Pacific water shrew (Sorex bendirii) COSEWIC assessment and status report: chapter 8

The Pacific water shrew is at the northern limits of its range in Canada, where it inhabits a restricted area in the lower Fraser River basin. Biological factors that determine its limited range in Canada are essentially unknown but competitive exclusion with the ecologically similar common water shrewprobably contributes to its distributional limits. Although this species is not at risk in the United States portion of its range, in Canada its distribution coincides with a heavily urbanized area undergoing rapid development and habitat change (Figure 6).

In 2004, the total population for the Greater Vancouver Regional District and the Fraser Valley Regional District was about 2.4 million (Table 4). This represents a population increase from 1996 of about 11%. There are no data on the amount of land used for this population growth, but Environment Canada (1992) estimated that growth and development from 1987 to 1992 in the Fraser Basin required about 934 ha of land per year. Most of the land for urban expansion was agricultural land (Moore 1990). Although the rate of conversion of rural to urban land has been declining because of more intensive use of available land, the supply of new developable land is finite and diminishing in the region. According to Environment Canada (1992), future population growth and development pressures will increase proposals for the development of remaining agricultural lands, wetlands, foreshore areas, forested land, and mountain slopes.

Figure 6. The amount of urbanized area within the range (extent of occurrence) of the Pacific water shrew (Sorex bendirii) in Canada.

Figure 6. The amount of urbanized area within the range (extent of occurrence) of the Pacific water shrew (Sorex bendirii) in Canada.

Rarity, coupled with its restriction to riparian and wetland habitats, makes the Pacific water shrew susceptible to habitat loss, fragmentation, and degradation. Although agriculture and forest harvesting activities impact this species, development is likely its greatest threat. Housing, commercial, recreational (such as golf courses), and industrial development reduces forested areas and riparian habitats that border streams or wetlands and degrade habitat from run-off and storm water management (Craig and Vennesland 2004 a). More research is needed to determine the distances and habitats that this shrew will disperse through in natural and modified landscapes. Its use of culverts and anthropogenic habitats such as agricultural drainage ditches also is unknown. But highways and roads are major barriers that would limit movements and increase fragmentation. No research has been done on the effect of water quality on this shrew. Changes in water quality resulting from sedimentation or contaminants could impact aquatic invertebrates that are eaten by the Pacific water shrew. Water contaminants such as oil would be expected to reduce the insular efficiency of its pelage. With much of this shrew’s range on private land, development activity in riparian areas will mostly be regulated by municipalities (see next section). The Best Management Practices Guidelines for Pacific water shrew in Urban and Rural Areas (Craig and Vennesland 2004b) recommend 100-m buffers around wetlands, streams, and creeks. Given the potential conflict between these recommendations and future development proposals, municipalities may choose to ignore the guidelines.

Table 4. Changes in population estimates for the Greater Vancouver Regional District and the Fraser Valley Regional District from 1996-2004. Data from BC Stats, ServiceBC, Ministry of Management Services
Regional District 1996 2004 1996-2004 % change
Greater Vancouver
1,906,492
2,132,697
10.6
Fraser Valley
230,976
260,247
11.2

Potential threats to this species include mortality from accidental captures in minnow traps and domestic cat predation. Pacific water shrews occasionally drown in minnow traps used by fisheries biologists for fish surveys. The scale of this accidental mortality is probably minimal. Galindo-Leal and Runciman (1994) emphasized the role of domestic cats as a major predator of Pacific water shrews in urban and agricultural landscapes. Because no studies have been on domestic cat diets in the lower Fraser River valley, the impact of this predator cannot be assessed.

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