Grey fox (Urocyon cinereogenteus) COSEWIC assessment and update update status report: chapter 10

Limiting factors and threats

Undoubtedly, the most important factor limiting grey fox populations in the U.S. is harvesting by humans (Fritzell 1987). In the mid 1970s, it was estimated that almost one half of Wisconsin’s grey fox population was harvested annually (Petersen et al. 1977). However, this harvest pressure has apparently been sustainable, because the grey fox continues to be harvested in large numbers in this state (WIDNR 2000). In addition, the grey fox has increased in number in Minnesota (B. Berg, pers. comm.), Wisconsin (J. Olson, pers. comm.), Michigan (T. Reis, pers. comm.) and New Hampshire (E. Orff, pers. comm.), despite there being no limits on the numbers harvested in those states (Table 1). The few grey foxes trapped each year in Canada are incidental captures by trappers targeting red fox. The effect of this grey fox by-catch on the grey fox population is unknown. Intraguild competition with other canids is widely cited but rarely demonstrated (see Nutrition section), and it is likely that grey foxes are able to avoid close competition with canids such as coyotes and red foxes, since they are often found in sympatry (e.g. Cypher 1993; Layne and McKeon 1956). Predation on grey foxes by larger predators such as coyotes, bobcats or golden eagles is not thought to be important to population changes (Fritzell 1987). Palmer (1956) thought that the extension of the grey fox’s range into the northeastern United States was associated with the range extension of one of its prey species, the eastern cottontail. However, the opportunistic nature of the grey fox’s diet seems to argue against this hypothesis. Diseases such as canine distemper and rabies are fatal to grey foxes and have been reported in significant numbers of wild individuals (Davidson et al. 1992; Gier 1948). In an epizootic situation these diseases could represent significant limiting factors. Climate is another factor that may be important to long-term trends in grey fox populations. Waters (1964) suggested that past warming and cooling trends have caused the range of the grey fox to expand and contract. Adverse environmental and nutritional conditions at the northern periphery of the grey fox’s range have been implicated in lower productivity (Root and Payne 1985). Also, the northward expansion of the grey fox’s range may be limited by the energy costs of locomotion in deep snow (M. Crête, pers. comm.). Deforestation may limit grey fox populations by reducing the availability of dense cover and the variety of habitats that are preferred by U. cinereoargenteus (Fritzell 1987). Another factor, which may have adverse effects on grey fox populations in Canada, is road mortality. Grey foxes in central New York were slightly less likely than red foxes to be shot by hunters or hit by vehicles, possibly indicating that their more secretive lifestyle enabled them to avoid contact with humans better than red foxes (Tullar and Berchielli 1982).

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