Spatial ecology of female bighorn sheep in a prairie landscape in Nebraska

Understanding the spatial ecology of small populations at the periphery of their range is important for identifying factors limiting population growth and developing sound management strategies. Bighorn sheep were reintroduced to Nebraska in 1981 and persist in a small population at the easternmost extent of the distribution of the species, where 1 of the 2 subpopulations is declining. Bighorn sheep space use and movement has been studied extensively in mountain and desert populations, but information is sparse from prairie populations in the Northern Great Plains. We investigated the spatial ecology of female bighorn sheep in Nebraska, USA, with global positioning system (GPS) telemetry. We tested the hypothesis that space use and movements would vary across seasons, years, and individuals but predicted that migration would involve relatively short distances in this translocated population (relative to native populations) occupying a fragmented landscape. Overall, females used smaller seasonal home ranges (3.3-7.8 km(2)) than most of those reported previously for the species and exhibited a high degree of variability in space use and movements across seasons, subpopulations, and individuals. Most females (92-100%) exhibited fidelity to their home ranges across seasons and years. Six females migrated between spatially distinct core lambing and winter ranges, although the distances (range = 7.9-8.7 km) and mean elevations (range = 31-41 m) between these core seasonal ranges were less than those reported for most native, migratory bighorn sheep populations. After accounting for variation in season, subpopulation, and years, home range size was positively associated with road density in both subpopulations (P < 0.001), suggesting that females incur greater energetic costs associated with greater space use in areas of higher fragmentation. Our results establish the basic spatial ecology of female bighorn sheep in Nebraska where their behavior appears to reflect the isolated nature of suitable habitat in this fragmented prairie landscape.