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Arborimus pomo

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA CHORDATA MAMMALIA RODENTIA CRICETIDAE

Scientific Name: Arborimus pomo
Species Authority: Johnson & George, 1991
Common Name(s):
English California Red Tree Mouse, Sonoma Tree Vole
Taxonomic Notes: This species is represented by Californian populations previously recognized as Arborimus (or Phenacomys) longicaudus. Recognition as a species distinct from A. longicaudus is based on chromosome differences, smaller overall size, and certain skull and muscle differences (Johnson and George, 1991). Musser and Carleton (in Wilson and Reeder 1993, 2005) and Baker et al. (2003) accepted A. pomo as a species distinct from A. longicaudus.

There is no consensus on the proper generic allocation for this species. It was placed in the genus Arborimus by Johnson and George (1991), Musser and Carleton (in Wilson and Reeder 1993, 2005), Hayes (1996), Adam and Hayes (1998), and Baker et al. (2003); included in the genus Phenacomys by Carleton and Musser (1984), Repenning and Grady (1988), and Verts and Carraway (1998). Bellinger et al. (2005) noted that recognition of Arborimus as a distinct genus is subject to interpretation of data.

Assessment Information [top]

Red List Category & Criteria: Near Threatened ver 3.1
Year Published: 2008
Date Assessed: 2008-06-30
Assessor(s): Blois, J. & NatureServe (Clausen, M.K., Hammerson, G. & Cannings, S.)
Reviewer(s): Amori, G. (Small Nonvolant Mammal Red List Authority) & Chanson, J. (Global Mammal Assessment Team)
Justification:
Listed as Near Threatened because its extent of occurrence is not much greater than 20,000 km², its range is severely fragmented and there is a continuing decline in the extent and quality of its forest habitat making it close to qualifying for Vulnerable under criterion B1.
History:
1996 Data Deficient

Geographic Range [top]

Range Description: This vole inhabits northwestern California, from Freestone, Sonoma County, north through Mendocino, Humboldt, and western Trinity counties to the South Fork of the Smith River, Del Norte County (Johnson and George 1991). There is some disagreement over the specific identity (pomo versus longicaudus) of tree voles at the northern extent of the range in northern California (see Hayes 1996).
Countries:
Native:
United States (California)
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: The total adult population size is unknown but probably exceeds 10,000. The species is relatively common in some areas, but the number of nests found can vary greatly by location and appears to be highest closer to the coast (up to several nests per acre); farther inland toward drier slopes the density drops off dramatically (Gordon Gould pers. comm. 1998). According to Brylski (1990), this species is reported to be rare to uncommon throughout the range, but the difficulty of locating nests and capturing individuals makes abundance hard to assess.

According to Gordon Gould (California Department of Game and Fish), there are currently 900 observations of nests or animals. Approximately 875 of these observations are from the mid-to-late 1990s and the remainder are from the early 1990s. Observations are mapped by the California Department of Game and Fish into occupied legal sections. These data probably will result in the mapping of about 325 occupied sections in California. Gould also stated that most surveys were completed at the request of the timber industry. This species does occupy areas that are not of interest to the timber industry. As a result, there are gaps in distributional information, and many more sections may be occupied than currently indicated (Gordon Gould pers. comm. 1998).

Information on current population trend is not unavailable, but area of occupancy and habitat quality probably are declining (likely at less than 30% over 10 years or three generations). The area of occupancy and abundance probably have declined compared to the historical situation. Evidence of persistence was poor in a Del Norte County population revisited in 1985 (Johnson and George 1991).
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology: The species' habitat consists of mixed evergreen forests; optimum habitat appears to be wet and mesic old-growth Douglas-fir forest, but this species also occurs in younger forests (e.g., Douglas-fir 47 years old). This vole is primarily arboreal but exhibits some terrestrial activity. It nests in trees, 2-50 m above ground; it may use old nests of birds, squirrels, or woodrats. Nests usually are in Douglas-fir trees but sometimes may be in other conifer or in Pacific madrone (Meiselman and Doyle 1996, Vrieze 1980; all as cited in Adam and Hayes 1998). Meiselman and Doyle (1996) found that nests were most abundant in old-growth forests; the species was associated with large-diameter Douglas-fir, high percentage canopy cover, high stump density, low snag density, shorter snags and logs, and lower elevation; all nests were in Douglas-fir, mostly adjacent to the trunk on the south side.

