BC Conservation Data Centre: Conservation Status Report
Crotalus oreganus
Western Rattlesnake
| Scientific Name: | Crotalus oreganus |
|---|---|
| Scientific Name Synonyms: |
Crotalus viridis
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| English Name: | Western Rattlesnake |
| Provincial Status Summary | |
| Status: | S2S3 |
| Date Status Assigned: | March 30, 2018 |
| Date Last Reviewed: | March 30, 2018 |
| Reasons: | Western Rattlesnake is restricted to dry, low elevation valleys in southern British Columbia. This species faces significant threats including road mortality, persecution by people and habitat loss and fragmentation. |
| Range | |
| Range Extent: | F = 20,000-200,000 square km |
| Range Extent Estimate (km2): | 28,838 |
| Range Extent Comments: | For the most part, rattlesnakes are restricted to the dry lower valleys of the Okanagan and Thompson river basins and the hotter portions of the Kettle, Nicola, Similkameen and Fraser (Lytton-Lillooet area) valleys (Charland et al. 1993). It is found as far west as Lytton, north to Chase and Kamloops, all through the Okanagan region, and east to Christina Lake (Matsuda et al. 2006; COSEWIC 2015b). COSEWIC (2015b) calculated the extent of occurrence (EOO) of dens to be 26,853 ? 26,923 km2. This included den sites observed to be occupied since 1998 (low value) and for all den observations since 1960 (high value). If all observations of snakes, not just den sites, are used, then the EOO is 28,838 km2. The B.C. CDC calculated the range extent to be 27,283 km2, based occurrences mapped as of 2017. |
| Area of Occupancy (km2): | F = 126-500 |
| Area of Occupancy Estimate (km2): | 356 |
| Area of Occupancy Comments: | COSEWIC (2015b) calculated the Index of Area of Occupancy (observations and dens since 1988) to be 356 2x2 grid cells, or, 1,424 km2. |
| Occurrences & Population | |
| Number of Occurrences: | D = 81 - 300 |
| Comments: | Currently there are 368 hibernaculum records in the provincial den database (Sarell and Hobbs unpubl. data 2013), not including 12 dens from which the snakes are believed to be extirpated (COSEWIC 2015b; Hobbs 2015). This translates to 100 element occurrences mapped by the B.C. Conservation Data Centre as of 2017 (dens that are close to each other may be considered one occurrence). |
| Number of Occurrences with Good Viability / Ecological Integrity: | U = Unknown |
| Number of Occurrences Appropriately Protected & Managed: | D = 13 - 40 |
| Comments: | Suitable habitat and some denning sites are known to be protected in provincial parks (Kalamalka Lake Prov. Park, White Lake Grasslands Prov. Park, Okanagan Mountain Prov. Park and Lac Du Bois Grasslands Park), Ecological Reserves (Haynes Lease Ecological Reserve), and Vaseux-Bighorn National Wildlife Area, land owned by Nature Trust of BC and managed for conservation. These habitat should be stable although there is a case where the proximity to a road and increased use by people may be harmful to the Rattlesnake population. Lac Du Bois Grasslands Park contains summer habitat but the hibernacula are on municipal or private lands just outside the park (COSEWIC 2004). This species is listed as Identified Wildlife under the Forest and Range Practices Act. There are 31 established and 20 proposed Wildlife Habitat Areas (WHA's) which provide limited protection (COSEWIC 2015b). |
| Population Size: | EF = 2,500 - 100,000 individuals |
| Comments: | The number of Western Rattlesnakes in British Columbia is uncertain. Research by Hobbs (2013) and Aquila Conservation & Environment Consulting (2014, in COSEWIC 2015b), estimates numbers less than 10,000 to just over over 56,000 individuals. |
| Threats (to population, occurrences, or area affected) | |
| Degree of Threat: | B = High |
| Comments: | "The main threats to the species are road mortality, persecution by people, and habitat loss and fragmentation from urban and agricultural developments. If dens are destroyed, entire subpopulations can collapse. Information on snake population size and structure at dens and the viability of the dens themselves is limited. Lack of knowledge and fear of rattlesnakes have resulted in persecution and killing of snakes." (COSEWIC 2015b) |
| Trend (in population, range, area occupied, and/or condition of occurrences) | |
| Short-Term Trend: | E = Decline of 30-50% |
| Comments: | Short term trends are unknown but suspected to be greater than 30% based on habitat loss and road mortality. Rattlesnakes from 12 dens in the provincial den database, which extends back to about 1980, are considered extirpated (COSEWIC 2015b; Hobbs 2015). |
