                  FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

NORTHWEST ECOYSTEM ALLIANCE;            
CENTER FOR BIOLOGICAL DIVERSITY;
TAHOMA AUDUBON SOCIETY,
               Plaintiffs-Appellants,
                 v.                          No. 04-35860
UNITED STATES FISH AND WILDLIFE
SERVICE; DAVE ALLEN, Regional                 D.C. No.
                                            CV-03-01505-PA
director of U.S. Fish and Wildlife
                                              OPINION
Service; STEVE WILLIAMS, Director
of U.S. Fish and Wildlife Service;
GALE NORTON, Secretary of the
Department of the Interior,
             Defendants-Appellees.
                                        
        Appeal from the United States District Court
                 for the District of Oregon
         Owen M. Panner, Senior Judge, Presiding

                 Argued and Submitted
           November 15, 2006—Portland, Oregon

                   Filed February 2, 2007

 Before: Alfred T. Goodwin, Diarmuid F. O’Scannlain, and
            Raymond C. Fisher, Circuit Judges.

                 Opinion by Judge Goodwin




                             1215
1218      NORTHWEST ECOYSTEM ALLIANCE v. USFWS


                        COUNSEL

Brent Plater, Center for Biological Diversity, San Francisco,
California; Stephanie M. Parent, Pacific Environmental Advo-
cacy Center, Portland, Oregon, for the plaintiffs-appellants.

M. Alice Thurston, U.S. Department of Justice, Washington,
D.C., for the defendants-appellees.


                         OPINION

GOODWIN, Circuit Judge:

  The United States Fish and Wildlife Service (the “Service”)
denied a petition to classify western gray squirrels in Wash-
             NORTHWEST ECOYSTEM ALLIANCE v. USFWS         1219
ington state as an endangered “distinct population segment”
(“DPS”) under the Endangered Species Act (“ESA”), 16
U.S.C. § 1531 et seq. Plaintiff-appellants Northwest
Ecosystem Alliance, Center for Biological Diversity, and
Tahoma Audubon Society (collectively, the “Alliance”)
sought review of the Service’s decision in the district court,
which entered summary judgment upholding the Service’s
determination. The Alliance filed a timely notice of appeal.
We affirm.

I.    BACKGROUND

     A.   The Endangered Species Act

   Congress enacted the ESA to “provide a means whereby
ecosystems upon which endangered species and threatened
species depend may be conserved, [and] to provide a program
for the conservation of such endangered species and threat-
ened species.” 16 U.S.C. § 1531(b). The ESA requires the
Service to identify and list species that are “endangered” or
“threatened.” 16 U.S.C. § 1533. The Service may list a spe-
cies, on its own initiative, through notice-and-comment rule-
making. 16 U.S.C. § 1533(b)(5). Alternatively, a species may
become listed through the petition process provided by the
Administrative Procedure Act (“APA”), 5 U.S.C. § 553(e).
Any interested person may petition the Service to add or
remove a species from the list. Id.; 16 U.S.C. § 1533(b)(3)(A).
Upon receiving such a petition, the Service must promptly
determine whether the petition is supported by “substantial
scientific or commercial information.” 16 U.S.C. § 1533(b)
(3)(A). If so, the Service is to “commence a review of status
of the species concerned.” Id. The Service is required to make
a finding on the status of the species within twelve months
and publish its finding in the Federal Register. 16 U.S.C.
§ 1533(b)(3)(B). The Service must make its decision “solely
on the basis of the best scientific and commercial data avail-
able.” 16 U.S.C. § 1533(b)(1)(A). If the Service finds that a
petitioned action is warranted, it must promptly publish a pro-
1220       NORTHWEST ECOYSTEM ALLIANCE v. USFWS
posed regulation to implement its finding. 16 U.S.C.
§ 1533(b)(3)(B)(ii). A decision by the Service to deny a peti-
tioned action is subject to judicial review. 16 U.S.C.
§ 1533(b)(3)(C)(ii).

