Landscape effects on black-tailed prairie dog colonies

Abstract

Black-tailed prairie dogs (Cynomys ludovicianus) increasingly compete for available habitat with human development in the Colorado Front Range. Because the effects of increased urbanization on prairie dog colonies are unknown, we studied how landscape context affects prairie dog density in Boulder County, Colorado, USA. We used burrow density as a proxy for prairie dog density because these variables were correlated at our study sites ( r=0.60). Using remotely sensed data and a GIS, we quantified percent urbanization, road density, and the percentage of other prairie dog colonies in the surrounding landscape at 200, 1000, and 2000 m from the perimeter of 22 prairie dog colonies, and compared burrow density with each landscape variable at each scale. We also calculated Akaike's information criterion (AIC) to determine the most parsimonious models predicting burrow density. Ranges of burrow densities and prairie dog densities in Boulder County were higher than in other studies using similar methodology. Within Boulder County, burrow density was significantly higher in colonies surrounded by greater density of roads. The degree to which prairie dog colonies were immediately surrounded by unsuitable habitat, i.e. the “boundedness” of the colony, was negatively correlated with colony area and positively correlated with burrow density. A model based on boundedness, the density of roads at the 2000 m scale, and the amount of prairie dog colonies at the 200 m scale explained 73% of the variance in prairie dog burrow density. However, a non-linear model including boundedness and the squared term of road density at the 2000 m scale had the lowest AIC value of all linear and non-linear models, indicating a possible threshold effect of urbanization on prairie dog density. Urbanization may have several implications for prairie dog persistence. Increased prairie dog density in urbanized landscapes may be related to the Refuge effect, i.e. decreased predator abundance. If higher prairie dog density increases competition for available resources, habitat quality may decline leading to population decline in highly urbanized landscapes. Furthermore, if dispersal is reduced in urbanized landscapes, then these colonies may not be recolonized after local extirpation from plague epizootics. Alternatively, urbanized colonies may be effectively isolated from plague vectors and reservoir hosts, which could result in a lower frequency of plague epizootics when compared to non-urbanized colonies.