Jan Zejda

Small rodent population fluctuations: The effects of age structure and seasonality

Despite more than 50 years of effort, the causes and mechanisms of small rodent population fluctuations remain unknown. The two major questions are as follows: (1) what is the cause of population decline and (2) what is the cause of cyclicity and its geographical variation? At present, no hypothesis can provide answers to both these questions. Recently, progress has been made by Boonstra (1994), who proposed the senescence hypothesis to explain the cause of cyclic decline in population numbers. Here, we tested the main prediction that voles in decline are older than in other phases of the cycle, by analysing changes in age structure in a fluctuating population of the bank vole (Clethrionomys glareolus). The results generally support this prediction; however, the differences in absolute age seem to be too small to explain the occurrence of senescent animals exclusively in declines. We propose a new model to explain changes in age structure and the mechanisms behind the decline and geographic variation in cyclicity. It is based on the idea that voles are oldest in declines, developed independently of Boonstra. However, it differs in three respects: (1) it is more general and thereby applicable to the whole cycle; (2) density-dependent changes in age structure are based on the bimodality in a females age at first reproduction; and (3) it stresses developmental rather than physiological changes in the quality of decline of animals as being relevant to the rate of senescence. We propose that seasonality of the environment is a principal candidate to explain geographical variation in cyclicity. We present substantial theoretical and empirical evidence to indicate that in more seasonal environments with shortened vegetation periods, population dynamics is inevitably less stable due to increased variation in two critical parameters – age at first reproduction and the length of the breeding season – which determine population growth rates. Any external perturbation may then easily destabilize population numbers. The general applicability of the seasonality-senescence hypothesis to other mammalian species decreases with declining r and increasing life span. The hypothesis is falsifiable, and testable predictions are provided.

Density‐dependent life histories in female bank voles from fluctuating populations

Using long-term data sets from two lowland floodplain forest bank vole populations in central Europe, we tested two predictions that with increasing densities: (i) proportions of nonparous females in winter populations would increase; and (ii) age of both nonparous and parous females would increase. These two predictions follow from the assumption that changes in age structure are driven by density-dependent shifts in age at first reproduction. Both populations were sampled by snap trapping between 1956 and 1976. For each year, we examined samples of animals collected between November and April for proportions of females with and without breeding experience and analysed their variation in age relative to the population density. The presence or absence of placental scars was used to discriminate between the parous and nonparous animals. Age was determined by measuring the length of the first mandibular molar (M1 ) roots. Both populations exhibited multi-annual fluctuations in numbers closely resembling those in northern Fennoscandia. The proportion of nonparous females in our total sample was 0.73, suggesting that it is uncommon for parous female bank voles to breed in two successive years and that their life histories are largely designed for breeding in one season only as a major reproductive strategy. Using a logistic-binomial regression model, we found that the probability of females being nonparous or parous at capture varied significantly with time, space and population density. The final model producing the best fit to data predicted that the proportion of nonparous females would be slightly larger over winter and substantially larger after high-density breeding seasons, which is consistent with the tested prediction. With increasing densities during the breeding seasons, both the nonparous and parous females became older at the onset of winter. Again, the field evidence was consistent with the predicted pattern. The age of parous females in the autumn at high densities was older, mainly because of an increased proportion of females entering their second winter. We propose a new intrinsic mechanism by which a delayed density dependence in age-specific fitness components can arise in microtines. This mechanism, based on an increased bimodality in age at first reproduction towards northern latitudes, has a potential to explain the south-north gradient in cyclicity observed in microtine populations in Fennoscandia.

Precocious breeding in female common voles and its relevance to rodent fluctuations

Age at first reproduction has been recognized as the most significant life-history parameter in determining population growth rates. Females in fluctuating rodent populations show great phenotypic variation in this parameter with precocious breeding, defined as pre-weaning fertile mating, being among the most extreme cases of early reproduction in mammals. Here we present the results of experiments in which 14-d-old female common voles (Microtus arvalis) were paired with adult males. Precocious breeding was common and coupled with unexpectedly large litters. We consider this reproductive feature to be adaptive in strictly seasonal environments with greater variation in length of the breeding season, with high resource availability, and a low level of intraspecific competition. Under these conditions, linkage between precocious breeding and high fertility has evolved as part of the most effective life-history strategy which produces the high intrinsic rates of population increase typical of small rodent populations. Seasonality of the environment was suggested to be the main factor behind the geographic pattern in small rodent cyclicity

