BC Conservation Data Centre: Conservation Status Report
Sorex rohweri
Olympic Shrew
| Scientific Name: | Sorex rohweri |
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| English Name: | Olympic Shrew |
| Provincial Status Summary | |
| Status: | S2 |
| Date Status Assigned: | January 18, 2024 |
| Date Last Reviewed: | January 18, 2024 |
| Reasons: |
This is recently discovered species (2007) is known from a limited range with few occurrences in the lower Fraser Valley, where their forest habitat is threatened by development. |
| Range | |
| Range Extent: | C = 250-1,000 square km |
| Range Extent Comments: |
This species is restricted to southwestern British Columbia in the Fraser Lowland and Northwestern Cascade Ranges Ecosections. Confirmed occurrences have found it on the south side of the Fraser River in British Columbia from Burns Bog as far east as Chilliwack Lake in the Chilliwack Valley. Precise eastern limits of its range can not be determined because of sampling inadequacies, but it may be as far east as the Skagit valley (Nagorsen and Panter 2024). In Washington State a survey in 2019 found this species on the dry east slopes of the Cascades for the first time, with identifications verified with genetic sampling (Ryckman and Ernst 2022). That they may occur to the east of the Cascades in B.C. is a possibility. Nagorsen (2007) demonstrated that S. rohweri and S. cinereus are allopatric with S. cinereus replacing S. rohweri on the north side of the Fraser River and east of the Chilliwack Valley. Contact zones where the two species may co-occur are the Skagit Valley and mid elevations in the Cascade Mountains where S. cinereus likely replaces S. rohweri at higher elevations in the Cascades. The BC population appears to be disjunct from the WA populations in the Olympic Peninsula. Using an alpha hull [following the methodology of Burgman and Fox (2003)], with all known occurrences, Sorex rohweri has a calculated Range Extent of ~670 sq. km. Nagorsen and Panter (2009, 2024) quote a range extent of 360 sq. km., evidently obtained using a more restrictive mapping methodology. |
| Area of Occupancy (km2): | D = 6-25 |
| Area of Occupancy Comments: | Using all known occurrences, there are eight 2x2 km grid squares occupied by Sorex rohweri. If historical occurrences are removed from the analysis only four 2x2 km grid squares are occupied, however the historical occurrences are in relatively intact areas. As this is a recently describes species and no targeted surveys have been done, there are likely more areas occupied by the species within its range. Insufficient habitat data exist to develop a habitat model for this shrew. |
| Occurrences & Population | |
| Number of Occurrences: | BC = 6 - 80 |
| Comments: | A survey of existing BC museum specimens by Nagorsen (2007) revealed 18 museum specimens of Sorex rohweri from 13 localities - they represent 8 element occurrences (EOs). These EOs include both historical museum specimens (originally identified on their tags as Sorex cinereus or Sorex vagrans) dating back to the 1930s and recent (1992-2004) vouchers taken by Zuleta and Galindo-Leal (1994), Fraker et al. (1999), and Roberston Environmental Services (2006) that were originally identified as S. cinereus and subsequently re-identified as S. rohweri (see Nagorsen 2007, Nagorsen and Panter 2009). More surveys for this species likely will result in more EOs, however as of the latest assessment (2024), there have not been any new occurrences confirmed (Nagorsen and Panter 2024). |
| Number of Occurrences with Good Viability / Ecological Integrity: | U = Unknown |
| Comments: | Because the population size at each EO is unknown, viability has to be based on ecological integrity. Habitat assessments need to be done at EOs to assess the quality of occurrences. Historical EOs will be problematic as most have no associated coordinates and their locations can only be estimated to the nearest 1-10 km. |
| Number of Occurrences Appropriately Protected & Managed: | C = 4 - 12 |
| Comments: | Two EOs are in protected areas. One EO (1992-1999) is partly in the Burns Bog Ecological Conservancy Area; and one historical EO (1977) is in Cultus Lake Provincial Park. The range extent of S. rohweri includes 2 provincial parks and 9 regional parks. |
| Population Size: | U = Unknown |
| Comments: | No population estimates exist for this species in BC or Washington. In the Olympic Peninsula of WA, Rausch et al. (2007) reported that "populations are evidently not great". The species is rare in collections of museum specimens from the BC range. However, until a directed survey is done focused on this species rarity cannot be assessed. |
| Threats (to population, occurrences, or area affected) | |
| Degree of Threat: | U = Unknown |
| Comments: | This species appears to be forest dependent (Nagorsen and Panter 2024), with potentially some flexibility with respect to successional stage (Nagorsen and Panter 2009). Threats will be loss of forests due to urban and agricultural development and forestry throughout the Fraser Valley. There are isolated patches of forest, but with these few patches of forest, there are issues with fragmentation and non-connectivity. Until more research is done on habitat requirements it is difficult to evaluate threats fully. Rausch (pers. comm.) suggested that the lack of occurrences in the Puget Sound Lowlands of Washington was the result of habitat loss. |
| Trend (in population, range, area occupied, and/or condition of occurrences) | |
| Short-Term Trend: | U = Unknown |
| Comments: | No data are available to quantify the short term (10 years) trend in populations size, extent of occurrence, area of occupancy, number of occurrences and/or condition of occurrences. There has been no systematic inventory throughout the BC range in the past 10 years and no systematic assessment of habitat conditions at recent or historical occurrence sites. Nevertheless, based on human population growth rates of ~11% over the past decade for the Greater Vancouver Regional District and Fraser Valley Regional District and the associated loss of habitat from development it is likely that the amount of habitat has declined in the past decade. |
| Long-Term Trend: | DE = Decline of 30-70% |
