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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.

Predation has been invoked as a factor synchronizing the population oscillations of sympatric prey species, either because predators kill prey unselectively (the Shared Predation Hypothesis; hereafter SPH), or because predators switch to alternative prey after a density decline in their main prey (the Alternative Prey Hypothesis; APH). A basic assumption of the APH is that the impact of predators on alternative prey depends more on the density of main prey than on the predator/alternative prey ratio. Both SPH and APH assume that the impact of predators on alternative prey is at least periodically strong enough to depress prey populations. To examine these assumptions, we utilized data from replicated field experiments in large areas where we reduced the breeding densities of avian predators during three years and the numbers of least weasels (Mustela nivalis) in two years when vole populations declined. In addition, we reduced the breeding densities of avian predators in two years when vole populations were high. The reduction of least weasels increased the abundance of their alternative prey, small birds breeding on the ground, but did not affect the abundance of common shrews (Sorex araneus). In years when vole populations declined, the reduction of avian predators increased the abundance of their alternative prey, common shrews and small birds. Therefore, vole-eating predators do at least periodically depress the abundance of their alternative prey. At high vole densities, the reduction of avian predators did not increase the abundance of common shrews, although the ratio of avian predators to alternative prey was similar to years when vole populations declined, which supported, APH. In contrast, the abundance of small birds increased after the reduction of avian predators also at high vole densities, which supported SPH. The manipulations had no obvious effect on the number of game birds, which are only occasionally killed by these small-sized predators. We conclude that in communities where most predators are small or specialize on a single prey type, the synchronizing impact of predation is restricted to a few similar-sized species.

Reproductive activities, including signalling with scents, may increase the risk of predation. Mammalian predators, like small mustelids, find voles by using odour cues of scent marks. Scent marks are also visible in ultraviolet light, and at least some diurnal raptors are attracted to these markings in the field. We performed a field experiment to find out whether manipulation of scent markings affects the density, survival and mobility of free-living voles, and the activity of mammalian and avian predators. A total of 20 plots (each 1 ha) were randomly divided into two treatments: scent manipulation and control plots. Scent manipulation plots were treated with vole scent-liquid and control plots with water. From each plot 1-2 voles were radio-collared and tracked for three weeks. Predators hunted more often on scent manipulation plots than on control plots leading to lower survival time of voles. Although scent manipulation attracted more avian predators, small mustelids were the main predators of voles. The density or the mobility of voles did not differ significantly between manipulation and control plots. Our results suggest that odour of scent marks may be a larger risk to voles than UV visibility of scent marks.

The hypothesis that the regular multiannual population oscillations of boreal and arctic small rodents (voles and lemmings) are driven by predation is as old as the scientific study of rodent cycles itself. Subsequently, for several decades, the predation hypothesis fell into disrepute, possibly because the views about predation and rodent dynamics were too simplistic. Here we review the work that has been done on the predation hypothesis primarily in Fennoscandia over the past decade. Models of predator-prey interaction have been constructed for the least weasel (Mustela nivalis) and the field vole (Microtus agrestis), which are considered to be the key specialist predator and the key prey species in the multispecies communities in the boreal forest region in Fennoscandia. The basic model has been parameterized with independent field data, and it predicts well the main features of the observed dynamics. An extension of the model also including generalist and nomadic avian predators predicts correctly the well-documented and striking geographic gradient in rodent oscillations in Fennoscandia, with the amplitude and cycle period decreasing from north to south. These geographic changes are attributed to the observed latitudinal change in the density of generalist and nomadic predators, which are expected to have a stabilizing effect on rodent dynamics. We review the other observational, modeling, and experimental results bearing on the predation hypothesis and conclude that it accounts well for the broad patterns in rodent oscillations in Fennoscandia. We discuss the application of the predation hypothesis to other regions in the northern hemisphere. The predation hypothesis does not make predictions about multiannual and latitudinal changes in body size, behavior, and demography of rodents, which may have some population-dynamic consequences. With the current evidence, however, we consider it unlikely that the phenotypic and genotypic composition of populations would be instrumental for generating the broad patterns in rodent oscillations.

1. Predation has been suggested as a major cause affecting survival, reproductive success and behaviour in vertebrate prey populations. The breeding season is a critical phase because the essential difference between selecting the proper nest site may be a question of breeding success and failure. 2. Islands may serve as refuges for ground nesting birds against natural mammalian predators. However, many of the bird species breeding on islands are currently threatened by introduced predators because they may lack mechanisms to confront the threats of the new predator. 3. We studied the combined effects of an introduced generalist predator, American mink (Mustela vison Schreb.), and island isolation and size on species richness, abundance and equitability of birds breeding on small islands in the outer archipelago of SW Finland, Baltic Sea. The study comprised two mink removal areas (one during 1993-2001 and the other during 1998-2001) and two comparable control areas (during 1994-2001 and 1998-2001), each covering 72-130 km2of which between 1·07 and 1· 27 km2were land area. The mean island size in all areas was < 2 ha. 4. In the two control areas, both the species richness and abundance were highest on the most isolated islands, while isolation did not have obvious effects on these variables in mink removal areas. Species equitability was not influenced by mink removal or island isolation. 5. The distribution of a maritime species, the razorbill (Alca torda L.), has changed dramatically: in the 1973-74, before mink invaded the area, the species was found commonly on less isolated islands than in 1994, after mink invasion. 6. We suggest that in the presence of mink birds may have changed their breeding site selection and started to breed on the most isolated islands, which are not visited by mink as frequently as less isolated islands. Therefore, our results indicate that increasing island isolation may increase the number of bird species and their numbers, because island isolation slows down mink dispersal.

