A transect survey of small land carnivore and red fox populations on a subarctic fell in Finnish Forest Lapland over 13 winters

1. What they did

Small rodents in the subarctic and northern forests of Finland had been studied for decades, but little was known about the small carnivores living alongside them. Starting in the winter of 1968/69, the author counted the tracks of four predators on a fell (a rounded mountain) in eastern Finnish Forest Lapland: red fox, pine marten, stoat and pygmy weasel.

• The transect. Once a week during the snowy season, he skied along a 5970 m observation line running west from the summit of Värriötunturi. The line crossed seven habitats, from a treeless summit through mountain birch, spruce, pine and juniper forests to bogs and a marsh.
• What was counted. Tracks crossing the line, only when they were at most two days old, from November to April, for 13 winters (1968/69 to 1980/81). If the same animal crossed the line more than once within a few minutes between two recording sticks 20 m apart, it counted as one crossing. Counts were standardized per 100 m of line and per four recordings a month.
• Rodent numbers. As an index of small rodent numbers, he used published counts of tree saplings damaged by voles in northern Finland, backed by field observations of the research station staff.
• Trailing foxes. In two winters he also followed red fox trails for 281 km in total, recording visits to carcasses, digging in the snow and kills.

2. What they found

Activity through the winter

Track numbers stayed within a narrow range from November to April, with no clear trends. Even January, the coldest month, did not change the activity of these carnivores.

Habitat use

• Pine marten: clearly favoured spruce-dominated forests (1.26 tracks per 100 m) and avoided openings. No marten tracks were recorded on the treeless summit.
• Red fox: used all habitats, most often the juniper-characterized mixed forest (0.99), open bog (0.79) and pine-dominated forest (0.65), then spruce forest and mountain birch, and least the treeless summit (0.15). Unlike the marten, foxes crossed the summit.
• Stoat: found in all habitats, most often in the juniper-characterized mixed forest (0.77) and spruce-dominated forest (0.70). Stoats used the spruce forest mainly early in the study, and pine martens later.
• Pygmy weasel: found in every habitat except the treeless summit, often only in the winter of 1978/79.
• Only one red fox track and one pine marten track were recorded in 13 winters on the short stretch of willow bog, so it was left out.

Numbers over 13 winters

• Red fox and stoat tracks peaked three times (1969/70, 1974/75 and 1978/79), pine marten twice (1974/75 and 1978/79) and pygmy weasel twice (1975/76 and 1978/79). The third peak, during a very marked rodent peak, coincided in all four species.
• When rodents were really scarce (1971/72, 1972/73, 1976/77 and 1979/80), stoat and weasel tracks were few or absent, while fox and marten tracks did not fall so sharply.
• After the rodent crash of 1978/79, the predators crashed or dispersed. Martens enlarged their home ranges; stoats stayed few and pygmy weasels were perhaps absent in 1980/81, even though rodents had begun to recover.

What red foxes did on the trail

Over 281 km of fox trails, foxes visited carcasses 0.22 and 0.25 times per km in the two winters, and dug in the snow 0.43 and 0.25 times per km. In 182.5 km they killed only one arctic hare, and in 98.5 km only one willow grouse. Foxes followed the trails of semi-domestic reindeer between grazing sites and, in earlier times, the tracks of wolverines, where carcasses might be found. Pine martens did not use these game trails.

3. What they concluded

• The food generalists (red fox and pine marten) fluctuated less than the rodent specialists (stoat and pygmy weasel), which are more vulnerable to crashes in rodent numbers. The pygmy weasel was especially vulnerable, vanishing in some poor rodent winters, perhaps by dispersing to refuges far away.
• Each species used the habitats differently, which reduces competition between them. The author links these patterns to food (for example, voles that foxes hunt live in grassy juniper forest and bogs) and to avoiding predators (the brown marten on white snow is visible to eagle owls and golden eagles, so it avoids open ground).
• Life under the snow matters, especially for the pygmy weasel, whose basal metabolism is two to three times higher than expected for a mammal of its size.
• Carcasses of reindeer and moose are essential for the generalists to survive these winters and give food reserves to specialists too. With wolves and wolverines gone, reindeer now die mostly from starvation on over-grazed pastures. The author suggests wolves, which belong to this ecosystem, would make it more functional, because they kill ungulates all year.

4. How tracking was or was not used

The whole study rests on snow tracking. The author identified and counted fresh tracks (no more than two days old) of four carnivores crossing a fixed ski transect each week, and judged when one animal had crossed several times. He also trailed red foxes for 281 km, recording carcass visits, digging in the snow and kills, and refers to his earlier tracking of pine martens to estimate home ranges and habitat use. He notes that when trailing a red fox, one soon sees its trail crossing those of the other three species.

5. Tracker or observer certification

Not reported. The author appears to have done the counts himself (“The present author began a census”), but nothing is said about who identified and aged the tracks or how. The results depend on telling apart stoat and pygmy weasel tracks and on judging track age in snow.

6. Other technologies, skills or innovations

• Recording sticks every 20 m along the transect, to decide whether repeated crossings were made by one animal.
• Vole damage to tree saplings as a practical index of small rodent numbers.

7. Implications for conservation (from the paper)

• Large herbivores, especially reindeer but also moose, are essential to the winter survival of foxes and martens in these subarctic forests, and their carcasses also help the specialists.
• The presence of wolves would make the ecosystem more functional, because they produce carcasses throughout the year.
• Stoats and especially pygmy weasels are vulnerable to rodent crashes and may be slow to return once they have dispersed.