Arctic fox versus red fox in the warming Arctic: four decades of den surveys in north Yukon

Abstract

During the last century, the red fox (Vulpes vulpes) has expanded its distribution into the Arctic, where it competes with the arctic fox (Vulpes lagopus), an ecologically similar tundra predator. The red fox expansion correlates with climate warming, and the ultimate determinant of the outcome of the competition between the two species is hypothesized to be climate. We conducted aerial and ground fox den surveys in the northern Yukon (Herschel Island and the coastal mainland) to investigate the relative abundance of red and arctic foxes over the last four decades. This region has undergone the most intense warming observed in North America, and we hypothesized that this climate change led to increasing dominance of red fox over arctic fox. Results of recent surveys fall within the range of previous ones, indicating little change in the relative abundance of the two species. North Yukon fox dens are mostly occupied by arctic fox, with active red fox dens occurring sympatrically. While vegetation changes have been reported, there is no indication that secondary productivity and food abundance for foxes have increased. Our study shows that in the western Arctic of North America, where climate warming was intense, the competitive balance between red and arctic foxes changed little in 40 years. Our results challenge the hypotheses linking climate to red fox expansion, and we discuss how climate warming’s negative effects on predators may be overriding positive effects of milder temperatures and longer growing seasons.

Appendix

Procedure comparing proportions in overlapping samples (from Bland and Butland, unpublished)The procedure by J. M. Bland and B. K. Butland comparing proportions in overlapping samples (http://www.users.york.ac.uk/~mb55/overlap.pdf, accessed on March 4, 2011) calculates two differences of proportions: one for paired data (in our case, dens surveyed in each of the compared periods) and another for unpaired data (in our case, dens only surveyed in one of the periods). It then combines the calculated differences in a weighted average to produce a single difference of proportions. We used the procedure to obtain 95 % confidence intervals to decide whether differences between proportions were statistically significant. We pooled the survey data within the periods 1984–1990 and 2003–2010 and classified each den as having been used or not by foxes within each of the two survey periods. We describe below the calculations supporting Bland and Butland’s procedure, using symbols described in Table 2.

Table 2 Representation of paired and unpaired data each divided into two datasets according to usage of dens by foxes

The difference (d) in the paired (p) dataset between the number of dens used in the first sample but unused in the second sample (n ₁₀), and the number of dens unused in the first sample but used in the second sample (n ₀₁) is:

$$ d_{p} = \frac{{n_{10} - n_{01} }}{n} $$

with variance (Var):

$$ Var\left( {d_{p} } \right) = \frac{{n_{10} + n_{01} }}{{n^{2} }} - \frac{{(n_{10} - n_{01} )^{2} }}{{n^{3} }} $$

while the difference in the number of dens used in the first (n _x ) and second (n _y ) samples in the unpaired (u) dataset is:

$$ d_{u} = \frac{{n_{x} }}{k} - \frac{{n_{y} }}{m} $$

where k and m are the respective sample sizes of the two samples, with variance:

$$ Var\left( {d_{u} } \right) = \frac{{n_{x} (k - n_{x} )}}{{k^{3} }} - \frac{{n_{y} (m - n_{y} )}}{{m^{3} }} $$

Combining these two difference estimates using a weighted (w) average, we get:

$$ d = \frac{{w_{p} d_{p} + w_{u} d_{u} }}{{w_{p} + w_{u} }} $$

where:

$$ w_{p} = \frac{1}{{Var(d_{p} )}} $$

and:

$$ w_{u} = \frac{1}{{Var(d_{u} )}} $$

with variance:

$$ Var\left( d \right) = \frac{1}{{\frac{1}{{Var(d_{p} )}} + \frac{1}{{Var(d_{u} )}}}} $$

The 95 % confidence interval is thus:

$$ d - 1.96\sqrt {Var(d)} \le d \le d + 1.96\sqrt {Var(d)} $$