Western harvest mouse (Reithrodontomys megalotis) COSEWIC assessment and update report: chapter 8

Limiting Factors and Threats

Harvest mice are likely susceptible to the direct effects of fire (i.e., mortality; Kaufman et al. 1988) as well as indirect effects that fire has on habitat (i.e., removal of cover and food). Kaufman et al. (1988) and McMillan et al. (1995) noted population declines following fire in ungrazed tall prairie in Kansas. In BC, a 1993 fire which burned the entire Hayne’s Lease Ecological Reserve near Osoyoos Lake and a 2003 fire that burned most of Okanagan Mountain Park likely reduced harvest mouse populations in these areas. However, densities of up to 13 western harvest mice/ha were recorded 3 years following a fire in the southern Okanagan (W. Klenner, pers. comm.), suggesting that the impacts of fire may be short-lived. In addition, Masters et al. (1998) found that prescribed burning of dense pine stands in Oklahoma resulted in an increase in herbaceous-forage production and an increase in densities of fulvous harvest mouse (Reithrodontomys fulvescens), a species with similar habitat requirements.

The use of rodenticides to control vole (Microtus spp.) and northern pocket gopher (Thomomys talpoides) populations in old-field and orchard habitats in BC may have significant impacts on local populations of western harvest mice. However, orchards are not the preferred habitat for the western harvest mouse; thus, mortality from poisoning is assumed to be low (Sullivan and Sullivan 2005).

Cattle and horse grazing, agriculture, and urbanization are likely the greatest threats to the habitat of western harvest mice. The only recent records of harvest mice within Alberta all come from the Suffield National Wildlife Area (Reynolds et al. 1999), an area with relatively little livestock or other agricultural activities, suggesting that in Alberta, the harvest mouse may be particularly sensitive to agricultural disturbance. In BC, conversion of grasslands to orchards, cultivated fields, and more recently, urban development and vineyards has eliminated large areas of shrub-steppe habitats important to this mouse (British Columbia Ministry of Environment, Lands and Parks 1998, Wood 2003). The use of linear edge habitats may be particularly important for the dispersal of this species among suitable habitat fragments. Because western harvest mice have been caught in roadside habitats (Whitaker and Mumford 1972, Ford 1977) and along cultivated fields (Nagorsen 1995), mowing these habitats also poses a significant threat. The creation of new roads may result in a significant barrier to dispersal and movement by harvest mice. Kozel and Fleharty (1979) found that no western harvest mice returned after having been transported to the other side of a road on the edge of their home range.

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