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, 63720-63762 [2011-25818]

Agencies

• Fish and Wildlife Service
• DEPARTMENT OF THE INTERIOR

[Federal Register Volume 76, Number 198 (Thursday, October 13, 2011)]
[Proposed Rules]
[Pages 63720-63762]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2011-25818]



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Vol. 76

Thursday,

No. 198

October 13, 2011

Part II





Department of the Interior





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Fish and Wildlife Service





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50 CFR Part 17





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; Proposed Rule

Federal Register / Vol. 76 , No. 198 / Thursday, October 13, 2011 / 
Proposed Rules

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DEPARTMENT OF THE INTERIOR

Fish and Wildlife Service

50 CFR Part 17

[Docket No. FWS-R1-ES-2008-0086; 92210-5008-3922-10-B2]


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

AGENCY: Fish and Wildlife Service, Interior.

ACTION: Notice of 12-month petition finding.

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SUMMARY: We, the U.S. Fish and Wildlife Service (Service), announce a 
12-month finding on a petition to list a distinct population segment of 
the red tree vole (Arborimus longicaudus) as endangered or threatened 
and to designate critical habitat under the Endangered Species Act of 
1973, as amended (Act). The Petition provided three listing options for 
the Service to consider: Listing the dusky tree vole subspecies 
throughout its range; listing the North Oregon Coast population of the 
red tree vole (Arborimus longicaudus) as a distinct population segment 
(DPS); or listing the red tree vole because it is endangered or 
threatened in a significant portion of its range.
    After review of the best available scientific and commercial 
information, we have determined that listing the North Oregon Coast 
population of the red tree vole as a DPS is warranted. However, the 
development of a proposed listing rule is precluded by higher priority 
actions to amend the Lists of Endangered and Threatened Wildlife and 
Plants. Upon publication of this 12-month petition finding, we will add 
this DPS of the red tree vole to our candidate species list. We will 
develop a proposed rule to list this DPS of the red tree vole as our 
priorities allow. We will make any determination on critical habitat 
during development of the proposed listing rule. In any interim period, 
we will address the status of the candidate taxon through our annual 
Candidate Notice of Review (CNOR).

DATES: This finding was made on October 13, 2011.

ADDRESSES: This finding is available on the Internet at https://www.regulations.gov. Supporting documentation we used in preparing this 
finding is available for public inspection, by appointment, during 
normal business hours at the U.S. Fish and Wildlife Service, Oregon 
Fish and Wildlife Office, 2600 S.E. 98th Ave., Suite 100, Portland, OR 
97266; telephone 503-231-6179; facsimile 503-231-6195. Please submit 
any new information, materials, comments, or questions concerning this 
finding to the above street address.

FOR FURTHER INFORMATION CONTACT: Paul Henson, Ph.D., Field Supervisor, 
U.S. Fish and Wildlife Service, Oregon Fish and Wildlife Office (see 
ADDRESSES section). If you use a telecommunications device for the deaf 
(TDD), call the Federal Information Relay Service (FIRS) at 800-877-
8339.

SUPPLEMENTARY INFORMATION:

Background

    Section 4(b)(3)(B) of the Endangered Species Act (Act) (16 U.S.C. 
1531 et seq.) requires that, for any petition to revise the Federal 
Lists of Endangered and Threatened Wildlife and Plants that contains 
substantial scientific and commercial information indicating that 
listing may be warranted, we make a finding within 12 months of the 
date of receipt of the petition on whether the petitioned action is: 
(1) Not warranted; (2) warranted; or (3) warranted, but the immediate 
proposal of a regulation implementing the petitioned action is 
precluded by other pending proposals to determine whether species are 
endangered or threatened, and expeditious progress is being made to add 
or remove qualified species from the Federal Lists of Endangered and 
Threatened Wildlife and Plants. Section 4(b)(3)(C) of the Act requires 
that we treat a petition for which the requested action is found to be 
warranted but precluded as though resubmitted on the date of such 
finding; that is, requiring a subsequent finding to be made within 12 
months. We must publish these 12-month findings in the Federal 
Register.

Previous Federal Actions

    On June 22, 2007, we received a petition dated June 18, 2007, from 
the Center for Biological Diversity and six other organizations and 
individuals (hereafter, ``the petitioners''), requesting that we list 
the dusky tree vole as an endangered or threatened species and 
designate critical habitat. The petitioners requested that if we found 
the dusky tree vole was not a listable entity as a subspecies, we 
either list the North Oregon Coast population of the red tree vole as a 
distinct population segment (DPS), or list the red tree vole because it 
is endangered or threatened in a significant portion of its range, 
including the North Oregon Coast population. On September 26, 2007, we 
sent a letter to Noah Greenwald, Center for Biological Diversity, 
acknowledging our receipt of the petition and providing our 
determination that emergency listing was not warranted for the species 
at that time.
    On October 28, 2008, we published a 90-day finding for the dusky 
tree vole in the Federal Register (73 FR 63919). We found that the 
petition presented substantial information indicating that listing one 
of the following three entities as endangered or threatened may be 
warranted:
    (1) The dusky tree vole subspecies of the red tree vole;
    (2) The North Oregon Coast DPS of the red tree vole; or
    (3) The red tree vole because it is endangered or threatened in a 
significant portion of its range.
    As a result of that finding, we also initiated a status review of 
the species, including an evaluation of the North Oregon Coast 
population of red tree vole and the red tree vole throughout its range. 
This notice constitutes our 12-month finding for the petition to list 
the dusky tree vole as endangered or threatened.

Species Information

    As a putative subspecies, the dusky tree vole is a member of the 
red tree vole taxon. Some of the scientific literature is specific to 
the ``dusky tree vole,'' but much of it describes the red tree vole and 
does not distinguish among subspecies. For that reason, available 
information on the red tree vole is presented below with the assumption 
that it also applies to the dusky tree vole. If the information source 
makes distinctions between the two, they are noted, as appropriate. 
Published literature on the red tree vole also includes work conducted 
on the closely related Sonoma tree vole (Arborimus pomo). Prior to 
1991, these taxa were both considered red tree vole (Johnson and George 
1991, entire). Where pertinent information is lacking or limited for 
the red tree vole, information on the Sonoma tree vole is presented 
because there have been no ecological or life-history differences noted 
for the two species (Smith et al. 2003, p. 187).
    Tree voles are small, mouse-sized rodents that live in conifer 
forests and spend almost all of their time in the tree canopy. Tree 
voles rarely come to the ground, and do so only to move briefly between 
trees. They are one of the few animals to persist on a diet of conifer 
needles, which is their principal food. When eating, tree voles strip 
away the resin ducts within conifer needles and eat the remaining 
portion; resin ducts contain terpenoid chemicals that make

[[Page 63721]]

them unpalatable to most species. Red tree voles live singly (or with 
young, in the case of females) in nests made of vegetation and other 
materials. Swingle (2005, p. 2) summarized the sizes of red tree vole 
nests as ranging from ``very small ephemeral structures about the size 
of a grapefruit, to large old maternal nests that may be nearly as 
large as a bushel basket and completely encircle the trunk of the tree 
(Taylor 1915; Howell 1926; Verts and Carraway 1998).'' Nests of females 
tend to be larger than those of males. Males and females live separate 
lives once leaving the nest, only coming together to breed. Further 
details of the life-history characteristics of tree voles are presented 
below.

