Arctic fox (Vulpes lagopus)

Arctic fox late winter in Svalbard
Photo: Stein Ø. Nilsen / Norwegian Polar Institute

The arctic fox is indigenous to the Arctic tundra and an apex predator and scavenger in Svalbard. It is functionally important because it affects prey species on land, through ground nesting birds such as grouse and geese and reindeer carcasses, and marine resources such as seabirds, seal cubs and seal carcasses, as well as being harvested annually and being the main host for dangerous zoonoses. Rapid climate change impacts on the population dynamics of the arctic fox through multiple drivers, such as access to reindeer carcasses, marine subsidies and zoonoses

What is being monitored?

Percentage of known dens with cubs

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The proportion of known dens in the Kongsfjorden area containing cubs has varied considerably during the monitoring period, from 0 to 100%. It is assumed that variations in the availability of food are chiefly responsible for this. At Kongsfjorden arctic foxes search for food on and in the close vicinity of bird cliffs. In Adventdalen and Sassendalen, breeding in all or none of the known dens has never been known. During the monitoring period, the proportion of occupied dens has fluctuated between 10% and 48%. Den sites show a variation from those located in the vicinity of bird cliffs to those in the major valleys.

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. 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 remained relatively stable, varying annually by between 10% and 70%, but never with a reduction as dramatic as after 1995.

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. 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. Sassendalen, where both geese and reindeer are available, is regarded as richer than Adventdalen, which only has reindeer. 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.

We have begun using integrated population models (IPM) to examine the effects of multiple drivers on the arctic fox population trend such as climate warming and changes in land-based and marine resources, increasing reindeer carcasses and geese, and declining access to seal cubs and seal carcasses as a result of diminishing sea ice. Despite rapid environmental changes caused by climate warming, the size of the arctic fox population remained relatively stable in the Adventdalen/Sassendalen area between 1997 and 2019

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 has also been shown in later studies.

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 coastal dens and breeding frequency amongst the inland dens, access to carcasses during the winter may be an 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.

Thus, the population dynamics of arctic foxes have been largely driven by access to reindeer carcasses which are affected by “rain-on-snow events”, which cause increased mortality amongst reindeer as their pastures become covered with ice. Furthermore, the numerous seabird colonies represent a stable nutrient contribution which has a stabilising impact on the arctic fox populations.  In Nater et al. (2021), we analysed the response of multiple drivers in the ecosystem and show that, despite increasing breeding frequency, the estimated population dynamics of arctic foxes in the Adventdalen/Sassendalen monitoring area are relatively stable and driven by non-conformant trends in various vital rates (e.g. increased breeding frequency, but reduced cub survival). Balanced contributions from survival versus breeding, and immigration versus local demographics play a stabilising role as regards population size, which is explained by the fact that the exploitation of resources from different ecosystems buffers arctic foxes against climate change in Svalbard.

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. 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.

Identifying the effects of multiple drivers such as access to reindeer carcasses, marine subsidies and zoonoses on the arctic fox population enables certain conclusions to be drawn concerning the ecological status of Svalbard. The possibility of exploiting resources from different ecosystems (land and marine) helps to buffer the arctic fox population against climate change. However, the future fate of Svalbard’s arctic fox populations will depend on how climate change affects their most important prey – reindeer, geese and seabirds, as well as other marine food sources such as seals, which are dependent on the distribution of the sea ice, in addition to dangerous zoonoses. Ischnocera have recently been discovered in the arctic fox population in Svalbard, and it is uncertain how this situation will develop or impact on the arctic fox population in the long term

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. Because reindeer and geese are important food resources for arctic foxes, the management of these species must also be taken into consideration in the management of arctic fox populations.
  2. Arctic foxes have been trapped or shot for several hundred years and hunting still takes place every year. . In an ecosystem which is exposed to the world's fastest rate of climate change, monitoring is vital in order to provide management advice concerning the overall impact of hunting and climate change, and whether the population trend indicates that hunting will be sustainable in a new climate scenario and able to counteract the effects of arctic fox predation on vulnerable prey species such as Svalbard ptarmigan and other ground nesting species.
  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

The breeding population of arctic foxes is monitored in two areas of West Spitsbergen using den monitoring. In a study area covering 220 km2 in the Brøggerhalvøya/Kongsfjorden area, six to nine breeding dens have been monitored annually since 1993 through to the present day.

In the second study area covering an area of 900 km2 in Adventdalen/Sassendalen, up to 32 breeding dens have been monitored over two periods, between 1982 and 1989 and from 1997 to the present day. These dens are visited every year (in late June to early August), and arctic fox activity (the presence of adult foxes and prey remains), the number of cubs (during the breeding season) and disturbances by humans are recorded.

Relations to other monitoring

Monitoring programme
International environmental agreements
Voluntary international cooperation
Related monitoring