Food-niche overlap between arctic and red foxes

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Elmhagen et al. 1283sample was small in some years, the overall pattern indicatesthat arctic and red foxes would use similar prey bases similarly.Differences between the species at the same altitudeswere more likely due to temporal variations in prey availability,since they were never found at the same altitude inthe same year.Food-niche breadths varied substantially among years forboth species. Arctic foxes showed a higher degree of specializationthan red foxes when the lemming population wasin the increase and crash phases. During lemming low phases,both fox species were relatively generalistic. These variationsin food-niche breadth are probably consequences ofopportunistic feeding behaviours. When lemmings are abundant,both species use them to a large extent, but when lemmingdensities are low they find other prey. Since arcticfoxes were found at altitudes where lemmings should makeup a relatively larger proportion of the prey community, theyshowed more pronounced shifts between specialized andgeneralized feeding strategies.The opportunistic feeding behaviour of arctic foxes is welldocumented. Arctic foxes in coastal habitats have access tomore types of prey than inland foxes do, and have a more diversifieddiet (Chesemore 1968; Fay and Stephenson 1989;Birks and Penford 1990; Hersteinsson and Macdonald 1996;Kapel 1999; Anthony et al. 2000). Foxes near bird coloniesincrease their consumption of birds and eggs upon the arrivalof migrating birds (Stickney 1991; Bantle and Alisauskas1998), while foxes in areas where prey are locally abundantadjust their use of habitat according to seasonal changes inprey distribution (Jepsen et al. 2002).Red foxes in southern Sweden have more diversified dietsthan those in northern Sweden (Englund 1965; Lindström1982). Red fox populations are also relatively stable in thesouth. No direct relationship has been demonstrated betweenreproduction and the availability of any particular prey species,and southern red fox populations are likely to be sociallyregulated. In contrast, the sizes of northern populations varymarkedly with the vole cycle (Englund 1970; Lindström 1989).Thus, just as arctic foxes in inland and coastal habitats havedifferent feeding strategies, there appears to be a south–north gradient in the feeding strategies of red foxes. Alongthe gradient of decreasing productivity in Sweden, from northernforests to low-production mountain tundra, red foxesshould have progressively less varied diets. This is supportedby Englund’s (1965) finding that red foxes on the mountaintundra consumed fewer birds than those in the forest.Because of their larger body size, red foxes require higherprey densities to sustain themselves than arctic foxes do(Hersteinsson and Macdonald 1992), and red foxes are usuallyfound in dens at lower altitudes, i.e., in more productiveareas. However, if arctic and red foxes are equally opportunisticand there are no differences between their virtual foodniches, it is difficult to see why arctic foxes would notendeavour to use the lower parts of their former range. Further,avoidance of lower altitudes, which has also been indicatedby radio-tracking of arctic foxes in Fennoscandia (Landa etal. 1998), is the opposite of the behaviour of arctic foxes onSvalbard, where productive habitats are favoured and morebarren areas avoided (Jepsen et al. 2002). Instead, competitionwith red foxes may explain why arctic foxes have retreatedto higher altitudes, and the high degree of food-nicheoverlap indicates that arctic and red foxes should competefor the same areas, as suggested by Hersteinsson and Macdonald(1982, 1992).What competitive mechanisms, then, may be important?Arctic and red foxes are territorial towards each other(Tannerfeldt et al. 2002) and it is unlikely that red foxes severelyreduce prey densities in areas outside their territorialborders. Exploitative competition should therefore be of littleconsequence on a larger scale. Instead, observations inthe wild and experiments involving penned individuals haveshown that red foxes predominate over arctic foxes and cankill both adults and juveniles (Rudzinski et al. 1982; Schameland Tracy 1986; Frafjord et al. 1989; Korhonen et al. 1997;Tannerfeldt et al. 2002). Arctic foxes also seem to avoid reproducingin dens near reproducing red foxes, which indicatesthat there is a competitive mechanism by which redfoxes can influence the distribution pattern of arctic foxes(Tannerfeldt et al. 2002). Interference competition, mediatedthrough territorial and encounter competition (Schoener 1983),could therefore be a mechanism of habitat segregation, withred foxes occupying the more productive habitats. The restrictedprey base in unproductive habitat will make arcticfoxes more dependent on lemmings than red foxes are.Therefore, competition between red and arctic foxes must beconsidered in efforts to conserve the arctic fox population inFennoscandia (Löfgren and Angerbjörn 1998).We conclude that arctic and red foxes on the Swedishmountain tundra have similar virtual food niches. Observeddifferences in arctic and red fox diets appear to be consequencesof spatial segregation that may, in turn, be causedby interference competition. The red fox is the strongercompetitor and might exclude the arctic fox from more productiveareas. Thus, an expansion of the range of the red foxduring the 20th century might have led to a reduction in theactual niche of the arctic fox.AcknowledgementsWe are grateful for funding from Oscar and Lili Lammsstiftelse, Carl Tryggers stiftelse, foundations at the RoyalSwedish Academy of Science, Magnus Bergvalls stiftelse,European Union LIFE-Nature, and World Wildlife Fund Sweden.Fjällräven AB, AB Dogman, Campbell’s, Milko, CloettaFazer, and Tågkompaniet supplied products that were usedin this project. We are also grateful for all the hard work putin by voluntary field workers.ReferencesAbrams, P. 1980. Some comments on measuring niche overlap.Ecology, 61: 44–49.Angerbjörn A., Tannerfeldt, M., and Erlinge, S. 1999. Predator–prey relations: arctic foxes and lemmings. J. Anim. Ecol. 68:34–49.Angerbjörn, A., Tannerfeldt, M., and Lundberg, H. 2001. 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Food-niche overlap between arctic and red foxes

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