We investigated the association between parental effort and susceptibility to haematozoan parasites in vole-eating Tengmalm's owls (Aegolius funereus) from 1993 to 1995. In a poor vole year (1993), almost all breeding Tengmalm's owls were infected with Trypanosoma avium, whereas in a good vole year (1994), only a few owls were infected. In a moderate vole year (1995), we found an intermediate prevalence of trypanosomes. In the moderate vole year, trypanosome-infected females were in poorer condition than were uninfected females. In the same moderate vole year, high parental effort was associated with increased susceptibility to haematozoan parasites for both genders, whereas in a good vole year no such association was found. In two breeding seasons (1996-1997) of relatively low vole abundance we tested whether supplementary food decreased parasite loads. In accordance with correlative data, trypanosome prevalence was lower among supplemented than control females. Our results support a hypothesis of a trade-off between parental effort and immunocompetence, and emphasize the importance of varying environmental conditions and physical condition of individuals on susceptibility to haematozoan infections.

Mathematical models have suggested that population cycles of northern voles are generated by a combined effect of delayed and direct density-dependent mechanisms. Predation is considered to be the most likely mechanism affecting vole populations in a delayed density-dependent manner. We conducted a replicated two-factor experiment with the field vole (Microtus agrestis) during 1999-2001 in western Finland, manipulating both predation rate and winter food supply to evaluate whether a shortage of winter food has the potential to limit the growth of vole populations in a direct density-dependent manner. Vole populations in fenced predator exclosures rapidly attained higher densities than in unfenced areas, with the difference persisting until the end of the experiment. In the first winter, food supplementation increased vole population growth in fenced areas, but not in unfenced areas. The growth of vole populations in both supplemented and nonsupplemented fenced areas became limited in a direct density-dependent manner during the first winter. During the second winter, food supplementation prevented the crash of vole populations within fences, whereas again no obvious effect was found in the areas exposed to predation. Furthermore, supplemental winter food increased the overwinter survival of voles in fenced areas in both winters. Our results indicate that Microtus vole populations that have succeeded in escaping regulation by predators are limited in growth by a lack of winter food. This factor is thus a strong candidate for the direct density dependence inherently necessary for the occurrence of population cycles.

The mechanisms driving short-term (3-5 yr) cyclic fluctuations in densities of boreal small rodents, and especially, those causing a crash in numbers, have remained a puzzle, although food shortage and predation have been proposed as the main factors causing these fluctuations. In the first large-scale vertebrate predator manipulation experiment with sufficient replication, densities of small mustelids (the least weasel Mustela nivalis and the stoat M. erminea) and avian predators (mainly the Eurasian Kestrel Falco tinnunculus and Tengmalm's Owl Aegolius funereus) were reduced in six different areas, 2-3 km2 each, in two crash phases (1992 and 1995) of the 3-yr cycle of voles (field vole Microtus agrestis, sibling vole M. rossiaemeridionalis, and bank vole Clethrionomys glareolus). The reduction of all main predators reversed the decline in density of small rodents in the subsequent summer, whereas in areas with least weasel reduction and in control areas without predator manipulation, small rodent densities continued to decline. That only reduction of all main predators was sufficient to prevent this summer crash was apparently because least weasels represent <40% of vole-eating predators in western Finland. These results provide novel evidence for the hypothesis that specialist predators drive a summer decline of cyclic rodent populations in northern Europe.

Nest predation is the main cause of nest failures in many bird species. To counter this, birds have evolved different behavioural strategies to decrease the visibility of their nests, thus reducing the probability of nest detection. We manipulated the long-term perception of nest predation risk in pied flycatchers (Ficedula hypoleuca) by experimentally increasing the nest vulnerability to predators. We placed treatment and control nest-boxes for breeding pied flycatchers that appeared identical during the initial phase of breeding. But after the removal of a front panel, treatment boxes had an enlarged entrance hole, almost twice the initial diameter. This treatment increases actual predation risk and presumably parental perception of risk. Control boxes presented instead an entrance hole of the same size both before and after the manipulation. When breeding in enlarged entrance holes, females doubled the vigilance at the nest while males reduced the time spent at the nest, compared to pied flycatchers breeding in control boxes. Increased vulnerability of the nest site to predation risk, thus, induced pied flycatcher parents to increase nest vigilance while reducing their activity at the nest. These results highlight the existence of plasticity in incubation behaviours under long-term experimentally increased nest predation risk.

1. Voles in northern Europe have been shown to exhibit cyclic population dynamics, with a latitudinal gradient in cycle length, amplitude and interspecific synchrony. 2. Previous studies have been based on a relatively sparse network of sampling sites. In the absence of spatially comprehensive long-term records of vole dynamics, we analysed a proxy of vole density, bird-ringing data on vole-eating avian predators, Tengmalm's owl (Aegolius funereus L.), the Ural owl (Strix uralensis Pall.), the long-eared owl (Asio otus L.) and the rough-legged buzzard (Buteo lagopus Pontoppidan) to study spatial population dynamics of voles. 3. We demonstrate that the breeding success of the avian predators is highly dependent on the abundance of voles, which is also reflected in the numbers of nestlings ringed in a particular area in each year. 4. Our results show the expected increase in cycle length from south to north in Finland, but also from west to east, and in contrast to previous studies, increasing irregularity of the cyclic dynamics towards the north. 5. Fluctuations of vole populations have been synchronous over large distances, up to several hundred kilometres. Such large-scale synchrony is more likely to be caused by movements of vole-eating predators and/or by climatic perturbations than by dispersal of voles. 6. We could not conclusively verify the recent suggestion that vole population dynamics have become less regular across Finland, although certain long-term changes are apparent.