Western harvest mouse (Reithrodontomys megalotis) COSEWIC assessment and update report: chapter 5

Habitat

Habitat requirements

Little is known about the detailed habitat requirements of the western harvest mouse. Information about habitat preference has been inferred from the types of habitats where the mouse is most often captured. This species prefers habitats characterized by tall herbaceous cover (Kaufman and Fleharty 1974, Moulton et al. 1981, Davis et al. 2000). In the western US and Mexico, these mice inhabit grasslands, shrub-steppe, edge habitats bordering agricultural areas, coastal salt marshes, and riparian habitats (Webster and Jones 1982). Although western harvest mice have been found at elevations up to 4,000 m in Mexico, in Canada, the mouse is confined to lower elevations (Nagorsen 1994).

British Columbia

In BC, the western harvest mouse is restricted to valley bottoms, or south-facing slopes (up to 780 m) of the South Okanagan Basin, South Okanagan Highland, Okanagan Range, and North Okanagan Basin ecosections (British Columbia Ministry of Environment 2005a, Meidinger and Pojar 1991). Suitable habitats within these ecosections include the very dry and hot subzones of the Bunchgrass, Ponderosa Pine, and Interior Douglas-fir Biogeoclimatic Zones (Nagorsen 1995, Meidinger and Pojar 1991). These areas are shrub-steppe grasslands with abundant tall grasses such as bluebunch wheatgrass (Pseudoroegneria spicata) and shrubs such as sagebrush (Artemisia tridentata), and antelope bitterbrush (Purshia tridentada) (Nagorsen 1994, 2005).

Nagorsen (1995) reported that most western harvest mice were captured in dry gullies with dense shrub cover that border grassland and shrub-steppe rangeland (n=16 animals).

At Prairie Valley (near Summerland, BC), 58.9% of the 321 western harvest mice were captured in old fields, 20.6% in sagebrush habitats, 15.6% in conventional apple orchards, 2.2% in riparian areas, 1.6% in Ponderosa pine (Pinus ponderosa) forests, 0.9% in hedgerows, and 0.3% in dwarf apple orchards (Sullivan 2004, Sullivan and Sullivan 2006a; Table 1). Western harvest mice occurred at densities of up to 10/ha in old fields and an unmanaged apple orchard, and up to 5/ha in sagebrush habitats (Sullivan and Sullivan 2005 and 2006b). Mean density during fall-winter months ranged from 54.3/ha in old fields, which had abundant alfalfa (Medicago sativa) and grass cover, to 4.7/ha in old fields with no alfalfa and limited grass cover. Consequently, habitat quality is inferred to increase with a well-developed and abundant herb layer. In a study at the Agriculture Canada Summerland Research Station, western harvest mice were captured on two irrigated old fields; abandoned for ≥25 years (Sullivan and Sullivan 2004; Table 1).

W. Klenner (unpubl. data; Table 1) found that western harvest mice occurred at relatively high densities (up to 22/ha) within ungrazed shrub-steppe habitats dominated by antelope bitterbrush (livestock excluded for at least 25 years). The amount of this habitat in the Okanagan is limited. Similar habitats that had been burned in 1991 (3 years prior to sampling) supported a western harvest mouse population, although at reduced densities (up to 13/ha). The mouse was also frequently captured in heavily grazed habitats (little grass cover) dominated by sagebrush or antelope bitterbrush (W. Klenner, unpubl. data). These results suggest that, although preferring well-developed grass cover found in habitats without heavy grazing, the western harvest mouse is able to persist, or at least disperse, through grazed habitats, as long as there is enough cover provided by shrubs such as antelope bitterbrush or sagebrush.

Alberta

The only information on habitat associations of western harvest mice within Alberta comes from small mammal surveys conducted within the Suffield National Wildlife Area (Reynolds et al. 1999; Table 1). Based on these records, this mouse is strongly associated with low relief, flat, or gently undulating grasslands with sandy soils and an associated dense cover of vegetation, particularly shrubs. While most captures were within grasslands, six mice were captured within a cottonwood forest with dense shrub cover. The mouse appears to require habitats with abundant overhead cover. No captures were made in wet habitats.

Habitat trends

British Columbia

The amount of shrub-steppe habitat available for the western harvest mouse in the Okanagan Valley has been declining over the past 70 years because of the combined effects of cattle grazing, agriculture, and urbanization. Ina recent analysis of antelope bitterbrush shrub-steppe habitat, Wood (2003), calculated the rate of loss (primarily due to vineyard development) to be 90 ha/year during 1995-2001. The rate of habitat loss increased to 220 ha/year during 2001-2003. Based on this rate, antelope bitterbrush habitat will be completely eliminated from the South Okanagan - Similkameen within the next 18 years (Wood 2003). Grazing presumably affects habitat quality by altering natural grassland plant species composition and cover (Tisdale 1947). Within the southern Okanagan, intensive grazing occurred in the late 1880s (Cannings et al. 1987) and continues today (Wikeem and Wikeem 2004). Vegetation management practices associated with agriculture (e.g., mowing and herbicide application) are assumed to decrease the habitat quality for western harvest mice, which are associated with abundant cover. However, this mouse appears somewhat flexible in habitat use and has been captured within conventional and organic orchards in the Okanagan, although at lower densities compared to habitats with abundant cover such as old fields (Sullivan 2004, Sullivan and Sullivan 2005, 2006a and b). Habitat loss related to the rapidly expanding cities of Penticton, Kelowna, and Vernon likely poses the most significant threat to western harvest mouse habitat. Over the past 30 years, the human population of the Okanagan-Similkameen river basin has more than doubled, which represents the fastest growth rate among the 23 major river basins in Canada (Statistics Canada 2003). From 1971 to 2001, the human population in the Okanagan-Similkameen basin increased by 137% to 285,145 (Statistics Canada 2003). It is anticipated that the Regional District of Okanagan-Similkameen will experience considerable growth in the next 20 years. The projected trend is for a 30% increase in population during 2004-2021 (South Okanagan Regional Growth Strategy 2006).

