Herd Units
Based on historical evidence of occupation by sheep and knowledge of habitat use patterns of extant populations, the Recovery Plan for the Sierra Nevada Bighorn Sheep defined sixteen areas as herd units geographically. Those herd units range from Olancha Peak in the south to the Twin Lakes in the north. The recovery plan designated twelve herd units that must be occupied in order for the subspecies to be downlisted to “threatened” status and ultimately removed from the endangered species list (delisted). At the time the Recovery Plan was drafted seven herd units were known to be occupied (Mount Warren, Mount Gibbs, Wheeler Ridge, Sawmill Canyon, Mount Baxter, Mount Williamson, Mount Langley). Since then there have been two natural colonizations. First was to Bubbs Creek on the west side of the Sierra, where there was no historical evidence of a past population. The Bubbs Creek herd unit was defined as a result and inserted into the final version of the Recovery Plan, but not included as one required for downlisting and delisting. Second, the Convict Creek herd unit was colonized by members of the Wheeler Ridge herd unit to its immediate south. Restoration actions during 2013-15 moved sheep to three additional herd units (Olancha Peak, Big Arroyo, and Laurel Creek). Following those actions, all of the twelve required herd units are now occupied to some degree. However, one (the Taboose Creek herd unit) appears to be another natural colonization, but has not had more than a few females documented in it, and the three newly restored herd units (Olancha Peak, Big Arroyo, and Laurel Creek) will need close monitoring for some years to determine if those populations establish themselves adequately. In addition to those herd units, Sierra bighorn were moved in 2015 to the west side of the Cathedral Range in the southern Yosemite National Park to a region not defined as a herd unit in the Recovery Plan but thought to be capable of supporting another herd. If newly created herds and some additional herds grow adequately, it is possible that downlisting criteria in the Recovery Plan could be met by 2020-25.
