Sierra Nevada Bighorn Sheep: Recovering the “Bravest Mountaineer” in the Yosemite Area. By Dr. Tom Stephenson, program leader for Sierra Nevada bighorn sheep recovery, CA Dept of Fish & Game. In the visitor center auditorium in Yosemite Valley, noon to 1:00pm. Sierra Nevada bighorn sheep are a federally endangered species that spends most, and in some cases all, of the year in the alpine. Sierra bighorn are dependent upon, and well adapted to, a landscape above 11,000 feet during both summer and winter. Sierra Nevada bighorn sheep are an umbrella species that represents the health and landscape integrity of the Sierra Nevada; they are dependent upon a contiguous, expansive wilderness landscape. When bighorn sheep existed at historical numbers in the Sierra Nevada they also likely functioned as a keystone species that mediated nutrient cycling in vegetation communities and functioned as an important prey species for carnivores and scavengers.