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Swift Fox

Limiting Factors

Swift FoxOnce widely distributed across the Great Plains of North America, from southern Canada to Texas, the Swift Fox range began to decline in the late 1800s. This decline has been attributed primarily to human-related factors, such as habitat fragmentation, Coyote abundance and range management. 

Despite the predominance of human-induced limiting factors, one natural limiting factor, climatic conditions, deserves mention. Due to its small size, the Canadian Swift Fox population may be extremely vulnerable to stochastic climatic conditions which may affect prey abundance and availability. Swift Foxes suffered lower survival and reproduction rates following a dry summer and a moderate to severe winter in 1988. A severe drought in the 1930s has also been implicated in the demise of the Swift Fox in Canada.

Conversion of native prairie to agricultural lands was one of the Agricultural Development major factors contributing to the decline of the Swift Fox in Canada. Within Canada, approximately 24% of the mixedgrass prairie region remains uncultivated. Despite severe fragmentation of this ecosystem, several large expanses of native grassland suitable for Swift Fox habitation exist in southern Alberta. Additional conversion of grassland to cropland would threaten this remnant habitat and the survival of resident Swift Foxes. Currently, range improvement on public land is marginal. However, conversion of private rangeland to cultivated land is often financially driven. Should the financial incentive for cultivating cropland increase, conversion of native grasslands would again threaten Swift Fox habitat and populations in the Canadian prairies.

Oil and natural gas exploration also fragment existing grasslands and increase road traffic and access by humans. Impacts of this type of disturbance on Swift Foxes are unknown, but both positive and negative effects may be expected. On the positive side, prey abundance for Swift Foxes may increase in the vicinity of roads. However, loss of local habitat, increased mortality due to road kills, trapping and accidental shooting may also result.

Coyotes are the primary predator of Swift Foxes, and are perhaps the most important factor limiting successful establishment of the species in Canada. With the extirpation of the Wolf (Canis lupus) on the Canadian prairies, Coyote densities have increased despite ongoing fur harvest and persecution by landowners. In southeastern Alberta, aerial surveys indicate that Coyote abundance increased 135% from the 1977-1989 survey period to the 1995-1996 survey period. As direct competitors and predators, Coyotes may restrict range expansion by Swift Foxes, particularly in the few grassland expanses which are suitable for fox colonization.

Little is known of the effects of range management on Swift Foxes, however several management practices may adversely affect the species. Use of pesticides may reduce prey availability (insects, rodents), or result in the accumulation of toxins in prey species. Consumption of contaminated prey may then affect Swift Fox reproductive success and survival. Overgrazing may also affect habitat quality and prey abundance.

Habitat quality, availability and Swift Fox survival are affected by a variety of factors, the majority of which are human-induced. Change of native prairie to agricultural land has had the greatest negative influence on Swift Fox populations, causing habitat loss and increased Coyote abundance.

Reprinted from Alberta Wildlife Status Report No. 7 (1997), with permission from Alberta Sustainable Resource Development.

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