Western harvest mouse (Reithrodontomys megalotis) COSEWIC assessment and update report: chapter 4

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Distribution

Global range

The western harvest mouse is endemic to North America and found throughout much of west-central Mexico and the west and central US. The species occurs at its northern limit in southwestern Canada (Hall 1981; Fig. 1).

Figure 1. Distribution of western harvest mouse within North America (adapted from Hall 1981 and Reynolds et al. 1999).

Canadian range

The Canadian distribution of the western harvest mouse is restricted to the intermontane grasslands of south-central BC (R. m. megalotis) and the dry mixed grass prairie of southeastern Alberta (R. m. dychei; Fig. 2). The Canadian range accounts for less than 1% of the total range. The BC and Alberta populations account for only 0.3 and 1.7% of the total range of R. m. megalotis and R. m. dychei, respectively. The Extent of Occurrences (EO) for R. m. megalotis and R. m. dychei, respectively are estimated at 2,850 and 459km² based on the locations at which they have been trapped recently and historically (COSEWIC Secretariat based on data from P.M.F. Lindgren). Likewise, the Area of Occupancies (AO) is approximately 108 and 156 km².

British Columbia

Western harvest mice occur throughout the Okanagan Valley, as far north as Vernon, and in the Similkameen River valley, as far north as Keremeos (Nagorsen 1995). The mouse has not been captured in the central Okanagan near Kelowna, suggesting that the Vernon population is isolated from populations to the south (Fig. 2). The mouse is also absent from adjacent valleys, including the Thompson and Kettle River valleys (Nagorsen 1995). However, low trapping success (Sullivan and Sullivan 2004) makes the absence of records an unreliable means of assessing distribution. For example, absence from the Kelowna area is based on four transects and 600 trap-nights, from the Thompson River Valley based on two transects and 300 trap-nights, and from the Kettle River Valley based on four transects and 600 trap-nights (Nagorsen 1995; Table 1).

Alberta

Little is known about the distribution of western harvest mice within Alberta. The mouse has been documented at only four localities. Prior to 1994, its distribution was based on three museum specimens collected near Milk River, Medicine Hat, and in the Pinhorn Grazing Reserve near Manyberries (Moore 1952, Smith 1993). During 1994 and 1995, 95 western harvest mice were captured along the South Saskatchewan River within the Canadian Forces Base Suffield National Wildlife Area (Reynolds et al. 1999; Table 1), extending the distribution range north by 80 km (Engley and Norton 2001).

With the exception of the Suffield records (Reynolds et al. 1999), the species has not been documented in Alberta since 1966. The lack of intervening records stands in spite of numerous surveys (D. Gummer, pers. comm.; Table 1) and extensive analysis of owl pellets (Schowalter 2004, R. Poulin and R. Schmelzeisen, pers. comm.) collected from within the supposed range of this species in Alberta. The analysis of owl pellets represents an excellent means of sampling for these mice. These data suggest that western harvest mice are extremely rare in Alberta and that the harvest mice sampled within the Suffield National Wildlife Area may be part of a disjunct northern population, isolated from the Montana population by at least 150 km (Fig. 2).

Figure 2. Distribution of the western harvest mouse in Canadabased on trapping records. Historical localities in Alberta were documented in 1951 and 1966.