| Long-Term Trend: | EF = Decline of 10-50% |
| Comments: |
2018: From COSEWIC (2015b): "Overall, approximately 15% of BC?s southern interior grasslands were lost to human development, mainly agriculture and urbanization, from mid-1800 to 1990 (Grasslands Conservation Council of BC 2004; BC Ministry of Environment 2007)." "The rate of loss has varied both among areas and grassland types. Most land conversion is concentrated in valley bottoms, which also contain habitat that is most suitable for rattlesnakes. Regionally, the highest losses have taken place in the northern Okanagan Basin, where over 47.6% of grasslands had been converted to agriculture (vineyards, orchards, other crops, irrigated pasture) or urban/industrial land uses by 2005 (Grasslands Conservation Council of BC 2004; BC Ministry of Environment 2007). High losses of grassland have also occurred in South Okanagan Highlands (38.6%), South Okanagan Basin (20.5%), and Thompson Basin (20.0%). 2012: The population has declined in the past by direct persecution, road mortality and habitat loss or degradation (Charland et al. 1993). The loss of native grassland varies from 11-45% in the Okanagan-Similkameen region to 10-20% in the Thompson region (COSEWIC 2004r) so declines in this range could be estimated based on grassland data. |
| Other Factors | |
| Intrinsic Vulnerability: | A=Highly vulnerable |
| Comments: | At the northern limit of its range, reproductive rates are lower than they are further south, increasing vulnerability (Macartney and Gregory 1988; Macartney et al. 1990; Charland et al. 1993). Females mature at 7-9 years and give birth to small litters of 2-8 young every 3-4 years; generation time is approximately 15 years (COSEWIC 2015b). Rattlesnakes do not survive well in developed landscapes. Furthermore, they are not known to recolonize areas where populations have been reduced. |
| Environmental Specificity: | B=Narrow. Specialist or community with key requirements common. |
| Comments: | Rattlesnakes are restricted to dry grassland habitat. They have a home range that include one or more winter dens, a transient area and a summer range. They overwinter in dens on steep slopes in rock outcrops, along talus slopes, or in earth-covered outcrops. Because they congregate at communial dens, they are vulnerable to disturbances. In the summer they require suitable basking and retreat sites and riparian areas (COSEWIC (2004; 2015b): |
| Other Rank Considerations: | |
| Information Gaps | |
| Research Needs: | |
| Inventory Needs: | Surveys for den sites, particularly at the edge of the range should be conducted. Annual monitoring of selected sites are needed to determine trends. |
| Stewardship | |
| Protection: | Areas that encompass all of the life stages/requirements should be protected, the priority being given to communal dens. Dens and important habitat on private lands can currently be protected using Development Permits, where local governments have incorporated this strategy into bylaws. Rattlesnake dens and critical habitat on Federal lands are technically protected by the Species at Risk Act. |
| Management: | |
| Version | |
| Author: | Cannings, S. and L. Ramsay (1998); L. Westereng and M. Sarell (2007) and L. Gelling (2011, 2017) |
| Date: | March 30, 2018 |
| References | |
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Okanagan Historical Society, 1965. Okanagan Historical Society Reports, 1965. https://okanaganhistoricalsociety.org |
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Atkins, M.C.P. 2021. Temporal and spatial changes in a Western Rattlesnake (Crotalus oreganus) population in British Columbia. M.Sc. Thesis, Thompson Rivers Univ., Kamloops, BC. 87 pp.
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BC Conservation Data Centre: Conservation Data Centre Mapping Service [web application]. 2006. Victoria, British Columbia, Canada. Available: http://maps.gov.bc.ca/imf50/imf.jsp?site=cdc.
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Charland, M.B., K.J. Nelson, and P.T. Gregory. 1993. Status of the Northern Pacific Rattlesnake in British Columbia. B.C. Minist. Environ., Lands and Parks, Wildl. Branch. Working Rep. WR-54. 23pp.
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COSEWIC. 2004. COSEWIC assessment and status report on the western rattlesnake Crotalus oreganus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 26 pp.
