Swift Foxes and Ideal Free Distribution: Relative Influence of Vegetation and Rodent Prey Base on Swift Fox Survival, Density, and Home Range Size

Abstract

Swift foxes (Vulpes velox) are an endemic mesocarnivore of North America subject to resource and predation-based pressures. While swift fox demographics have been documented, there is little information on the importance of top-down versus bottom-up pressures or the effect of landscape heterogeneity. Using a consumable resource-based ideal free distribution model as a conceptual framework, we isolated the effects of resource-based habitat selection on fox population ecology. We hypothesized if swift fox ecology is predominantly resource dependant, distribution, survival, and space use would match predictions made under ideal free distribution theory. We monitored survival and home range use of 47 swift foxes in southeastern Colorado from 2001 to 2004. Annual home range size was 15.4 km 2 , and seasonal home range size was 10.1 km 2 . At the individual level, annual home range size was unrelated to survival. Estimates of fox density ranged from 0.03 to 0.18 foxes/km 2 . Seasonal survival rates were 0.73 and 1.0 and did not differ seasonally. Foxes conformed to the predictions of the ideal free distribution model during winter, indicating foxes are food stressed and their behavior governed by resource acquisition. During the rest of the year, behavior was not resource driven and was governed by security from intraguild predation.

Winter movement behavior by swift foxes (Vulpes velox) at the northern edge of their range

“…In particular, individuals face increased energetic demands due to increased metabolism to counteract heat loss owing to cold temperatures (Prestrud 1991) and increased costs of locomotion owing to snow depth (Crête and Larivière 2003). At the same time, winter is typically associated with decreased prey availability (Michener 1998) and accessibility (Halpin and Bissonette 1988), which can negatively impact energy budgets for carnivores, particularly for smaller carnivores that must balance energetic needs with predation risk from larger carnivores (Thompson and Gese 2012). Collectively, these stressors can lead to decreased overwinter survival.…”

Is time partitioning the currency of coexistence for a grassland canid community?

“…reduced food acquisition). As with other carnivore communities (Dröge et al 2017), swift foxes and coyotes may only coexist intermittently as populations of the dominant predator (Jachowski et al 2020) or resource availability (Palomares and Caro 1999, Caro and Stoner 2003, Valeix et al 2007) fluctuate across space and time (Thompson and Gese 2012).…”

“…The reduction in temporal breadth and increased temporal overlap could reflect a convergence by all three canid species to coincide foraging activity with the temporal availability and vulnerability of prey to increase foraging efficiency (Monterroso et al 2014). For swift foxes, however, if a broad niche breadth is an adaptive response to the risk of interference competition, the reduction in niche breath suggests that the energetic needs of parental care are substantial enough to outweigh the costs of interference competition (Thompson and Gese 2012). Given that the cost of interference competition is mortality, such a trade-off seems unlikely, unless the risk of interference competition also changes seasonally.…”

Habitat selection in a fluctuating ground squirrel population: Density‐dependence and fitness consequences

“…The ideal‐free distribution (IFD) theorizes that animals take this into account when self‐assorting among habitat types; as population density increases in high‐quality habitat and competition for limited resources becomes intense, animals preferentially settle in lower quality habitat, where they are equally likely to survive or reproduce due to lack of competition (Fretwell & Lucas, 1969 ). Some studies criticized the validity of the underlying assumptions, noting that factors such as differential access to resources among individuals (Conradt et al, 1999 ; Parker & Sutherland, 1986 ), predation risk (Garshelis, 2000 ; Morris, 1989 ; Thompson & Gese, 2012 ), lack of accurate information (Abrahams, 1986 ; Hemingway et al, 2018 ), movement costs (Abrahams & Labelle, 2020 ; Matsumura et al, 2010 ), or ecological traps (Robertson et al, 2013 ; Schlaepfer et al, 2002 ) might dominate settlement decisions. Nonetheless, most criticisms remain untested in the field, in part because accurate assessments of habitat quality should involve calculations of animal performance (Mosser et al, 2009 ; Van Horne, 1983 ), yet few investigations tie direct measurements of survival or reproduction to resource selection decisions by individual animals (Chalfoun & Martin, 2007 ; Gaillard et al, 2010 ; Pulliam & Danielson, 1991 ).…”