Swift fox (Vulpes velox) COSEWIC assessment and status report 2009: chapter 8


Limiting Factors and Threats

The primary natural factor causing Swift Fox mortality in Canada is predation by Coyotes and Golden Eagles (Aquila chrysaetos), with habitat loss, degradation and disturbance from development activities and disease being additional escalating concerns. Competition with Coyotes and Red Foxes are an additional potential limiting factor. Other threats to Swift Foxes in Canada include poisoning, vehicle collisions and trapping.

Predation

Coyotes are the principal cause of Swift Fox mortality (Covell 1992; Carbyn et al. 1994; Sovada et al. 1998; Kitchen et al. 1999; Olson and Lindzey 2002; Andersen et al. 2003; Ausband and Foresman 2007). Of the 89 Alberta and Saskatchewan Swift Fox carcasses examined by Carbyn et al. (1994) between 1983 and 1992, 38% were thought to be killed by Coyotes, 8% were likely killed by avian predators (of which 71% were Golden Eagles), and 7% were known or suspected American Badger kills. Moehrenschlager et al. (2007b) found that 31% of radio–collared Swift Foxes were killed by Coyotes and 33% by Golden Eagles (n=39) and also compared intraguild predation (i.e., killing and eating among potential competitors) of Swift Foxes (including Montana) and Mexican Kit Foxes. Coyote:fox home range size ratios in Canada were approximately four times what they were in Mexico. Disparity in ratios of home range area and availability of escape holes was attributed to differences in prey diversity and abundance. Moehrenschlager et al. (2007b) suggested that this pattern would lead to a higher rate of encounters with Swift Foxes, and thus, increased Swift Fox mortality. They concluded that Canadian Swift Foxes were twice as susceptible to predation in their 51–99% kernel use area than in their 50% core area. In Texas, increases in Swift Fox survival rates and recruitment occurred following Coyote control, although how long this effect persists is unknown (Kamler et al. 2003b). Significant reductions in Coyote populations in the Alberta–Saskatchewan–Montana border region decreased predation and/or competition; however, predation by Golden Eagles kept Swift Fox survival rates low (Moehrenschlager et al. 2007b). Coyote predation has hindered Swift Fox reintroduction efforts in Canada (Scott–Brown et al. 1987; Carbyn et al. 1994).

Habitat Alteration and Disturbance

The level of oil and gas development within the Canadian range of Swift Fox has markedly increased since 1993 (Moehrenschlager and Moehrenschlager 2006). Swift Fox den use, and survival were not adversely affected by development activities, as long as dens were not destroyed (Moehrenschlager 2000), although there was some evidence of negative effects on reproduction during pipeline construction. Swift Fox pairs along an oil pipeline corridor (n=2) did not produce any kits, whereas all pairs in the control area (n=8) did. In the year after pipeline completion, kits were produced by all ten pairs (Moehrenschlager 2000). Analyses of trapping data suggest that habitat fragmentation negatively affects fox abundance (Moehrenschlager, unpub. data).

Habitat issues lie at the root of many mortality factors for Swift Fox discussed in this section. For example, habitat conversion through agriculture (crops and/or livestock) has had indirect effects on Swift Fox populations. The extirpation of Wolves which was at least partially due to agriculture on the Canadian prairies led to Coyotes expanding in range and number (Sargeant et al. 1993) leading to increased predation on and competition with Swift Foxes. As pastureland increased, so did human persecution of American Badgers. This human activity likely reduced Badger predation on Swift Foxes, but in turn decreased availability of dens and escape holes to Swift Foxes.

Climate change may affect Swift Fox populations by causing spatial shifts in suitable habitat. Indirect effects of climate change will be on conditions that influence prey availability (Carbyn 1998).

Disease

To date, there have been no recorded disease outbreaks in Swift Fox populations (Moehrenschlager and Sovada 2004). Regardless, exposure to pathogens can threaten recovery efforts, particularly for small populations (Miller et al. 2000). Disease is well–documented in other endangered canids, with viruses such as canine parvovirus (CPV), canine distemper virus (CDV), and rabies (to name a few) having suddenly and swiftly killed large portions of small populations of Ethiopian Wolves (Canis simensis), Island Foxes (Urocyon littoralis), and African Wild Dogs (Lycaon pictus) in recent times (Laurenson et al. 1998; Woodroffe and Ginsberg 1999; Timm et al. 2000).

