Swift fox (Vulpes velox) COSEWIC assessment and status report 2009: chapter 5


Habitat

Habitat Requirements

Preferred habitat is short– or mixed–grass prairie on level terrain or gently rolling hills (Cutter 1958a; Kilgore 1969; Hillman and Sharps 1978; Hines 1980). Swift Foxes occasionally occur on fallow cultivated lands, usually adjacent to short–grass prairie (Cutter 1958a; Kilgore 1969; Floyd and Stromberg 1981; Jackson 1997; Sovada et al. 1998; Matlack et al. 2000). They usually select areas where vegetation is short (25 cm or less) and sparse, and with minimal topographic features such as canyons, steep hills, or coulees (Whitaker Hoagland 1997). This habitat use likely allows for long sight–lines that reduces predation risk (Pruss 1999).

In Canada, Swift Foxes prefer native grassland over cropland (Carbyn 1998). While survival rates of Swift Foxes living in fallowed sunflower fields in Kansas were similar to those of foxes living in prairie habitat (Sovada et al. 1998; Matlack et al. 2000), farmland in Canada does not appear to be similarly usable, likely due to the different nature of agricultural systems. Availability of mixed–grass prairie in Alberta and Saskatchewan therefore appears critical for reintroduction success and survival (Carbyn 1998; Smeeton et al. 2003). In the US, Swift Foxes sometimes occur in badlands (Wyoming; Lindberg 1986; Wooley et al. 1995), sand hills (Nebraska; Blus et al. 1967), and piñon–juniper habitat (Colorado; Covell 1992). While Swift Foxes can occur in a variety of habitats, ranging from mixed agricultural areas (Kilgore 1969; Hines 1980; Sovada et al. 2003) to sagebrush steppe to short–grass prairie transition (Olson et al. 1997), distributional patterns of this species in Canada suggest that the context in which such habitats are situated greatly influences their relative quality.

Suitable habitat is also related to prey availability (particularly the density of fossorial mammals) and low predator abundance. There is evidence that Swift Foxes do not require standing water sources (Pruss 1999); however, proximity to permanent water does enhance habitat suitability (Mamo 1994). The presence of fossorial species, like American Badgers (Taxidea taxus) and Ground Squirrels (Spermophilus spp.) is presumably desirable, providing Swift Foxes with burrows, which they readily modify (Carbyn 1998). However, when soil type allows, they will excavate their own dens (Carbyn 1998). Density and distribution of dens is also an important habitat feature (Herrero et al. 1991), particularly for avoiding Coyote predation or Red Fox competition (Tannerfeldt et al. 2003).

Moehrenschlager et al. (2007a) used analyses of population census data to assess critical habitat characteristics that predict Swift Fox occurrence. They developed model parameters from winter live–trap survey data collected in Alberta, Saskatchewan, and Montana between 15 October 2005 and 15 February 2006. They also used satellite image data from August 1999 and September/October 2000 to assess 15 environmental variables in the trapping area that might be useful in predicting Swift Fox distribution. Four habitat parameters successfully explained variation in Swift Fox presence: moisture, topography, cropland, and habitat fragmentation. Swift Foxes primarily occurred in relatively dry areas and tended to avoid areas with large elevation changes, preferring habitats with gently sloping terrain. Swift Fox dens are often located on slightly raised areas (Pruss 1999; Tannerfeldt et al. 2003). Cropland and the presence of Swift Fox were negatively associated. There are reports of Swift Foxes inhabiting agricultural areas (Sovada et al. 1998; 2001), but at sites that tend to be left fallow in alternate years. Swift Fox also avoided fragmented landscape, edge habitats, and roads. Habitat parameters affected the occurrence of Swift Foxes more than 5 km from their release site, likely owing to home range size (Moehrenschlager et al. 2007a). From October 2008 – April 2009, A. Moehrenschlager conducted surveys with scent–post camera–traps in 32 previously sampled and 28 unsampled townships in the western half of the Canadian Swift Fox range. Results from this effort suggest that the model is not only predictive of fox presence/absence among censuses, but also highly predictive of Swift Fox presence in previously unsurveyed regions (A. Moehrenschlager 2009).

Habitat Trends

Swift Foxes inhabit some of the most modified landscapes in North America. The Prairie Ecozone represents about 5% of Canada’s land area, of which 52% lies in Saskatchewan and 34% in Alberta (Gauthier and Wiken 2003). Current estimates indicate that only 25–30% of native Canadian grassland habitats remain, concentrated in southeastern Alberta and southwestern Saskatchewan (Hammermeister et al. 2001).

