Survival and Cause‐Specific Mortality of Red Foxes in Agricultural and Urban Areas of Illinois

Abstract

Range expansion and population increase by coyotes (Canis latrans), reduced hunting and trapping, and intensified agricultural practices in the Midwest have altered red fox (Vulpes vulpes) mortality, although relative impacts of these factors are unknown. We examined mortality causes and survival of red foxes in urban and rural agricultural areas of Illinois, using radio telemetry data from 335 foxes (Nov 1996 to May 2002). We used Akaike's Information Criterion to evaluate six survival models for foxes reflecting 1) environmental effects, 2) intrinsic effects, 3) temporal effects, 4) behavioral effects, 5) social effects, and 6) a global model. Environmental and intrinsic models of survival were optimal for adult foxes. Adult foxes with low (0‐20%) and high (80‐100%) percentages of row crops in their home ranges had higher survival than adults with moderate percentages (40‐70%). Heavier adults at capture also survived better. A global model (all covariates) was optimal for juvenile foxes. Higher juvenile survival associated with larger litters, lower body fat, and reduced dispersal time. Yearly survival ranged from 0.18 for rural male juveniles to 0.44 for rural female adults. Adult survival rates (0.35) were 11% higher than juvenile survival rates (0.24). Yearly survival varied for urban foxes due to cyclic outbreaks of sarcoptic mange (Sarcoptes scabei). Thus, summer survival (May‐Sep) of urban juveniles ranged from 0.10 (mange present) to 0.83 (no mange recorded). Mange was the most common (45% of all fatalities) source of mortality for urban foxes, followed by road kill (31%). We recorded only 4 mange fatalities (2%) for rural foxes. Rural foxes experienced low hunting mortality (7%) and equivalent road kill and coyote predation fatalities (40% each). Sources of mortality for midwestern foxes have dramatically changed since the 1970s when hunting was the major cause of mortality. Coyote predation has effectively replaced hunting mortality, and cyclic patterns of mange outbreaks in urban fox populations might indicate a dynamic source or sink relationship to surrounding rural fox populations. Absent mange, urban areas might provide refugia for red foxes where coyote populations persist at high densities in rural areas. Managers of sympatric urban and rural wildlife populations must understand survival dynamics influencing the population at the landscape level.

Wildlife Population Dynamics in Urban Landscapes

“…Perhaps owing to the difficulty in studying small populations, few studies have documented lower survival rates for species that are negatively associated with urbanization (but see Price et al 2011). In contrast, several studies provide evidence that elevated survival rates in cities (e.g., Gosselink et al 2007;McCleery et al 2008;Lehrer et al 2012) promote high densities of species, as with raccoons (Prange et al 2003), fox squirrels (McCleery et al 2008), eastern long-necked turtles (Chelodina longicollis; Rees et al 2009), and many birds (e.g., Canada geese (Branta canadensis; Balkcom 2010); northern mockingbirds (Mimus polyglottos; Stracey and Robinson 2012), peregrine falcons (Falco peregrines anatum; Kauffman et al 2003), and great tits (Parus major; Horak and Lebreton 1998)). On the other hand, a review of carnivores showed equivocal comparisons of survival rates between urban and rural populations, with some species exhibiting greater survival with urbanization (i.e., kit foxes (Vulpes macrotis), raccoons, coyotes, stone marten (Martes foina)) and others with no change or negative trends (i.e., striped skunks, red foxes, bobcats, mountain lions (Puma concolor; Iossa et al 2010)).…”

“…For example, woodchucks ( Marmota monax; Lehrer et al 2012) and fox squirrels (McCleery et al 2008) are more likely to die from predation in rural areas, but mortality risk shifts from predators to vehicle collisions with urbanization. Likewise, coyote killing of foxes, due to intraguild competition, decreases with urbanization in those cities where coyotes are not as abundant as adjacent rural areas (Gosselink et al 2007;Cypher 2010). The noise of cities also may indirectly reduce predation in cases where predators avoid the loud environments (Francis et al 2009).…”

“…For example, rabies is an important disease that can dramatically reduce host density in urban areas and other notable epizootics in urban areas have been reported for raccoons, striped skunks, and red foxes (Riley et al 1998;Rosatte 2000). Much like rabies, epizootics of sarcoptic mange ( Sarcoptes scabei) impact high-density canid populations, and these can be dramatic in urban areas where populations are subsidized with anthropogenic foods and buffered from predation (Gosselink et al 2007;Soulsbury et al 2010).…”

“…Sarcoptic mange often becomes epizootic at high host densities, and urban red fox populations may cycle as source and sink, depending on the prevalence of this disease (Gosselink et al 2007). In peak years of the mange cycle, urban populations may serve as sinks, where mortality rates exceed reproduction and limited dispersal occurs, but during periods of low mange incidence survival in urban areas is relatively high and urban populations serve as sources with considerable juvenile dispersal into rural areas (Gosselink et al 2007).…”

Infectious Disease and Contaminants in Urban Wildlife: Unseen and Often Overlooked Threats

“…Similarly, the red fox population in Bristol, England, which had also reached high densities, was reduced by sarcoptic mange (Soulsbury et al 2007). In a study spanning urban, rural, and edge areas in Illinois, high density red fox populations in urban areas were affected most by sarcoptic mange, while foxes in rural areas, where coyote ( Canis latrans) predation predominated and fox density was lower, had the lowest mange incidence (Gosselink et al 2007;Crooks et al 2010). Epizootics like sarcoptic mange may be more common, and potentially more damaging, in high-density urban wildlife populations.…”