|
|
|
With no evidence for population declines, the Western Harvest Mouse does not warrant a threatened or endangered status. However, because it occurs in low population numbers, has a restricted range and low dispersal abilities, and there is evidence for habitat fragmentation, this species should remain on the provincial Blue List as a sensitive species. This would be consistent with
the COSEWIC designation of Vulnerable.
Image Credits: Top sketch by Michael Hames for the Royal British Columbia Museum. Bottom sketch in Mammals of British Columbia, Cowan and Guiguet, 1978. Historical facts The Western Harvest Mouse was first discovered in Okanagan valley of British Columbia in 1942. The Western Harvest Mouse is the smallest mouse in British Columbia with an average total length of 136 mm and weight of 11 g. It even rivals the Olive Backed Pocket Mouse for the title of smallest rodent in Canada. Both are about the size of the larger shrews. It is often confused with the Deer Mouse and House Mouse, two species that coexist with the harvest mouse in southern British Columbia. It can be distinguished from these two species by: more buffy dorsal colour, a smaller size, and the outer surface of each of the upper incisors have distinct shallow grooves. This species is a slim, long-tailed mouse that has prominent, naked ears, a slender sparsely haired tail, grey above and whitish below. The harvest mouse has three pelages: juvenile, sub-adult, and adult. The juvenile pelage is rather woolly and dull grey. The adult pelage is the brightest. Moulting starts on the ventral surface and spreads over the flanks to meet on the back; then the new fur spreads fore and aft. A second point of origin is on the muzzle, the new coat spreading back to form a moult-line behind the ears. There is one annual moult each summer. It has four toes on the forefeet and five on the hind feet. A small murid rodent, the Western Harvest Mouse is a naturally rare species associated with the grassland rodent communities of western North America. Although it has been extensively studied in the United States, little is know about its biology in Canada, where it reaches the northern limits of its range.
Map
British Columbia
North America Across North America, the Western Harvest Mouse inhabits sagebrush steppe and agricultural areas in areas below elevations of 500 m. It forages in grasslands bordering riparian areas such as irrigation right-of-ways, coastal salt marshes, streams or lakes; and in deciduous ravines of willow, rose and trembling aspen. In British Columbia, they are mainly associated with edge habitats bordering agricultural areas and rangeland. Shrub or grass overstory with tall lush herbaceous cover and natural debris are important for cover and concealing nests. Typical shrub species in the Western Harvest Mouse habitats of British Columbia include: wild rose, mock orange, Saskatoon berry, chokecherry, squaw currant, Douglas maple, snowberry, and black hawthorn. Giant wildrye was present at many R. megalotis capture sites. Undisturbed grassy edges habitats are also exploited. It has been captured at Okanagan Landing from roadside habitats with high grass and weeds, such as thistle. In the Similkameen Valley, a population was found in disturbed edge habitat of smooth brome, thistle, giant wildrye, and goldenrod that bordered hay and alfalfa fields. All known populations in British Columbia are associated with intermontane grasslands of the Okanagan Basin Ecosection in the Southern Interior Ecoprovince. The elevational range extends from 300 m to 780 m, with most occurrences below 600 m. Populations found at higher elevations (>600 m) were associated with south-facing slopes. Most occurrences are in the very dry hot subzone of the Bunchgrass biogeoclimatic zone. This zone, which is confined to the hot valley bottoms, is dominated by bunchgrass and big sage. Greasewood replaces big sage in the sandy soils of the south Okanagan. A few occurrences in the south Okanagan were in the Ponderosa Pine biogeoclimatic zone, and, in the north Okanagan. The Western Harvest Mouse is associated with the Interior Douglas-fir biogeoclimatic zone. Interestingly, this species is absent from the Bunchgrass zone in the Thompson Basic Ecosection. The Western Harvest Mouse may have more than one nest within its home range to use as rest sites. The nest site is about the size of and shape of a baseball, consisting of grass lined with fine plant materials and can be found on the ground in clumps of grass, shrubs or logs or hanging from vegetation. There is a tiny entrance on