An epizootic of sarcoptic mange was prevalent among Scandinavian red foxes (Vulpes vulpes) during the late 1970s and 1980s. By substantially reducing the population density of foxes, the epizootic created a natural experiment on the importance of fox predation for prey density. The fox population started to recover during the late 1980s. We monitored the populations of the fox and its prey [voles (Cricetidae), mountain hare (Lepus timidus), European hare (L. europaeus), Capercaillie (Tetrao urogallus), Black Grouse (T. tetrix), Hazel Grouse (Bonasa bonasia), and roe deer (Capreolus capreolus)] throughout the event, on a local (101—102 km2), a regional (104 km2), and a national scale. Methods included den counts, snap—trapping, pellet/dropping counts, counts of displaying birds, young/adult ratio from incidental observations of deer, regional questionnaires, and national hunting records. The study revealed red fox predation as a crucial factor in limiting the numbers of hares and grouse as well as fawns per doe of roe deer in autumn, and in conveying the 3—4 yr cyclic fluctuation pattern of voles to small game. The classical view, that predators take but a doomed surplus of their prey, was false for these species in Scandinavia.

Suggestions of collapse in small herbivore cycles since the 1980s have raised concerns about the loss of essential ecosystem functions. Whether such phenomena are general and result from extrinsic environmental changes or from intrinsic process stochasticity is currently unknown. Using a large compilation of time series of vole abundances, we demonstrate consistent cycle amplitude dampening associated with a reduction in winter population growth, although regulatory processes responsible for cyclicity have not been lost. The underlying syndrome of change throughout Europe and grass-eating vole species suggests a common climatic driver. Increasing intervals of low-amplitude small herbivore population fluctuations are expected in the future, and these may have cascading impacts on trophic webs across ecosystems.

Cyclic vole populations, defined as showing fairly regular 3–4 yr density fluctuations but with variable amplitudes, were monitored in boreal Sweden in spring and fall 1971–2002, starting in fall 1971. Voles were snap‐trapped on permanent sampling plots at the landscape level within a 100 by 100 km study area north of Umeå. The predominating species trapped were Clethrionomys glareolus, C. rufocanus and Microtus agrestis. In addition to the 3–4 yr cycles, there was a long‐term decrease in numbers and amplitude of the fluctuations, which was especially conspicuous in C. rufocanus. In this latter species there was a persistent decline of both spring and fall densities, apparently bringing the population close to extinction in the area. However, the decline of spring densities from the 1970s to the 1980s and onwards was also evident in C. glareolous and M. agrestis. The declines in numbers and amplitude were largely linked to an increased frequency and/or accentuation of winter declines, which more or less neutralized or even overrode the density increase during the reproductive season in the previous summer, especially so in the second year of the cycles. Thereby the gradual two‐large‐step build‐up of high spring densities, very much founding the base for the very large peak densities and amplitudes in the 1970s, was successively replaced by a one‐smaller‐step build‐up of more modest spring densities, leading to lower peak densities and amplitudes in the 1980s, 1990s and early 2000s. Understanding the causes of the increased frequency and/or severeness of winter declines appears critical to understanding the observed long‐term changes in numbers. However, the underlying causes of the increase of winter declines and the decrease of densities and amplitudes are unknown, but some hypotheses are presented and discussed here. Also, some implications from the decreased vole abundance for reproduction and densities of predators on the voles, and on predators’ alternative prey species, are briefly discussed.

Largely synchronous population fluctuations of Clethrionomys glareolus, C. rufocanus, and Microtus agrestis were monitored by snap—trapping in spring and autumn in 1971—1988 in a strongly seasonal environment near Umea, northern Sweden. All species were cyclic in the sense that they showed fairly regular (3—4 yr) fluctuations, but amplitudes (Nmax/Nmin) varied, averaging °200—fold in each species. This conclusion was supported by autocorrelation and spectral analysis, and by fitting time series data to a model for phase—forgetting cycles. By contrast, data did not conform to a model for phase—remembering cycles (with fixed period and amplitude). The transition between cycles, i.e., from the low to increase phase, was characterized by a distinct shift in rate of change in numbers from low to high or markedly higher values both in summer and winter. Generally, rate of change in summer declined continuously from the increase phase through each cycle. Moreover, there was a similar decrease of rate of change in winter, although rate of change (mainly in C. rufocanus and M. agrestis) first frequently increased early in the cycle. Rate of changes was delayed density dependent in all species both in summer and winter, as revealed by high negative correlations with density in previous autumn and spring for summer and winter changes, respectively. These new finding of delayed density dependence (DDD) support the suggestion that vole cycles are generated by a time—lag mechanism. Possible mechanisms of the DDD are discussed. Regression analyses of rate of change in the different voles suggest that, besides the strong dependence on previous density, rate of change in numbers was also affected by current seed supply (in C. glareolus) and/or weather variables (temperature and precipitation sums) that may have affected the quantity or quality of food.

