Ecotypic variation in recruitment of reintroduced bighorn sheep: Implications for translocation

Abstract

European settlement led to extirpation of native Audubon's bighorn sheep (formerly Ovis canadensis auduboni) from North Dakota during the early 20th century. The North Dakota Game and Fish Department subsequently introduced California bighorn sheep (formerly O. c. californiana) that were indigenous to the Williams Lake region of British Columbia, Canada, and Rocky Mountain bighorn sheep (O. c. canadensis) that were indigenous to the Sun River region of Montana. Although California bighorn sheep are no longer recognized as a distinct subspecies, they are smaller and adapted to a milder climate than either the native bighorn sheep of North Dakota or introduced bighorn sheep from Montana. Because reintroductions still play a key role in the management of bighorn sheep and because local adaptation may have substantial demographic consequences, we evaluated causes of variation in recruitment of bighorn sheep reintroduced in North Dakota. During 2006–2011, Montana stock recruited 0.54 juveniles/adult female (n = 113), whereas British Columbia stock recruited 0.24 juveniles/adult female (n = 562). Our most plausible mixed‐effects logistic regression model (53% of model weight) attributed variation in recruitment to differences between source populations (odds ratio = 4.5; 90% CI = 1.5, 15.3). Greater recruitment of Montana stock (fitted mean = 0.56 juveniles/adult female; 90% CI = 0.41, 0.70) contributed to a net gain in abundance (r = 0.15), whereas abundance of British Columbia stock declined (fitted mean = 0.24 juveniles/adult female; 90% CI = 0.09, 0.41; r = − 0.04). Translocations have been the primary tool used to augment and restore populations of wild sheep but often have failed to achieve objectives. Our results show that ecotypic differences among source stocks may have long‐term implications for recruitment and demographic performance of reintroduced populations. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

Translocated to the fringe: genetic and niche variation in bighorn sheep of the Great Basin and northern Mojave deserts

“…Furthermore, translocations from multiple source areas often brought previously isolated populations into close proximity, potentially facilitating interbreeding between distinct groups. In particular, early on, many translocated populations experienced poor recruitment (Krausman, ), potentially the result of demographic and genetic effects (Ramey et al ., ; Hedrick et al ., ; Whittaker et al ., ), and poor consideration of ecotypic variation (Whiting et al ., ; Wiedmann & Sargeant, ). Nevertheless, repatriation programmes have led to successful re‐establishment of extirpated populations, especially across the Great Basin (NDOW, ).…”

Ecotypic variation in population dynamics of reintroduced bighorn sheep

“…We used a hand‐held net‐gun fired from a helicopter to capture individual bighorn sheep (Krausman et al ) except in 2014, when we used a drop‐net to capture bighorn sheep in Alberta, Canada. Marked bighorn sheep included individuals captured from herds throughout our study area (Wiedmann and Bleich , Wiedmann and Sargeant ) and bighorn sheep translocated to North Dakota from Oregon, USA (2003 and 2004), Montana (2006 and 2007), and Alberta (2014). We marked captured bighorn sheep with very high frequency radio‐telemetry collars equipped with mortality sensors, which we used to monitor survival from 3 November 2000 to 14 May 2016.…”

“…We used survival estimates with age ratios reported by Wiedmann and Sargeant () to parameterize a matrix population model ( T ) and explore implications of survival and fecundity for population structure, population growth, and bighorn sheep management. Our model featured 18 states, representing age classes 1 (young‐of‐the‐yr) to >18 (Fig.…”

“…We estimated transition probabilities ( p 2 : p 15 ) via local polynomial regression (loess smoothing) of annual survival versus age, extended the curve to estimate survival for ages 1 and 16–18, and applied the survival rate for 18‐year‐old bighorn sheep to older bighorn sheep as well. We estimated products of maternity ( m ) and first‐year survival ( p 0 ) by iteratively adjusting mp 0 until lamb:female ratios matched results of Wiedmann and Sargeant (). We used the dominant eigenvector of T (i.e., stable age distribution) to initialize age ratios for projections.…”

“…Vital rates of bighorn sheep are of interest to biologists because adult survival and juvenile recruitment can have variable and important effects on population growth and, ultimately, on the success or failure of translocated populations. Recently, Wiedmann and Sargeant () described differences in recruitment rates between sympatric bighorn sheep that originated in British Columbia (BC ecotype; = 0.24 juveniles/adult F) and Montana (MT ecotype; = 0.56 juveniles/adult F). The Montana ecotype comprised just 19.7% of the female population but recruited 31.6% of juvenile bighorn sheep over the long term.…”