservation. It is vital that the principles underlying the recovery mechanisms are known for both pest control and conservation to align management methods to either maintain animal numbers at a permanent minimum level or increase population size. Collapses can be caused naturally, as in the declining phase of multi-annual fluctuations and after natural disasters, or by man-made events, such as pesticide application. In general, there are three ways population recovery can occur: (1) in situ survival and multiplication of a small remaining fraction of the population; (2) immigration; or (3) a combination of the two. The recovery mechanism strongly depends on life history strategy, social behaviour and density-dependent processes in population dynamics of the species in question. In addition, the kind of disturbance, its intensity and spatial scale, as well as environmental circumstances (e.g. the presence and distance of refuge areas) have to be taken into account. Recovery time can vary from a couple of days to several years depending on the reproductive potential of the species and the type of disturbances, regardless of whether the collapse is man made or natural. Ultimately, most populations rebound to levels equal to numbers before the collapse. Based on current knowledge, case-by-case decisions seem appropriate for small-scale conservation. For pest control, a large-scale approach seems necessary. Further investigations are required to make sound, species-specific recommendations.

Diet quality limits summer growth of field vole populations

Marked variation occurs in both seasonal and multiannual population density peaks of northern European small mammal species, including voles. The availability of dietary proteins is a key factor limiting the population growth of herbivore species. The objective of this study is to investigate the degree to which protein availability influences the growth of increasing vole populations. We hypothesise that the summer growth of folivorous vole populations is positively associated with dietary protein availability. A field experiment was conducted over a summer reproductive period in 18 vegetated enclosures. Populations of field voles (Microtus agrestis) were randomised amongst three treatment groups: 1) food supplementation with ad libitum high protein (30% dry weight) pellets, 2) food supplementation with ad libitum low protein (1% dry weight; both supplemented foods had equivalent energy content) pellets, and 3) control (no food supplementation), n = 6 per treatment. Vole density, survival, demographic attributes and condition indicators were monitored with live-trapping and blood sampling. Highest final vole densities were attained in populations that received high protein supplementation and lowest in low protein populations. Control populations displayed intermediate densities. The survival rate of voles was similar in all treatment groups. The proportion of females, and of those that were pregnant or lactating, was highest in the high protein supplemented populations. This suggests that variation in reproductive, rather than survival rates of voles, accounted for density differences between the treatment groups. We found no clear association between population demography and individual physiological condition. Our results demonstrate that dietary protein availability limits vole population growth during the summer growing season. This suggests that the nutritional quality of forage may be an underestimated source of interannual variation in the density and growth rates of widely fluctuating populations of herbivorous small mammals.

Bears are simply voles writ large: social structure determines the mechanisms of intrinsic population regulation in mammals

The literature reveals opposing views regarding the importance of intrinsic population regulation in mammals. Different models have been proposed; adding importance to contrasting life histories, body sizes and social interactions. Here we evaluate current theory based on results from two Scandinavian projects studying two ecologically different mammal species with contrasting body sizes and life history traits: the root vole Microtus oeconomus and the brown bear Ursus arctos. We emphasize four inter-linked behavioral aspects—territoriality, dispersal, social inhibition of breeding, and infanticide—that together form a density-dependent syndrome with potentially regulatory effects on population growth. We show that the two species are similar in all four behaviors and thus the overall regulatory syndrome. Females form matrilineal assemblages, female natal dispersal is negatively density dependent and breeding is suppressed in philopatric young females. In both species, male turnover due to extrinsic mortality agents cause infanticide with negative effects on population growth. The sex-biased and density-dependent dispersal patterns promote the formation of matrilineal clusters which, in turn, leads to reproductive suppression with potentially regulatory effects. Hence, we show that intrinsic population regulation interacting with extrinsic mortality agents may occur irrespective of taxon, life history and body size. Our review stresses the significance of a mechanistic approach to understanding population ecology. We also show that experimental model populations are useful to elucidate natural populations of other species with similar social systems. In particular, such experiments should be combined with methodical innovations that may unravel the effects of cryptic intrinsic mechanisms such as infanticide.

Concurrent effects of age class and food distribution on immigration success and population dynamics in a small mammal

During the settlement stage of dispersal, the outcome of conflicts between residents and immigrants should depend on the social organization of resident populations as well as on individual traits of immigrants, such as their age class, body mass and/or behaviour. We have previously shown that spatial distribution of food influences the social organization of female bank voles (Myodes glareolus). Here, we aimed to determine the relative impact of food distribution and immigrant age class on the success and demographic consequences of female bank vole immigration. We manipulated the spatial distribution of food within populations having either clumped or dispersed food. After a pre-experimental period, we released either adult immigrants or juvenile immigrants, for which we scored sociability and aggressiveness prior to introduction. We found that immigrant females survived less well and moved more between populations than resident females, which suggest settlement costs. However, settled juvenile immigrants had a higher probability to reproduce than field-born juveniles. Food distribution had little effects on the settlement success of immigrant females. Survival and settlement probabilities of immigrants were influenced by adult female density in opposite ways for adult and juvenile immigrants, suggesting a strong adult-adult competition. Moreover, females of higher body mass at release had a lower probability to survive, and the breeding probability of settled immigrants increased with their aggressiveness and decreased with their sociability. Prior to the introduction of immigrants, resident females were more aggregated in the clumped food treatment than in the dispersed food treatment, but immigration reversed this relationship. In addition, differences in growth trajectories were seen during the breeding season, with populations reaching higher densities when adult immigrants were introduced in a plot with dispersed food, or when juvenile immigrants were introduced in a plot with clumped food. These results indicate the relative importance of intrinsic and extrinsic factors on immigration success and demographic consequences of dispersal and are of relevance to conservation actions, such as reinforcement of small populations.