   The definition of the term “species” is at the heart of the
instant appeal. The ESA defines “species” to include “any
subspecies of fish or wildlife or plants, and any distinct popu-
lation segment of any species of vertebrate fish or wildlife
which interbreeds when mature.” 16 U.S.C. § 1532(16)
(emphasis added). Thus, a population of wildlife that does not
constitute a taxonomic species may nevertheless qualify for
listing as a DPS. The statute does not expressly define the
term “distinct population segment.” The Service and the
National Marine Fisheries Service (“NMFS”) have jointly
adopted a policy statement to guide their evaluation of
whether a population group should be treated as a DPS. Pol-
icy Regarding the Recognition of Distinct Vertebrate Popula-
tion Segments Under the Endangered Species Act, 61 Fed.
Reg. 4722 (Feb. 7, 1996) (“DPS Policy”). The DPS Policy
sets forth two factors for consideration: the “[d]iscreteness of
the population segment in relation to the remainder of the spe-
cies to which it belongs,” and the “significance of the popula-
tion segment to the species to which it belongs.” Id. at 4725.
Discreteness is satisfied if a population segment is “separated
from other populations of the same taxon as a consequence of
physical, physiological, ecological, or behavioral factors,” or
if a population’s boundaries are marked by international bor-
ders. Id. Significance, in turn, is analyzed under four non-
exclusive factors: (1) whether the population persists in a
unique or unusual ecological setting; (2) whether the loss of
the population would cause a “significant gap” in the taxon’s
range; (3) whether the population is the only surviving natural
occurrence of a taxon; and (4) whether the population’s
genetic characteristics are “markedly” different from the rest
of the taxon. Id. A population qualifies as a DPS if it is both
discrete and significant. Id. If a population is deemed to be a
           NORTHWEST ECOYSTEM ALLIANCE v. USFWS                1221
DPS, the inquiry then proceeds to whether it is endangered or
threatened. Id.

  B.   Western Gray Squirrels in Washington

   Sciurus griseus griseus, a subspecies of the western gray
squirrel,1 is the largest native tree squirrel in the Pacific
Northwest. Status Review and 12-Month Finding for a Peti-
tion To List the Washington Population of the Western Gray
Squirrel, 68 Fed. Reg. 34,628, 34,629 (June 10, 2003) (“Final
Finding”). Members of the subspecies are “silvery-gray with
dark flanks and creamy white underneath.” Id. They live in
trees, rarely venture into open spaces, and subsist principally
on acorn and nuts. Historically, the western gray squirrel was
widespread throughout Washington, Oregon, California, and
western Nevada. Id. at 34,630. Today, the western gray squir-
rel is fairly common in California, where it is a regulated
game species, with an estimated population of eighteen mil-
lion. Id. at 34,631. In Oregon, the subspecies is not rare and
is legally hunted, but its distribution appears to be much
reduced from historical levels. Id. at 34,632. In Nevada, the
western gray squirrel is rare and has been classified as a “pro-
tected species” under state law. Id. at 34,631.

   In Washington, the western gray squirrel once ranged from
the Puget Sound to the Columbia River, and from the Cascade
Mountains to Lake Chelan. Id. at 34,632. The population has
long been separated from the rest of the subspecies by the
Columbia River. During the last century, its distribution has
been reduced to three geographically isolated populations: the
Puget Trough population, the North Cascades population, and
the South Cascades population.

