Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Fish and Wildlife Biology

Department or School/College

College of Forestry and Conservation

Committee Chair

L. Scott Mills

Commitee Members

Thomas R. Stephenson, Mark Hebblewhite, Daniel H. Pletscher, Michael S. Mitchell, Gordon Luikart


bighorn sheep, conservation, endangered species, Ovis canadensis sierrae, population models, vital rates


University of Montana


An extinction vortex is one of the greatest threats to endangered species; when demographic, environmental, and genetic stochasticity interact with each other and with deterministic factors, such as habitat quality, to reinforce the demise of a small population. To successfully escape an extinction vortex and enable species recovery, all processes that affect endangered populations should be comprehensively assessed and incorporated into conservation plans. For my dissertation, I worked in conjunction with California Department of Fish and Game to develop a comprehensive research program to guide recovery efforts for federally endangered Sierra Nevada bighorn sheep, the rarest subspecies of mountain sheep in North America. I initiated a combination of demographic, habitat and genetic analyses to identify the stochastic and deterministic factors limiting the recovery of this subspecies, examine the relative and synergistic impacts of these factors on the performance of Sierra Nevada bighorn sheep, and the benefits of different management activities for stimulating recovery efforts. Just as the extinction vortex predicts, I found that small populations of Sierra Nevada bighorn sheep were driven by a number of stochastic and deterministic processes. Demographic, habitat, climate, predation, and genetic factors operated singly and in concert to shape the overall viability of this subspecies. The interaction of factors led to atypical demographic patterns that deviated from theoretical expectations and increased extinction risk. To alleviate extinction processes, I found that management strategies must be tailored to population-specific dynamics, targeting those vital rates and ecological drivers which have the greatest power to increase performance. Results from this study have elucidated critical aspects of Sierra Nevada bighorn sheep ecology, provided a recovery strategy for this subspecies, and supplied new quantitative tools for examining the dynamics of small and endangered populations. Ultimately, this work offers an example of assessing population viability, not in terms of probability of extinction, but in terms of quantifying conservation measures that will alleviate extinction dynamics and achieve endangered species recovery goals.



© Copyright 2010 Heather Elizabeth Johnson