1.5. The Siberian flying squirrel as a landscape ecological object

The Siberian flying squirrel (Pteromys volans) is distributed through the Eurasian boreal taiga zone from Hokkaido, Japan, to the Baltic countries and Finland (Ognev 1940, Wilson & Reader 1993, Dobson 1994). In Finland, the species range extends from southern country to southern parts of the Finnish Lapland. In the south the range seems to be rather continuous whereas in northern Finland the species is more common in western Kainuu and Koillismaa than along the eastern border of the country. There are no recent documented observations of the species from northern Ostrobothnia (Reunanen 1998). No permanent populations appear to have been established on the Swedish side of the Tornio river although some historical observations from Finnish Lapland exist (Hokkanen et al. 1982).

The Siberian flying squirrel is a nocturnal arboreal species. The species prefers mature mixed spruce forests and tends to favour forest sites that are conspicuously rich in deciduous trees, especially aspen (Populus tremula), and distinctively large spruces (Picea abies; Hanski 1998). The species is omnivorous but its diet principally constitutes of leaves in summer and catkins and buds of alder (Alnus sp.) and birch (Betula sp.) in winter. Supplementary food items such as seeds and buds of conifers are consumed frequently (Mäkelä 1996, own observations). Hence, food availability does not seem to be a limiting factor restricting its space use of in forest landscapes. The Siberian flying squirrel is predated mainly by large owls and the goshawks (Accipiter gentilis), and to some extent by martens (Martes martes). However, due to relatively low population densities of the species it appears to be of minor importance to predators and only occurs occasionally as a prey item in their diet (see e.g. Huhtala et al. 1976) unlike north American flying squirrels which are an important prey for some avian predators (Carey et al. 1992). The occupied forest sites apparently provide the species sufficient food reserves and cavities for safe breeding and roosting. Also, red squirrel dreys are frequently used for roosting (Hanski et al. 2000, own observations). In addition to forest habitats, the Siberian flying squirrel also occurs in cultural environments in southern Finland (see Sulkava et al. 1994, Wistbacka et al. 1996).

Home range sizes differ remarkably between the sexes. Mean home range size for males is close to 60 hectares and this area contains several separate habitat patches that are visited regularly. Females are more site tenacious and concentrate their activities principally on one habitat patch. The average home range size for females is 8.3 hectares. Home ranges for females do not overlap and they inhabit their own habitat patches; comparatively males frequently include more that one female in their range and, therefore, move regularly among sometimes rather distant habitat patches (Hanski 1998, Hanski et al. 2000, own observations). Juveniles disperse in late summer and establish their own home ranges within a mean distance of two kilometres. However, some individuals, more often females, disperse distances of up to eight kilometres (Selonen & Hanski 2000). During the dispersal they can cross open areas up to 100-150 meters (own observations).

The Siberian flying squirrel has been listed as an endangered species in Finland (Rassi et al. 2000). Public and game inquiries in the late 1970’s indicated that the decline of the species has apparently been continuous since the 1950’s (Hokkanen et al. 1982). This negative population trend was recently documented also in smaller, more intensively studied areas in southern Finland (Anon. 2001). The decline of the species was attributed to habitat loss and general degradation of remaining habitat patch quality. Also, fragmentation of forest areas was seen as a process that creates unfavourable landscape patterns for the local long term persistence of the species (Hokkanen et al. 1982, Rassi et al. 2000).