Taxonomy and Description

    Tree voles are less than 8.2 inches (in) (209 millimeters (mm)) 
long and weigh up to 1.7 ounces (oz) (49 grams (g)) (Hayes 1996, p. 1; 
Verts and Carraway 1998, p. 301; Forsman 2010, pers. comm.). Pelage 
(fur) color ranges from brownish red to bright brownish-red or orange-
red (Maser et al. 1981, p. 201). The darker coat color has been 
attributed to the dusky tree vole (Bailey 1936, p. 198; Maser et al. 
1981, p. 201). Melanistic (all black) forms of the dusky (Hayes 1996, 
p. 1) and red tree vole (Swingle 2005, p. 46), as well as cream-colored 
red tree voles (Swingle 2005, p. 82), rarely occur.
    Howell (1926, p. 35) described several physical differences between 
voles described as dusky tree voles and red tree voles. These 
differences include coat color, as well as skull and dental 
characteristics. However, Howell (1926, p. 34) based his description of 
the red tree vole on the observations of 40 tree voles, 32 of which 
were from California. At least 28 of the California tree voles were 
collected from Carlotta, Humboldt County, within the range of what is 
now considered the Sonoma tree vole (Howell 1926, p. 41; Blois and 
Arbogast 2006, pp. 953-956). Howell's description of the red tree vole 
was therefore based on a collection that was actually comprised 
primarily of Sonoma tree voles, rendering the comparison to dusky tree 
voles of questionable value.
    The taxonomic history of red and dusky tree voles is complex; a 
comprehensive description can be found in Miller et al. (2010, pp. 64-
65). The red tree vole was first described from a specimen collected in 
Coos County, Oregon (True 1890, pp. 303-304), and originally placed in 
the genus Phenacomys. The dusky tree vole was first described from a 
dead specimen found in Tillamook County and originally classified as a 
distinct species, P. silvicolus (Howell 1921, entire), later renamed P. 
silvicola (Miller 1924, p. 400). Taylor (1915, p. 156) established the 
subgenus Arborimus for tree voles, which Johnson (1968, p. 27; 1973, p. 
243) later proposed elevating to full generic rank, although this genus 
has not been universally adopted (e.g., Verts and Carraway 1998, pp. 
309-311). For the purpose of this finding, we use the generic 
classification, Arborimus, adopted by the petitioners.
    Johnson (1968, p. 27) concluded that analysis of blood proteins and 
hemoglobin from dusky and red tree voles ``* * * suggested combining 
the named forms of Arborimus into a single species * * *''. Hall (1981, 
p. 788) cited Johnson (1968, p. 27) as suggesting a ``subspecific 
relationship of the two taxa,'' and others have cited Johnson as well 
in supporting the classification of the dusky tree vole as a subspecies 
(e.g., Maser and Storm 1970, p. 64; Johnson and George 1991, p. 1). 
However, based on a lack of detectable genetic differences and a lack 
of consistently verifiable morphological differences between dusky and 
red tree voles, Bellinger et al. (2005, p. 207) suggested subspecific 
status of the dusky tree vole may not be warranted.
    Miller et al. (2006a, entire) analyzed mitochondrial DNA sequences 
from red tree voles throughout their range in Oregon. This study was 
not designed to address red tree vole taxonomy, but rather, how 
historical processes may have affected the genetic diversity and 
structure of the red tree vole across much of its range. The authors 
found significant genetic discontinuities based on unique haplotypes 
that result in three genetically distinct groupings of red tree voles. 
A primary discontinuity divided the red tree vole's range into a 
northern and a southern region in terms of genetic makeup as determined 
from mitochondrial DNA. Some overlap of these two genetic groups 
occurred, but in general, red tree voles north of Douglas and 
southeastern Lane Counties were genetically different from tree voles 
to the south (Miller et al. 2006a, Fig.1, pp. 146, 151-152). There are 
no known geographic or geological features that coincide with this 
genetic discontinuity that might explain this genetic break. The 
northern genetic group was further subdivided by a secondary 
discontinuity that coincided with the Willamette Valley, a non-forested 
barrier currently separating individuals in the northern Oregon Coast 
Range to the west from the Cascade Range to the east (Miller et al. 
2006a, Fig.1, pp. 146, 151-152).
    Although Miller et al. (2006a, entire) found genetic 
discontinuities in the red tree vole in Oregon, the authors did not 
comment on the taxonomic status of the species. Subsequent 
conversations with the geneticists who authored this paper indicated 
that the genetic differences described in Miller et al. (2006a, entire) 
were substantial enough to potentially warrant taxonomically 
classifying the three genetically distinct groups as separate 
subspecies if there were corresponding differences in other traits, 
such as behavior or morphology, to provide additional support (Miller 
and Haig 2009, pers. comm.). Recent review of external morphological 
characters by Miller et al. (2010, entire) did not distinguish dusky 
tree voles from red tree voles, but the authors noted that additional 
analysis of other physical characteristics (e.g., fur color) would be 
required to better determine the dusky tree vole's taxonomic status. 
The Integrated Taxonomic Information System (ITIS), a database 
maintained by a partnership of U.S., Canadian, and Mexican agencies, 
other organizations, and taxonomic specialists to provide 
scientifically credible taxonomic information, does not recognize the 
dusky tree vole as a subspecies of the red tree vole (information 
retrieved 15 March 2011, from the ITIS database). Wilson and Reeder 
(2005, entire) is the industry standard for mammalian taxonomy. 
Subspecies were not recognized until the most recent edition, published 
in 2005. Although Wilson and Reeder (2005, pp. 962-963) recognize the 
dusky tree vole as a subspecies, the more recent research on tree vole 
genetics and analyses attempting to clarify the taxonomic status of the 
dusky tree vole have only become available subsequent to that review, 
and therefore were not considered at the time that volume was 
published.

Range and Distribution

    Tree voles are endemic to the humid, coniferous forests of western 
Oregon and northwestern California (Maser 1966, p. 7). The red tree 
vole occurs in western Oregon from below the crest of the Cascade Range 
to the Pacific coast (Hayes 1996, p. 2; Verts and Carraway 1998, pp. 
309-310), with a geographic range covering approximately 16.3 million 
acres (ac) (6.6 million hectares (ha)) across multiple ownerships (USDA 
and USDI 2007, p. 287) (Figure 1).
BILLING CODE 4310-55-P

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[GRAPHIC] [TIFF OMITTED] TP13OC11.000

BILLING CODE 4310-55-C
    The southern boundary of the range of the red tree vole borders the 
range of the Sonoma tree vole, which Johnson and George (1991, p. 12) 
classified as a separate species from the red tree vole. Johnson and 
George (1991, pp. 11-12) suggested the break between the ranges of 
these two species was the Klamath Mountains along the Oregon-California 
border. Murray (1995, p. 26) considered the boundary between the two 
species to be the Klamath River in northwestern California. A recent 
mitochondrial DNA analysis supports the classification of tree voles in 
northwestern California (Del Norte County) as Arborimus longicaudus 
(Blois and Arbogast 2006, pp. 956, 958).
    The red tree vole has not been found north of the Columbia River 
(Verts and Carraway 1998, p. 309), but the actual northern limit of its 
historical distribution in northwestern Oregon is unclear. Within the 
Oregon Coast Range, the northernmost tree vole collection site was in 
the vicinity of Saddle Mountain in central Clatsop County (Verts and 
Carraway 1998, pp. 310, 546; Forsman and Swingle 2009, pers. comm.). 
Although no tree voles have been detected in recent search efforts in 
northern Clatsop and Columbia

[[Page 63723]]