| Comments: | No data are available to quantify the long term trend in populations size, extent of occurrence, area of occupancy, number of occurrences and/or condition of occurrences. However, if this shrew is strongly associated with forested habitats as the limited data suggest, its habitat has been declining since the mid 1800s with the loss of forested habitats from agriculture and urban growth (Boyle 1997). |
| Other Factors | |
| Intrinsic Vulnerability: | U=Unknown |
| Comments: | Specific life history information is not available for this recently recognized species. But it likely matures quickly with a generation time of one year, and produces multiple young per litter similar to other shrews. |
| Environmental Specificity: | U=Unknown |
| Comments: | Nagorsen (2007) citing personal comments from Robert Rausch and habitat data associated with a few BC captures suggested that this species may be a forest dependent. Based on extensive trapping in the Olympic Peninsula, Rausch found that most captures were in forest, although it occupies various successional stages. |
| Other Rank Considerations: | A new species only described to science in 2007. Only two scientific papers have been published on this species, the Rausch et al. (2007) taxonomic description and more recently Nagorsen and Panter (2009) which contains some information on habitat. |
| Information Gaps | |
| Research Needs: | 1) The mtDNA sequencing by Rausch et al. (2007) was based on only 2 specimens from the Olympic Peninsula. mtDNA sequences from British Columbian samples should be assessed to verify that British Columbian populations share a similar unique mtDNA sequence. Nuclear DNA (microsatellites) should also be assessed to test if the British Columbian populations are genetically divergent from the Washington populations. Given the isolation of the BC population from the population in the Olympics, it would be expected to show genetic divergence. 2) Currently any captures of S. rohweri require the taking of voucher specimens to confirm their identification from skull and dental traits. A simple non-destructive method for taking DNA samples from live shrew captures that can be applied by consultants doing small mammal assessments should be developed and tested. |
| Inventory Needs: | There have been no inventories in BC specifically focused on this shrew. Our knowledge of habitat and distribution in BC is based on historical museum records and opportunistic vouchers from 3 environmental assessments (Zuleta and Galindo-Leal 1994; Fraker et al. 1999; Roberston Environmental Services 2006). More inventory applying pitfall traps is essential to determine the eastern limits of the range, the elevational range in the lower mainland, habitat associations, and fragmentation of the range. Sumas Mountain, Campbell Valley Regional Park, and the Skagit and Chilliwack valleys are areas where inventory could be focused. |
| Stewardship | |
| Protection: | Much of this species' range is on private land. The greatest concentration of recent (since 1990) occurrences is in Burns Bog. Except for a Vedder Mountain capture (originally identified as S. cinereus) in 1992 by Zuleta and Galindo-Leal (1994), all recent EOs are from Burns Bog. Three are within the Burns Bog Ecological Conservancy Area, but three other EOs are on private land outside the conservancy boundaries near 72nd Street (see Nagorsen 2007). |
| Management: | Until more research is done on habitat requirements and forest dependency, a Best Management Plan cannot be developed for this species. |
| Version | |
| Author: | Nagorsen, D., L. Ramsay and A. Teucher with updates by L. Ramsay (2024) |
| Date: | January 06, 2024 |
| References | |
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Boyle, C. A., L. Lavkulich, H. Schreier, and E. Kiss. 1997. Changes in land cover and subsequent effects on lower Fraser Basin ecosytems from 1827 to 1990. Environmental Management 21:185-196.
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Fraker, M., C. Bianchini, and I. Robertson. 1999. Burns Bog ecosystem review: small mammals. Robertson Environ. Serv. Ltd., Langley, BC. 58pp.
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Greater Vancouver Regional District. 2004. Burns Bog Ecological Conservation Area. Available: http://www.gvrd.bc.ca/burnsbog/ (accessed 4 June 2007).
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Nagorsen, D. and N. Panter. 2024. Shrews and Moles of British Columbia. Second Edition. Royal BC Museum Handbook. 239 pp.
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Nagorsen, D.W. 2007. Status of Rohwer's Shrew (Sorex rohweri) in British Columbia. Report submitted to BC Ministry of Transportation and Hemmera Consulting. 11 pp.
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Nagorsen, D.W. and N. Panter. 2009. Identification and status of the Olympic Shrew (Sorex rohweri) in British Columbia. Northwestern Naturalist 90:117-129.
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Rausch, R.L., J.E. Feagin and V.R. Rausch. 2007. Sorex rohweri sp. nov. (Mammalia, Soricidae) from northwestern North America. J. Mamm. Biol. 72:93-105.
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Robertson Environmental Services. 2006. South Fraser Perimeter Road. Vegetation and wildlife impact assessment. Technical Volume 12 of the Environmental Assessment Application. Prepared for the BC Ministry of Transportation, 254 pp.
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Ryckman, J.D. and K.A. Ernst. 2022. Genetic analysis leads to range extension of the Olympic Shrew. (Sorex rohweri) to the eastern slopes of the Cascade Range in Washington State. Western Wildlife 9:17?23.
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Zuleta, G.A., and C. Galindo-Leal. 1994. Distribution and abundance of four species of small mammals at risk in a fragmented landscape. B.C. Minist. Environ., Lands and Parks, Wildl. Branch. Working Rep. WR-64. Victoria, British Columbia. 80pp.
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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. 2024. Conservation Status Report: Sorex rohweri. B.C. Minist. of Environment. Available: https://a100.gov.bc.ca/pub/eswp/ (accessed Jun 7, 2026).