Pacific water shrew (Sorex bendirii) COSEWIC assessment and status report: chapter 4

Global range

The Pacific water shrew ranges from northern California through western Oregon and Washington to southwestern British Columbia in Canada (Figure 2). A Pacific coastal species, this shrew is restricted to coastal lowlands and lower elevations in the coastal mountain ranges. 

Figure 2. Global range of the Pacific water shrew (Sorex bendirii).

Figure 2.  Global range of the Pacific water shrew (Sorex bendirii).

Canadian range

In Canada, the Pacific water shrew is confined to the lower Fraser River valley region in extreme southwestern British Columbia. This distributional area is limited to the Lower Mainland Ecoregion of the Pacific Maritime Ecozone. More surveys are required to delimit the precise boundaries of its range. But the known historical and recent occurrence records (Figure 3) suggest that this species ranges as far east as the Chilliwack River and Harrison Lake. Craig and Vennesland (2004a) reported Pacific water shrew sightings by naturalists further east in the Skagit River valley. However, given that the Skagit River is within the range of the common water shrew (Nagorsen 1996), identifications based on captures are required to verify the presence of the Pacific water shrew in this valley. Northern limits of the range are lower elevations (below 800 m) in the Coast Mountains on the north side of the Fraser River.

Figure 3. Canadian range of the Pacific water shrew (Sorex bendirii) based on all known historical and recent occurrences.

Figure 3.  Canadian range of the Pacific water shrew (Sorex bendirii) based on all known historical and recent occurrences.

There are about 142 occurrence records (historical museum specimens, sightings, recent captures) for this species in Canada. Of the 142 records, 99 (70%) are historical museum specimens taken from 1888-1957. The 142 occurrence records represent about 44 distinct locations or sites.

The extent of occurrence based on historical and recent occurrences is about 3,350 kmĀ². This represents about 5% of the global range. The area of occupancy is unknown. Based on a comparison of recent (< 20 years ago) and historical records, Craig and Vennesland (2004a) concluded that the extent of occurrence of the Pacific water shrew has declined 16-20%. But there has been no systematic survey of the entire historic range to assess temporal range changes, and the reputed range decline described by Craig and Vennesland (2004a) may be an artifact of recent sampling. For example, no recent surveys have been done in the municipality of Vancouver (compare Figure 3 and Figure 5). 

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