Swift fox reintroductions in Canada were originally planned at two separate locations in large part because of concerns about the potential for disease. Ironically, the success of reintroduction efforts from a demographic and genetic point of view through the effective connection of the two populations has enhanced the threat of disease through the creation of one epidemiological unit. Indeed, the potential threat is well illustrated by a limited seroepidemiological survey conduced in 2001 on 21 Canadian Swift Foxes (A. Moehrenschlager 2009). Over half tested positive for CDV antibodies, with high titres indicative of infection, all 21 had antibodies for CPV, with evidence of infection in 15. Its shared range with several domestic and free–ranging carnivores heightens the possibility of disease transfer between Swift Foxes and any number of sympatric species (Pybus and Williams 2003).Domestic dogs, in particular, are known to pose a significant risk as reservoirs for infectious diseases, especially for wild canids (Aguirre 2009). Although Swift Foxes have not been known to be reservoirs of rabies in the core of their range (Miller et al. 2000), this virus is enzootic in Skunks in the Canadian Prairies (Rosatte 1998) and has had well–documented devastating effects on Ethiopian Wolves and African Wild Dogs (Laurenson et al. 1998).

Sylvatic plague is a flea–transmitted disease caused by the bacterium Yersinia pestis (Olsen 1981). It causes epizootics resulting in 90–100% mortality in Prairie Dog colonies (Cully and Williams 2001; Gasper and Watson 2001). Canids are infected by flea bites or by consuming infected rodents (Thomas et al. 1989). Sylvatic plague is endemic (Humphreys and Campbell 1947; Leighton et al. 2001) to areas coincident with Swift Fox range. Antibody prevalence in Swift Foxes was 100% in northwestern Texas (n=12; McGee et al. 2006b), 24% in northeastern Colorado (n=61; Salkeld et al. 2007), 51% in southeastern Colorado (Gese et al. 2004) and 6% in New Mexico (n=16; Harrison 2003).

Poisoning

Historically, poisoning for predator control had serious impacts on Swift Fox populations, likely contributing to their decline in the early 20th century (Scott–Brown et al. 1987). Poisoning of Swift Foxes is now illegal in Canada and therefore has declined as a direct threat. A 1972 ban on predator toxicant use on US federal lands aided the recovery of Swift Foxes (Pruss et al. 2008). Swift Foxes readily consume poisoned baits laid out for Wolves, Coyotes, Striped Skunks (Mephitis mephitis), and Ground Squirrels (Spermophilus richardsonii) (Pruss et al. 2008). Despite legal protection Swift Foxes are still occasionally poisoned. Two of 39 Swift Fox carcasses examined in a Canadian study (including Montana) died from poisoning (Moehrenschlager et al. 2007b).

Even when Swift Foxes are themselves not targeted, the potential risk of mortality through encounter of poison baits directed at coyotes or secondary poisoning from consuming poisoned rodents is evident. The extent to which either occurs is unknown although it would stand to reason that the many poisons in use, both legally and illegally, constitute a risk factor for Swift Foxes. For example, many sheep ranchers perceive Coyotes to be a problem predator and Compound 1080 (sodium monofluoroacetate) can be used for their control in both Alberta and Sasktchewan, although the laws limiting their use have become much more stringent than in the past. Strychnine is also used to control Coyotes in Alberta and Saskatchewan. In Alberta it comes in tablet form (50% strychnine) and may be handled only by authorized, trained provincial or municipal personnel (Pest Management Regulatory Agency [PMRA] 2005). The tablets are mixed with bait and covered with loose snow or dirt. As a precaution, product labels indicate that application is not permitted if species at risk occur in the area (PMRA 2005). Strychnine is used to control Striped Skunks (as the primary vector of rabies in Canada; Fehlner–Gardner et al. 2008) and ground squirrels in Alberta and Saskatchewan. Saskatchewan implemented temporary emergency control measures for Richardson’s Ground Squirrel (Spermophilus richardsonii) in 2007. It should be noted that producers have access to 2% liquid strychnine concentrate, thereby increasing the opportunity for off label use and stockpiling. Until July 2009, PMRA began to allow use of strychnine–laced grain by pest control operators, agricultural producers, and authorized personnel from government–approved pest control programs, in rural areas with high densities of Richardson’s Ground Squirrel (PMRA 2008). In Alberta, the use of 2% strychnine for the same purpose was reinstated in 2009. Besides restrictions on use where species at risk are present, the only other safeguard against Swift Fox poisoning is product labelling.

Zinc phosphide has been used as a rodenticide in Alberta and Saskatchewan. It comes in a solid, granular or pellet form. Control of ground squirrels calls for placement of bait on the highest part of the mound, 15–20 cm from the burrow opening (PMRA 2006a). This makes baits accessible to Swift Foxes. Sodium cyanide is also used to control rodents in prairie agricultural areas. To protect Swift Foxes from accidental poisoning in Alberta, labels on sodium cyanide containers must now include a website address for a Swift Fox range map and notification that users must consult the Alberta Fish and Wildlife office in Medicine Hat for approval (PMRA 2006b).

