Disentangling the importance of interspecific competition, food availability, and habitat in species occupancy: Recolonization of the endangered Fennoscandian arctic fox

Abstract

Ecosystems alterations can profoundly affect species occurrence and distribution, thereby influencing trophic and/or competitive interactions. Arctic foxes have been fully protected in Fennoscandia following their drastic decline in the early 19th century, and their lack of recovery has been hypothesized to result from increased interspecific competition with red foxes that have colonized the arctic fox’s range and/or changes in prey dynamics due to increased variation in climatic conditions. We used a large-scale spatio-temporal study combined with an extensive red fox removal intervention to test these two hypotheses by evaluating the relative importance of diverse environmental factors affecting the recolonization of arctic foxes in Northern Norway. Arctic foxes were never observed at sites where just a few daily observations of red foxes were recorded, and the probability of recolonization was zero at sites where no red fox was removed. The probability of recolonization increased when lemming abundance was very high, but the relative importance of this variable was half that of red foxes. Thus, our results lend support to both hypotheses, but they clearly highlight the presence of red foxes as the factor most limiting arctic fox recolonization in this part of its distribution. As the abundance of arctic foxes has remained extremely low even after more than 70years of full protection, direct conservation actions seem to be needed to promote the recovery of arctic fox populations. Our research therefore highlights that conservation actions targeting a major reduction in red fox numbers are needed because the competitive interaction with red foxes is the most important factor limiting recolonization of this species.