  The Puget Trough population, which is found near the
  1
   The two other subspecies are Sciurus griseus nigripes and Sciurus
griseus anthonyi. Final Finding, 68 Fed. Reg. at 34,629. Only Sciurus
griesus griesus is at issue here.
1222       NORTHWEST ECOYSTEM ALLIANCE v. USFWS
Puget Sound, lives in a transitional ecological setting. Id. at
34,633. The population’s habitat of Oregon white oak wood-
lands is nestled between upland Douglas-fir forests and prai-
ries. The habitat is wetter, flatter, and contains fewer mast-
producing trees than the rest of the subspecies’ range. Conse-
quently, the Puget Trough population is more dependent on
the Oregon white oak for sustenance than populations in eco-
logically more diverse habitats. “Although the western squir-
rel was once common on the partially wooded prairies
adjacent to the Puget Sound, the surviving Puget Trough pop-
ulation is now centered on Fort Lewis,” a military reservation.
Id. “During intensive surveys in 1998 and 1999, only 6 west-
ern grey squirrels . . . were detected in over 4,000 hours of
survey effort.” Id. Some researchers have concluded that the
Puget Trough population is “at a high risk of extirpation.” Id.

   The North Cascades population is found in Chelan and
Okanogan Counties. Id. at 34,632. Unlike the Puget Trough,
the North Cascades habitat lacks oaks, the main source of
winter foods for the western gray squirrel in most of its range.
Id. at 34,635. Instead, the North Cascades population subsists
on seeds and nuts produced by pine trees, big leaf maples, and
English walnut trees. A survey in 2000 detected only three
remnants out of the eighty-nine nests recorded in a 1996 sur-
vey, and found eighteen previously unreported nests. The
reduced number of nests suggests a corresponding population
decline.

   The South Cascades population, which constitutes the larg-
est remaining population of western gray squirrels in Wash-
ington, is found in Skamania, Klickitat, and Yakima counties.
Id. at 34,632, 34,634. One study has found western gray
squirrels in Klickitat to have substantially larger body mea-
surements than elsewhere in the subspecies’ range. Id. at
34,637. The study also concluded that the Klickitat population
have substantially larger home range sizes and more nests per
squirrel than elsewhere. Surveys in 2000 and 2001 produced
population density estimates of 0.08-0.13 squirrel per hectare
           NORTHWEST ECOYSTEM ALLIANCE v. USFWS              1223
in the Klickitat Wildlife Area, as compared with 1.37 per
hectare in Lake County, California, or with 2.47 per hectare
in Yosemite Valley in California. Id. at 34,634.

  C.   Procedural History

   On January 4, 2001, the Service received a petition filed by
the Alliance requesting an emergency rule to list the Wash-
ington population of the western gray squirrel as an endan-
gered or threatened species. On October 29, 2002, the Service
published its initial finding that the petition presented substan-
tial information to indicate that one or more distinct popula-
tion segments of western gray squirrels may exist in
Washington. 90 day Finding for a Petition To List the Wash-
ington Population of the Western Gray Squirrel as Endan-
gered or Threatened, 67 Fed. Reg. 65,931. The Service
proceeded with a twelve-month status review. An early draft
decision prepared by the Service’s staff scientists recom-
mended listing the Washington population as an endangered
DPS. However, the Service ultimately denied the petition in
a June 2003 decision published in the Federal Register. Final
Finding, 68 Fed. Reg. at 34,628. The Service determined that
the Washington population was not significant, under the DPS
Policy, to the taxon to which it belonged.

   On November 3, 2003, the Alliance filed a complaint in the
District of Oregon seeking declaratory and injunctive relief
against the Service and its officials. The Alliance contended
that the Service’s decision was arbitrary and capricious. On
August 2, 2004, the district court granted summary judgment
for the Service.

   This appeal presents two issues: (1) whether the Service’s
construction of the term “distinct population segment” is enti-
tled to Chevron deference, and if so, whether the Service’s
construction is reasonable; and (2) whether the Service’s
denial of the petition was arbitrary and capricious.
1224       NORTHWEST ECOYSTEM ALLIANCE v. USFWS
II.    JURISDICTION AND STANDARD OF REVIEW

   The district court had subject matter jurisdiction under 28
U.S.C. § 1331 and 16 U.S.C. § 1540(c) & (g). We have appel-
late jurisdiction pursuant to 28 U.S.C. § 1291.