A previously unknown picornavirus was isolated from bank voles (Clethrionomys glareolus). Electron microscopy images and sequence data of the prototype isolate, named Ljungan virus, showed that it is a picornavirus. The amino acid sequences of predicted Ljungan virus capsid proteins VP2 and VP3 were closely related to the human pathogen echovirus 22 (approximately 70% similarity). A partial 5' noncoding region sequence of Ljungan virus showed the highest degree of relatedness to cardioviruses. Two additional isolates were serologically and molecularly related to the prototype.

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Northern voles and lemmings are famous for their spectacular multiannual population cycles with high amplitudes. Such cyclic vole populations in Scandinavia have shown an unexpected and marked long-term decline in density since the early 1970s, particularly with a marked shift to lower spring densities in the early 1980s. The vole decline, mainly characterized by a strongly decreased rate of change in numbers over winter, is associated with an increased occurrence of mild and wet winters brought about by a recent change in the North Atlantic Oscillation. This has led to a decrease in winter stability and has shortened the period with protective snow cover, the latter considered as an important prerequisite for the occurrence of multiannual, high-amplitude cycles in vole populations. Although the vole decline is predicted to be negative for predators' reproduction and abundance, empirical data showing this are rare. Here we show that the dynamics of a predator-prey system (Tengmalm's owl, Aegolius funereus, and voles), have in recent years gradually changed from 3-4 yr, high-amplitude cycles towards more or less annual fluctuations only.

1. The population fluctuations in time in northern Sweden are examined for the following species: voles, mountain hare, willow grouse, black grouse, capercaillie, hazel hen, red fox, long-eared owl, Tengmalm's owl, and tularemia. Necessary population data have been obtained from the period 1963-1975/76 as revealed by catches, literature survey, hunting statistics, bird ringing, and obligatory reporting of tularemia in man. 2. The populations of the species under consideration are found to fluctuate synchronously in time and show a 3- or 4-year cycle for the period 1963-1975. Population peaks have occurred in connection with the peak densities of voles in the winters 1963-1964, 1966-1967, 1969-1970 and 1973-1974. 3. Voles caused extensive forest damage (mainly bark-eating) in at least the latter three peak winters. From consideration of the available literature it is apparent that bark is a marginal food. Thus, increased bark-eating during peak densities of voles in winter should be interpreted as a shortage of preferred food. 4. The species studied appear to form a unit (subsystem) within the boreal forest ecosystem. This idea is supported by the connecting predatorprey relationships and the demonstrated synchronous population fluctuations. The subsystem contains herbivores, their food vegetation, and predators. Tularemia is regarded as only one among other predators on voles and mountain hares. 5. It is postulated that voles play a central role in causing the overall synchronism in the population fluctuations of the subsystem. 6. The synchronous population fluctuations described can be explained by the following model for their regulation: a) An initial decline in vole numbers is brought about by food shortage at winter peak densities. b) Predator populations (built up with the help of the rich supply of voles) cooperate with food shortage and at some critical point predators alone are able to fulfil the decrease in vole numbers. c) Because of the decrease in vole numbers the predators are forced into a decline themselves and must turn to alternative prey species. Mountain hare and gamebird populations represent a low biomass compared with vole populations and predation thus causes the decline in numbers of these small game. d) Low numbers of predators and excessive food supply then allow voles, mountain hares, and gamebirds to increase again. e) The building up of vole populations sets the stage for another increase in the number of predators and a new cycle is started.