Academic journal article The American Midland Naturalist

Survival of Female Bighorn Sheep (Ovis Canadensis) in the Black Hills, South Dakota

Academic journal article The American Midland Naturalist

Survival of Female Bighorn Sheep (Ovis Canadensis) in the Black Hills, South Dakota

Article excerpt

ABSTRACT.-

Bighorn sheep (Ovis canadensis) were re-introduced into the Black Hills, South Dakota, U.S.A. in 1965. To date limited information exists concerning vital rates of this population. From 2010 to 2013, we estimated survival and cause-specific mortality of 55 adult female bighorn sheep in three herds in the east-central Black Hills. We documented 21 mortalities. Of those, pneumonia (19%) and predation (19%) accounted for most known causes of mortality: however, we were unable to ascertain cause of death for 47.6% of mortalities. We used a known fate analysis in Program MARK to estimate monthly survival; our best approximating model indicated survival differed during May-Jun compared with the remainder of the year. Monthly survival estimates for May-Jun were 0.95 (95% CI = 0.91-0.97) compared with 0.99 (95% CI = P.98-0.99) for Jul-Apr, and overall annual survival was 0.81 (95% CI = 0.72-0.87). We found little support for the hypothesis that survival was influenced by body mass or nutritional condition (ingesta-free body fat). Our results indicated disease, predation, and other factors predisposing ewes to mortality, especially during and shortly after parturition, were contributors to the current demographic status of this population.

INTRODUCTION

Throughout North America, bighorn sheep ( Ovis canadensis) have declined dramatically since European settlement (Buechner, 1960) because of an array of environmental and demographic factors (Singer et al, 2001; Wehausen et al, 2011). Restoration efforts aimed at restoring native populations have been mixed, with many populations experiencing repeated declines and extinctions {e.g., Berger, 1990). Primarily, these fluctuations have been attributed to disease (Hobbs and Miller, 1992; Cassirer and Sinclair, 2007) or predation (Kamler et al, 2002; Rominger et al, 2004). Pneumonia, typically Pasturella spp., has been particularly devastating across North America resulting in partial or complete die offs in numerous herds (e.g., Cassirer et al, 1996; Jorgenson et al, 1997), although bighorn sheep have been documented with the pathogens and not exhibiting clinical signs (Onderka and Wishart, 1988). These die offs are often attributed to contact with domestic sheep and typically are followed by years of depressed lamb recruitment (Singer et al, 2000; Monello et al, 2001).

Cougar (Puma concolar) predation also has been implicated as a cause of bighorn declines (Hayes el ai, 2000; Rominger et ai, 2004), with increased impacts during declines in their primary prey, mostly mule deer (Odocoileus hemionus; Kamler et ai, 2002; Rosas-Rosas et ai, 2003). McKinney et ai (2006) observed no influence of cougar abundance on predation of bighorn sheep and concluded predation may be a function of-learned behavior by individuals (e.g., Ross et ai, 1997). In addition to disease and predation, poor nutrition can predispose individuals to mortality factors, especially in temperate environments with seasonal fluctuations in abundance and quality of forage (Hobbs, 1989; Festa-Bianchet et ai, 1997; Monteith et ai, 2013). In ungulates heavier females often exhibit higher survival (Gaillard et ai, 2000), yet, nutritional condition (i.e., percent body fat) may be more sensitive to habitat factors experienced by the individual than body mass (Monteith et ai, 2014). Assessing the relative contribution of extrinsic factors on patterns of survival is therefore contingent on an understanding of the nutritional limitations within a population (Monteith et ai, 2014).

Similar to other regions, native bighorn sheep were extirpated from the Black Hills in the early 1900's [South Dakota Department of Game, Fish and Parks (SDGF&P), 2007] and in western South Dakota around 1925 (Zimmerman, 2008). Transplants beginning in 1965 resulted in the establishment of four herds in the Black Hills region (SDGF&P, 2013). Since 2006 annual surveys conducted by SDGF&P have indicated significant declines in lamb:female ratios as well as general population declines in the east-central Black Hills. …

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