Habitat loss is the major phenomenon threatening species in the boreal forests. One example of a threatened species affected by habitat loss is the Siberian flying squirrel (Pteromys volans). We used mark-recapture data from 1992 to 2004 for 3 flying squirrel populations to estimate the demographic parameters of these populations. Average adult survival was 0.43-0.53, and probability of surviving and staying near the natal territory through the first winter was 0.23-0.30 in different populations. These values are low compared to survival estimates presented in the literature for other Sciurid species. The estimated population growth rates indicated population decline in all areas. In one of the areas, we found a decline in adult survival and population growth rate during the study. This decline was demonstrated to be linked to ongoing habitat loss due to logging. Low survival and emigration probabilities suggest that these populations are likely to be sinks. This highlights the need to ensure dispersal between the remaining patches of suitable habitat for successful conservation of this and other species associated with old-growth forests. The situation is worrisome as the studied populations are situated in an area where flying squirrel population densities have traditionally been high.

24To devise effective conservation actions, it is important to know which factors are associated 25 with the population parameters of a declining population. Using mark-recapture methods, we 26 estimated the annual population size, growth rate and survival probability of an ear-tagged flying 27 squirrel population over a 15-year period in a 4,500 ha study area in western Finland. The species is 28 considered vulnerable, but detailed knowledge concerning population sizes or trends is lacking. The

Climatic conditions, trophic links between species and dispersal may induce spatial synchrony in population fluctuations. Spatial synchrony increases the extinction risk of populations and, thus, it is important to understand how synchrony-inducing mechanisms affect populations already threatened by habitat loss and climate change. For many species, it is unclear how population fluctuations vary over time and space, and what factors potentially drive this variation. In this study, we focus on factors determining population fluctuations and spatial synchrony in the Siberian flying squirrel, Pteromys volans, using long-term monitoring data from 16 Finnish populations located 2–400 km apart. We found an indication of synchronous population dynamics on a large scale in flying squirrels. However, the synchrony was not found to be clearly related to distance between study sites because the populations seemed to be strongly affected by small-scale local factors. The regularity of population fluctuations varied over time. The fluctuations were linked to changes in winter precipitation, which has previously been linked to the reproductive success of flying squirrels. Food abundance (tree mast) and predator abundance were not related to population fluctuations in this study. We conclude that spatial synchrony was not unequivocally related to distance in flying squirrels, as has been observed in earlier studies for more abundant rodent species. Our study also emphasises the role of climate in population fluctuations and the synchrony of the species.Electronic supplementary materialThe online version of this article (10.1007/s00442-019-04537-3) contains supplementary material, which is available to authorized users.

Accurate estimates of population size and distribution are a prerequisite for effective management of populations, but for most species a reliable estimation of the absolute population size is very difficult. In 1998, the Finnish Ministry of Environment set up a working group to plan a national-level survey for estimation and monitoring of the population size of Siberian flying squirrel (Pteromys volans). In 2006, the population size was reported to be 143 000 females. However, evaluation of the magnitude of possible biases was not attempted. Our aim was to test the population size estimate by conducting a resampling study with the methods of the national survey on an eartagged population of flying squirrels. We found that the methods used in the national survey provide greatly upward biased estimates. Moreover, the magnitude of the bias in the method seems to be sensitive to the true population sizes rendering the method unsuitable for population trend monitoring purposes. Therefore, we caution against using the national survey method in monitoring or in making conclusions about the population size of the Siberian flying squirrel.