Space use in the Siberian flying squirrel, Pteromys volans, in fragmented forestsVesa Selonen, Ilpo K. Hanski and Paul Stevens
Dept. of Ecology and Systematics, Div. of Population Biology, University of Helsinki, Finland
One of the most important individual response in the context of habitat fragmentation is space use. Space use is influenced for example by landscape structure and mating system of the species. The landscape contains several different features that influence how animals can move, for example landscape connectivity and ecological corridors.
We studied the space use of Siberian flying squirrel, which has declined in Finland probably due to modern forestry The flying squirrel is an arboreal, herbivorous rodent living in Eurasian taiga forests. Flying squirrels prefer spruce-dominated forests for moving, foraging and reproduction.
The study was done in fragmented forest landscapes in southern Finland in 1996-1999. 24 adult males, 23 adult females and 40 juveniles were radio tracked in three different study areas. In Anjalankoski study area flying squirrels were living in nest boxes, where as in Iitti and in Nuuksio study areas they were living in cavities and dreys.
Radio-tagged individuals were located once a night during summer for home-range analysis and followed continuously for two hours to study the movement paths used by individuals and to study the significance of ecological corridors for movement. Radio locations of flying squirrels were placed on landscape maps (Fig 1).
Effect of fragmentation on space use was studied by comparing number of used patches, and size of home range to the landscape variables patch size, distance to nearest neighbour patch, area of spruce-dominated forests, and area of other habitats with trees. The last two variables were measured from a buffer zone around the center of the home range. Other habitats with trees connected spruce and deciduous forest patches in a matrix of open areas.
Nine adult male, three female, and nine juvenile (before dispersal) flying squirrels regularly used forest corridors for movement between patches. The corridors connected spruce forest patches in a matrix of fields, clear-cuts, and sapling stands. In addition, when corridors were not available six males and ten juveniles (during dispersal) crossed saplings or semi open areas wider than 50 m. Semi open and open areas less than 50 m were crossed by several flying squirrels. Juveniles moved mainly in spruce dominated forest before dispersal, but during dispersal moved also in other habitats with trees (Fig 1). Two juveniles were observed to cross fields wider than 100 m.
Adult males used several spruce patches, while females usually stayed in one patch. According to step-wice regression model, the patch size was the most important landscape variable determining the home range size in males (Fig 2), meaning that males living in small patches used several different patches and moved in larger areas than males living in one large patches. For females, none of the measured variables correlated with the home-range size.
Male home-range sizes were smaller in the nest box study area, than in the other study areas, where flying squirrels lived in cavities and dreys (Fig 3). Female home-range sizes were similar in the different study areas. The difference in male home ranges was due to a higher density of females in the nest box study area than in the other study areas.
Flying squirrels prefer to use spruce forest for movement and can use tree corridors for patch change. However, flying squirrels can also move in other habitats with trees (even saplings) and do not necessarily need spruce forest connection for patch change. Trees are needed for crossing open areas wider than 100-150 m. The home range size of adult male flying squirrels was affected by the size of the occupied patch and the density of the females. Space use of females was not influenced by the landscape structure.
Whether the lack of nest holes or the size of living patch is more important reason than the landscape connectivity, for the decline of the flying squirrel population in Finland, remains unclear. However, we feel that the quality (specially amount of cavities) and area of spruce patches is more important than connectivity, because flying squirrels, and especially juveniles during dispersal, use all kind of habitats with trees for movement, and can colonize spruce patches that are not totally isolated by open areas.
Figure 1. Movement routes used by two adult males, one female and tree juveniles, which used corridors for movement (dashed line) and movement route for one dispersing juvenile, which did not use corridors (straight line). Dark green = spruce forest, light green = young forest, light brown = sapling, dark brown = pine forest, red = clear cut, yellow = field, blue = other open.
Figure 2. Home-range size in relation to size of occupied spruce patch in male and female flying squirrels (males n=17, r= -0.56, p=0.02 and females n=23, r=0.21, p=0.33).
Nest box area (n=6) Other areas (n=18)
Figure 3. The home range sizes of male flying squirrels in nest box study area (Anjalankoski) and in other study areas (Nuuksio and Iitti) , where flying squirrels were living in cavities and dreys (ANOVA, F=3.63, p=0.04). The density of nest boxes in Anjalankoski (0.6 boxes/ha of forest land) was much higher than density of cavities in natural conditions.