It breeds throughout the year. Ovulation is induced by copulation (Adam and Hayes 1998). Females may breed within 24 hours of giving birth. It sometimes exhibits delayed implantation. Gestation period is 27 to 48 days, with an average of 31 days (Hamilton 1962). Litter size usually is two, with a range of one to five (Adam and Hayes 1998). Newborns are altricial, weaned at 25 to 46 days (Hamilton 1962).

This species is thought to have very limited dispersal capability (Thomas et al. 1993). Predators include Spotted Owls (A. longicaudus made up almost 50% of prey items of Spotted Owls in Oregon), and probably other owls, Raccoons, and Fishers (Adam and Hayes 1998). This vole feeds primarily on Douglas-fir needles. It also eats needles of Grand or Lowland White Fir, Sitka Spruce, and Western Hemlock. It may eat inner bark of twigs as well (Benson and Borell 1931). Usually feeds inside or on top of its nest.
Systems: Terrestrial

Threats [top]

Major Threat(s): Threats include forest fragmentation and habitat loss (Maser et al. 1981; Thomas et al. 1993; Gordon Gould pers. comm. 1998). Although the species is locally common in the foothills of mountains on the east edge of the coastal plain in Humboldt County, loss and fragmentation of habitat has been extensive everywhere within the range (Williams 1986). Timber harvest and clearing of trees for agriculture and home sites have significantly reduced available habitat and fragmented populations (Maser et al. 1981). Construction of roads and power lines has also contributed to the loss of habitat and fragmentation and isolation of populations. Furthermore, these trends are likely to continue at an accelerated pace in the future (Williams 1986).

Previously it was believed that this vole requires old-growth mesic forest and, according to Gould (1987), projections were that all old-growth forests capable of sustaining a commercial harvest in California would be cut within 25 years. Obviously such a situation would be a grave threat to the tree vole. However, recent surveys have found many nests in secondary forest and on drier inland slopes. The vast majority of recent observations come from secondary growth forest, some of which is 70-80 years old and fairly well developed (Gordon Gould pers. comm., 1998). A 1984-1985 study conducted nest surveys in nine 20-hectare sites of young, mature, and old-growth Douglas-fir forests in northern California. A total of 79 nests were found with 39 nests occurring in old-growth forest, 22 nests in mature forest, and 18 nests in young forest (Meiselman 1987). Thus this species appears to be more adaptable and probably better able to disperse through various habitats than was previously believed.

This vole may have limited dispersal capabilities and thus may be vulnerable to habitat loss or fragmentation (Huff et al. 1992, cited by Adam and Hayes (1998).

Conservation Actions [top]

Conservation Actions: This vole occurs on a few protected sites. Less than 5% of known sites are protected (Gordon Gould pers. comm. 1998). The Survey and Manage program required the U.S. Forest Service to survey for certain logging-sensitive species (including the tree vole) throughout areas subject to timber sale and to provide adequate no-logging buffers if such species are found. Some vole protection was eliminated in March 2004 when the Survey and Manage provisions of the Northwest Forest Plan were withdrawn. Subsequent litigation may affect the ultimate outcome of this situation.

It is necessary to determine the total extent of range and abundance of this species, and to monitor populations to determine trends and further investigate the effect of fragmentation. This species may benefit from existing/proposed conservation measures for the spotted owl (Thomas et al. 1993).

Citation: Blois, J. & NatureServe (Clausen, M.K., Hammerson, G. & Cannings, S.) 2008. Arborimus pomo. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 21 October 2014.
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