The substantial conversion of native Canadian prairie habitats to agricultural uses was the most important contributor to the historic contraction of Swift Fox range (Hillman and Sharps 1978). In areas of prime cropland in Saskatchewan, less than 2% of native prairie remains (Prairie Conservation Action Plan [PCAP] Partnership 2003).

In recent times, the dominant agent of Swift Fox habitat loss has shifted from agriculture to industrial energy development and associated road networks. In both Alberta and Saskatchewan, rapidly expanding energy–related development (e.g., roads, wells and service trails) is encroaching on many native prairie remnants, contributing to continued fragmentation of the landscape (Carbyn 1998; Moehrenschlager 2000; Forrest et al. 2003; Samson et al. 2004). Previously isolated native grassland areas are being increasingly targeted for such activities. Moehrenschlager and Moehrenschager (2006) commented on the “unprecedented levels” of oil and gas industrial development during the latest census as compared to the early 1990s. There are also a number of significant wind energy development projects at various stages of approval and construction in the region. This includes a recent resolution sponsored by Cypress County to seek permission from the Alberta government to install wind energy facilities on Crown lease lands in the heart of Swift Fox range (Binder 2009).

Continued demand for fossil fuels likely means that an increasing number of areas will be targeted for exploration with negative consequences for Swift Foxes. Issues associated with this type of disturbance include general habitat degradation and loss, reductions in prey availability, increasing risk of predation by, and interspecific competition, with other predators, significant increases in traffic and road–induced mortality, auditory disturbance, and increase in structure in grassland habitats providing perches for raptors (Moehrenschlager and Sovada 2004; Doherty et al. 2008).

In addition to physical changes in Swift Fox habitat from grassland conversion, the prairies have also experienced other significant biological changes. Plains Bison (Bison bison) were the predominant ungulate grazer until about 1870 and have been replaced by cattle, which differ in their grazing patterns. From 1956 to 1976, grazing pressure on mixed–grass prairie increased by one–third in Saskatchewan and one–half in Alberta (Coupland 1987). Changes in grazing patterns and intensity have altered vegetative cover and plant composition, and consequently, the composition and distribution of Swift Fox prey. For example, Swift Foxes prefer areas with sparse vegetation, interspersed with sites suitable for their small mammal prey (Carbyn 1998). In heavily cultivated areas of southern Saskatchewan, small mammals were abundant in all habitat types except pastures (Sissons et al. 2001), which likely negatively affected Swift Foxes. Well–managed rangeland with grazing regimes that increase rodent abundance should provide suitable year–round habitat for Swift Foxes (ASFRT 2007).

Climate change may also stimulate habitat modifications. Most models predict the Canadian prairies will experience mean annual temperature increases, resulting in droughts of increased extent and severity (e.g., Sauchyn et al. 2002; Bonsal and Regier 2007). Increased drought could alter the structure and composition of vegetation and thus Swift Fox populations (Lawton 1993). Additionally, altered landscapes could lead to changes in the abundance and/or distribution of Swift Fox prey, predators, and interspecific competitors, potentially leading to population decline.

The historic extent of potential Swift Fox habitat in Canada has decreased by 58% Gauthier and Wiken 2003). Within the remaining areas, Swift Foxes avoid those with greater topographic relief and shrubland, which are least likely to be converted to cropland. Thus, optimal Swift Fox habitat constitutes only a fraction of the remaining grasslands (Carbyn 1998). Results from recent habitat modelling and validation efforts indicate little potential Swift Fox habitat beyond that which is currently occupied. Existing Swift Fox range is slowly degrading in certain regions primarily due to expanding oil and gas exploration and associated road/trail networks (A. Moehrenschlager. 2009).

Habitat Protection/Ownership

Of the remaining native grasslands on the Canadian prairies, 45% are potentially arable and at risk of being cultivated (Erickson et al. 2004). Saskatchewan has 28% of its grasslands protected in conservation areas but Alberta has only 2% (Gauthier and Wiken 2003). However, Alberta Crown lands under grazing disposition are managed with a no cultivation policy, thereby minimizing the danger of being ploughed for annual crops.

Although the Prairie Farm Rehabilitation Administration (PFRA) Community Pastures Program has returned more than 145,000 ha of poor–quality cultivated lands to grass cover (PFRA 2008), exotic forage species likely compromise its suitability as Swift Fox habitat. In addition, the Saskatchewan Representative Areas Network Program aims to conserve representative and unique examples of varied and diverse landscapes. Since being launched in 1997, more than two million hectares of both Crown and privately owned land have been added to the nearly three million hectares of existing protected lands (e.g., parks, ecological reserves; Saskatchewan Environment 2006). Legislation in Saskatchewan under the WildlifeHabitat and Protection Act currently prohibits the breaking of native grasslands on about two million hectares of Crown land.

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