Counties (Forsman and Swingle 2009, unpublished data), the area 
historically had extensive forests with large Douglas-fir (Pseudotsuga 
menziesii) and western hemlock (Tsuga heterophylla) trees conducive to 
tree vole habitat (Robbins 1997, pp. 205-206). Therefore, we believe it 
is reasonable to assume that tree voles were present in those areas 
prior to the late 1800s and early 1900s when virtually all old forests 
in the region were clear-cut or burned. The Columbia River was 
considered Oregon's most productive logging center in the late 1800s 
(Robbins 1997, p. 220), and it is likely that virtually all of the 
suitable tree vole habitat in Clatsop, Columbia, and Washington 
Counties was removed before tree vole occurrence could be recorded. 
Whether tree voles may persist undetected in Columbia County and 
northern Clatsop County is not known at this time; although not 
detected in the most recent search efforts, tree voles may be 
overlooked if they are sparsely distributed or few in number.
    Farther east, the red tree vole occurs in the Columbia River Gorge 
from Wahkenna Creek to Seneca Fouts State Park, 4 miles (mi) (6 
kilometers (km)) west of Hood River (Forsman et al. 2009b, p. 230). The 
red tree vole range had been described as west of the crest of the 
Cascade Range in Oregon (Corn and Bury 1986, p. 405). However, recent 
surveys have also found them just east of the Cascade Range crest, in 
the headwaters of the Lake Branch of Hood River, 19 mi (30 km) 
southwest of the town of Hood River (Forsman et al. 2009b, p. 227).
    Surveys conducted for red tree voles by the Forest Service and the 
Bureau of Land Management as part of the Survey and Manage program 
under the Northwest Forest Plan (NWFP) have provided additional 
information on the distribution of the red tree vole (USDA and USDI 
2007, p. 289). These surveys indicate red tree voles are uncommon and 
sparsely distributed in much of the northern Coast Range and northern 
Cascade Range of Oregon. Forsman et al. (2004, p. 300) reached the same 
conclusion based on remains of red tree voles in pellets of northern 
spotted owls (Strix occidentalis caurina), although data were sparse 
from the northern Oregon Coast Range compared to the rest of the red 
tree vole's range. Based on these surveys and data from owl pellets, 
the eastern limit of red tree vole distribution in southwestern Oregon 
appears to include forested areas in Josephine County and a narrow band 
along the western and northern edges of Jackson County (Forsman et al. 
2004, pp. 297-298; USDA and USDI 2007, p. 289).
    Red tree voles are generally restricted to lower elevation 
coniferous forests, although there are a few records of this species 
above 4,265 feet (ft) (1,300 meters (m)) (Manning and Maguire 1999, 
entire; Forsman et al. 2004, p. 300). Hamilton (1962, p. 503) suggested 
red tree voles may be limited to lower elevations because their nests 
do not provide adequate insulation during winter. Because tree voles 
are active throughout the year, it is also possible they are absent 
from high-elevation areas because they find it difficult to forage on 
limbs covered with snow and ice during winter (Forsman et al. 2004, p. 
300).
    The range of the putative dusky tree vole is less clear than that 
of the red tree vole. Johnson and George (1991, p. 12) described its 
range as restricted to the western slope of the Coast Range in 
Tillamook and Lincoln Counties. However, Maser (1966, p. 16) summarized 
collection and nest records for the dusky tree vole from locations east 
of the crest of the Coast Range down to the western edge of the 
Willamette Valley in Washington, Yamhill, Polk, Benton, and Lane 
Counties. Maser (2009, pers. comm.) believed the southern limit of the 
dusky tree vole to be in the vicinity of the Smith or Umpqua Rivers 
(western Douglas County) based on a shift in vole behavior and habitat 
type. Brown (1964, p. 648) mentioned four dusky tree vole museum 
specimens collected near Molalla in Clackamas County east of the 
Willamette Valley. Howell (1926, p. 34) referred to Stanley Jewett, a 
fellow naturalist, finding ``unmistakable evidence'' of red tree voles 
in old nests near Bonneville, in far eastern Multnomah County at the 
foot of the Cascade Range, and then goes on to say, ``Though this sign 
may possibly have been of longicaudus, it is considered more likely to 
have been of silvicola.'' However, he did not elaborate on why he 
concluded that it was indicative of the dusky tree vole. Maser (1966, 
p. 8) observed that tree voles historically collected north of Eugene 
and west of the Willamette Valley were typically classified as dusky 
tree voles, while those collected north of Eugene and east of the 
Willamette Valley were almost all identified as red tree voles.

Home Range and Dispersal

    The only published data on home range sizes and dispersal come from 
red tree voles radio-collared in the southern Coast Range and southern 
Cascades of Douglas County in southwestern Oregon (Swingle 2005, pp. 
51-63, 84-89; Swingle and Forsman 2009, entire). Of 45 radio-collared 
red tree voles, 18 had home ranges consisting of their nest tree and a 
few adjacent trees, whereas the remainder occupied up to 6 different 
nests spaced up to 532 ft (162 m) apart in different trees (Swingle and 
Forsman 2009, p. 277). Mean and median home ranges were 0.43 ac (0.17 
ha) and 0.19 ac (0.08 ha), respectively (Swingle and Forsman 2009, p. 
278). Home range sizes did not differ among gender, age, or among voles 
occurring in young (22-55 years old) versus old (110-260 years old) 
forests (Swingle and Forsman 2009, pp. 277-279). An unpublished study 
conducted by Brian Biswell and Chuck Meslow found mean male home ranges 
of 0.86 ac (0.35 ha) and mean female home ranges of 0.37 ac (0.15 ha) 
(Biswell and Meslow, unpublished data referenced in USDA and USDI 
2000b, p. 8). Dispersal distances of nine subadults ranged from 10 to 
246 ft (3 to 75 m) (Swingle 2005, p. 63). The longest known straight-
line dispersal distance was for a subadult male who traveled 1,115 ft 
(340 m) over the course of 40 days (Biswell and Meslow, unpublished 
data referenced in USDA and USDI 2000b, p. 8).

Habitat

    Red tree voles are found exclusively in conifer forests or in mixed 
forests of conifers and hardwoods (Hayes 1996, p. 3). Throughout most 
of their range, they are principally associated with Douglas-fir for 
foraging and nesting (Jewett 1920, p. 165; Bailey 1936, p. 195). 
However, their nests have also been documented in Sitka spruce (Picea 
sitchensis) (Jewett 1920, p. 165), grand fir (Abies grandis), western 
hemlock, Pacific yew (Taxus brevifolia), and non-conifers such as 
bigleaf maple (Acer macrophyllum) and golden chinquapin (Castanopsis 
chrysophylla) (Swingle 2005, p. 31). Hardwoods are generally not 
recognized as an important habitat component (USDA and USDI 2002, p. 
1). Tree vole nests are located in the forest canopy and are 
constructed from twigs and resin ducts discarded from feeding, as well 
as fecal pellets, lichens, dead twigs, and conifer needles (Howell 
1926, p. 46; Clifton 1960, pp. 53-60; Maser 1966, pp. 94-96; Gillesberg 
and Carey 1991, p. 785; Forsman et al. 2009a, p. 266). On the occasions 
when tree voles nest in non-conifers or snags, they are virtually 
always in trees that have limbs interconnected with adjacent live 
conifers where the voles can obtain food (Maser 1966, p. 78; Swingle 
2005, p. 31). Within the northern Oregon Coast Range, primarily in the 
Sitka spruce plant series (see Distinct Vertebrate Population Segment 
Analysis for plant