Effects of deforestation on structure and diversity of small mammal communities in the Moravskoslezske Beskydy Mts [Czech Republic]

During 1970s and 1980s, a large area of mountains in the Czech Republic was influenced by long-term industrial air pollution. Among the most degraded areas were the peaks of the Moravskoslezské Beskydy Mts, where vast clearings resulted from emissions and subsequent forest destruction. This study is aimed at determining the degree of deforestation that is necessary to cause changes in structure and species diversity of small mammal communities that were observed previously. Communities of rodents and insectivores were monitored for a minimum of 3 years at two mountain ranges of the Moravskoslezské Beskydy Mts (Czech Republic) by standard mouse snap-traps. The localities (Smrk and Kněhyně) differ by the degree of human disturbance. Clearings on Smrk Mt are very large (> 30 ha) with no remaining original forest growth as a result of intensive air pollution, unlike the same habitat type at Kněhyně Mt, where the clearings are minor (< 3 ha) and contain living solitary trees. Structure and diversity of small mammal communities in clearings were compared with those from original forests and other mountain habitats. Communities of small mammals at clearings in Smrk Mt (with dominatingMicrotus agrestis) are structurally very different from all other habitats, while structure of communities at Kněhyně clearings are very similar to those of original mountain forest (Complete linkage clustering based on Renkonen index). The community of the original mountain spruce forest at Kněhyně had the highest species diversity (according to Shannon-Weaver, Brillouin, and Simpson indices, Shannon evenness, and rarefaction), while species diversity at clearings of Smrk was the lowest. Shannon diversity of community at Kněhyně primeval forest is similar to that of Kněhyně clearings, while at Smrk Mt the forest diversity is higher than that of clearings. The species diversity of mountain forest and clearings at Kněhyně Mt was significantly higher than that in the same habitats at Smrk Mt. Our results obtained in disturbed habitats at Kněhyně and Smrk Mts suggest that the degree of deforestation may influence the presence and/or the degree of community changes. If the forest destruction is relatively small (clearings < 3 ha), the structure and diversity of small mammal communities do not differ from those of original forest.

Distribution of the common hamster in the Czech Republic after 2000: retreating to optimum lowland habitats

Abstract. Abundances of the common hamster in western Europe declined dramatically over the last 30 years. Recently, severe restrictions in distribution range have also been reported from central European countries. Here we update knowledge of the hamster distribution range in the Czech Republic based on information from six independent sources: (1) monitoring programme for the common vole carried out by State Phytosanitary Administration, (2) monitoring carried out directly by us, (3) questionnaire data from farming companies, (4) questionnaire data from district museums, (5) data from a public server BioLib for mapping species distribution, and (6) observations from nonrelated research activities and persons taking interest in hamsters. The comparison of locations detected after 2000 with those from the last survey > 30 years ago suggests that the range is severely reduced with hamsters retreating to the optimum lowland habitats along large rivers. These results suggest that the demographic mechanisms causing population decline in western populations operate in central Europe as well.

Comparison of enumeration and Jolly-Seber estimation of population size in the common voleMicrotus arvalis

Capture-recapture data on common volesMicrotus arvalis (Pallas, 1779) in central Europe have been almost exclusively analysed by means of the enumeration technique (minimum number alive or calendar of catches). Here we compare enumeration and Jolly-Seber (JS) estimation of population size in the common vole using live-trapping data from an alfalfa field-population in southern Moravia, Czech Republic. Over the entire study the enumeration estimate of the population size was smaller by an average of 28% than the JS estimate. The negative bias increased with density, decreased with both capture probability and the survival rate, and was more pronounced in males at high density. We conclude that the method of direct enumeration is not reliable for estimating population size in the common vole.

Yellow-necked mouse attacked a dead bank vole

An unique observation was made when an adult yellow-necked mouseApodemus flavicollis (Melchior, 1834) attacked a dead bank voleClethrionomys glareolus (Schreber, 1780).