(Map may reflect historical as well as recent sightings)
The red tree vole is a small rodent inhabiting older, coniferous forests in western Oregon and northwestern California. A certain population of the red tree vole – the North Oregon Coast “distinct population segment” – became a candidate for federal Endangered Species Act protection in October 2011. The State of Oregon lists the red tree vole as a sensitive-vulnerable species in the Coast Range Ecoregion.
The candidate North Oregon Coast population of the red tree vole (green area in above map) inhabits about a quarter of the red tree vole's full range. The range of the North Oregon Coast distinct population segment encompasses the Oregon Coast Range from the Siuslaw River north to the Columbia River. It encompasses Clatsop, Columbia, Tillamook and Lincoln Counties; western portions of Washington, Yamhill, Polk, Benton and Lane Counties; and very small areas of western Multnomah (less than 17,000 acres) and northern Douglas Counties (less than 7,000 acres west of Cottage Grove).
Overall, the full range of the red tree vole includes areas in these Oregon Counties: Benton, Clackamas, Clatsop, Columbia, Coos, Curry, Douglas, Hood River, Jackson, Josephine, Lane, Linn, Marion, Multnomah, Polk, Tillamook, Washington, Yamhill counties.
(The Endangered Species Act provides that a species, subspecies, or “distinct population segment” can qualify for protection. Distinct population segments are determined by such factors as a population’s genetic or behavioral discreteness and significance to the rest of the species.)
The North Oregon Coast population of the red tree vole is on the Federal List of Candidate Species, meaning the U.S. Fish and Wildlife Service may propose to list this population under the Endangered Species Act at a later date. In June 2007, the Fish and Wildlife Service received a petition to list the dusky tree vole (Arborimus longicaudus silvicola), a supposed subspecies of the red tree vole that occurs in the northern Oregon Coast Range. As part of a status review, we did not find sufficient evidence to conclude that the dusky tree vole was a valid subspecies (more information on this determination is below, under Taxonomy). However, we did find that the North Oregon Coast population of the red tree vole qualifies as a distinct population segment. On October 13, 2011, we published a 12-month finding on the petition, concluding that listing the North Oregon Coast population is warranted, but doing so at this time is precluded by higher priority listing actions.
The taxonomy of the red tree vole is complex and not yet resolved. Early researchers concluded that the dusky tree vole, found in the northern Oregon Coast Range in Lincoln, Tillamook, and Clatsop Counties, was a subspecies of the red tree vole. More recent research indicates the complexity in reaching an unequivocal conclusion on the subject. One study could not find genetic differences between red tree voles and those classed as dusky tree voles. Conversely, another study found that red tree voles break out into three genetically distinct groups, one of which overlaps the range of the putative dusky tree vole. The most recent research on this topic, comparing body measurements between red tree voles and those classed as dusky tree voles, found significant differences between the groups; the differences were subtle, however, and the variables measured could not be used to reliably discern red tree voles from dusky tree voles. Although the Fish and Wildlife Service did not find that the dusky tree vole was a valid subspecies as part of its status review, this does not resolve the taxonomic status of the species.
Description and Life History
Appearance. Red tree voles are small, furry rodents less than 8 inches (206 millimeters) long, weighing up to 2 ounces (50 grams). A long, fur-covered tail accounts for about 50 percent of the vole's total length. Their thick coats range in color from reddish-brown to orange-red. Melanistic (all black) and cream-colored forms of the red tree vole also occur.
Diet. Red tree voles live in tree tops and rarely come to the forest floor. They are one of the few animals that can persist on a diet of conifer needles, their principle food. As a defense mechanism, conifer trees have resin ducts in their needles that contain chemical compounds (terpenoids) that make them unpalatable to animals. Tree voles, however, are able to strip away these resin ducts and eat the remaining portion of the conifer needle. Piles of resin ducts on the ground may be seen under trees where tree voles have foraged.