During grasshopper outbreaks, landowners often increase application of insecticides. During such outbreaks, Swift Foxes consume a higher proportion of grasshoppers as part of their diet (Kilgore 1969; Egoscue 1979), which could lead to increased poisoning. For example, Sovada et al. (1998) reported that 22% (n=18) of dead radio–collared Swift Foxes in Kansas were poisoned by insecticide.

Vehicle Collisions

Swift Foxes often den near roads (Hillman and Sharps 1978; Hines and Case 1991; Carbyn 1998; Pruss 1999) making them susceptible to vehicle collisions. Road kill is an important source of mortality, particularly for juveniles (Pruss 1994; Sovada et al. 1998; Herrero 2003; Kamler et al. 2003a; Ausband and Foresman 2007). Kits are especially vulnerable when dens are close to highways (Carbyn 1998). Of 89 Canadian Swift Fox carcasses (1983–1992), five were likely road kills (Carbyn et al. 1994). Within the last year or so, there have been at least five confirmed road–killed Swift Foxes in southeastern Alberta (J. Nicholson comm. 2009). Necropsies of 39 Swift Foxes from the border population confirmed three deaths due to vehicle collisions (Moehrenschlager et al. 2007b). In Montana, vehicle collisions caused 15% (n=33) of mortalities in radio–collared Swift Foxes (Ausband and Foresman 2007).

Recent expansion of the oil and gas industry on the Canadian prairies means increased road development, increasing the risk of road kill (Pruss et al. 2008). There is currently a proposal to open the Wild Horse US/Canada border crossing for 24–hour service and expand Highway 41 in southeastern Alberta (Fig. 6) as a north–south transportation corridor. This would certainly result in significant increases in traffic volume and 24–hour heavy truck traffic through one of the highest density Swift Fox areas in Alberta (Hildebrand 2008).

Interspecific Competition

Interspecific competition, particularly with other canids may limit Swift Fox populations, particularly when mediated by habitat changes. Competitive interaction strength is likely related to prey abundance (Creel et al. 2001). Given that Coyotes seldom consume Swift Foxes after killing them is evidence for interference competition (Sovada et al. 1998; Kitchen et al. 1999; Matlack et al. 2000; Kamler et al. 2003b). The extirpation of Wolves (Canis lupus) on the Canadian prairies led to Coyote range expansion (Riley et al. 2004), raising the potential for negative impacts on Swift Fox populations.

Red fox have also recently expanded their range on the Canadian prairies, stimulated at least in part by habitat fragmentation and the expansion of the human footprint (Kamler and Ballard 2002). This relatively novel competitor with Swift Foxes may be even more significant than Coyotes because: 1) in rural areas, Coyotes typically avoid areas of high human activity (Roy and Dorrance 1985; Pruss 2002), whereas Red Foxes, like Swift Foxes, will associate with humans (Nowak 1999); 2) anthropogenic changes on native prairie, such as urbanization and fragmentation (particularly from oil and gas development), have facilitated Red Fox range expansion into Swift Fox range (Kamler and Ballard 2002); 3) while there is considerable dietary overlap between Swift Foxes and Coyotes (Kitchen et al. 1999; Kamler et al. 2007b), the overlap is even greater between sympatric foxes (Moehrenschlager and Sovada 2004); and 4) Red Foxes tend to occur at higher densities than Coyotes (Knowlton 1972; Baker and Harris 2004) meaning that encounter likelihood increases. The interaction between Coyotes and Red Foxes may affect the strength of competition between Red Foxes and Swift Foxes (Carbyn 1998; Allardyce and Sovada 2003; Cypher et al. 2003). In southern Colorado, Coyote population reduction temporarily increased survival rates of both juvenile and adult Swift Foxes, although effects on adults were highly dependent on the timing of coyote control (Karki et al. 2007). Robinson (1961) and Linhart and Robinson (1972) found no increase in Swift Foxes following Coyote control.

Trapping

Today, the endangered status of the Swift Fox in Canada affords the species legal protection from trapping, although incidental injuries or mortalities in traps or snares set for other species are known to occur (Moehrenschlager 2000). At one time, Swift Foxes were important for the North American fur trade, with 117,025 pelts traded between 1853 and 1877 (Herrero et al. 1986). However, trapping for Swift Fox dramatically declined as a result of declining populations and low pelt prices (US$3–10 [Stephens and Anderson 2005]).

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