Sierra Nevada Bighorn Sheep - Alex Cain

The Climb to Recovery - Sierra Nevada Bighorn Sheep

Alex Cain - BIO 227

Description and Ecology(1,2)

Wild Sheep first populated North America after they crossed the Bering land bridge from Siberia.  In North America, wild sheep have diverged into two extant species, thinhorn and bighorn. The Sierra Nevada Bighorn Sheep are unique in that they have become a genetically isolated subspecies due to their divergence from bighorn sheep to the East and North.  These sheep are roughly five feet long and 3 feet tall(measured to shoulders), can weigh approximately 100-220 pounds, and live on average 10-15 years.  The most prominent feature of Sierra Nevada Bighorn Sheep is the large curled brown horns that continue to grow throughout their lives.

The Sierra Nevada Bighorn Sheep rely on two characteristics for survival in their environment.  First, their ability to to move swiftly across rocky slopes.  Second, their excellent eyesight at distance.  Both of these allow them to evade predators.  Therefore, Sierra Nevada Bighorn Sheep gravitate towards visually open, steep, rocky terrain.

Sierra Nevada Bighorn Sheep behavior also centers around mechanisms to avoid succumbing to predators.  First, they live in groups(divided by gender most of the year) as a way to have more eyes and ears looking out for predators.  This allows the group more time to feed and less time individually surveying surroundings.  Second, they are primarily active during the day.  Again, this allows these groups to utilize their eyesight as a means of avoiding predators.  

Geographic and Population Changes(1)

During the 1970’s Sierra Nevada Bighorn sheep persisted in only two areas.    The contiguous Mount Baxter and Sawmill Canyon which consisted of a total of 220 sheep, 150 in the Mount Baxter herd and 70 in the Sawmill Canyon herd.  However, the Mount Williamson herd contained only 30 sheep.  Due to the larger size of the Mount Baxter and Sawmill Canyon herds, they were used as a sources of reintroduction stock.  Throughout the 1980’s this stock was used to reestablish populations at Wheeler Ridge, Mount Langley, Lee Vining Canyon, and the South Warner Mountains.  Unfortunately, the Warner Mountains herd died out in 1988 as a result of contact with domestic sheep.

The time between 1985 and 1995 saw a decline of about 60% of the total Sierra Nevada Bighorn sheep population.  This was thought to be partially due to the harsh winter of 1995 and sheep attempting to live at the high elevations.  However, the population has bounced back and reached and estimated size of 350 in 2004.

Listing, Cause, Main Threats(1)

Sierra Nevada Bighorn sheep were listed as an endangered species on January 3, 2000.  These sheep have received a priority number of 3 due to the small population size, fragmented distribution, high recovery potential, and because they are a distinct subspecies.  The main threats to the survival of the Sierra Nevada Bighorn sheep include:

•  Disease: there is a potential for the transfer of disease organisms through contact with domestic sheep.  The most  significant disease threat is pneumonia.  This is considered a significant source of mortality and requires management.
• Predation: mountain lion predation on winter ranges has accounted for most of the known mortalities since the late 1970’s.  
• Population: independent groups of females, some containing less than 5, have been observed.  This small female population size alone make these sheep vulnerable to extinction.

Description of Recovery Plan(1)

The Recovery Plan for the Sierra Nevada Bighorn sheep is designed to address the main threats to their survival and can be summarized as follows:

Protect bighorn sheep habitat: 

• Acquire key habitat areas from landowners
• Maintain quality of existing habitat

Increase population growth: 

• Temporarily protect sheep from predation losses
• Increase use of low elevation winter ranges
• Reduce risk of of sheep contracting diseases (prevent contact between bighorn sheep and domestic sheep)

Increase number of herds:

• Execute a strategy for the translocation of sheep 
• Develop sources of translocation stock

Implement a genetic management plan to maintain the genetic diversity of Sierra Nevada Bighorn sheep

Establish a monitoring system that watches status and trends of bighorn sheep herds:

• Monitor predators, vegetation, disease exposure 

Continue needed research:

• Genetic population structure
• Develop population viability analysis
• Bighorn habitat use patterns and relative human use patterns
• Future introduction sites
• Effects of climate change on Bighorn sheep

What Can You Do?

There are many individuals who have dedicated themselves to the conservation of the Sierra Nevada Bighorn Sheep and so can you!  A great way to get involved in through the Sierra Nevada Bighorn Sheep Foundation.  You can become and member or simply donate.  Please see the link below to join the movement helping the Sierra Nevada Bighorn sheep down their road to recovery!

http://sierrabighorn.org/

Work Cited
1) Recovery Plan for the Sierra Nevada Bighorn Sheep. Rep. Sacramento: U.S. Fish and Wildlife Service, 2007. Print.

"      2) Recovery Program." Sierra Nevada Bighorn Sheep Facts - California Department of Fish and Wildlife. California Department of Fish and Wildlife, n.d. Web. 29 Nov. 2016.