   We review de novo the district court’s grant of summary
judgment. United States v. City of Tacoma, 332 F.3d 574, 578
(9th Cir. 2003). As discussed below, we review the Service’s
interpretation of the ESA, as expressed in the DPS Policy,
under the analytic framework laid out in Chevron U.S.A., Inc.
v. Natural Res. Def. Council, Inc., 467 U.S. 837 (1984). We
review the Service’s decision on the Washington gray squirrel
under the APA, which provides that an agency action may be
set aside only if it is “arbitrary, capricious, an abuse of discre-
tion, or otherwise not in accordance with law.” 5 U.S.C.
§ 706(2)(A). This standard of review is “highly deferential,
presuming agency action to be valid and affirming the agency
action if a reasonable basis exists for its decision.” Indepen-
dent Acceptance Co. v. California, 204 F.3d 1247, 1251 (9th
Cir. 2000) (citations omitted). We may not consider informa-
tion outside of the administrative record, Love v. Thomas, 858
F.2d 1347, 1356 (9th Cir. 1988), and may not “substitute [our]
judgment for that of the agency.” Citizens To Preserve Over-
ton Park v. Volpe, 401 U.S. 402, 416 (1971), abrogated on
other grounds by Califano v. Sanders, 430 U.S. 99, 105
(1977). Our task is simply to ensure that the agency “consid-
ered the relevant factors and articulated a rational connection
between the facts found and the choices made.” Nat’l Ass’n
of Home Builders v. Norton, 340 F.3d 835, 841 (9th Cir.
2003) (quoting Baltimore Gas & Elec. Co. v. Natural Res.
Def. Council, Inc., 462 U.S. 87, 105 (1983)); see also Blue
Mountain Biodiversity Project v. Blackwood, 161 F.3d 1208,
1211 (9th Cir. 1998) (court must determine whether the
agency decision was “based on a consideration of the relevant
factors”) (citation omitted).
             NORTHWEST ECOYSTEM ALLIANCE v. USFWS                       1225
III.    DISCUSSION

  A.     Whether the DPS Policy Is Entitled to Chevron
         Deference

   The Alliance contends that the DPS Policy’s requirement
that a population be significant to its taxon is unlawfully
restrictive. The Alliance does not seek to invalidate the DPS
Policy on its face, but only as applied here.2

   A court reviewing an administrative interpretation of a stat-
ute must first ascertain whether Congress has spoken clearly
on the issue. Chevron, 467 U.S. at 842-44. If the statute is
clear, we “must give effect to the unambiguously expressed
intent of Congress” regardless of the agency’s view. Id. at
843. If the statute is ambiguous, however, we do not simply
impose our own independent interpretation. Id. Rather, we
must determine how much deference to give to the adminis-
trative interpretation. Id.; United States v. Mead Corp., 533
U.S. 218, 227-31 (2001). The precise degree of deference
warranted depends on the statute and agency action at issue.
Mead, 533 U.S. at 227-31. Under Chevron’s classic formula-
tion,

       [i]f Congress has explicitly left a gap for an agency
       to fill, there is an express delegation of authority to
  2
    We may nevertheless consider the general validity of the element of
the DPS Policy — i.e., the significance requirement — that was used by
the Service to deny the Washington gray squirrel a listing as an endan-
gered DPS. We may not, however, evaluate elements of the DPS Policy
that were not applied against the Alliance. We note that the Service does
not argue on appeal that the validity of the significance requirement can-
not be evaluated in an as-applied suit. We also note that, before the district
court, the Service did not challenge the Alliance’s first claim for relief. It
is this claim, that the Service “had a nondiscretionary duty to list the
Washington populations of the western gray squirrel under the ESA upon
determining they are imperiled distinct population segments,” that objects
to the existence of the significance requirement rather than merely its
application to the Washington gray squirrels.
1226       NORTHWEST ECOYSTEM ALLIANCE v. USFWS
    the agency to elucidate a specific provision of the
    statute by regulation. Such legislative regulations are
    given controlling weight unless they are arbitrary,
    capricious, or manifestly contrary to the statute.