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COSEWIC. 2004r. COSEWIC assessment and update status report on the Eastern and Western Yellow- bellied Racers, Coluber constrictor flaviventris and Coluber constrictor mormon in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vii + 35 pp. (www.sararegistry.gc.ca/status/status_e.cfm)
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COSEWIC. 2015b. COSEWIC assessment and status report on the Western Rattlesnake Crotalus oreganus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. xi + 44 pp.
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Hobbs, J. 2013. Den Survey and Population Assessment of the Northern Pacific Rattlesnake in BC. Report prepared for BC Ministry of Forests, Lands and Natural Resource Operations, Victoria, BC. 39 pp.+ appendices.
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Hobbs, J. 2015. Electronic spreadsheet of snake dens for the province of BC (excluding those on Indian Reserves and private land). Hemmera Envirochem Inc., Victoria, BC.
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Hobbs, J. and M. Sarell. 2000. Gopher Snakes (and cohorts): an assessment of selected den sites in the south Okanagan. Report prepared for Ministry of Environment, Lands and Parks. Kamloops, BC. 32pp.
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Hobbs, J. and Sarell, M. 2002c. An assessment of Racer and Gopher snake habitat in the Williams Lake and 100-Mile Forest Districts. Unpubl. rep. prepared for Minist. Environ., Lands and Parks, Williams Lake, BC. 48pp.
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Hobbs, J., and M. Sarell. 2001. Gopher snakes (and cohorts): An assessment of selected den sites in the Penticton, Vernon, Merritt and Boundary Forest Districts. Unpubl. rep. prepared for Minist. Environ., Lands and Parks, Kamloops, BC. 47pp.
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Hobson & Associates. 2006. Environmental issues and options for the South Okanagan Regional Growth Strategy. Prepared for the Environmental Advisory Committee, Regional District of Okanagan-Similkameen.
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Iredale, F.J. 2006h. Fall inventory for Northern Pacific Rattlesnake, Racer, and Gopher Snakes 2006 MOE Regions 3 and 8. BC Conservation Core report prepared for Ministry of Environment, Kamloops, BC. 38pp.
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Iredale, F.J. and G. Ferguson. 2007. Spring/fall inventory for Northern Pacific Rattlesnake, Racer, and Gopher Snakes 2007 MOE Regions 3. BC Conservation Corps report prepared for Ministry of Environment, Kamloops, BC. 20pp.
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Kirk, D.A., Karimi, S., Maida, J.R., Harvey, J.A., Larsen, K.W. and Bishop, C.A., 2021. Using ecological niche models for population and range estimates of a threatened snake species (Crotalus oreganus) in Canada. Diversity, 13(10), p.467.
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Macartney, J. M., and P. T. Gregory. 1988. Reproductive biology of female rattlesnakes (Crotalus viridis) in British Columbia. Copeia 1988:47-57.
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Macartney, J.M., P.T. Gregory, and M.B. Charland. 1990. Growth and sexual maturity of the western rattlesnake, Crotalus viridis, in British Columbia. Copeia 1990:528-542.
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Matsuda, B.M., D.M. Green and P.T. Gregory. 2006. Royal BC Museum handbook amphibians and reptiles of British Columbia. Royal B.C. Mus., Victoria, BC. 266pp.
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Sarell, M. 2003. An inventory strategy for the Gopher Snake (Pituophis catenifer deserticola) in south-central British Columbia. Report prepared for the Ministry of Water, Lands, and Air Protection, Penticton, BC.
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Spruyt, J., 2024. Effectiveness of road mortality mitigation in a northern community of snakes (Doctoral dissertation, Thompson Rivers University).
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Winton, S. A. 2018. Impacts of road mortality on the Western Rattlesnake (Crotalus oreganus) in British Columbia. M.Sc. Thesis, Thompson Rivers University, Kamloops, B.C. 94 pp.
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Please visit the website Conservation Status Ranks for information on how the CDC determines conservation status ranks. For global conservation status reports and ranks, please visit the NatureServe website http://www.natureserve.org/.
Suggested Citation:
B.C. Conservation Data Centre. 2018. Conservation Status Report: Crotalus oreganus. B.C. Minist. of Environment. Available: https://a100.gov.bc.ca/pub/eswp/ (accessed Aug 2, 2026).