Food-niche overlap between arctic and red foxes

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Elmhagen et al. 1275al. 1999). To assess the importance of competition betweenarctic and red foxes, it is therefore necessary to study theirniche overlap, and food niche is one dimension.Arctic foxes in Fennoscandia and other inland habitatstypically use lemmings as their primary prey, but they alsoinclude voles, birds, eggs, hares, insects, berries, and carcassesof reindeer or caribou (Rangifer tarandus) in theirdiet (Macpherson 1969; Angerbjörn et al. 1999; Strand et al.1999; Dalerum and Angerbjörn 2000; Elmhagen et al. 2000).Voles are usually the most important prey for red foxesin Sweden and northern Europe, but birds, eggs, roe deer(Capreolus capreolus), lagomorphs, insects, fruit, and berriesare also fed upon (Englund 1965; Lindström 1989; Goszczynski1974; Leckie et al. 1998; O’Mahony et al. 1999). In a review,Hersteinsson and Macdonald (1982) concluded thatboth arctic and red foxes are highly opportunistic, and suggestedthat the more limited prey base in arctic areas is theonly reason why arctic fox populations often have a less varieddiet than red fox populations. This assumption is supportedby the results of a feeding experiment in which foxescould choose between already dead prey of different species:neither red foxes nor arctic foxes showed a preference forvoles or lemmings (Barth et al. 2000). On the contrary, threedietary studies involving sympatric arctic and red foxes showedthat arctic foxes generally eat more lemmings and fewervoles than red foxes do, and it was suggested that red foxescould be more generalistic feeders (Smits et al. 1989; Frafjord1995; Frafjord 2000). However, the samples in the two earlierstudies were very small and none of the studies providedany data on prey availability. Thus, it is not possible to determinewith certainty what causes the observed dietary differences.Either arctic and red foxes could have differenthunting behaviours that result in different prey preferences,or they could be segregated on a small spatial scale andtherefore have access to different prey bases in areas wherethey are regarded as sympatric. Differences in prey preferencesindicate that there is food-niche separation betweenthe fox species and that competition may be of less importance.However, if there are no fundamental food-niche differences,spatial segregation could result from interspecificcompetition.The best way to determine if two species compete is tocompare “actual” and “virtual” niche overlaps, these beingthe local population equivalents of the realised and fundamentalniches of an entire species (Colwell and Futuyma1970). For example, food competition between mink (Mustelavision) and otter (Lutra lutra) was investigated throughcomparisons of food-niche breadths and food-niche overlapsunder sympatric and allopatric conditions (Clode and Macdonald1995). In Fennoscandia there are no areas of mountaintundra where red foxes are absent to the extent that wecould study the “virtual diet” of the arctic fox. However, wecan test whether there is small-scale spatial segregation betweenthe species with respect to altitude, an important habitatvariable. Further, lemming and vole populations fluctuateover time and relative abundances change from year to yearat all altitudes (Oksanen et al. 1999). Thus, we can alsostudy how arctic and red foxes respond to differences in preyspecies composition. If there are species-specific differencesin prey preferences, i.e., if arctic and red foxes have differenthunting behaviours, they will respond differently to changesin the composition of the prey community. Consequently,the degree of niche overlap will vary between years. If, instead,they use prey similarly, niche overlaps should be relativelyconstant in time. There are four possible scenariosdepending on the existence of spatial segregation and (or)species-specific prey use: (1) No spatial segregation, no differencesin prey preferences: arctic and red foxes have thesame diets, i.e., food-niche overlap is 100%. (2) No spatialsegregation, different prey preferences: arctic and red foxeshave different diets and the degree of food-niche overlapchanges between years. (3) Spatial segregation, no differencesin prey preferences: arctic and red foxes have differentdiets, owing to differences in the composition of the preycommunity at different altitudes, but the degree of foodnicheoverlap is consistent between years. (4) Spatial segregation,different prey preferences: arctic and red foxes