467 U.S. at 844. If Chevron deference is inapplicable because
Congress has not delegated interpretative authority to the
agency, the agency’s views still “constitute a body of experi-
ence and informed judgment to which courts and litigants
may properly resort for guidance.” Skidmore v. Swift & Co.,
323 U.S. 134, 140 (1944). The “fair measure of deference”
may then range from “great respect” to “near indifference,”
depending on “the degree of the agency’s care, its consis-
tency, formality, and relative expertness, and . . . the persua-
siveness of the agency’s position.” Mead, 533 U.S. at 228.

   [1] The ESA does not expressly define “distinct population
segment,” and the parties agree that the term has no generally
accepted scientific meaning. Because the statutory term is
elastic, we must decide whether the DPS Policy is entitled to
deference under the Chevron standard, or under the less defer-
ential Skidmore standard.

   [2] Chevron deference applies “when it appears that Con-
gress delegated authority to the agency generally to make
rules carrying the force of law, and that the agency interpreta-
tion claiming deference was promulgated in the exercise of
that authority.” Mead, 533 U.S. at 226-27. “[D]elegation of
such authority may be shown in a variety of ways, as by an
agency’s power to engage in adjudication or notice-and-
comment rulemaking, or by some other indication of a com-
parable congressional intent.” Id. at 227. “It is fair to assume
generally that Congress contemplates administrative action
with the effect of law when it provides for a relatively formal
administrative procedure tending to foster the fairness and
deliberation that should underlie a pronouncement of such
force.” Id. at 230. In the ESA, Congress expressly delegated
authority to the Service to develop criteria for evaluating peti-
             NORTHWEST ECOYSTEM ALLIANCE v. USFWS                     1227
tions to list endangered species. Under 16 U.S.C.
§ 1533(h)(2), the Service is required to publish, in the Federal
Register, guidelines on “criteria for making findings . . . with
respect to petitions.” The Service must also “provide to the
public notice of, and opportunity to submit written comments
on, any guideline (including amendment thereto) proposed to
be established under this subsection.” 16 U.S.C. § 1533(h). In
substance, the formality § 1533(h) requires for policy state-
ments is indistinguishable from notice-and-comment rulemak-
ing under the APA. Compare 16 U.S.C. § 1533(h) with 5
U.S.C. § 553.3 This fact weighs in favor of affording Chevron
deference.

   The Alliance contends that the DPS Policy is an informal
policy statement that lies “beyond the Chevron pale.” Mead,
533 U.S. at 234; see Christensen v. Harris County, 529 U.S.
576, 587 (2000) (“[I]nterpretations contained in policy state-
ments, agency manuals, and enforcement guidelines, all of
which lack the force of law[,] do not warrant Chevron-style
deference.”). However, one important reason for denying
Chevron deference to policy statements is that they are gener-
ally exempt from the public notice-and-comment procedures
required by § 553 of the APA. 5 U.S.C. § 553(b)(3)(A); cf.
Mead, 533 U.S. at 232-234 (denying Chevron deference to
letter rulings adopted without public notice and comment);
Christensen, 529 U.S. at 587 (advisory opinion letter adopted
without notice and comment); Reno v. Koray, 515 U.S. 50, 61
(1995) (internal agency guideline adopted without notice and
comment). In contrast, § 1533(h) of the ESA expressly
requires public notice and comment for both the creation and
the modification of the DPS Policy. These procedural rigors,
combined with the express congressional command to the
Service to develop guidelines,4 distinguish the DPS Policy
  3
    The only noteworthy difference is that whereas § 553(e) of the APA
gives interested citizens the right to petition for the issuance, amendment,
or repeal of a rule, § 1533(h) of the ESA does not expressly recognize
such a right for guidelines.
  4
    Cf. EEOC v. Arabian American Oil Co., 499 U.S. 244, 256-58 (1991)
(no Chevron deference to agency guideline adopted without express con-
gressional delegation of authority).
1228         NORTHWEST ECOYSTEM ALLIANCE v. USFWS
from garden-variety policy statements that do not enjoy Chev-
ron status.5

