Arctic fox (Vulpes lagopus)

What is being monitored?

Status and trend

The arctic fox is a predator and a scavenger at the top of the food web in Svalbard. No estimates are available for the total arctic fox population, but they are abundant and found across most the archipelago.

Arctic fox populations which live in areas without small rodents (lemming) are more stable than those on the mainland, where the arctic fox population is affected by the population cycle of lemmings. There are no small rodents in Svalbard, with the exception of a local population of the southern vole (Microtus levis) of varying distribution on both sides of the species’ core area in Grumantbyen, west of Longyearbyen. As prey, small rodents are therefore insignificant in Svalbard.

The breeding population of arctic fox is monitored in two areas of West Spitsbergen and the arctic fox population is relatively stable overall. However, data based on live catches, the observation of foxes and the monitoring of arctic foxes from one area the Brøgger Peninsula - Kongsfjorden area (ca 250 km²) in 1990–2003, however, revealed local variations in the arctic fox populations in Svalbard, too (Fuglei et al. 2003). All the dens in this area are located near the coast.

From 1990, the arctic fox population in the Kongsfjord area grew rapidly and breeding took place in all the known dens in the summer of 1995. A sudden drop occurred in autumn 1995, and the population remained low in 1996 and 1997, when none of the dens were used for breeding. The population then rose again and reached a relatively high level (71%) in 2002. Since then, the breeding frequency has varied from year to year between 10% and 70%, but there has never been such a dramatic reduction as in 1995. Another good reproduction year in 2014.

Even though there are no good population estimates for the whole of Svalbard, the population density in the other monitoring area, Adventdalen/Sassendalen (ca 900 km²), is estimated at 1–1.5 foxes per 10 km². Breeding activity varied in 1982–1989 and 1997–2001, and litters varied in size from 4 to 7.5 cubs (Eide et al. 2012). The dens here are located along the coast and in two large valleys with varying access to food.

More breeding was recorded in the coastal dens, which are close to nutrient-rich seabird colonies, than in the dens in the valleys, where less food was available (Eide et al. 2012). Sassendalen, where both geese and reindeer are available, is regarded as richer than Adventdalen, which only has reindeer (Eide et al. 2004). This picture is somewhat more nuanced today, as pink-footed geese are increasingly nesting in inner Adventdalen.

About half of the cubs were produced in the dens along the coast, and the remainder were produced in Sassendalen (30%) and Adventdalen (24%). No difference in breeding frequency was recorded in the two valleys, even though differing amounts of food are available there.

Causal factors

The variations in the breeding in the Brøgger Peninsula - Kongsfjorden area are thought to be related to the increased availability of food in the winter of 1993–1994 owing to extreme icing of the reindeer grazing due to rainfall in December 1993. The amount of precipitation was identical with the annual average for Ny-Ålesund and it caused high reindeer mortality, so that many reindeer carcasses were available to the foxes. More food was also available in the summer due to a growing population of barnacle geese in the area.

This shows that in some areas lacking small rodents, large variations in an arctic fox population may, nevertheless, occur from year to year, especially within small areas, and this may, in turn, influence the production of local prey populations such as geese. No explanation has been found for the sudden decline in the arctic fox population from 1995, with no breeding in the two succeeding years.

The proportion of arctic fox dens where breeding is taking place in a particular year thus varies with the availability of food during the winter.

This has also been observed in the other area, Adventdalen and Sassendalen, where dens used for breeding are being monitored. The proximity of the dens to seabird colonies, along with the availability of reindeer carcasses in winter, is decisive for the number of arctic fox dens being used for breeding. This is because more food is available, and with greater predictability, from the bird cliffs along the coast.

The variation in breeding from year to year in the inland dens was related to the number of reindeer carcasses on the tundra. As there was also a link between the use of the coastal dens and the frequency of breeding in the inland dens, the availability of carcasses in winter seems to be the most important factor driving the population dynamics of arctic foxes in Svalbard.

The reason why there was no difference in the breeding frequency between the two valleys, despite the difference in the availability of food there (geese and reindeer versus only reindeer), may be that the production of cubs is determined by the availability of food in late winter before the cubs are born. Seabirds and reindeer carcasses, both of which influence the breeding frequency of the arctic fox, are both available early in the breeding season of the foxes (before the cubs are born), whereas the migrating geese reach Svalbard about the same time as the cubs are born and thus have no effect on the breeding frequency.

Consequences

The occurrence of seabirds and reindeer carcasses is unlikely to be dependent upon the density in the fox population. We therefore believe that the population dynamics of the arctic fox in Svalbard are driven by "bottom-up" processes, i.e. they are regulated from lower in the food chain, from plants to herbivores.

The herbivores are affected by the climate through an increased frequency of rain on the snow, with consequent icing of the tundra, which makes grazing plants unavailable and results in higher mortality among reindeer and more occurrence of carcasses.

In view of the function of the arctic fox as a predator and a carrion eater at the top of the food chain in the tundra ecosystem, a rise in the fox population will greatly affect the breeding success of ground-nesting birds like geese and ptarmigan, as well as seabirds breeding on the bird cliffs. Even though more geese are nesting in Svalbard, this does not seem to be decisive for the breeding frequency in the foxes, but the greater availability of food will potentially be important for the summer survival of the cubs.

The winter climate in Svalbard, with periods of mild weather and precipitation in the form of rain, has been found to affect all the herbivores in the ecosystem – ptarmigan, reindeer and southern voles – synchronously (Hansen et al. 2013). Reduced access to food, due to grazing plants being covered by ice, results in higher mortality in winter and lower production of offspring.

Whereas the reduction in the populations of the three herbivores occurs completely synchronously and always in winters when rain occurs, summers when the arctic fox population drops are delayed by one year. This is mainly because high reindeer mortality one winter is generally followed by less mortality and fewer carcasses the next winter because weak animals have already been removed from the population (Hansen et al. 2013).

About the monitoring

Arctic foxes are one of 3 terrestrial mammal species that live the whole year in Svalbard and they are found almost all over Svalbard.

They are protected on Bjørnøya. Until 2005, only a few foxes were living there, but since then active dens have been found almost every year.

The reasons for monitoring arctic foxes in Svalbard are:

1. The Arctic fox is a top predator that has a large influence on processes in the terrestrial ecosystem
2. Arctic foxes have been trapped or shot for several hundred years and hunting still takes place every year. The monitoring is important to enable management advice to be given concerning the effects of hunting and whether the population trend indicates that the hunting is sustainable.
3. The Arctic fox is a vector for zoonoses (wildlife diseases that can be transmitted to humans) like rabies, the parasite toxoplasmosis and the parasite found in mice, Echinococcus multilocularis. Monitoring the prevalence of these is a concern for human health.

Places and areas

• Brøggerhalvøya
• Bjørndalen
• Adventdalen
• Sassendalen

Relations to other monitoring

Monitoring programme

Climate-Ecological Observatory for Arctic Tundra (COAT)

International environmental agreements

None

Voluntary international cooperation

None

Related monitoring

None