Pacific water shrew (Sorex bendirii) COSEWIC assessment and status report: chapter 2

Pacific Water Shrew
Sorex Bendirii

Species information

The Pacific water shrew is the largest North American species of Sorexwith a total length of 154 mm and a body mass of 10.6 g. The dorsal pelage ranges from dark brown to black; the undersides are dark brown. The tail is a unicoloured dark brown. The large hind feet have a stiff fringe of hairs up to 1 mm in length.

Distribution

The Pacific water shrew ranges from northern California through western Oregon and Washington to southwestern British Columbia in Canada. In Canada, the Pacific water shrew is confined to the lower Fraser River valley region in extreme southwestern British Columbia, where it ranges as far east as the Chilliwack River and Harrison Lake. Northern limits of the range are lower elevations (below 800 m) in the Coast Mountains on the north side of the Fraser River.The 142 known occurrence records represent about 44 distinct locations or sites. The extent of occurrence based on historical and recent occurrences is about 3,350 km².

Habitat

The Pacific water shrew is associated with skunk cabbage marshes, red alder riparian and stream habitats, and dense wet forests of western redcedar.  Most captures are in riparian habitats in close proximity to water. But individuals have been captured 25-350 m away from streams in forest. Although forested habitats are important, it has been found in non-forested grassy habitats bordering ditches and sloughs in British Columbia. There are no specific data on habitat trends and no quantitative data exist for the rate of habitat loss in the past 10 years. But wetlands and forested habitats have declined over the past 100 years. Protected areas are limited to 26 provincial and regional parks. Federal lands within the known range include 4 Department of National Defence properties and about 62 Indian Reserve lands. The amount of habitat on provincial Crown land is small, probably less than 20%.

Biology

The basic biology of this species has been little studied. It is primarily insectivorous eating insect larvae, slugs, snails, mayfly naiads, and earthworms. At least 25% of the prey is aquatic. The Pacific water shrew is semi-aquatic and capable of swimming and diving in water. It can swim continuously for up to 3.5 minutes and dive for up to 60 seconds. The breeding season extends from February to August with females producing two to three litters of five to seven young. The generation time is about one year.

Population sizes and trends

No estimates of population density exist for this species from any part of its range and the total number of individuals and the number of mature animals in Canada is unknown. The Pacific water shrew appears to be rare throughout its range. Nothing is known about yearly population fluctuations. Long-term population trends can only be inferred from anecdotal accounts and evidence from historical captures.

Limiting factors and threats

In Canada, the Pacific water shrew’s distribution coincides with a heavily urbanized area undergoing rapid development and habitat change. Rarity, coupled with its restriction to riparian and wetland habitats, make this species susceptible to habitat loss, fragmentation, and degradation from development. 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.

Special significance of the species

The Pacific water shrew is a member of the Pacific Coastal faunal element, a group of coastal mammals that range from northern California to British Columbia and southeast Alaska.

Existing protection

This species was designated Threatened by COSEWIC in 1994. The status was re-examined and confirmed in May 2000. British Columbia has designated this species as S1S2 (critically imperiled or imperiled). The Pacific water shrew is protected from killing or collecting under the provincial Wildlife Act; it is listed as an Identified Wildlife Species under the British Columbia Provincial Forest and Range Practices Act. Riparian habitat protection is also provided by the Riparian Areas Regulation of the provincial Fish Protection Act. The Department of Fisheries and Oceans (DFO) also has a set of guidelines for establishing Fisheries Sensitive Zones that protect fish and fish habitat. The provincial Riparian Areas Regulation and the DFO regulations apply only to streams, creeks, ditches, or wetlands with fish or that are connected to fresh-water ecosystems with fish. None of the Pacific water shrew’s range is in a national park. Its known distributional area includes 5 provincial parks and 26 regional parks. Protected areas on the south side of the Fraser River are small with no connectivity.

COSEWIC History

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list.  On June 5th 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species. 

Definitions (2006)

Wildlife Species
A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and it is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)
A wildlife species that no longer exists.

Extirpated (XT)
A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)
A wildlife species facing imminent extirpation or extinction.

Threatened (T)
A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)Footnotea
A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)Footnoteb
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)Footnotec
A category that applies when the available information is insufficient (a) to resolve a species' eligibility for assessment or (b) to permit an assessment of the species' risk of extinction.

 

Canadian Wildlife Service

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

 

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