Western harvest mouse (Reithrodontomys megalotis) COSEWIC assessment and update report: chapter 11

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Technical Summary

Reithrodontomys megalotis megalotis

Western harvest mouse megalotis subspecies – Souris des moissons de la sous-espèce megalotis

Extent and Area Information^a

^aArea information was estimated based on present (i.e., post-1970) locations (see Fig. 2). AO was estimated following IUCN guidelines and used a 2 x 2 km (4 km²) grid.

Population Information

List populations with number of mature individuals in each:

Threats (actual or imminent threats to populations or habitats)

• urban development,
• cultivation and agricultural activity,
• domestic livestock grazing,
• fire

Rescue Effect (immigration from an outside source)

Status of outside population(s)?

Current Status

Status and Reasons for Designation

Reasons for Designation

This subspecies has a limited range, and a small extent of occurrence and area of occupancy. However, the extent of occurrence and area of occupancy appear to be constant. Its principal native habitat in the Okanagan as well as old fields is declining. Furthermore, old apple orchards where the mouse has been caught are being converted to vineyards. Dispersal distance is limited and the likelihood of rescue effect is small. Extensive sampling has revealed the occurrence of the mouse at more localities. 63,000 hectares of suitable habitat is protected.

Applicability of Criteria

Criterion A: (Declining Total Population): There is no direct evidence of a declining population trend and there are no good data on population size.  It appears to be naturally rare and occurs at low densities.

Criterion B: (Small Distribution, and Decline or Fluctuation): EO < 3,000 km²; AO &lt; 110 km². Close to meeting threatened B2ab but the population is not fragmented and there are greater than 10 localities.

Criterion C: (Small Total Population Size and Decline): Total population size is unknown and there may be limited habitat available.

Criterion D: (Very Small Population or Restricted Distribution): There is likely limited availability and high demand for exploitation of the remaining habitat, and the remaining populations appear to be fragmented.

Criterion E: (Quantitative Analysis): Not applicable.

Reithrodontomys megalotis dychei

Western harvest mouse dychei subspecies – Souris des moissons de la sous-espèce dychei

Extent and Area Information^a

^aArea information was estimated based on present (i.e., post-1970) locations (see Fig. 2).  AO was estimated following IUCN guidelines and used a 2 x 2 km (4 km²) grid.

^bAll present records are from within a single large locality, the Suffield National Wildlife Area (459 km²).

Population Information

List populations with number of mature individuals in each:

Threats (actual or imminent threats to populations or habitats)

• urban development,
• cultivation and agricultural activity,
• domestic livestock grazing,
• fire

Rescue Effect (immigration from an outside source)

Status of outside population(s)?

Current Status

Status and Reasons for Designation

Reasons for Designation

This subspecies has a limited range and has been found at only one location in the past 40 years; this location is isolated from others. Dispersal distance is limited and the population fluctuates. This species is commonly found in owl pellets in the USA, but none have been reported in owl pellets (including burrowing owls) in Suffield or other areas in southeast Alberta, despite substantial sampling. Owl pellet analysis is an excellent means of sampling for these mice.

Applicability of Criteria

Criterion A: (Declining Total Population): There is no direct evidence of a declining population, but there are no good data on population size.  There appears to be only one isolated population. 

Criterion B: (Small Distribution, and Decline or Fluctuation): Endangered under B1ab(iii)+2ab(iii)c(iv): Small extent of occurrence and area of occupancy. Fewer than 5 locations and decreasing EO, AO and number of populations. Fluctuation in number of individuals.

Criterion C: (Small Total Population Size and Decline): Would fit Threatened under C2ii if there were fewer than 10,000 adults during population 'lows' and there was only the single population.

Criterion D: (Very Small Population or Restricted Distribution):  Appears to be only one isolated population.

Criterion E: (Quantitative Analysis): Not applicable.