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Red Foxes have Not Replaced Arctic Foxes in the Northern Yukon

Reference
Gallant, D., Slough, B.G., Reid, D.G. and Berteaux, D. Arctic fox versus red fox in the warming Arctic: four decades of den surveys in north Yukon. Polar Biology 35: 1421-1431.
Both the red fox (Vulpes vulpes) and the arctic fox (Vulpes lagopus) occupy similar ecological niches and consume similar prey, including small rodents and scavenged kills of larger carnivores. The red fox is at least twice as large as the arctic fox and is therefore large enough to kill the smaller foxes if disputes over territory or food arise. However, both species can coexist because the arctic fox is so much better adapted to high arctic conditions. Over the last century or so, the red fox has expanded into traditional arctic fox territory, in Canada as well as Eurasia, as the climate has warmed and vegetation shifted, raising concerns that the red fox will eventually out-compete the arctic fox (Hersteinsson and Macdonald 1992; Killengreen et al., 2007).

In order to investigate whether red foxes have become more common over the last forty years, Gallant and colleagues chose a study site in the northern Yukon, where warming due to climate change has been most intense. Their research area stretched from the Alaska-Yukon border east to the Blow River along the coastline of the Arctic Ocean, including Herschel Island (virtually all of the study area was within Ivvavik National Park). The authors undertook a survey of fox dens within this region between July 2008 and the summer of 2010 and compared the results to 13 previous surveys (1971-1972; 1984-1990; 2003).

They conducted a helicopter survey in 2008 of the mainland sites, landing at all dens to document den status. Herschel Island was surveyed in 2008 on foot in June and again in July. In 2009, the Herschel Island sites were visited monthly from May to August and the mainland sites during July. Many of the dens were checked again during 2010 by volunteers. Altogether, 164 different den sites were visited in 2008-2010 and the team were successful in relocating 74 of 136 dens known from past surveys (with 90 new dens located).

The fox species using each of the dens was verified by direct sightings or presence of winter fur, in part to determine whether occupation of individual dens had changed over time. They categorized each of the dens as one of the following: changed from red to arctic fox, changed from arctic to red fox, used by arctic fox once, used by red fox once, used by arctic fox several times, used by red fox several times, inactive, and used by unknown species.

After analyzing their data, Gallant and colleagues (2012) found that "the relative abundance of red and arctic foxes changed little over four decades, with recent survey results falling within the range of past ones." They also found that "a total of 62.3% of dens for which we have ≥ 4 years of data spanning ≥ 18 years (n = 61) have been occupied by arctic foxes at least once without any detection of red fox usage...the percentage of dens that changed from arctic fox to red fox occupation (8.2%, 5 dens) was similar to that of dens that changed from red fox to arctic fox occupation (9.8%, 6 dens)."

The authors state: "We reject our hypothesis that climate warming has led to increasing dominance of red fox over arctic fox in tundra habitats of the north Yukon during the last four decades" even though there has been significant warming and an increase in primary productivity documented for this region.

In addition, they point out that "our results spanning four decades of climate warming do not support Hersteinsson and Macdonald's (1992) proposition that climate warming triggers a bottom-up chain of increased productivity leading to increased abundance of the larger red fox. Also, we observed unchanged red fox abundance as winters became warmer..."

The authors suggest that "climate warming in the Arctic also had several negative effects on red foxes, and that these negative effects overrode the positive effects potentially arising from longer growing seasons and milder winters" and conclude that "it is reasonable to consider red foxes as an integral part of the north Yukon tundra ecosystem, rather than as an invasive species whose presence has been enabled by climate change."

Additional References
Hersteinsson, P. and Macdonald, D.W. 1992 . Interspecific competition and the geographical distribution of red and arctic foxes V. vulpes and A. lagopus. Oikos 64: 505-515.

Killengreen, S.T., Ims, R.A., Yoccoz, N.G., Bråthen, K.A., Henden, J.-A. and Schott, T. 2007. Structural characteristics of a low Arctic tundra ecosystem and the retreat of the Arctic fox. Biological Conservation 135: 475-488.

Archived 2 October 2012