[[Page 63724]]

series description), tree vole diet and nest tree species selection 
favors western hemlock and Sitka spruce (Walker 1930, pp. 233-234; 
Forsman et al. 2008, Table 2; Forsman and Swingle 2009, pers. comm.; 
Maser 2009, pers. comm.), although some vole nests have been found in 
Douglas-fir in this plant series (Howell 1921, p. 99; Jewett 1930, pp. 
81-83; Forsman and Swingle 2009, pers. comm.).
    Based on their study of small mammal habitat associations in the 
Oregon Coast Range, Martin and McComb (2002, p. 262) considered red 
tree voles to be habitat specialists. In that study of forests of 
different patch types, red tree voles selected ``conifer large 
sawtimber patch types'' and landscapes that minimize fragmentation of 
mature conifer forest (Martin and McComb 2002, pp. 259, 261, 262). The 
vegetation classification scheme used by Martin and McComb (2002, p. 
257) defines the conifer large sawtimber patch type as forest patches 
with greater than 70 percent conifer composition, more than 20 percent 
canopy cover, and mean diameter at breast height (dbh) of greater than 
21 in (53.3 cm) (it should be noted that studies where researchers 
actually measured the canopy cover of stands used by red tree voles 
indicate the minimum canopy cover requirements of red tree voles are 
much higher, on the order of 53 to 66 percent (e.g., Swingle 2005, p. 
39)). Red tree voles were most abundant in contiguous mature conifer 
forest (unfragmented landscapes), and were negatively affected by 
increasing patch densities at the landscape scale (Martin and McComb 
2002, p. 262).
    Although red and Sonoma tree voles occur and nest in young forests 
(Jewett 1920, p. 165; Brown 1964, p. 647; Maser 1966, p. 40; Corn and 
Bury 1986, p. 404; Thompson and Diller 2002, entire; Swingle and 
Forsman 2009, p. 277), most comparisons of relative abundance from 
pitfall trapping and nest presence data show increased occurrence in 
older forests throughout the range of these species (Corn and Bury 
1986, p. 404; Corn and Bury 1991, pp. 251-252; Ruggiero et al. 1991, p. 
460; Meiselman and Doyle 1996, p. 38; Gomez and Anthony 1998, p. 296; 
Martin and McComb 2002, p. 261; Jones 2003, p. 29; Dunk and Hawley 
2009, entire). The occurrence of active nests in remnant older trees in 
younger stands indicates the importance of legacy structural 
characteristics (USDA and USDI 2002, p. 1). Although the bulk of the 
evidence points to forests with late-successional characteristics as 
important to the red tree vole, we lack specific data on the minimum 
size of trees or stands required to sustain populations of the red tree 
vole over the long term.
    There is no single description of red tree vole habitat and a wide 
variety of terms have been used to describe the older forest stands the 
tree voles tend to select (e.g., late-successional, old-growth, large 
conifer, mature, structurally complex). Where these terms appear in 
cited literature, or where specific ages are referred to, we refer to 
them in this analysis. Otherwise, we use the term ``older forest'' when 
collectively referring to these stand conditions. In using the term 
``older forest,'' we are not implying a specific stand age that 
represents tree vole habitat. Rather, we use the term to represent the 
mixture of old and large trees, multiple canopy layers, snags and other 
decay elements, understory development and biologically complex 
structure and composition often found in forests selected by tree 
voles.
    The most extensive and intensive analysis of red tree vole habitat 
associations on Federal lands throughout the vole's range found a 
strong association between tree vole nest presence and late-
successional and old-growth forest conditions (forests over 80 years 
old), with optimal red tree vole habitat being especially rare (Dunk 
and Hawley 2009, p. 632). Throughout their range on Federal land, the 
probability of red tree vole nest presence (Po) in the highest quality 
habitat (forest exhibiting late-successional structural 
characteristics) was 7 times more than expected based on the 
proportional availability of that habitat, whereas in lowest quality, 
early-seral forest conditions, Po was 7.6 times less than expected 
based on availability (Dunk and Hawley 2009, p. 632). In other words, 
red tree voles demonstrated strong selection for nesting in stands with 
older forest characteristics, even though that forest type was 
relatively rare across the landscape. Conversely, tree voles avoided 
nesting in younger stand types that were much more common across the 
landscape.
    Trees containing tree vole nests are significantly larger in 
diameter and height than those without nests (Gillesberg and Carey 
1991, p. 785; Meiselman and Doyle 1996, p. 36 for the Sonoma tree 
vole). Other forest conditions associated with red tree vole habitat 
include the number of large trees and variety of tree size distribution 
(Dunk and Hawley 2009, p. 632). Carey (1991, p. 8) suggested that tree 
voles seem especially well-suited to the stable conditions of old-
growth Douglas-fir forests (multi-layered stands over 200 years old, 
with decay elements). Old-growth trees may be optimum tree vole habitat 
because primary production is high and needles are concentrated, 
providing maximum food availability (Carey 1991, p. 8). In addition, 
old-growth canopy buffers weather changes and has high water-holding 
capacity, providing fresh foliage and a water source (Gillesberg and 
Carey 1991, pp. 786-787), as well as numerous cavities and large limbs 
that provide stable nest substrates.
    As noted above, tree voles can be found in younger forests, 
sometimes at fairly high densities (Howell 1926, pp. 41-45: Maser 1966, 
pp. 216-217; Thompson and Diller 2002, p. 95). It is not understood how 
younger forests influence the abundance, persistence, or dispersal of 
red tree voles. Carey (1991, p. 34) suggested younger forests were 
population sinks for red tree voles. Based on surveys in young forests 
(22-55 years old) and observations of radio-collared tree voles, 
Swingle (2005, pp. 78, 94) and Swingle and Forsman (2009, pp. 283-284) 
concluded that some young forests may be important habitat for tree 
voles, particularly in landscapes where old forests have largely been 
eliminated or currently exist in isolated patches. However, Swingle 
(2005, pp. 78, 94) cautioned against using the occasional presence of 
tree voles in young forests to refute the importance of old forest 
habitats to tree voles. Young forest stands may serve as interim 
habitat for tree voles and may provide connectivity between remnant 
patches of older forest, but whether younger forests are capable of 
supporting viable populations of tree voles over the long term is 
uncertain. The limited evidence available suggests that tree vole 
occupation of younger forest stands may be relatively short-lived 
(Diller 2010, pers. comm.) or intermittent (Hopkins 2010, pers. comm.).
    After weighing all of the best available information, we conclude 
that although red tree voles may use younger forest types to some 
degree, the preponderance of evidence suggests red tree voles 
demonstrate strong selection for forests with older forest conditions, 
as well as contiguous forest conditions. Whether tree voles can 
potentially persist in younger forests over the long term is unknown 
(USDA and USDI 2007, p. 291). However, although the data are limited, 
the available evidence suggests that red tree voles likely do not 
maintain long-term or consistent populations in younger stands (Diller 
2010, pers. comm.; Hopkins 2010, pers. comm.). There is a relatively 
large body of evidence, on the other hand, that red tree voles exhibit 
strong selection for areas of contiguous habitat exhibiting

