Food-niche overlap between arctic and red foxes

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