Sierra Nevada Bighorn Sheep

Quinn Bredl

Dr. Needles

BIO 227, M/W 12-2PM

Source: National Park Service

Organism Description and Ecology

About 600,000 years ago, bighorn sheep populations began to diverge into two species: thinhorn and bighorn sheep. While there was much debate as to the classification of Sierra Nevada bighorn sheep and whether they were distinguishable from those bighorn sheep in surrounding areas, there has been genetic evidence to support the assertion that Sierra Nevada bighorn sheep are their own subspecies.

In order to survive in their habitat, bighorn sheep require both agility and keen eyesight. This is to ensure that they are able to identify and evade predators. With short legs and stocky builds, they can easily navigate rocky terrain. Their optimal habitat will be in visually open area that contains precipitous rocks. Bighorn sheep habitat is naturally patchy and fragmented.

In the Sierra Nevada region, bighorn sheep utilize a variety of elevations, ranging from 4,760 to 13,120 feet. With such a wide range of elevations, vegetation varies from Great Basin sagebrush-bitterbrush-bunchgrass scrub, pinyon-juniper woodland and mountain mahogany scrub, mid-elevation and subalpine forest, woodlands and meadows and alpine meadows and other alpine habitats.

In order to increase the likelihood of spotting predators and therefore increase the time allocated to feeding, bighorn sheep have turned to group living. Because bighorn sheep depend on their keen eyesight to spot predators, they are diurnal (active during the day). At night, they typically inhabit rocky slopes.

For much of the year, bighorn sheep sexes live in separate ranges. However, during the wintertime they rejoin for mating season.

Geographic and Population Changes

Bighorn sheep herds were once scattered along the alpine crest of the Sierra Nevada from the Sonora Pass area south to Olancha Peak. Prior to their settlement in the Sierra Nevada, bighorn sheep populations probably exceeded 1,000 individuals. Population loss apparently began after European immigrants settled in the region during the mid-1800s, which continued on through most of the 20^th century. By 1948, bighorn sheep were thought to inhabit only five areas, with an estimated total population of 390. By the 1970s, the number of areas dropped to two. Specific causes for the population drop are unknown. However, market hunting, as well as die-offs from pneumonia contracted from domestic sheep may have played a role.

Known herd locations

In 1978, it was discovered that two herds contained a cumulative population of 220. Annual monitoring verified that the population persisted at high numbers. Because of this, these herds were used to repopulate other areas, with some being successful while others suffered from high mortality rates.

After increasing from 250 sheep in 1978 to almost 300 in 1985, the population declined 60% to just over 100 sheep in 1995. This can mainly be attributed to the harsh winters that the sheep were exposed to, especially at high elevations.

Listing Date and Listing Type

Sierra Nevada bighorn sheep was listed in the on April 20, 1999. The reason for their listing mainly falls under “Disease or Predation”.

Reason for Listing

Due to the dramatic decrease in bighorn sheep populations from the transmission of pneumonia from domestic sheep to bighorn sheep, as well as mountain lion predation, the low population poses a risk to the continued existence of the subspecies. At the time, the existing efforts to improve the population were unsuccessful due to the U.S. Forest Service’s inability to reduce contact between domestic sheep and bighorn sheep, as well as lacking the lawful authority to remove mountain lions from the region.

Source: National Park Service

Description of Recovery Plan

The population recovery includes three key components: population dynamics, genetic variation, and ecosystem integrity. In regard to population dynamics, the goal is to manage population processes in order to reduce variation in reproduction. This can be accomplished by maintaining a higher number of females within the population. Regarding genetics, the fundamental goal is to maintain genetic variation since a reduction in genetic variation could lead to a reduction in fitness. Lastly, ecosystem integrity should be maintained by providing suitable habitat conditions and safe access to that habitat. This includes the assurance that adequate vegetation exists in winter ranges, where bighorn sheep have struggled to maintain populations.

Potential Personal Action

Because of the relatively miniscule impact that human consumption has had on bighorn sheep populations, there doesn’t seem to be direct personal action that we can take to improve their populations in the Sierra Nevada.

Additional Resources

Counting Sheep - A documentary about Sierra Nevada bighorn sheep and their endangered status

Works Cited

Recovery Plan for the Sierra Nevada Bighorn Sheep. Rep. Sacramento: California/Nevada Office of the U.S. Fish and Wildlife Service, 2007. Web. 25 Nov. 2015.

"Sierra Nevada Bighorn Sheep." National Park Service. National Park Service, n.d. Web. 25 Nov. 2015.