havedifferent diets and the degree of food-niche overlap changesbetween years.In this study we investigated the summer diets of sympatricarctic and red foxes over 4 years to find out which scenariois enacted on the Fennoscandian mountain tundra. We testfor dietary differences between arctic and red foxes, foraltitudinal segregation between these species, and for an effectof altitude on the occurrence of different prey species intheir diets. We also study whether food-niche overlap andfood-niche breadth change between years. Finally, we discussthe implications of arctic and red fox food niches forthe degree of interspecific competition.Materials and methodsStudy areaThe study area covered 1300 km 2 of mountain tundra inthe Vindelfjällen Nature Reserve in Swedish Lapland (66°N,16°E). The main vegetation types were dry heath, grass heath,and dry fen. Plant-species diversity was low, except for somerelatively rich patches (Rune 1981). Arctic foxes bred onlyabove the tree line, which was situated at 750 m asl. Mostred foxes breed below the tree line, in birch and coniferousforests, but all red foxes included in this study inhabited formerarctic fox dens on the mountain tundra. These dens arehereinafter referred to as red fox dens. Prey species availableto the foxes were Norwegian lemming (Lemmus lemmus), fieldvole (Microtus agrestis), bank vole (Clethrionomys glareolus),grey-sided vole (Clethrionomys rufocanus), shrews (Sorex spp.),mountain hare (Lepus timidus), reindeer, rock ptarmigan(Lagopus mutus), willow ptarmigan (Lagopus lagopus), passerines(Passeriformes), ducks (Anseriformes), gulls, and waders(Charadriiformes).Lemming and vole populations are generally cyclic in arcticand mountain tundra areas (Elton 1924; Hanski et al.1991; Angerbjörn et al. 2001), but there have been occasionalprolonged interruptions in the cycle in Fennoscandia(Angerbjörn et al. 2001). During these, lemming populationsfluctuate with a periodicity resembling the normal cycle, butthe amplitude is smaller than usual. The interruptions havetherefore been described as periods without peak years, sincethe highest lemming densities are about 10 times lower thanduring “real” peaks (Angerbjörn et al. 2001; Tannerfeldt etal. 2002). The last lemming peak in Sweden occurred in1982, consequently our study was carried out during one of© 2002 NRC Canada
1276 Can. J. Zool. Vol. 80, 2002these periods. However, trapping data from our study areashowed that lemming abundance fluctuated between yearsand the highest summer density was at least 20 times higherthan during population low phases (Ecke et al. 2001). Theabundance of nesting birds does not change significantly betweenyears, although nesting success can vary (Svensson etal. 1984). We have no information on fluctuations in thereindeer and hare populations. Other predators in the areathat could interact with the fox species were wolverines(Gulo gulo) and golden eagles (Aquila chrysaëtos).Collection of scats and scat analysisWe have surveyed arctic fox dens in our study area since1985. We regard a den as inhabited when we observe foxcubs and (or) adult foxes regularly returning to and stayingat it. We also regard a den as inhabited when the amount offresh scats, prey remains, recently excavated openings, andwear and tear on the vegetation surrounding the openings indicatesthat the den is used to this extent, even though nofoxes are seen. Arctic and red fox families occasionally moveduring summer (Tannerfeldt et al. 2002). In such cases weonly used scats from one of the dens in our analyses. Thus,within the same summer, scats collected at different densrepresent different foxes.In the summers of 1993 and 1996–1998, we collectedscats from all dens inhabited by arctic or red foxes. Lemmingnest counts and snap-trapping data showed that thelemming population was in the low phase in 1993, increasedin 1996, decreased (crashed) in 1997, and remained low in1998 (Ecke et al. 2001; Tannerfeldt et al. 2002). Scats werealso collected in 1994 and 1995, but most were unfortunatelylost during storage, owing to a non-understandingmother. Most of the arctic fox scats collected in 1996 and1997 were also included in an earlier study on arctic fox diet(Elmhagen et al. 2000).We separated fresh scats, i.e., collected during the currentsummer, from older ones by their colour, position in the vegetation,and smell (Macpherson 1969; Birks and Penford1990). We made visual observations of foxes at all arctic foxdens but at only 6 of 10 red fox dens. When we did not seeany foxes, we relied on the presence