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？km2, and seasonal home range size was 10.1？km2. At the individual level, annual home range size was unrelated to survival. Estimates of fox density ranged from 0.03 to 0.18？foxes/km2. 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. 1. Introduction Swift foxes (Vulpes velox) are a mesocarnivore endemic to the Great Plains region of North America. Historically, swift foxes occupied the extensive shortgrass prairie regions from central Canada into New Mexico and Texas and from the Rocky Mountains east into Iowa [1, 2]. Today, they are found in a variety of landscapes, ranging from shrub steppe to agricultural to ranchland to native shortgrass prairie [3, 4]. While they are capable of exploiting a wide range of habitats, reported population parameters such as density and survival vary widely indicating variation in habitat quality. There is little information on landscape structure or disturbance thresholds that control swift fox abundance or lead to exclusion [5]. The ideal free distribution [6] has been used to study the influence of predation on prey distribution [7]. According to the tenets of the ideal free distribution, high-quality habitat should be occupied to a certain threshold at which point competition or social structure forces animals into poorer-quality areas. In this case, survival and reproductive success would be similar across landscapes though density and home range size would vary. However, most applications of the ideal free distribution to
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