[[Page 63725]]

conditions characteristics of older, mature forests (Corn and Bury 
1986, p. 404; Corn and Bury 1991, pp. 251-252; Ruggiero et al. 1991, p. 
460; Meiselman and Doyle 1996, p. 38; Gomez and Anthony 1998, p. 296; 
Martin and McComb 2002, p. 261; Jones 2003, p. 29; Dunk and Hawley 
2009, entire). We therefore further conclude that unfragmented forests 
with late-successional characteristics are thus most likely to provide 
for the long-term persistence of the species, and in this finding we 
consider these older forest types as representative of high-quality 
habitat for the red tree vole.
    Tree voles may tolerate some forest fragmentation, but the point at 
which forest gaps become large enough to impede their movements or 
successful dispersal is not known. Howell (1926, p. 40) suggested that 
``considerable'' expanses of land without suitable trees are a barrier 
to tree vole movements. However, as noted earlier, known dispersal 
distances for red tree voles are quite short, ranging from 10 to 246 ft 
(3 to 75 m) (Swingle 2005, p. 63), with 1,115 ft (340 m) being the 
longest known dispersal distance (Biswell and Meslow, unpublished data 
referenced in USDA and USDI 2000b, p. 8). This suggests that relatively 
small distances, roughly less than 1,200 ft (366 m) between forest 
patches, may serve as effective barriers to dispersal or recolonization 
for red tree voles. Radio-collared tree voles crossed logging roads, 
first-order streams, and canopy gaps up to 82 ft (25 m) wide (Biswell 
and Meslow, unpublished data referenced in USDA and USDI 2000b, p. 8; 
Swingle and Forsman 2009, p. 283). Some of these crossings occurred on 
multiple occasions by a single vole. This suggests that ``small forest 
gaps'' (Swingle 2005, p. 79) may not greatly impair tree vole movement, 
but increasing gap size may be expected to limit tree vole movement. In 
addition, Swingle (2005, p. 79) suggested that the necessity of 
descending to the ground to cross openings may reduce survival. There 
are three records of red tree voles captured in clearcuts (Borrecco 
1973, pp. 34, 36; Corn and Bury 1986, pp. 404-405; Verts and Carraway 
1998, p. 310), in one case over 656 ft (200 m) from the forest edge. In 
two of these instances, the authors suggested the individuals were most 
likely in the act of dispersing.
    In summary, based on our evaluation of the best scientific and 
commercial data available, as detailed above, for the purposes of this 
finding we consider older forests with late-successional 
characteristics to represent high-quality habitat for red tree voles, 
and younger forests in early-seral condition to represent low-quality, 
transitional habitat for red tree voles. In addition, we consider it 
likely that younger forests only play a role as interim, low-quality 
habitat for red tree voles if they occur in association with older 
forest patches or remnants.

Reproduction

    Red tree vole litter sizes are among the smallest compared to other 
rodents of the same subfamily, averaging 2.9 young per litter (range 1 
to 4) (Maser et al. 1981, p. 205; Verts and Carraway 1998, p. 310). 
Clifton (1960, pp. 119-120) reported that captive tree voles became 
sexually mature at 2.5 to 3.0 months of age. Females breed throughout 
the year, with most reproduction occurring between February and 
September (Swingle 2005, p. 71). Red tree voles are capable of breeding 
and becoming pregnant immediately after a litter is born (Clifton 1960, 
p. 130; Hamilton 1962, pp. 492-495; Brown 1964, pp. 647-648), resulting 
in the potential for females to have two litters of differently aged 
young in their nests (Swingle 2005, p. 71; Forsman et al. 2009a, p. 
270). Captive tree voles may have litters just over a month apart 
(Clifton 1960, p. 130). Forsman et al. (2009a, p. 270) observed two 
female voles in the wild that produced litters at 30 to 35 day 
intervals. Young tree voles develop more slowly than similar-sized 
rodents of the same subfamily (Howell 1926, pp. 49-50; Maser et al. 
1981, p. 205), first exiting the nest at 30 to 35 days old, and not 
dispersing until they are 47 to 60 days old (Swingle 2005, p. 63; 
Forsman et al. 2009a, pp. 268-269).

Diet

    Tree voles are unique in that they feed exclusively on conifer 
needles and the tender bark of twigs that they harvest from conifers. 
In most of their range, they feed primarily on Douglas-fir (Howell 
1926, p. 52; Benson and Borell 1931, p. 230; Maser et al. 1981, p. 
205). In portions of the northern coastal counties of Oregon (Lincoln, 
Tillamook, and Clatsop), tree voles also consume needles from western 
hemlock and Sitka spruce, and in some parts of their range they feed on 
grand fir, bishop pine (Pinus muricata), and introduced Monterrey pine 
(P. radiata) (Jewett 1920, p. 166; Howell 1926, pp. 52-53; Walker 1930, 
p. 234; Wooster and Town 2002, pp. 182-183; Forsman and Swingle 2009, 
pers. comm.; Swingle 2010, pers. comm.). Conifer needles contain 
filamentous resin ducts that are filled with terpenoids, chemicals that 
serve as defensive mechanisms for trees by making the leaves 
unpalatable. Tree voles have adapted to their diet of conifer needles 
by stripping away these resin ducts and eating the more palatable 
portion of the needle (Benson and Borell 1931, pp. 228-230; Perry 1994, 
pp. 453-454; Maser 1998, pp. 220-221; Kelsey et al. 2009, entire). 
Resin ducts typically run the length of the needle, but may be located 
in different portions of the needle, depending on the tree species; 
this forces the tree vole to behave differently depending on the tree 
species on which they forage. As an example, the resin ducts in 
Douglas-fir needles are located along the outer edges of the needle, so 
tree voles remove the outside edge and consume the remaining middle 
portion of the needle. Conversely, the resin ducts of western hemlock 
are located away from the outside edges along the midline of the 
needle. Thus, voles foraging on hemlock needles will consume the outer 
edge of the needle and discard the center (Clifton 1960, pp. 35-45; 
Forsman and Swingle 2009, pers. comm.; Kelsey et al. 2009, entire; 
Maser 2009, pers. comm.).
    Within the Sitka spruce plant series of the northern Oregon Coast 
Range of Oregon, tree voles appear to prefer, and perhaps require, a 
diet of western hemlock and Sitka spruce needles (Walker 1930, p. 234; 
Forsman and Swingle 2009, pers. comm.; Maser 2009, pers. comm.;). Voles 
in the Sitka spruce plant series rarely forage on Douglas-fir, even 
where it is available; foraging on Douglas-fir only becomes more 
evident where the Sitka spruce plant series transitions into the 
adjacent western hemlock series (Forsman and Swingle 2009, pers. comm.; 
Forsman and Swingle 2009, unpublished data). Maser (2009, pers. comm.) 
observed that tree voles adapted to a diet of western hemlock starved 
to death in captivity because they would not eat the Douglas-fir 
needles they were offered. Because the resin ducts of western hemlock, 
Sitka spruce, and Douglas-fir needles are in different locations on the 
needle, their removal requires a different behavior depending on which 
species is being eaten (Clifton 1960, pp. 35-49; Kelsey et al. 2009, 
entire). Maser (2009, pers. comm.) suspected that voles raised in 
stands of western hemlock never learned the required behavior for 
eating Douglas-fir, although Walker (1930, p. 234) observed a captive 
vole raised on hemlock needles that preferred hemlock but would eat fir 
or spruce in the absence of hemlock. Conversely, voles taken from 
Douglas-fir stands have been observed to eat both Douglas-fir and 
western hemlock in captivity (Clifton

[[Page 63726]]

1960, p. 44; Maser 2009, pers. comm.), although voles appear to be 
reluctant to switch between tree species (Walker 1930, p. 234; Forsman 
2010, pers. comm.).
    Tree voles appear to obtain water from their food and by licking 
water off of tree foliage (Clifton 1960, p. 49; Maser 1966, p. 148; 
Maser et al. 1981, p. 205; Carey 1996, p. 75). In keeping captive 
Sonoma tree voles, Hamilton (1962, p. 503) noted that it was important 
to keep leaves upon which they fed moist, otherwise the voles would 
lose weight and die. The need for free water in the form of rain or dew 
on foliage may explain why the distribution of tree voles is limited to 
relatively humid forests in western Oregon and California (Howell 1926, 
p. 40; Hamilton 1962, p. 503). However, there are no quantitative data 
on water consumption by tree voles, and some forests in which they 
occur (e.g., portions of southwestern Oregon) have little rain or dew 
during the summer months. How they are able to persist under such 
conditions is unclear.

