Western harvest mouse (Reithrodontomys megalotis) COSEWIC assessment and update report: chapter 15

Executive Summary

Western harvest mouse

Reithrodontomys megalotis

Megalotis subspecies
(Reithrodontomys megalotis megalotis)

Dychei subspecies
(Reithrodontomys megalotis dychei)

Species information

The western harvest mouse (Reithrodontomys megalotis) has a body mass of approximately 11 g and averages 136 mm in total length, half of which is its tail. This brownish mouse has a faint dark dorsal stripe which runs the length of its body from head to tail, and has whitish fur on its belly. This species has prominent naked ears, a tail that is sparsely furred and white feet. It is similar in appearance to the larger and more common deer mouse and house mouse; however, juvenile deer mice are most often grey in colour and house mice have tails that are completely naked.

Distribution

The western harvest mouse occurs at its northernmost limit within the prairie grasslands of southern British Columbia (BC) and southeastern Alberta (AB). It is widespread throughout the central and western US, as well as Mexico.

Habitat

The western harvest mouse is associated with dry shrub-steppe habitats, apparently preferring areas with extensive cover, either in the form of tall grasses or shrubs such as antelope bitterbrush or sagebrush. This mouse occurs in many habitats, including dry gullies bordering grasslands and shrub-steppe rangelands, old fields, ponderosa pine forests, and both grazed and ungrazed sagebrush and antelope bitterbrush habitats.

Biology

This nocturnal mouse is omnivorous, but mainly eats seeds, new plant growth, and invertebrates such as caterpillars and moths. Western harvest mice live above ground and build small grass nests on the ground or up to 1 m above ground in shrubs. Females can breed at the age of four months and may have up to five litters per season with an average litter size of 3. Although they can live for 18 months, most do not live past six months. Western harvest mice appear to be able to enter torpor to cope with cold temperatures. Some researchers speculate that they hibernate, although this may not occur within southern BC as this species has been captured during all months of the year. In Canada, owls are the only formally confirmed predator of western harvest mice, but other likely predators include prairie rattlesnakes, hawks, jays, shrikes, raccoons, foxes, weasels, skunks, badgers and coyotes. The dispersal distance of western harvest mouse is generally restricted to less than 300 m. Dispersal has been documented along road right-of-ways; however, roads appear to be avoided, and may limit dispersal.

Population sizes and trends

In the US, the western harvest mouse is a dominant member of grassland small mammal communities, reaching densities of 60 animals/ha within suitable habitat. In Canada, this species is naturally rare and occurs at low densities, typically comprising less than 10% of the small mammal community. Population densities as high as 80 animals/ha have been recorded in BC. Western harvest mice populations appear to peak in late fall or early winter and decline to low levels in midsummer. There are no data available on population size or trends at either a provincial or national level.

Limiting factors and threats

Western harvest mice are susceptible to habitat change resulting from fire, but populations can recover quickly provided there is suitable unburnt habitat nearby. Habitat fragmentation and loss caused by grazing, cultivation and other agricultural activities, and urban development, are likely the most significant threats to populations in Canada.

Special significance of the species

The BC and Alberta populations of western harvest mice are the northernmost distributions of two distinct subspecies separated by the Rocky Mountains. Although these subspecies are common and widespread throughout much of the US, marginal populations may contribute unique genetic attributes to a species’ gene pool, and therefore enhance the genetic diversity and ability of the entire species to adapt to new conditions.

Existing protection

In 1994, COSEWIC assessed the BC subspecies (R. m. megalotis) as Special Concern and the Alberta subspecies (R. m. dychei) as Data Deficient in Alberta in 1994. Due to its limited range and apparent rarity, the BC government lists the western harvest mouse as Vulnerable (i.e., Blue Listed). The provincial rank for this species is S2S3, implying that this mouse is imperiled to uncommon. BC has over 63,000 ha of suitable habitat within protected areas (including parks, reserves, and land trusts). However, many of these areas may be too small and/or separated beyond the dispersal distance of the mouse and, consequently, the viability of populations within this network of protected areas is unknown. Several Indian Reserves may contain significant amounts of western harvest mouse habitat. A lack of information in Alberta has resulted in the provincial government classifying the species as Status Undetermined. The few localities with confirmed presence of western harvest mice results in a provincial rank of S1, implying that this mouse is critically imperiled. Protected habitats are found within the Suffield National Wildlife Area (45,900 ha) and Writing-On-Stone Provincial Park (1,718 ha). Prior to the mid-1990s when western harvest mice were trapped in the Suffield National Wildlife Area, this mouse had not been recorded in the province since 1966.

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 5, 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

Species
Any indigenous species, subspecies, variety, or geographically defined population of wild fauna and flora.

Extinct (X)
A species that no longer exists.

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

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

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

Special Concern (SC)*
A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events.

Not at Risk (NAR)*
A species that has been evaluated and found to be not at risk.

Data Deficient (DD)***
A species for which there is insufficient scientific information to support status designation.

* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
** Formerly described as “Not In Any Category”, or “No Designation Required.”
*** Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994.

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.

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

Page details

Date modified: