Arctic fox

Distribution

The arctic fox has a circumpolar distribution and is found in a wide variety of tundra habitats. It reproduces in mountain tundra habitats in Fennoscandia, on the arctic tundra of Eurasia and North America, Greenland, Iceland, Svalbard and other barren arctic islands outside the coast of Siberia, Canada and the Bering Strait. Arctic foxes are abundant throughout their distribution, with the exception of Fennoscandia, Medny Island (in the Komandor Islands of Russia) and the Pribilof Islands in Alaska, where they have undergone serious declines. They are red Listed as highly endangered in Fennoscandia, where the species is the subject of intense conservation efforts.

Genetic studies show that there is considerable contact between populations; some foxes move over vast distances, sometimes using sea ice to move between arctic continents and islands.

In Svalbard arctic foxes occur almost everywhere on the archipelago from the highest mountain ridges to the coasts and even on the drift ice. Arctic foxes were not seen on the islands of Bjørnøya or Jan Mayen for many years, but recently they have been naturally reintroduced over the sea ice to Bjørnøya where they now reproduce annually.

General ecology

The Arctic fox is both a predator and a scavenger. Across the Arctic, in relation to the habitat they live in, two ‘ecotypes’ are recognised — the ‘lemming fox’ and the ‘coastal fox’.

In inland areas (in North America, Eurasia and East Greenland) where arctic foxes are characterised as lemming specialists, they rely on small rodent populations that are cyclic (fluctuating with periodicities of three to five years). In such environments, where prey availability is unpredictable, there is large variation between years in litter sizes.

Coastal foxes that occur in regions without lemmings like Svalbard, Iceland and West Greenland are generalists, preying on food items both from marine and terrestrial food webs. Their annual food supply is more predictable and stable, and is available in excess in summer, but is restricted during the winter. Coastal foxes have more stable litter size between years, produce fewer cubs every year and the population size is more stable compared to inland foxes.

With the exception of a local sibling vole population in Svalbard, there are no small rodents, so Svalbard foxes concentrate their forage efforts on other types of prey. In spring and summer ringed seal pups, seabirds, geese, bird eggs, and rock ptarmigan are the major prey items. In the winter their diet consists mainly of rock ptarmigan, stored food that was gathered and cached in the summer and autumn, carcasses of seals and reindeer. Some foxes also specialize on feeding on remnants of seals killed by polar bears.

Den occupancy rates are mainly determined by the availability of seabirds and reindeer carcasses in late winter when the foxes mate. Access to reindeer carcasses is the main factor driving the population dynamics of arctic foxes in Svalbard. Availability of reindeer carcasses during winter depends on rain on snow events, which synchronize the whole community of resident vertebrate herbivores in Svalbard, and cause a lagged correlation with their predator — the arctic fox.

Arctic foxes in Svalbard have no natural enemies or competitors. Predation has not been reported on either adults or youngsters. Starvation during the winter is likely the main cause of mortality.

Life history and reproduction

The Arctic fox is monogamous. Pairs are territorial in the breeding season. The home ranges of pairs are highly variable in size; along the coast, close to bird cliffs, where food is available in excess in the breeding season the home range is generally small (3–5 km2) compared to home range sizes in valley regions in central regions of Spitsbergen where prey is more scattered and unpredictable (60 km²).

Mating takes place from the end of February until mid April. After a gestation period of 52–54 days, the annual litter, which normally contains five or six young, is born in a den in May or early June. Newborn fox cubs have a body weight between 50–65 g. When the young are three or four weeks old they emerge from the den. At four or five weeks of age they are weaned. The young are able to leave their dens for longish periods at eight to ten weeks of age and the dens are abandoned during August.

Arctic foxes become sexually mature during their first year of life (10 months), but in Svalbard most of the females do not reproduce until they are 3 years old and both sexes have lower survival rates compared with other arctic fox populations. The mortality rate of young in the denning period is low, ca. 20–25% from birth until they abandon the den. Estimated mortality during the first winter is 74%, but this drops to 32% for adults. Average longevity is about three to four years, but a 16 year old individual has been recorded from Svalbard which is the highest age ever reported in the wild for this species.

Genetic studies show that some litter mates remain in proximity of each other during their first winter. This pattern is stronger for females than for males, indicating male-biased juvenile dispersal, and weaker for older animals. Females in an area are more related to each other than random chance would dictate, suggesting that females may remain in the areas where they are born.

Management status and monitoring

The fox population in Svalbard seems to be stable and abundant. There are no population estimates available for the archipelago as a whole, but the density in one area, Adventdalen-Sassendalen (ca. 900 km²), has been estimated to be 1–1.5 foxes per 10 km².

The arctic fox is an important vector for parasites/diseases that can be transferred to humans, such as rabies and the tape worm Echinococcus multilocularis as well as toxoplasmosis. The first outbreak of rabies in Svalbard was registered in 1980. A second outbreak came in 2011 with rabies prevalence in foxes of 0.3% between the two outbreaks. Since the outbreaks are so infrequent rabies is likely re-introduced by immigrating pulses of foxes from Arctic Russia to Svalbard. This is supported by the fact that rabies from Svalbard is closely related genetically to the rabies virus found in Siberia, Russia.

The Arctic fox is the final host of the tape worm Echinococcus multilocularis which has become established in the Grumant area in Isfjorden in association with a local population of sibling voles which is the intermediate host for this parasite. This parasite can be fatal to humans. Transmission is thought to occur via handling of fox faeces, so this activity should be conducted wearing gloves (if it takes place at all) or via handling of foxes through trapping. Toxoplasmosis is also highly prevalent (43%) and causes mortality in foxes, particularly in juveniles.

The Arctic fox is one of the most valuable furbearer in the arctic and has been trapped in Svalbard for several hundred years. Currently, most of the trapping occurs in Nordenskiöld Land in the hunting season that last from 1 November to 15 March.

New data shows that trapping has significant effects on the age and gender composition of the population (demography), with an increase of young foxes in hunted populations. The current take of an average of 120 foxes per year appears to be sustainable. But, there is a need to consider the harvesting level closely in new climatic settings.

As an apex predator that undergoes seasonal fat deposition and emaciation, one possible threat for coastal foxes is negative effects of high levels of persistent organochlorine (OC) pollutants.