Natural Range Expansions
For decades, bighorn sheep were viewed as a species that does not colonize unoccupied habitat, requiring translocation to reestablish extirpated populations. Only in the past 15 years has evidence to the contrary begun to erode this notion. We have documented several recent instances of natural colonizations among Sierra Nevada bighorn sheep.
In 2002 an observation at the base of Charlotte Dome by climbing guide S.P. Parker brought our attention to a group of bighorn females living west of the Sierra Nevada crest along Bubbs Creek, which lies between Kearsarge Pass and Cedar Grove. This discovery resulted in the delineation of a new herd unit in an area with no historic evidence of bighorn sheep use. The sheep that colonized this area are most likely derived from a small group of females that once lived year-round in the Kearsarge Peak area but disappeared after 1995. As many as 17 females may now inhabit the Bubbs Creek herd unit.
More recently, evidence has emerged of a small number of bighorn sheep living at the south end of the Convict Creek herd unit. This group was undoubtedly established via colonization across Mono Pass from the adjacent Wheeler Ridge herd unit. During the winter of 2011 skiiers on Esha Peak photographed three ewes, three lambs, and a young male, and the same group was observed by Recovery Program staff in June of 2011. There is widespread hope that this little group will grow in size and expand its range to the north.
Prior to the initiation of a restoration program in the late 1970s, only three native bighorn populations persisted in the Sierra Nevada. These are now known as the Mount Williamson, Mount Baxter, and Sawmill Canyon herd units. The 1979-88 translocation program resulted in the restoration of four herd units, and the recent natural colonizations described above bring the total occupied herd units to nine. It is hoped that the same natural process will soon add the Taboose Creek and Coyote Ridge herd units to the list.