The species
There existed three subspecies of Bighorn Sheep in North America but considering some analysis conducted in 1999, only two should be considered as separated : the Desert Bighorn Sheep and the California Bighorn Sheep. However in British Columbia, California Bighorn sheep and Rocky Mountain Bighorn sheep populations are still considered as different ecotypes.
The Bighorn Sheep population was assessed at 38 000 individuals in the Rocky Mountains. California Bighorn sheep were reintroduced in North America after a huge population decline due to disease and are now well established with 10 000 individuals estimated. In British Columbia they were also reestablished and the population was assessed at 3,030 to 3,625 individuals. However, they are still disseminated in disconnected herds across the province. Rocky Mountain Bighorn Sheep are disseminated in herds along British Columbia and some of these herds are in connection with herds in Alberta in some areas. The population encompassed 3,000 individuals in 1996.
The diet of Bighorn Sheep is composed of graminoids, forbs and shrubs in variable proportions particularly according to area and season. The rutting period varied according to habitat as the parturition should occurs when the food-resource availability is optimal. On average in British Columbia the rutting period lasts from November to the beginning of December. The observed reproduction rate of Bighorn sheep is high and ensures quick positive evolution of the population or the stability of populations already at the carrying capacity of their environment. Bighorn Sheep show high constancy to home range (75% for males and 90% for females). They are living in sexually separated herds. The number of male home areas varies from two to six (pre-rut, rutting, mid-winter, late-winter/spring and summer) whereas for female it diverges from two to four areas (winter, spring, lambing and summer). Home areas are mostly a portion or an entire mountain and their size varies highly according to season, area, gender or herd; from 0.03 to 32.9 square km. The average home range size usually covers a narrower range, for example in Montana the average is 5.41 square km for females and 7.98 square km for males. Distances travelled by Bighorn Sheep during seasonal migration vary according to herd and subspecies. The distance range for California Bighorn Sheep is ranging from a few hundred meters to more than 70 km and from 24 to more than 51 km for Rocky Mountain Bighorn Sheep. For this subspecies the migration distance differs also by gender. In October and in mid-March, males gather in fall (or pre-rut) areas. They spread for the rutting period which lasts from early November to the end of December. After that, some of them return to fall areas and others migrate to mid-winter area where they stay during 271 to 303 days. Females stay shorter in their wintering areas (240 to 268 days) which often corresponds to the fall areas of the males. At the end of May or during June they migrate to the lambing range which is frequently a less attractive area in order to reduce predation risks. At the end of June or beginning of July all Bighorn Sheep migrate to summer areas.
Within a home range, Bighorn Sheep can be found in a diversity of habitats. According to their changing needs and the particularities that the habitat offers, they move daily or seasonally from one habitat type to another. Habitat preferences and seasonal strategies varies with herds and subspecies. Two general important needs are mineral licks and watering pockets.
Conservation and Management
In British Columbia, Bighorn Sheep are considered at risk. The province counts five metapopulations of the California subspecies while the Rocky Mountain subspecies in British Columbia are only a small portion of a bigger metapopulation. The minimum number of individuals needed to have a sustainable population has been established and is used to point out populations that appear too small. The evolution of the Bighorn Sheep subspecies populations size, both declines and increases, is described within the article. By 2000, the California Bighorn Sheep population size was so reduced that the species became of special concern for British Columbia.
There are many different diseases, infections and others threats that can negatively affect Bighorn Sheep populations and that are described in the article. Those include livestock ranching, lungworm infection, human presence, predation and inappropriate livestock operations. In the same way, there exist many habitat damages that can limit Bighorn Sheep populations such as domestic sheep business, cattle and horses grazing, agriculture, urbanization, transportation constructions, resources extraction industries, recreation activities, fire suppression strategies, habitat competition with other wild species and the invasion of inedible plant species. Due to the overlapping of land use for human developments and for Bighorn Sheep at low elevation, the suitable habitat range for Bighorn Sheep has been dramatically reduced. The winter habitat loss is especially severe.
A diversity of habitat protection actions and programs are described in the article and some protected areas containing essential habitat range are listed. This includes protection by the province and by private land acquisition programs.
- Hunting is an useful tool to manage populations in order to stabilize them at carrying capacity and is very regulated and controlled.
- Protocols have been established to limit vegetation management projects to areas where no herds are observed and it is recommended that protocols includes limitations for the presence of domestic sheep at proximity to Bighorn Sheep herds.
- Transportation is also partially limited to protect Bighorn Sheep habitats based on sections of the Wildlife Act. However improvements to those access regulations need to be made.
- Unfortunately, the wildlife habitat improvement plan of the Region could not be implemented due to lack of funding.
- Weed management plans have been implemented by the Ministry of Forests.
A high-level, large-scale management plan should be created to reach sustainability of Bighorn Sheep population and habitat. Different recommendations are proposed in the article to be taken into account while creating such a plan. In summary they recommend to ensure Bighorn Sheep population sustainability in their historic areas ; to diminish the proximity of domestic cattle ; to limit perturbations during crucial periods and in crucial habitats (especially recreational activities and transportation access) ; to ensure existence and accessibility to migration corridors and crucial home areas and to improve, protect or recover suitable nutritional sources while limiting the presence of invasive plants. General and specific wildlife measures addressing these recommendations are proposed in the article. In addition, wildlife habitat areas can provide specific protection for crucial habitats where prescribed measures are incomplete.
- Source R.A. Demarchi. 2004. BIGHORN SHEEP Ovis canadensis. Accounts and Measures for Managing Identified Wildlife – Accounts V. 2004. 19p. Available at : http://www.env.gov.bc.ca/wld/frpa/iwms/documents/Mammals/m_bighornsheep.pdf [Accessed 03/2021].