of tracks and fur andthe size of openings of newly excavated dens to determinewhich species used the den. We collected scats only at inhabiteddens, so there was little risk of confusing arctic andred fox scats. There were cubs in all dens where we sawfoxes, except in 1997, when the lemming population was inthe crash phase. In that year, one cub that died early wasseen at one of three dens inhabited by arctic foxes, whilethere were cubs at one of two dens inhabited by red foxes.There can be differences between the prey eaten by adultfoxes themselves and those they bring to their cubs (Lindström1994). However, Frafjord (2000) found only one significantdifference in foxes’ diets between dens with and withoutcubs: insects occurred more often in scats from red fox denswith cubs, though insects were not an important prey item.We therefore assumed that the scats from each den gave afair representation of the prey caught by the adult foxes, independentlyof the proportion of adult to juvenile scats.Scats were dried at 105°C prior to analysis. Small or fragmentedscats were combined, so the analysed volume ofeach sample was approximately that of one medium-sizedscat. Such samples are hereinafter referred to as single scats.When more than 20 scats had been collected from a den, werandomly picked 20 for analysis. The total sample consistedof 293 arctic fox scats and 177 red fox scats from 15 and 9dens, respectively.We fragmented individual scats by hand under a magnifyingglass. We noted occurrences of different prey items andestimated the percent dry volume of mammal fur and bones,feathers, eggshells, insects, and vegetation by eye. We convertedthese volume estimates to “whole-scat equivalents” asdescribed by Angerbjörn et al. (1999). We summarized thepercent volumes of each food category in the scats collectedat the same den.Fur was identified as reindeer, hare, fox, or small mammalby means of a reference collection and microscopic charactersdescribed by Day (1966). We ignored single fox hairsand small tufts of fox hair that probably resulted from grooming.We determined lemming and vole species using descriptionsof molars in Niethammer and Krapp (1982). We didnot distinguish between bank vole and grey-sided vole, butused the category Clethrionomys spp. Shrews were also identifiedby the presence of teeth, but we did not discriminatebetween species. When teeth from more than one smallmammalspecies were found in the same scat, we used thenumber of teeth from each species to partition the volumesof fur and small-mammal bones among them. When therewere no teeth in a scat, small-mammal fur was classified as“unidentified small mammals” (Tables 1 and 3). For estimatingvolume, calculating niche overlap and niche breadth, andcorrelating the frequencies of occurrence of different preyitems with altitude, we assumed that the total volumes ofunidentified small mammals in the scats were in the sameproportions as the total volumes of small-mammal specieswhich had been identified at each den (Figs. 1 and 2, Table4). Consequently, unidentified small mammals were distributedin these categories accordingly.During the analyses we noted that teeth from small mammalsoccurred less frequently relative to the presence ofsmall-mammal fur in some years, and generally less often inred fox scats than in arctic fox scats (Tables 1 and 3). Thisappeared to be related to the proportion of voles to lemmings.If foxes handle lemmings differently from voles, theuse of teeth for estimating proportions of different smallmammals in the scats could result in a bias. However, if it isappropriate to use this method, the estimated proportions oflemmings, voles, and shrews should be approximately thesame if fur is used for identification. The fur of the Norwegianlemming is largely black and yellow, while the fur ofvoles and shrews is brown or greyish. We made coarse estimatesof the proportions of lemming and vole/shrew fur in25% steps from 0 to 100% for each scat. We compared theresults obtained through identifying teeth (where unidentifiedsmall mammals were distributed in the categories ofidentified small-mammal species) with the estimates obtainedfrom fur using Spearman’s rank-correlation tests, where eachden was treated as one unit. The two methods gave very similarresults, with significant correlations for both arctic foxes(lemmings: p < 0.001, r S = 0.85, n = 15; voles/shrews: p
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Food-niche overlap between arctic and red foxes

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