Alberta

Given the current state of knowledge about western harvest mice in Alberta, evaluation of habitat trends is only speculative. It is likely that Alberta’s history of cultivation and grazing (Coupland 1987), as well as urbanization around cities, will jeopardize habitat quality for small mammals in a similar manner as for BC.

Habitat protection/ownership

British Columbia

Suitable habitat for the western harvest mouse occurs in several protected areas (including parks, ecological reserves, and land trusts) located throughout the Okanagan and Similkameen River valleys. While these protected areas may be free from various forms of development and disturbance, except for the ecological reserves, many are still grazed by domestic livestock. These protected areas total 94,903 ha, with 67% of this area (63,567 ha) suitable for western harvest mice (Table 2). The six protected areas that contribute the majority of western harvest mouse habitat are Kalamalka Lake Protected Area (1,806 ha), Okanagan Mountain Park (6,247 ha), White Lake Grasslands Protected Area (3,741 ha), Vaseux Protected Area (1,983 ha), Snowy Protected Area (1,653 ha), and the South Okanagan Grasslands Protected Area (8,052 ha). In addition, approximately 38,700 ha are managed by the Nature Trust of British Columbia and include both privately owned and long-term lease agreements (2,700 ha) as well as Crown grazing tenures (36,000 ha). Many of these protected areas are likely separated by distances greater than the dispersal ability of the mouse (Nagorsen 1994).

Federal lands that contain western harvest mouse habitat include the 325 ha Agriculture Canada Summerland Research Station. Also, 17 Indian Reserves, totalling over 45,000 ha, may include habitat suitable for this mouse (Table 3). Nagorsen (1994) suggested that many of these reserves include low elevation grasslands that are relatively undisturbed and, therefore, have high potential to contain suitable habitat. The state of Indian Reserves, especially near Osoyoos is likely quite different now and the potential for finding mice has likely declined (D. Fraser, pers. comm.)

Table 2. Protected areas (PA) containing suitable habitat for western harvest mice within British Columbia.
Name of protected area Total area (ha) Area of suitable1 habitat (ha) % suitable1 habitat
Kalamalka Lake Park2
978
978
100
Kalamalka Lake PA2
3,231
1,806
56
Okanagan Lake Park2
98
65
66
Okanagan Mountain Park2
10,462
6,247
60
Inkaneep Park2
21
21
100
White Lake Grasslands PA2
3,741
3,741
100
Vaseux Protected Area2
2,015
1,983
98
Johnstone Creek Park2
38
14
37
Keremeos Columns Park2
57
4
7
Snowy PA2
25,889
1,653
6
South Okanagan Grasslands PA3
9,370
8,052
86
Trout Creek Ecological Reserve2
68
68
100
Hayne's Lease Ecological Reserve2
101
101
100
Campbell-Brown Ecological Reserve4
104
104
100
Mahoney Lake Ecological Reserve2
30
30
100
Nature Trust - private or long-term lease5
2,700
2,700
100
Nature Trust - Crown grazing tenure5
36,000
36,000
100
Total
94,903
63,567
67

1 Suitable habitat as defined by Nagorsen (1995) (i.e., biogeoclimatic units = BGxh1, PPxh1, IDFxh1, and IDFxh1a)
2 Adapted from Wikeem and Wikeem 2004
3 Adapted from British Columbia Ministry of Water, Land and Air Protection (2003a, b, c, and d)
4 Adapted from British Columbia Ministry of Environment (2005b)
5 C. McNaughton pers. comm.

Table 3. Indian Reserves within, or adjacent to, known distribution of western harvest mice within British Columbia 1.
Reserve name Total area (ha)
Blind Creek 6
161.0
Blind Creek 6a
0.1
Chopaka 7 & 8
1,573.8
Duck Lake 7
179.1
Keremeos Forks 12 & 12a
954.1
Lower Similkameen 2
1,293.7
Mission Creek 8
2.0
Narcisse’s Farm 4
750.3
Osoyoos 1
12,987.6
Osoyoos 3
64.7
Penticton 1
18,539.8
Penticton 2
13.1
Priest’s Valley 6
33.6
Range 13
6,768.1
Salmon River 1
1,559.3
Tsinstikeptum 9
339.0
Tsinstikeptum 10
641.8
Total
45,861.1

1Source = ATK coordinator, COSEWIC Secretariat.

Alberta

The Suffield National Wildlife Area likely supports a significant population of western harvest mice at the northern range limit for this species. Over 45,900 ha of western harvest mouse habitat is federally protected along the South Saskatchewan River within the Canadian Forces Base Suffield National Wildlife Area since its official designation in June 2003. In 1992, a small mammal survey (5 transects, 888 trap-nights) in Writing-On-Stone Provincial Park (1,718 ha) failed to capture any western harvest mice (D. Gummer, pers. comm.). However, this protected area should still be considered as providing potential habitat. The distance that separates the Suffield National Wildlife Area and Writing-On-Stone Provincial Park (over 150 km) makes dispersal between these two areas unlikely.

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