Nests. Red tree voles are mainly active at night. When they are outside their nests, they spend much of their time gathering live conifer branchlets, upon which they feed. They may feed away from the nest but more often they bring the branchlets back to the nest to cache or to feed themselves or their young. Nests are constructed of branchlets, discarded resin ducts, and other materials, ultimately shaped into a sphere with interior tunnels. As nests continue to be occupied, more material is added and they can become quite large, sometimes completely encircling the tree trunk. Nests of females tend to be larger than those of males.
Reproduction. Adult red tree voles lead solitary lives, with the males and females only coming together to breed. Red tree vole litters are small relative to similar rodent species, averaging just under 3 young per litter. Adult females are capable of becoming pregnant immediately after giving birth, resulting in some females having two litters of differently aged young in the nest. Juvenile tree voles develop more slowly than similar sized rodents, and don’t disperse from the nesting area until they are 47 to 60 days old.
Home range. Red tree voles have very limited home ranges, often less than half an acre, and their typical dispersal distance is often less than the length of a football field.
Distribution and Range
Red tree voles are native to the humid, coniferous forests west of the crest of the Cascade Mountains in Oregon and northwestern California. They are generally restricted to lower elevations. The red tree vole occurs in western Oregon from the Cascade crest to the Pacific coast, with a geographic range covering approximately 16.3 million acres. However, red tree voles now appear to be uncommon or absent in much of the North Coast Range and North Cascades of Oregon. A comparison of historical data with current surveys strongly suggests that the North Oregon Coast population has declined significantly. Recent survey results show that red tree vole populations in the North Oregon Coast Range are isolated with limited distribution.
The red tree vole is found primarily in late-successional (older, structurally complex) forests in western Oregon and northwestern California. Because of their exclusive diet of conifer needles, red tree voles are restricted to conifer forests. Though they use a variety of tree species, they principally feed on Douglas fir needles and nest in Douglas fir trees. However, red tree voles in a portion of the North Coast Range are associated with Sitka spruce and western hemlock forests. Red tree voles are sometimes found in younger forest stands but appear unlikely to persist in these areas. Research indicates they exhibit a strong preference for older trees and complex forested habitats. Nests tend to be found in the larger-diameter trees within a stand. Expanses of land without suitable forest cover can be a barrier to tree vole movement and population connectivity.
Reasons for Decline
As arboreal (tree dwelling) mammals, red tree voles are vulnerable to habitat loss and fragmentation from timber harvest, wildfire, development, recreation, roads, and other human-caused disturbances. Loss of habitat in the North Oregon Coast Range has caused habitat fragmentation, isolation of subpopulations, and small population sizes, which can cause negative genetic impacts to a population. These effects are further exacerbated by the vole’s naturally narrow habitat requirements, low mobility, low reproductive potential, and low dispersal ability to move among limited habitat.
On Forest Service and Bureau of Land Management lands in western Oregon, areas subject to timber harvest (except thinning activities in stands younger than 80 years old) must be surveyed for red tree voles and high-priority sites must be protected. Management goals for the red tree vole are generally compatible with those for the threatened northern spotted owl and other late-successional forest species. It is likely that the benefits of these management measures will overlap.
The Fish and Wildlife Service is working with federal and state partners and citizens on forest conservation and management activities that could help deter the need to list the North Oregon Coast vole population in the future. This is what we commonly do for candidate species. We don’t wait for a listing determination to be made for a candidate species before we start working to protect it, and proactive conservation measures can help deter the need to list a species.
For example, we work with federal, state, and private landowners and organizations on Candidate Conservation Agreements that promote species conservation while providing regulatory assurances if the species is listed in the future. Through our Partners for Fish and Wildlife program, we work with private landowners on voluntary conservation efforts on their property.
U.S. Fish and Wildlife Service. 2008. Endangered and Threatened Wildlife and Plants; 90-Day Finding on a Petition To List the Dusky Tree Vole (Arborimus longicaudus silvicola) as Threatened or Endangered. 73 FR 63919-63926
U.S. Fish and Wildlife Service. 2011. Endangered and Threatened Wildlife and Plants; 12-Month Finding on a Petition to List a Distinct Population Segment of the Red Tree Vole as Endangered or Threatened. 76 FR 63720-63762