Habitat Selection and Resource Use by Bighorn Sheep Following Translocation
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Authors
McCain, Sean Richard
Issue Date
2025
Type
Thesis
Language
en_US
Keywords
Bighorn Sheep Restoration , Conservation , Reintroduction , Translocation , Ungulate Recovery , Wildlife Ecology
Alternative Title
Abstract
Recovery of North American ungulates is one of the greatest conservation success stories of the 20th century. Following widespread extirpations resulting from novel diseases, habitat fragmentation, and overharvest leading up to the 20th century, numerous conservation measures were implemented by Federal and State governments to restore struggling populations of wild sheep, elk, deer, goats, and other wildlife. One of the most effective restoration tools to emerge from this era was translocation of individuals by physically moving them from healthy source populations to formerly occupied habitats. This process became an essential management tool, particularly for restoring isolated populations of ungulates. Yet, despite numerous successful translocations, the tool remains an imperfect solution. Translocated ungulates continue to face ecological and physiological challenges in adapting to new habitats. Asynchronous topographical, climatic, and environmental conditions between source and target habitats can hinder population establishment and expansion, leading to low survival, low recruitment, and even local extirpation. Understanding how translocated ungulates select resources and adapt to new habitats is critical for improving long-term outcomes of conservation. Few ungulates epitomize the challenges of reintroduction by translocation than bighorn sheep (Ovis canadensis). Bighorn sheep were once extirpated throughout much of their native range but have since been reintroduced to their historic habitat throughout the western United States, Mexico, and Canada. To date, over 22,000 individuals have been translocated in North America. While many of these translocations have been successful in reestablishing population, numerous translocations have failed or resulted in low recruitment, high mortality, and limited range expansion. These unsuccessful reintroductions have allowed conservation practitioners and scientists to improve methodology for translocations, which now includes consideration of habitat similarities, distance to domestic sheep, and genetic and physiological adaptations to local conditions.In our first chapter, we sought to determine the environmental and topographic conditions that make translocations successful for long-term establishment of populations of bighorn sheep. We quantified selection of resources in a unique population of translocated bighorn sheep in western North Dakota and its corresponding source population in north-central Montana. We used resource selection functions to test the hypothesis that these two populations would select habitat with high-quality forage during spring green up, when access to forage is crucial to recruitment and survival of young. Additionally, we tested the hypothesis that female bighorn sheep in both study areas engage in a similar trade-off between vegetation and terrain that is safer for raising young following parturition. In support of our hypothesis, both the source population in Montana and the translocated population in North Dakota selected habitat with access to nutrition pre-parturition and shifted to safer habitat post-parturition. Interestingly, some differences in selection emerged between the two populations, reflecting the distinct topographic and environmental characteristics of the two study areas, despite their geographic proximity.
In our second chapter, we examined the influence of ecotypic variation between two populations of bighorn sheep translocated from different regions to the same area. We hypothesized that ecotypic variation, defined as a set of characteristics that restrict a subgroup of animals to a narrow range of environmental conditions, would result in contrasting outcomes of selection of resources in the two translocated populations. To examine differences between translocated ecotypes, we quantified selection of resources by bighorn sheep translocated from Rocky Boy’s Reservation, Montana, and from Morenci, Arizona to the same release site in Antelope Island, Utah. The population in Antelope Island provided a unique opportunity to explore differences in selection of resources between two ecotypes of Rocky Mountain bighorn sheep. We used resource selection functions to test hypotheses that 1) selection of resources in Utah by bighorn sheep translocated from Montana would contrast with that of bighorn sheep translocated from Arizona, 2) bighorn sheep translocated from Montana would signal a phenological mismatch between the timing of parturition and availability of forage, and 3) bighorn sheep from Montana would select rugged, steep terrain more strongly than bighorn sheep translocated from Arizona.
In support of our first hypothesis, selection of resources by bighorn sheep translocated from Montana contrasted significantly with bighorn sheep translocated from Arizona. Our second hypothesis was supported by the limited selection of forage by Montana bighorn sheep, although there is reason to believe these bighorn sheep may adjust their selection of resources on Antelope Island. Finally, we found strong support for our hypothesis that bighorn sheep translocated from Montana consistently selected rugged, steep terrain more strongly compared to bighorn sheep translocated from Arizona. These findings further reinforce the necessity of considering ecotypic variation when translocating bighorn sheep.