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1 December 2005 Natal dispersal, adult home ranges and site fidelity of mountain hares Lepus timidus in the boreal forest of Sweden
Fredrik Dahl, Tomas Willebrand
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Abstract

Using radio-telemetry we investigated natal dispersal, adult home ranges and site fidelity of mountain hares Lepus timidus in northern Sweden. We captured 48 leverets from 20 litters while these were still suckling and radio-tracked them for up to 37 months. Leverets showed limited dispersal; < ⅓moved far enough to leave the home range of an average adult female. We radio-collared and tracked 73 adult hares to determine annual and seasonal home ranges along with site fidelity. Males had significantly larger annual and winter-spring home ranges than did females. Both sexes, and especially males, had significantly larger home ranges during winter-spring than during summer-autumn. Adults showed strong site fidelity. None of the adults dispersed from an established home range even though some long-distance excursions were recorded, especially for the males during the breeding season in spring. Limited natal dispersal and a high degree of adult site fidelity suggest that dispersal of mountain hares, as juveniles or as adults, occurs at a low rate in the boreal forest during prevailing conditions, i.e. during low densities and in a continuous population. We conclude that the mountain hare seems to be a philopatric species compared with other small game species in the same ecosystem.

The importance of spatial dynamics in management and conservation of wildlife has received increased attention in recent years (e.g. Sutherland et al. 2000, Clobert et al. 2001). Movements have the potential of buffering local variations in population numbers, and buffer zones could be used as simple means to reduce risks of potential over-harvest as suggested by Willebrand & Hörnell (2001) for willow grouse Lagopus lagopus. At the population level, dispersal is arguably the most important movement an animal makes, because it moves individuals from one area to another. Seasonal movements can create large temporal changes in abundance, even in species that would not normally be considered migratory, but do not move animals between populations (Begon et al. 1996, Clobert et al. 2001). Natal dispersal was defined by Greenwood (1980) as dispersal from the birth site to that of first reproduction or potential reproduction, and is often the only long-distance movement an animal will make (Dice & Howard 1951, Sutherland et al. 2000, Engen et al. 2002). Information on both natal dispersal rate and distance moved is needed to understand the effect of dispersal on population dynamics (Pulliam et al. 1992, Schumaker 1996), but such detailed information is known only for a limited number of species (Sutherland et al. 2000). Despite its importance only one previous study has been published on natal dispersal of any hare species, viz. the snowshoe hare Lepus americanus, tracking leverets from suckling until they had established adult home ranges (Gillis & Krebs 1999). Breeding dispersal, i.e. the subsequent movements of adults after their first reproduction (Greenwood & Harvey 1982), usually occurs at lower frequency than natal dispersal, but can be important for the lifetime reproductive success of individuals (Clutton-Brock 1988, Newton 1989). Although not within the scope of this paper, dispersal also has important consequences for population genetics (Aars & Ims 2000).

Home-range size can be used to determine the spatial scale to which dispersal distances should be weighted. Annual home-range sizes (MCP) are well known for the mountain hare Lepus timidus in Scotland, e.g., Hewson & Hinge (1990) found home ranges to be 113 ha and 89 ha for males and females, respectively. In Scandinavia, however, only a few limited studies have investigated home-range size, and none of them followed the hares for a whole year (Seiskari 1957, Olsson 1997).

The mountain hare is one of Sweden's most popular game species due to its extensive range; 40,000–200,000 have been bagged annually during the last 25 years. Several studies using bag data have pooled the mountain hare with capercaillie Tetrao urogallus and black grouse T. tetrix in a coherent group of small game showing similar patterns of population dynamics (Hörnfeldt 1978, Small et al. 1993). In Sweden, the synchronous fluctuation of the small game community in the boreal forest is attributed to the alternative prey hypothesis, where predators act as a link to the microtine 3–4 year cycle (Marcström et al. 1988, 1989, Lindström et al. 1994). It should be noted, however, that the periodicity and degree of synchrony may vary in different parts of Scandinavia and the former Soviet Union (Labutin 1960, Lindén 1988). Still, it is doubtful if bag data can be used to resolve any detailed pattern of mountain hare population dynamics at a local scale.

In this study we measured natal dispersal, and annual/seasonal home ranges of adult mountain hares in the boreal forest of northern Sweden. We also investigated the site fidelity of adult individuals. The aim of our study was to develop an understanding of the dispersal of the mountain hare in the boreal forest. That is, the way animals displace themselves over the landscape, both as juveniles and as breeding adults.

Material and methods

Study area

The 100-km2 large study area was situated in the boreal region of northern Sweden (64°20′N, 20°10′E). The area is dominated by intensively managed coniferous forest (Ball et al. 2000). The climate is continental with cold winters (−11°C in January) and moderately warm summers (15°C in July). The altitude ranges within 90–186 m. a.s.l. Annual precipitation ranges within 600–800 mm, and the ground is usually covered with snow from November to April. The mountain hare forms a continuous population all over the boreal forest of Sweden. However, population densities are substantially lower than for snowshoe hares in the boreal forest of North America (Keith 1981, Lindlöf & Lemnell 1981).

Capture and instrumentation

Adult hares were trapped in spring (February–April) using spring-loaded bow nets (Marcström et al. 1989). The traps were baited with hay, aspen twigs and salt block