the under side, which leads into a golfball-sized chamber, lined with the finest of plant material such as down or dandelion fluff. This species does not construct burrows, although it will use other small mammal burrows for shelter. Grassland habitats in southern British Columbia have been affected by cattle grazing, agriculture, and urbanization. The most intensive grazing in the Okanagan Valley occurred in the late 1880s. Over-grazing by cattle has altered the plant species composition and cover in British Columbia grasslands and, presumably, this has affected habitat quality and small mammal population densities. Heavy cattle grazing reduced cover in ravines and on the grasslands, causing threats to the habitat of the Western Harvest Mouse. Grazing also reduced the availability of food since the Western Harvest Mouse's diet consists primarily of seeds from grasses. In British Columbia, ongoing habitat loss through urbanization has had the greatest impact on this species. Historically, grasslands were distributed continuously throughout low elevations of the Okanagan and Similkameen valleys. The conversion of grasslands to irrigated orchards, vineyards, and cultivated fields, and recent urban development has eliminated much of the original grassland-steppe in these valleys and contributed to habitat fragmentation. The rapid growth of urban centres, such as Penticton, Kelowna, and Vernon, in the past decade has resulted in significant habitat loss and contributed to fragmentation of the range. The amount of protected (i.e., parks and ecological reserves) habitat in this region is small. The size of these protected areas may be insufficient to maintain minimum viable populations of the Western Harvest Mouse if these areas become isolated and surrounded by unsuitable habitat. A fire in July 1993 that burned the entire Hayne's Lease Ecological Reserve at Osoyoos Lake demonstrates the vulnerability of small reserves. It is also noteworthy that these protected areas are widely separated, probably beyond the dispersal distance of the Western Harvest Mouse. Large "non-protected" areas with significant habitat are the Inkaneep Indian Reserve on Osoyoos Lake, the Penticton and Okanagan reserves near Okanagan Lake, and the Upper and Lower Similkameen reserves. These reserves support large tracts of low elevation grassland that are relatively undisturbed. Because of its rarity in Canada, the Western Harvest Mouse is of little economic concern in agricultural areas. Rodentcides are routinely used by orchardists in British Columbia to control vole and pocket gopher populations, but the harvest mouse is uncommon in cultivated habitats and mortality from poisoning is probably low. Potential limiting factors other than those described above are: competition with other small mammals and habitat disturbance from hay mowing, cultivation and fire.
Breeding The young harvest mice weigh approximately 1.0 to 1.5 g at birth. They are pink, naked, and blind. Their lower incisors appear at four days; the pelage is visible at five days; the eyes open between the tenth and the twelfth day, and they begin to walk at about the same time. The young are weaned at 19 days and continue to develop rapidly
Behaviour The information on the sociability of this species is rather conflicting. It has been described as 'ferocious,' 'cannibalistic,' 'not gentle,' and 'nervous,' and it has been reported that it dislikes being handled. In colonies, harvest mice appear rather sedentary and spend much of their time clustered together. On the other hand, they are remarkably compatible in mixed colonies of house mice and deer mice. They often cluster together with these other species and even form integrated social hierarchies in the mixed group. Captive Western Harvest Mice can be induced to enter shallow torpor by exposure to temperatures below 10C. The ability to enter shallow torpor presumably is an adaptation for conserving energy during periods of stress from food deprivation, water shortage, or cool ambient temperatures. Torpor may be critical for the survival of Canadian populations because they are at the extreme northern limits of their range, where they may be exposed to cool temperatures. It is unknown if this species is capable of hibernation. It has been captured throughout the year on trapping grids at Summerland in the Okanagan Valley.
Diet or Growing requirements
Predators Related On-line Sites to Visit
Publications
Living Landscape Directory of Researchers and their projects this section sponsored by: |
|
Located at: 
|
All rights reserved
|
Industry Canada
