Northern Prairie Wildlife Research Center
Reported mortality rates of kit foxes (Vulpes macrotis), a species most similar to swift fox, also resemble our results. Ralls and White (1995) reported an annual mortality rate of 0.42 for a kit fox population on the Carrizo Plain Natural Area in California. Cypher and Scrivner (1992) reported annual estimates of 0.53 (1980-84) and 0.68 (1985-90) for kit foxes on the Naval Petroleum Reserves in California. Disney and Spiegel (1992) reported annual mortality rates of 0.60 (1990) and 0.32 (1991) for kit foxes in Kern County, California.
Predation, specifically by coyotes, was the main cause of swift fox mortality. Covell (1992) reported a similar finding in southern Colorado, where coyotes were responsible for 85% of all predation on swift foxes. Laurion (1988) also reported significant swift fox mortality attributed to coyotes. Coyotes have been reported as the major source of mortality among swift foxes reintroduced in southern Canada (Carbyn et al. 1994). Kit foxes also are preyed upon heavily by coyotes (O'Farrell 1987, Ralls and White 1995). Our observation that depredation of swift foxes always occurred away from dens and core activity areas suggests this species is more vulnerable to predation in peripheral areas of their home range.
Poisoning of swift foxes in this study was a localized incident. Information gathered by state authorities suggested the poisoning was an intentional act, but the target species was not believed to be swift fox. Primary or secondary poisoning may affect local swift fox populations, especially where rodenticides are used to control prairie dogs (Miller et. al. 1994). Poisoning also has been reported to cause mortality in kit fox populations (Schitoskey 1975, Standley et al. 1992). There is no evidence of widespread poisoning risks to swift fox populations, although predator control programs that included poisoning apparently contributed to the decline in swift fox numbers in the late 1800s and early 1900s (Johnson and Sargeant 1977, Zumbaugh and Choate 1985). If we remove poisoned foxes from our calculations, the mortality rate for adult swift foxes was 0.46 ± 0.08 ( ± SE) overall and 0.47 ± 0.12 in the Rangeland Area.
Swift foxes potentially have high reproductive rates, which may compensate for high mortality. We estimated litter size to average 3.25 ± 0.34, using observations of 8 dens at which we could count pups as they emerged (M. A. Sovada and C. C. Roy, unpublished data). Average litter sizes for swift foxes range from 3.4 to 5.7 (Kilgore 1969; Hillman and Sharps 1978; Rongstad et al. 1989. Ecology of swift fox on the Pinon Canyon Maneuver Site, Colorado. Unpublished final report to the U.S. Army, Fort Carson, Colorado, USA; Covell 1992; Carbyn et al. 1994).
Mortality rates of juveniles reported in this study could be misleading because of small sample sizes and lack of independence among sampled animals (i.e., sibling relationships), although radiocollared siblings never died together. The difference in juvenile mortality due to vehicle collision in Cropland and Rangeland areas may be attributed to 90% more roads in the Cropland Area. Moreover, juveniles are probably less wary than adults and may have been at greater risk to vehicle collision (Storm et al. 1976). Whether vehicle collisions are additive or a compensatory mortality factor for swift foxes is unknown.
Although highly cultivated landscapes are not considered suitable to support sustainable populations of swift foxes (Samuel and Nelson 1982), a few studies have indicated swift foxes inhabit areas of mixed agricultural use (Kilgore 1969, Hines 1980). Our results show no difference in mortality rates between swift foxes in the Cropland Area and Rangeland Area.
Changes in the canid community within the historic swift fox range seem to have been detrimental to swift fox populations. Swift foxes apparently thrived in the region when the canid community was dominated by gray wolves (Johnson and Sargeant 1977). Gray wolves, unlike coyotes, apparently paid little attention to the smaller swift fox, and chances of swift fox encounters with wolves probably were less than with coyotes, because of the larger home range size and lower overall density of wolves compared to coyotes (Johnson and Sargeant 1977). More importantly, wolves likely kept the numbers of coyotes significantly depressed (Johnson and Sargeant 1977, Sargeant et al. 1987). There is considerable evidence that interspecific competition, often as interference competition, acts as a mechanism regulating spatial distribution and population size among canid species (Carbyn 1982, Rudzinski et al. 1982, Sargeant et al. 1987, Bailey 1992, Ralls and White 1995). Scott-Brown et al. (1987) speculated that the decline of the swift fox may be a direct result of competition with and predation by coyotes.