Mortality

    In the only quantitative study conducted to date, Swingle et al. 
(2010, p. 258) found that weasels (Mustela spp.) were the primary 
predators of red tree voles. However, many other animals feed on tree 
voles, including ringtails (Bassariscus astutus) (Alexander et al. 
1994, p. 97), fisher (Martes pennanti) (Golightly et al. 2006, p. 17), 
northern spotted owls (Forsman et al., 1984, p. 40), barred owls (Strix 
varia) (Wiens 2010, pers. comm.), and a variety of other nocturnal and 
diurnal raptors (Miller 1933, entire; Maser 1965a, entire; Maser 1965b, 
entire; Forsman and Maser 1970, entire; Reynolds 1970, entire; Graham 
and Mires 2005, entire). Other documented predators include the 
Steller's jay (Cyanocitta stelleri) (Howell 1926, p. 60), a gopher 
snake (Pituophis catenifer) (Swingle et al. 2010, p. 258), domestic 
dogs (Canis familiaris) (Swingle et al. 2010, p. 258), and house cats 
(Felis catus) (Swingle 2005, pp. 90-91). In addition, Maser (1966, p. 
164) found tree vole nests that had been torn apart and inferred the 
destruction was likely caused by northern flying squirrels (Glaucomys 
sabrinus), raccoons (Procyon lotor), western gray squirrels (Sciurus 
griseus), or Douglas' squirrels (Tamiasciurus douglasii), apparently in 
search of young voles. Forsman (2010, pers. comm.) recorded video 
footage of northern flying squirrels, western gray squirrels, and 
Douglas' squirrels chasing tree voles or tearing into tree vole nests 
in what appeared to be attempts to capture voles.
    Swingle et al. (2010, p. 259) estimated annual survival of radio-
collared tree voles to be 15 percent. Little is known about the 
vulnerability of red tree voles to predators in different habitats. 
Swingle (2005, pp. 64, 90) found that of 25 documented cases of 
predation on radio-collared voles, most occurred in young (22-55 years 
old) forests (Forsman and Swingle 2009, pers. comm.). Predation by 
weasels, which accounted for 60 percent of the predation events, 
occurred only in the 22-55-year-old forests, and 80 percent of the 
weasel predation was on female voles. Most of the radio-collared sample 
consisted of females and were in young forest, so forest age and vole 
gender explained little of the variation in the data (Forsman 2010, 
pers. comm.; Swingle 2010, pers. comm.). Although there was no 
statistical difference in predation rates among forest ages and vole 
gender, Swingle et al. (2010, p. 260) suspected weasel predation on 
tree voles may be inversely proportional to nest height. Tree vole 
nests tend to be found in the lower portion of the tree crown 
(Gillesburg and Carey 1991, pp. 785-786; Swingle 2005, pp. 29-30), and 
tree vole nests tend to be higher above the ground in older stands or 
larger trees than in younger stands or smaller trees (Zentner 1966, pp. 
18-20; Vrieze 1980, pp. 18, 32-33; Meiselman and Doyle 1996, p. 38; 
Swingle 2005, pp. 29-30). Thus, tree voles could be more prone to 
predation in shorter trees that comprise younger stands and limit the 
height of nests above the ground. Swingle et al. (2010, p. 261) also 
suggested that female tree voles may be more susceptible to predation 
than males because they occupy larger, more conspicuous nests and spend 
more time outside the nest collecting food for their young.
    Other mortality sources include disease, old age, storms, forest 
fires, and logging (Maser et al. 1981, p. 206). Carey (1991, p. 8) 
suggested that forest fires and logging are far more important 
mortality factors than predation in limiting vole abundance.

Defining a Species Under the Act

    Section 3(16) of the Act defines ``species'' to include any species 
or ``subspecies of fish or wildlife or plants, and any distinct 
population segment of any species of vertebrate fish or wildlife which 
interbreeds when mature'' (16 U.S.C. 1532(16)). Our implementing 
regulations at 50 CFR 424.11 provide further guidance for determining 
whether a particular taxon or population is a species for the purposes 
of the Act: ``[T]he Secretary shall rely on standard taxonomic 
distinctions and the biological expertise of the Department and the 
scientific community concerning the relevant taxonomic group'' (50 CFR 
424.11(a)). As previously noted, we were petitioned to list the dusky 
tree vole as a subspecies of the red tree vole. The petitioners 
requested that if we found that the dusky tree vole was not a listable 
entity as a subspecies, then we subsequently consider whether it should 
be listed as the North Oregon Coast DPS of the red tree vole. 
Alternatively, the petitioners requested that the dusky tree vole be 
protected by listing the red tree vole because it is endangered or 
threatened in a significant portion of its range. The analysis to 
determine whether this is a viable subspecies or DPS according to 
section 3(16) of the Act follows.

Subspecies Analysis

    There is no universally accepted definition of what constitutes a 
subspecies, and the use of the term subspecies may vary among taxonomic 
groups (Haig et al. 2006, entire). To be operationally useful, 
subspecies must be discernible from one another (i.e., diagnosable), 
not merely exhibit mean differences (Patten and Unitt 2002, pp. 28, 
34). This element of ``diagnosability,'' or the ability to consistently 
distinguish between populations, is a common thread that runs through 
all subspecies concepts. It is important to use multiple sources of 
information when evaluating a taxon's status. The greater the 
concurrence among multiple morphological, molecular, ecological, 
behavioral, and physiological characteristics, the higher the level of 
confidence in the taxonomic classification (Haig et al. 2006, p. 1591). 
To assess subspecies classification for the dusky tree vole, we 
evaluated all the available data to determine whether the evidence 
points to a consistent separation of the putative dusky tree voles from 
the remaining population of red tree voles. If the assessment of these 
multiple characteristics provides a clear and consistent separation of 
the putative dusky tree vole subspecies from the remaining red tree 
vole population, such that any individual from the range of the dusky 
tree vole would likely be correctly assigned to that subspecies on the 
basis of the suite of characteristics analyzed, that evidence would be 
considered indicative of a likely valid subspecies.

