Arctic Fox (Vulpes lagopus)
A. Angerbjörn1, D. Berteaux2, R. Ims3
1Department of Zoology, Stockholm University
2Département de Biologie, Université du Québec à Rimouski
3Department of Biology, University of Tromsø, Norway
November 10, 2012
Highlights
- The current population of Arctic fox in Fennoscandia is estimated to be less than 200 individuals compared to over 15,000 in the mid-19th century.
- Arctic fox depend on lemmings as a primary food source; consequently fox population trends reflect changes in the lemming population cycle. Thus, for example, in Fennoscandia, Arctic fox reproductive peaks in 2011 were followed by reproductive failure in 2012, closely following lemming population trends.
- In North America over the last century, the Red fox has been expanding northward into historically arctic fox-only territories.
- Long term monitoring at Bylot Island, Nunavut, Canada, has shown relatively stable arctic fox populations and reproductive rates despite the presence of red foxes with low population density.
Introduction
The Arctic fox Vulpes lagopus (syn. Alopex) is a small circumpolar canid and opportunist predator inhabiting the arctic tundra. In many Arctic areas, it is the most abundant mammalian predator, affecting breeding success of migrating birds and possibly also lemming cycles (see the Lemming essay for more information about the lemming cycle and other topics). Where lemmings (Lemmus sp.) are present (Fennoscandia, Siberia, North America and parts of Greenland) the Arctic fox relies heavily on this prey, but also on other rodents (Microtus sp., Myodes sp.), ptarmigan, passerines and carcasses. Where lemmings are absent (Iceland, Svalbard and western Greenland), they live on other resources such as sea birds, seal carcasses and fish. Some populations switch between lemmings, migratory birds and marine resources depending on intra- and interannual variations in prey availability.
Lemming numbers fluctuate drastically, but normally with a regularity in a cycle with peaks every 3-5 years (lemming years). This variation in food availability is a key determinant of Arctic fox reproductive success. During a lemming population peak phase, an Arctic fox female can give birth to as many as 20 cubs (Tannerfeldt and Angerbjörn, 1998), while most females fail to reproduce during a low lemming phase, and those that do have only a few cubs. Arctic fox mortality is also dependent on the lemming cycle, with a higher survival in years with high density of lemmings. Juvenile mortality can be as high as 90% (due to starvation and predation) during their first year when lemming abundance is low.
The Arctic fox may also depend on remains of carrion left by larger predators such as the polar bear (Ursus maritimus), arctic wolf (Canis lupus), wolverine (Gulo gulo) and humans where seal hunting is common. Present low numbers of these predators in some areas may have reduced the amount of winter food available for Arctic foxes to scavenge (Angerbjörn et al., 2004). It is itself a victim of predation, mainly from Vulpes vulpes, Gulo gulo, Aquila chrysaëtos, and humans.
Arctic fox dens are usually situated in frost-free ground, often in low mounds or eskers (long winding ridges of stratified sand and gravel) on the open tundra. The dens have 4-250 entrances and a system of tunnels, which covers up to 1000 square meters (Dalerum et al., 2002). Some dens have been used for centuries by generations of foxes. Arctic foxes can start breeding in their first year of life and are considered to be essentially monogamous, although extra-pair paternity is frequent and they can increase group size at high population densities (Cameron et al., 2011; Norén et al., 2012). Territories are maintained by pairs during the breeding season or sometimes all year round, with size and shape determined by food availability. Foxes sometimes wander hundreds of kilometres from their territories during winter, both on land and on sea ice. Young foxes can sometimes stay in their parent's home range but are usually not allowed to breed. Mating usually occurs in April-May, and the young are born after a gestation of 52 days. At the age of three to four weeks the young start to appear outside the den and weaning takes place at the age of 5 to 9 weeks. The young gradually become independent during the following month. Some cubs may leave the den in their sixth week of life while others stay until early spring before they disperse. The average life span for animals that reach adulthood is around three years.
The Artic fox has developed many physical attributes that have allowed them to adapt to the Arctic environment and they do not hibernate during the winter months. The winter fur of the Arctic fox has the best insulative properties among all mammals. They further conserve body heat by having fur on the soles of their feet (Linnaeus named it lagopus, hare-foot), small ears, a short nose, and a well-developed ability to reduce blood flow to peripheral regions of the body. In autumn, they can put on more than 50% of their body weight as fat for insulation and as energy reserves. Arctic foxes change between summer and winter pelage and thereby adjust insulating properties and enhance camouflage.
Most information about population status and the ecology of the species is from Europe (in particular Fennoscandia) and North America.
Europe
In mainland Europe, the Arctic fox is found above the alpine tree line in the mountain tundra of Fennoscandia (Sweden, Finland, Norway) and north of the arctic tree-line in north-eastern Norway and north-western Russia. Today, the Fennoscandian Arctic foxes are distributed in four geographically separate areas (Fig. 4.5). Between these populations, the migration rate is very low or absent (Dalén et al., 2006). The present population status and its historical development is best known from mountain tundra of Fennoscandia located in the north-western regions of the Swedish counties (Fig. 4.5) of Jämtland (C), Västerbotten and Norrbotten (A) and in the northern regions of Finnish Lapland (Dalén et al., 2006). In Norway, it is found in the eastern, south-central and northern region.
