Swift fox use of black-tailed prairie dog towns in northwest Texas: with notes on movement

Due to their burrowing and selective foraging activities, black-tailed prairie dogs (Cviiomys ludovicianus) have the ability to alter biotic and abiotic characteristics of their environment. A number of studies have indicated that many species of birds, plants, and mammals are highly associated or dependent upon prairie dogs, and that prairie dogs are a keystone species. However, many land managers consider prairie dogs to be pests that degrade rangeland, compete with cattle, and impact crops. As a result several studies have been initiated to determine whether prairie dogs are a keystone species. Swift fox {Vulpes velox) are one of the species that have been reported to depend on prairie dog colonies for food and habitat.

The assumption that prairie dogs are a keystone species for swift fox may be plausible due to their overlapping distributions across the North American short and midgrass prairies systems. However, documenting dependence of swift fox on prairie dogs has been difficult because dependence can vary in space and time. It has been hypothesized that swift use prairie dogs as food, and use their burrows for escape habitat and for denning. The relatively few studies that have addressed these issues have been anecdotal in nature and were conducted within the mixed grass prairie region, which is the northern range of the swift fox. One mechanism for the hypothesized dependence is based upon the fact that prairie dogs create landscape patches that differ from the surrounding landscape, and vegetation patch structure can have distinct effects on wildlife distribution patterns. However, no studies have been conducted in short grass prairie areas, particularly in the Southern High Plains eco-region, where changes in vegetation structure are not as pronounced as those in tall grass areas.

Our objectives were to investigate relationships between swift fox and prairie dogs in the short-grass prairies of northwest Texas. We investigated whether swift fox occurred in black-tailed prairie dog colonies more often than in surrounding vegetation types. If swift fox selected prairie dog towns out of proportion to their availability this would lend support to the argument that prairie dogs serve as a keystone species for swift fox. However, if swift fox avoided prairie dog towns, or did not actively select these habitats, statements of swift fox dependence on prairie dogs would be questionable. We hypothesized that because of the vegetation cover and potential den sites created by prairie dogs, swift fox would select prairie dog towns more often than other available habitat types.

To examine this issue we captured and radio collared 72 swift fox in areas occupied by prairie dogs on the Rita Blanca National Grasslands (NG), and on fragmented private ranches (PR) in northwest Texas from January 2002 to April 2004. Previous research in these areas of Texas indicated that prairie dogs were a minor component of swift fox diet. Therefore, to address the question of swift fox dependency on prairie dog colonies we examined swift fox locations and den locations in relation to the location and annual distribution of prairie dog towns at each study site.

We overlaid swift fox locations on vegetation maps and found that swift fox used prairie dogs towns proportionately less than expected based upon availability at both study sites. On PR swift fox used prairie dog towns significantly less than expected in proportion to their availability in every year of the study (2002, x^2 = 35.576; 2003, x^2 = 70.817; 2004, x^2 = 77.167, P = 0.0001). On NG in 2002, swift fox used prairie dog towns as expected in proportion to availability (x^2 = 0.062, P = 0.999). However, in 2003 (x^2 = 16.633, P ¡Ü 0.0001) and 2004 (x^2 = 23.455, P ¡Ü 0.0001) swift fox on NG underutilized prairie dog towns. We also mapped swift fox den locations and found that den sites were located in prairie dog towns (2 of 106 den sites on NG and 2 of 87 on PR for all years pooled) as expected based upon availability (P = 0.539 and P = 0.339).

Prior research indicated that prairie dogs comprised a minor portion (i.e., <2%) of swift fox diet in our study area. Because we found that swift fox generally avoided prairie dog colonies during routine activities and that den sites were not located in these areas more than expected based upon prairie dog town availability, we must reject our original hypothesis. Our results indicate that northwest Texas and other areas similar to our study area swift fox are not dependent on prairie dogs and therefore, prairie dogs should not be considered a keystone species for swift fox.

Dispersal plays an important role in the population dynamics of many carnivores, yet little information exists about the dispersal and movement patterns of swift foxes (Vulpes velox). Howard (1960) defined dispersal as the movement an animal makes from its birthplace to where it reproduces or would have reproduced if it had survived and found a mate. Dispersal is risky for many individuals because of costs associated with exploring and establishing new territories (O'Neal et al. 1987). Landscape pattern can act differently on the movement of species with different degrees of influence. For example, those species that are more adaptable could potentially disperse greater distances than those that are rigid in their requirements.

Movement similar to dispersal is exploration or quasi-dispersal (i.e. individuals make short-term excursions outside of their established home ranges; Lidicker and Stenseth 1992). In many studies this behavior is also referred to as an extra-territorial foray. Exploratory movements can be motivated by the need to search for a mate, or efforts to locate additional resources, better forage, or better living conditions. These excursions may also be preludes to actual dispersal and occur commonly among younger age classes (usually juveniles).

We radio-collared and monitored 68 swift fox for dispersal at 2 study sites in northwest Texas from January 2002 to April 2004. Dispersal distance for juveniles (13.1 ¡À 0.3 km), adults (10 ¡À 4.7 km) and floaters (25.4 ¡À 9.1 km) did not differ by age class (P = 0.64) or sex (P = 0.24) but differed by study site (P = 0.04). Mean dispersal distance from private ranches (PR) was greater than from public grasslands (NG). Peak dispersal occurred during October-November (13 individuals) and January-February (7 individuals). Dispersal direction appeared to be in relation to land use practices (i.e., toward rangelands and away from anthropogenic features). Fox from NG did not have a significant direction of dispersal (P = 0.08), whereas fox from PR dispersed in a northwesterly direction (P < 0.001). Thirteen fox had extra-territorial explorations outside delineated home ranges. Distance of extra-territorial forays did not differ by sex (P = 0.93) or length of foray (P = 0.28). Knowledge of movement distances and patterns is important for managers in order to conserve and protect swift fox and their habitat.