Geography

    As described under Range and Distribution, there is no clear 
demarcation for the range of the putative dusky tree vole. All

[[Page 63727]]

descriptions include the western slope of the northern Oregon Coast 
Range, typically Tillamook and Lincoln Counties. Other descriptions 
expand this range to include the east slope of the Oregon Coast Range 
(Maser 1966, p. 16), and south to include the coastal portion of 
Douglas County (Maser 2009, pers. comm.). Still others suggest tree 
voles found in the foothills of the Cascade Range (Brown 1964, p. 648) 
and in the Columbia River Gorge (Howell 1926, p. 34) were dusky tree 
voles. Contemporary descriptions of the dusky tree vole range usually 
reference Johnson and George (1991, p. 12), who, despite not finding 
any strong morphometric or karyologic (chromosomal) differences between 
the subspecies, state the two taxa, ``* * * now can be properly 
delineated geographically.'' Johnson and George (1991, p. 12) go on to 
describe the dusky tree vole range as the Pacific slope of the Oregon 
Coast Range in Tillamook and Lincoln Counties without substantiating 
the basis for their geographic delineation. There is thus no clear and 
consistent description of what may constitute the range of the ``dusky 
tree vole.''

Blood Proteins

    Johnson (1968, p. 27) analyzed blood proteins of dusky tree voles, 
red tree voles, and heather voles (Phenacomys intermedius) to determine 
whether Arborimus should remain as a subgenus under Phenacomys or be 
elevated to a full genus. Multiple authors cite this work to support 
the classification of the dusky tree vole as a subspecies of the red 
tree vole (e.g., Maser and Storm 1970, p. 64; Hall 1981, p. 788; 
Johnson and George 1991, p. 1). However, we fail to reach this 
conclusion based on Johnson's (1968, p. 27) work. Johnson (1968, p. 27) 
describes his results as follows:

    The tree mice of the species Arborimus longicaudus (including A. 
silvicola) have in the past been included with the heather vole, 
Phenacomys intermedius. Two specimens of P. intermedius (of two 
subspecies) and 16 specimens of A. longicaudus (of two subspecies) 
were examined. In these two species the serum proteins and 
hemoglobins have suggested combining the named forms of Arborimus 
into a single species, and separating the genera Arborimus and 
Phenacomys.

    Although Johnson (1968, p. 27) concluded that the named forms 
longicaudus and silvicola should be combined, he did not make any 
further determination on whether or not silvicola should be retained as 
a subspecies. We therefore question whether Johnson (1968, p. 27) 
definitively designates silvicola as a subspecies. While Hall (1981, p. 
788) cited Johnson (1968, p. 27) as suggesting a ``subspecific 
relationship of the two taxa,'' he also notes that this designation is 
a ``provisional arrangement'' because of the existing uncertainty about 
the relationship of the two taxa.

Genetics

    In this section and the Summary section below we describe and 
analyze the research on tree vole genetics as it relates to answering 
the question of whether or not the dusky tree vole is a taxonomically 
valid subspecies of the red tree vole. This should not be confused with 
our analysis later in this document (see Distinct Vertebrate Population 
Segment Analysis) wherein we evaluate the genetics research as it 
relates to its contribution towards determining the discreteness and 
significance of a potential DPS of the red tree vole.
    Bellinger et al. (2005, p. 207) failed to find detectable genetic 
differences between dusky and red tree voles, suggesting that 
subspecific status may not be warranted. Miller et al. (2006a, p. 145) 
found three distinct genetic entities in their analysis of 
mitochondrial DNA of red tree voles throughout Oregon. For this 
analysis, we are interested in the genetic entity that Miller et al. 
(2006a, p. 151) labeled the ``Northern Coast range'' sequence. While 
Miller et al. (2006a, entire) do not describe specific boundaries for 
this entity, the sampling locations in this entity are distributed 
across the northern Oregon Coast Range, extending south to latitudes 
roughly equivalent with the cities of Eugene and Florence (see Figure 1 
for city locations). This genetic entity encapsulates most of the range 
descriptions of the putative dusky tree vole. Although the objective of 
Miller et al. (2006a, entire) was not to address the taxonomy of the 
dusky tree vole, in subsequent conversations with the authors, they 
concluded that the genetic differences between these groups were 
sufficient to potentially support subspecies recognition if there were 
congruent differentiations in other characteristics (Miller and Haig 
2009, pers. comm.).

Morphology

    The dusky tree vole has been described as darker than the red tree 
vole (Bailey 1936, p. 198; Maser et al. 1981, p. 201; Hall 1981, p. 
788; Johnson and George 1991, p. 12), but there has been no analysis to 
indicate an identifiable change in coat color either between the two 
entities or that corresponds with the boundaries of the haplotype 
groups found in Miller et al. (2006a, entire) (see Genetics, above). 
Maser (2007, pers. comm.; 2009, pers. comm.) postulated that the darker 
coat color in voles from the northern Oregon Coast Range was due to the 
denser, darker forests in which a darker coat provided a more cryptic 
coloration than a lighter coat color. Assuming this hypothesis is 
correct, because there is a gradual transition of tree species and 
forest composition as one progresses south in the Coast Range, it is 
reasonable to hypothesize that a corresponding change in coat color may 
also be gradual rather than abrupt and thus not easily discernable from 
the red tree vole. This needs to be evaluated using a consistent and 
repeatable method for comparing pelage color. Such an analysis is 
currently being conducted but is not available for this review (Forsman 
2010, pers. comm.).
    In measuring multiple morphometric features, Johnson and George 
(1991, p. 5) found statistical differences distinguishing Oregon tree 
voles from California samples, but were not able to easily detect 
discernable differences between samples within Oregon or California. 
Miller et al. (2010, p. 69) found statistically significant differences 
in some external morphological features between putative dusky tree 
voles and red tree voles. Although these differences were statistically 
significant in distinguishing between groups of tree voles, they were 
of little diagnostic utility because they were so subtle they could not 
be used to reliably classify an individual tree vole as a dusky tree 
vole or a red tree vole (Miller et al. 2010, p. 67). A possible 
explanation for the statistical difference, yet lack of diagnostic 
utility, is that the morphological features measured also exhibited a 
positive correlation with latitude; tree voles from the northern part 
of the range were larger than tree voles from the southern part of the 
range. This is a clinal pattern consistent with Bergmann's Rule, an 
ecological principle stating that larger forms of species tend to be 
associated with cooler climate and higher latitude (Miller et al. 2010, 
p. 69).

Behavior

    Tree voles within the narrow band of Sitka spruce found along the 
coastal portion of the northern Oregon Coast Range north of Newport 
exhibit a different diet than voles in the rest of the range, foraging 
on Sitka spruce or western hemlock rather than on Douglas-fir (Walker 
1930, p. 234; Forsman and Swingle 2009, pers. comm.) (see above under 
Diet). This diet requires a different treatment of needles

[[Page 63728]]

than in other areas because resin ducts in spruce and hemlock are 
located in different parts of the needle than in Douglas-fir (Kelsey et 
al. 2009, pp. 12-13). While this behavioral difference exists primarily 
in the Sitka spruce plant series of the northern Oregon Coast Range, it 
comprises only a small portion of the area within the northern Coast 
Range genetic sequence found by Miller et al. (2006a, pp. 150-151; see 
Genetics, above) and does not correspond to the general boundaries of 
that genetic entity, nor does it correspond to any of the various 
boundaries of the putative dusky tree vole's range.

