Population Dynamics of a Bighorn Sheep (Ovis Canadensis) Herd in the Southern Black Hills of South Dakota and Wyoming

By Parr, Brynn L.; Smith, Joshua B. et al. | The American Midland Naturalist, January 2018 | Go to article overview

Population Dynamics of a Bighorn Sheep (Ovis Canadensis) Herd in the Southern Black Hills of South Dakota and Wyoming


Parr, Brynn L., Smith, Joshua B., Jenks, Jonathan A., Thompson, Daniel J., The American Midland Naturalist


Introduction

Bighorn sheep (Oms canadensis) once numbered in the millions across the western United States (Buechner, 1960; Berger, 1990); however, due to uncontrolled harvest and diseases introduced from domestic sheep, bighorn numbers began to plummet around the turn of the 20th century (Buechner, 1960; Berger, 1990). Managing agencies have attempted numerous re- introductions across the western United States since the mid-1900s with varying levels of success (Berger, 1990; Singer et al, 2000a; Hedrick, 2014).

In South Dakota native sheep were extirpated by the early 1900s (South Dakota Game, Fish, and Parks, 2007 [SDGFP]; Zimmerman, 2008). South Dakota Department of Game, Fish and Parks (SDGFP) began bighorn sheep re- introductions in the 1960s and experienced the same array of success as other agencies (Singer et al., 2000a; SDGFP, 2007). In the mid-2000s, the Custer State Park and Rapid Creek bighorn sheep herds began to decline due to pneumonia outbreaks (SDGFP, 2007; Smith et al., 2014a). The Elk Mountain herd, located in the southern Black Hills of South Dakota and Wyoming, declined in size between 2009 and 2012; the specific cause was unknown (J. Kanta, SDGFP, pers. comm.).

To better understand the decline in the Elk Mountain bighorn sheep population, our primary objectives were to evaluate survival and cause-specific mortality of all sex and age classes of bighorn sheep as well as estimate population size for the herd. More specifically, our objectives were to: determine annual survival rates of adult bighorn sheep, weekly survival rates and recruitment rates of lambs, determine cause-specific mortality of all classes of bighorn sheep, and estimate population size of the Elk Mountain herd.

Methods

study area

Our study area, Elk Mountain, is located in the southern Black Hills in western South Dakota and eastern Wyoming, U.S.A. (43°43' N latitude and 104°02' W latitude; Fig. 1). The study area encompassed approximately 18,600 ha. Elevations ranged from 1132 m to 1728 m above mean sea level, and topography consisted of rock outcrops, rolling hills, steep ridges, and gulches (Froiland, 1990). Herbaceous cover (Bromus spp., Poa annua) dominated the landscape at 54.7% (USDA GeoSpatialDataGateway, 2014), whereas shrub/scrub (Artemisia spp.) covered 26.8%, and evergreen forest (17.7%; Pinus ponderosa) comprised the majority of the remaining landscape. Average annual precipitation was 42 cm; mean temperatures ranged from a low of -11 C in January to a high of 32 C in July [National Oceanic and Atmospheric Administration (NOAA), 2014]. Climate values were based on data collected at the Newcastle, Wyoming, weather station from 1981-2010 (NOAA, 2014). Other ungulates in the study area included mule deer (Odocoileus hemionus), white-tailed deer (O. virginianus), elk (Cervus elaphus), and pronghorn (Antilocapra americana). Potential predators on bighorn sheep at Elk Mountain included mountain lions (Puma concolor), bobcats (Lynxrufus; Parr et al., 2014), coyotes (Canis latrans), bald eagles (Haliaeetus leucocephalus), and golden eagles (Aquila chrysaetos).

ADULT CAPTURE AND DATA COLLECTION

From March 2012 to February 2014, we captured adult bighorn sheep inhabiting the Elk Mountain Region of South Dakota and Wyoming via drop net (Jessup et al., 1984; Kock et al, 1987) and helicopter net gun (Jacques et al., 2009). We aged females based on tooth eruption and wear (Hemming, 1969; Krausman and Bowyer, 2003) and males via horn annuli (Geist, 1966). We fitted adult rams (>4 у of age) and ewes (>1 у of age) with either very high frequency (VHF; Model M2520B, 154-155 MHz) or global positioning system (GPS; Model G2110B, 154-155 MHz) radio-collars (Advanced Telemetry Systems, Isanti, Minnesota). In addition to radio-collars, all captured ewes and immature rams (<3 у of age) received an ear tag for additional identification. We evaluated ewes for pregnancy status using a Bantam XLS portable ultrasound (E. …

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