The Arctic fox was one of the first mammal species to colonize Fennoscandia after the last ice age and is well known for its fluctuating population size due to the lemming cycle. Historically, the Arctic fox was an abundant species, with breeding populations reaching at least 15,000 individuals in lemming peak years during the mid-19th Century. However, fox numbers suffered a drastic decline due to over-harvesting by the fur industry at the beginning of the 20th Century. Despite legal protection, the population has remained at a low density for over 80 years. The situation deteriorated further in the 1980s and 1990s because of an absence of lemming peaks. In 2000, only 3 Arctic fox litters were found in Sweden, Finland and Norway, indicating that the population was close to extinction. It is a priority species according to the European Commission Habitats Directive.
At the present population size of less than 200 individuals in mainland Europe, even a small change in demographic parameters or pure "accidents" can dramatically affect the risk of extinction. As a rule of thumb, vertebrate populations smaller than 500 individuals are considered to be at considerable risk of extinction (Thomas, 1990), although considerably larger populations might be required to secure long-term persistence (Traill et al., 2007). Many young Arctic foxes have little chance of finding a non-related partner. The species is highly dependent on a regular pattern of population cycles of small rodents. The main threat is the small population size constrained by low food availability due to disruptions of the rodent cycle. The Red fox (Vulpes vulpes) poses an additional threat to the Arctic fox; this is discussed in more detail below.
More recently, in 2011, Fennoscandia experienced a peak lemming year, which favoured Arctic fox reproduction. Sweden observed about 65 Arctic fox litters that same year (Fig. 4.6). This was the most productive summer since 1970. However, lemming populations crashed in 2012 and resulted in complete reproductive failure in Sweden and Norway (Angerbjörn et al., unpublished). This is a clear demonstration of the current vulnerable state of the species in this part of Europe and how strongly Arctic foxes depend on lemming abundance.
It is believed that climate change has decreased snow cover, and increased growing season and primary productivity in northern Fennoscandia - see the Snow essay for more information on snow cover change, and the Vegetation essay for more information about changing growing season and primary productivity. Connected to this warming and possibly also land use change in tundra areas (Killengreen et al., 2011) is the impact of the Red fox. It is twice the size of the Arctic fox and has about twice the home range area. The Red fox is a dominant competitor, a predator on juveniles and adults, and is currently increasing its range above the tree line, taking over dens and excluding Arctic foxes from parts of their breeding range (Tannerfeldt et al., 2002) and carrion food resources during the winter (Killengreen et al., 2012). Consequently, the Arctic fox is forced to retreat to higher altitudes during summer to reduce the risk of Red fox predation on their cubs. Regarding the food niche, Red fox has a broader range of alternative prey sources that are less accessible to Arctic foxes during low lemming phases.
Projects such as Save the Endangered Fennoscanidan Alopex, SEFALO and SEFALO+ have successfully used a combination of supplemental feeding and Red fox control to increase the number of Arctic foxes. However, earlier conservation attempts did not reach major parts of the distribution area. For example, SEFALO+ only reached the B and C sub-populations, and the situation is still critical in the larger parts of the distribution range A (Fig. 4.5).
North America
In North America the Arctic fox is abundant and the overall population probably ranges in the tens of thousands of individuals (ABA unpublished). From the 1920s to the mid 1970s, Arctic fox fur represented the most important asset traded by Inuit in Canada to secure cash and other valuable goods (Sawtell, 2005). Currently, trapping is no longer a major economic activity and only a few hunters still trap foxes. The effects on populations of the heavy trapping period followed by a virtual cessation of trapping are unknown.
Over the last century, a northward expansion in the distribution of the Red fox has been observed in North America (Macpherson, 1964). For example, according to pelt records, Red foxes were observed for the first time in 1918 in southern Baffin Island, Nunavut, Canada, and continued their expansion northward to reach Pond Inlet and Arctic Bay in the late 1940s (Macpherson, 1964). Nowadays, Red foxes breed in the High Canadian Arctic, e.g., on Bylot Island, Nunavut. Coinciding with this northward spread of the Red fox, the Arctic fox abundance appears to have declined in some places, as documented through local traditional knowledge (Gagnon and Berteaux, 2009). There is now an extensive zone of overlap in the distribution of both species.
Detailed long-term monitoring of an Arctic fox population on Bylot Island began in 1993. The monitoring was first opportunistic but became systematic in 2003. About 100 dens covering approximately 600 km2 are now surveyed annually (Cameron et al., 2011). About 20-35 adults and 10-60 cubs are trapped and marked every year. The monitoring also involves automatic cameras placed at dens, systematic analysis of feeding regime through stable isotopes, and year-round monitoring of space use through satellite telemetry. The reproductive population varies greatly between years and is strongly dependent on the local lemming cycle. No long-term trend in abundance was observed in the last decade (Fig. 4.7). The Red fox breeds in the study area where it competes with and predates on arctic foxes, but the number of reproductive individuals is stable at 0-2 per year.
Arctic fox and Red fox distributions also overlap in western North America. Aerial surveys were conducted at Herschel Island, Yukon, Canada, and the coastal mainland to investigate the relative abundance of Red and Arctic foxes over the last 40 years. Although these areas have experienced intense climate warming trends in recent decades, little change in the relative abundance of the two species was observed (Gallant et al., 2012). Fox dens in northern Yukon are mostly occupied by Arctic fox, with active Red fox dens occurring sympatrically, i.e., the latter live in the same territory without interbreeding with Arctic fox. Although vegetation changes have been reported, there is no indication that secondary productivity and food abundance for foxes have increased, which may explain the population stability. Thus, as indicated for northernmost Fennoscandia, the expansion of Red fox at the expense of the Arctic fox may be related to other phenomena causing increased secondary productivity and resources to expanding Red fox populations (Killengreen et al., 2011).
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