Summary

    Bellinger et al. (2005, p. 207) concluded that the absence of 
detectable genetic differences between red tree voles and putative 
dusky tree voles, combined with the lack of consistently verifiable 
morphological differences, suggested that the subspecific status of the 
dusky tree vole might not be warranted. Miller et al. (2006a, entire) 
found evidence of marked genetic differences in the red tree vole that 
could indicate the existence of a possible subspecies, although they 
did not explicitly address the implications of their work on red tree 
vole taxonomy. Subsequent conversations with the authors, however, 
indicated that observed genetic differences were sufficient to 
potentially support recognition of the dusky tree vole as a subspecies 
if there were additional differentiations in identifiable 
characteristics and if the boundaries of those differentiations were 
congruent with the ``Northern Coast range'' genetic grouping identified 
in Miller et al. (2006a, p. 151). However, our review of the best and 
most current data on the genetics, behavior, morphology, and range of 
the putative dusky tree vole reveals no other characteristics of 
diagnostic utility that correspond with the ``Northern Coast range'' 
haplotype grouping identified by Miller et al. (2006a, p. 151). There 
is not a consistent and well-substantiated range description of the 
dusky tree vole. Although some morphological differences may occur 
between the red tree vole and the putative dusky tree vole, these 
differences have little diagnostic utility and may only represent a 
clinal variation, as would be expected between northern and southern 
populations of the red tree vole based on Bergmann's Rule (an 
ecogeographic principle that states that animals at more northerly 
latitudes tend to be larger than individuals of the same species at 
more southerly latitudes) (Miller et al. 2010, entire). The prevailing 
behavior of foraging on western hemlock and Sitka spruce within the 
Sitka spruce plant series does not correspond to the geographic range 
of the ``Northern Coast range'' genetic entity described by Miller et 
al. (2006a, p. 151), but comprises only a small portion of the range of 
that haplotype group. Presumptive differences in coloration, which 
served as one of the primary bases for the original subspecies 
distinction of the dusky tree vole, have never been quantified. Such a 
conventional approach to subspecies designation, used historically and 
frequently based on apparent geographic or clinal variation, is often 
not supported when tested by more rigorous analyses of multiple 
characters (e.g., Thorpe 1987, pp. 7, 9).
    Given the lack of diagnostic characteristics that correspond with 
the ``Northern Coast range'' haplotype group described by Miller et al. 
(2006a, p. 151) and the findings of Bellinger et al. (2005 entire) and 
Miller et al. (2010 entire) that there are no detectable genetic or 
morphological differences yet found between dusky tree voles and red 
tree voles, we do not believe there is sufficient evidence to indicate 
that the dusky tree vole is a distinct subspecies. Although the dusky 
tree vole was recognized as a subspecies in Wilson and Reeder's Mammal 
Species of the World (2005, pp. 962-963), we note that this reference 
did not recognize, or was published prior to, the availability of the 
work of Bellinger et al. (2005, entire) and Miller et al. (2006a, 
entire; 2010 entire). Subsequent to the publication of some of these 
latter works, the Oregon Natural Heritage Information Center ceased 
recognition of the dusky tree vole as a subspecies (ORNHIC 2007, p. 
17), as did the U.S. Forest Service and Bureau of Land Management's 
Survey and Manage program (USDA and USDI 2007, p. 289). Finally, the 
dusky tree vole is not recognized as a valid subspecies of the red tree 
vole in the Integrated Taxonomic Information System (ITIS 2011). 
Therefore, based on the best available scientific and commercial data, 
as described above, we have concluded that the dusky tree vole is not a 
valid subspecies, and therefore is not eligible for listing as such 
under the Act. We must next evaluate whether the North Oregon Coast 
population of the red tree vole is a DPS to determine whether it would 
constitute a listable entity under the Act.

Distinct Vertebrate Population Segment Analysis

    The Service and the National Marine Fisheries Service (now the 
National Oceanic and Atmospheric Administration--Fisheries), published 
the Policy Regarding the Recognition of Distinct Vertebrate Population 
Segments Under the Endangered Species Act (DPS Policy) in the Federal 
Register on February 7, 1996 (61 FR 4722) to guide the implementation 
of the DPS provisions of the Act. Under the DPS Policy, three elements 
are considered in the decision regarding the establishment and 
classification of a population of a vertebrate species as a possible 
DPS. These are applied similarly for additions to and removals from the 
Lists of Endangered and Threatened Wildlife and Plants. These elements 
are:
    (1) The discreteness of a population in relation to the remainder 
of the species to which it belongs;
    (2) The significance of the population segment to the species to 
which it belongs; and
    (3) The population segment's conservation status in relation to the 
Act's standards for listing, delisting, or reclassification (i.e., is 
the population segment endangered or threatened?).
    In the petition, we were asked to consider listing a DPS for the 
red tree vole in the North Oregon Coast portion of its range if we did 
not conclude that the dusky tree vole was a valid subspecies of the red 
tree vole. In accordance with our DPS Policy, this section details our 
analysis of the first two elements, described above, to assess whether 
the vertebrate population segment under consideration for listing may 
qualify as a DPS.
    Specific to red tree vole genetics, as we noted above (see 
Subspecies Analysis), in this section we have reviewed the research on 
red tree vole genetics and evaluated whether or not the genetics 
evidence supports identifying a population segment that meets the 
discreteness and significance standards described above. Although 
genetic research indicates that the putative dusky tree vole may not be 
a valid subspecies (e.g. Bellinger et al. 2005, entire; Miller et al. 
2010, entire), whether or not a population segment is discrete and 
significant is a different question and these works do not exclude the 
possibility that there is a discrete and significant population segment 
for the red tree vole.

Discreteness

    The DPS Policy's standard for discreteness requires an entity to be 
adequately defined and described in some way that distinguishes it from 
other representatives of its species. A population segment of a 
vertebrate species may be considered discrete if it

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satisfies either of the following two conditions:
    (1) It is markedly separated from other populations of the same 
taxon as a consequence of physical, physiological, ecological, or 
behavioral factors (quantitative measures of genetic or morphological 
discontinuity may provide evidence of this separation); or
    (2) It is delimited by international governmental boundaries within 
which significant differences in control of exploitation, management of 
habitat, conservation status, or regulatory mechanisms exist.
    The North Oregon Coast portion of the red tree vole range is 
markedly separated from the rest of the species' range based on the 
genetic discontinuities described by Miller et al. (2006a, pp. 150-
151). Miller et al. (2006a, entire) examined phylogeographical patterns 
by analyzing mitochondrial control region sequences of 169 red tree 
voles sampled from 18 areas across the range of the species in Oregon. 
In addition, they analyzed Cytochrome b sequences from a subset of 
these samples. Through phylogenetic network and spatial genetic 
analyses, the researchers found a primary genetic discontinuity 
separating red tree voles from the northern (areas A through F (Miller 
et al. 2006a, Figure 1, pp. 146, 151-152)) and southern (areas G 
through R (Miller et al. 2006a, Figure 1, pp. 146, 151-152)) sampling 
areas; a secondary discontinuity separated the northern sampling areas 
into eastern (areas B, E, and G (Miller et al. 2006a, Figure 1, pp. 
146, 151-152)) and western (areas A, C, D, and F (Miller et al. 2006a, 
Figure 1, pp. 146, 151-152)) subdivisions separated by the Willamette 
Valley (Miller et al. 2006a, pp. 150-153). Miller et al. (2006a, p. 
151) labeled the eastern subdivision as the ``Northern Cascade range'' 
sequence, and the western subdivision the ``Northern Coast range'' 
sequence, reflecting the associated mountain ranges. As described in 
the Taxonomy and Description section, above, genetic researchers 
considered the degree of genetic difference between the 3 groupings of 
red tree voles to be highly significant (Miller and Haig 2009, pers. 
comm.). We thus consider the population of red tree voles represented 
by the ``Northern Coast range'' haplotypes to be markedly separated 
from other populations of the taxon as evidenced by quantitative 
measures of genetic discontinuity.
    Red tree vole