Evaluation of an augmentation of bighorn sheep at Badlands National Park, South Dakota

Teresa J. Zimmerman

Abstract: The Audubon’s bighorn sheep (Ovis canadensis auduboni) was extirpated throughout its range by 1925.  In order to restore this native ungulate, 22 Rocky Mountain bighorn sheep (O. c.  canadensis) were reintroduced to Badlands National Park (BNP) in 1964.  Based on the estimated effective population size and analysis of molecular data , the population at BNP had undergone a bottleneck at founding.  In order to restore the genetic diversity, recommendations were to augment the BNP population from an outbred native source.  The BNP population also had been identified by the International Union for the Conservation of Nature as a population of conservation concern as they represented the eastern most extent of the species’ current and historic range.  In September 2004, 23 bighorn sheep were captured at Wheeler Peak, New Mexico and released at BNP to augment the existing population of approximately 68 individuals.  Because it has been recommended that introduced populations are regularly monitored to evaluate the success or failure of transplants, the objectives of this study were to: 1) document survivorship, natality, dispersal, and recruitment of introduced bighorn sheep females and their offspring, 2) estimate 95% and 50% adaptive kernel planimetric and surface area home ranges of introduced bighorn sheep females, 2) determine habitat selection of introduced bighorn sheep females, 4) compare home range size and habitat selection of introduced to resident bighorn sheep females, and 5) determine current and historic genetic heterozygosity.  As of August 2007, 18 of 23 introduced bighorn sheep survived/remained with the sub-population. In 2005 and 2006, the 3 month-old lamb to adult ewe ratios were 90:100 and 62:100, respectively.  In June 2006, 9 yearling male and female lambs dispersed from BNP with dispersal distances ranging from 43 to 524 km. In May 2007, 3 yearling male lambs dispersed from BNP with a dispersal distance of 25 km.  The 95% adaptive kernel and surface area estimates of the introduced bighorns decreased between years, but core home range size did not differ.  The 95% and 50% adaptive kernel planimetric and surface area home range estimates were greater in resident than introduced sheep.  Introduced and resident bighorn sheep differed in their use of habitat with introduced sheep selecting areas closer to roads, human use areas, and water. Since 1992, heterozygosity of resident bighorn sheep decreased 1.1% annually.  Heterozygosity increased from 0.374 pre-augmentation to 0.653 post-augmentation.  The observed reproductive rate and survival of bighorn sheep is similar to trends observed in increasing populations, but poor yearling recruitment is severely hampering population growth.  Introduced bighorn sheep increased occupied bighorn sheep habitat in BNP by utilizing different habitats than resident bighorns.  The acclimation of introduced bighorn sheep to humans may have contributed to their habitat use.  The long term viability of the BNP bighorn sheep population appears to be limited by population size not genetic diversity, therefore, management efforts should focus on increasing yearling recruitment.