   [3] Notwithstanding the robust process through which the
DPS Policy emerged, the Alliance submits that Chevron def-
erence is inappropriate because the DPS Policy does not have
the “force of law.” Mead, 533 U.S. at 227. The Alliance
emphasizes that the Service had considered adopting the DPS
Policy as a rule, but ultimately decided to adopt it as a policy
statement instead.6 This argument is unpersuasive. The DPS
Policy was not formulated in response to any party’s petition
but rather as a definitive statement of how the Service would
conduct all future “evaluation[s] of distinct vertebrate popula-
tion segments for the purposes of listing, delisting, and reclas-
sifying under the Act.” 61 Fed. Reg. at 4725. The Alliance
has presented no evidence that the DPS Policy has ever been
treated (by the Service or parties presenting petitions to list
species) as anything other than legally binding. We therefore
hold that the DPS Policy is entitled to Chevron deference.

  B.    Whether the DPS Policy Is a Reasonable Construction
        of the ESA

   An agency interpretation that enjoys Chevron status must
be upheld if it is based on a reasonable construction of the
statute. Chevron, 467 U.S. at 843-45. The Alliance argues that
the DPS Policy cannot withstand scrutiny even under the def-
erential Chevron standard.
  5
     The district court case that the Alliance cites involving the Habitat
Conservation Planning Handbook, National Wildlife Federation v. Bab-
bitt, 128 F. Supp. 2d 1274 (E.D. Cal. 2000), is inapt. The case never states
that the Handbook was issued pursuant to § 1533(h), but does say that
even though “[t]he Secretary argues that the HCP Handbook is not legally
binding,” “[t]here is room for argument on this point.” Id. at 1292. The
Handbook was also not pivotal to the case so the district court had no
occasion to examine fully the deference to which the Handbook is entitled.
   6
     The Service’s internal deliberation on whether to adopt a definition of
“distinct population segment” as a rule or as a policy is documented in the
record. (Excerpts of Record 375-76, 405, 407.)
           NORTHWEST ECOYSTEM ALLIANCE v. USFWS                1229
   [4] First, the Alliance challenges the DPS Policy’s require-
ment that a population be both discrete and significant in
order to qualify as a “distinct population segment.” The Alli-
ance asserts that the words “distinct” and “discrete” are syno-
nyms. Webster’s Third New International Dictionary 659
(2002). Thus, they contend, any requirement that a population
be significant in addition to being discrete is an additional
hurdle not contemplated by the statute. However, the term
“distinct” is not as limited in meaning as the Alliance sug-
gests. “Distinct” can mean “notable” or “unusual.” Id. It is not
inconsistent with common usage, nor is it unreasonable, for
the Service to construe “distinct” to mean both “discrete,” in
the sense of being separate from others, and “significant,” in
the sense of being notable.

   [5] Second, the Alliance claims that the significance
requirement conflates separate statutory definitions. The term
“significant” appears in the ESA’s definition of “endangered
species,” which is defined as “any species which is in danger
of extinction throughout all or a significant portion of its
range . . . .” 16 U.S.C. § 1532(6). The DPS Policy incorpo-
rates a separate significance requirement into the definition of
“distinct population segment,” which is in turn part of the
statutory definition of “species,” 16 U.S.C. § 1532(16). The
Alliance argues that the DPS Policy conflates the statutory
definitions of “species” and “endangered species,” reducing
the latter to mere surplusage. Actually, the two significance
requirements serve different functions. The significance
requirement in the DPS Policy pertains to whether a popula-
tion qualifies as a species, while significance in § 1532(6)
relates to whether a species is endangered. The two terms
overlap to some extent in application,7 but they are not identi-
cal. For example, a population may be significant under the
DPS Policy if it has distinctive ecological or biological traits,
  7
   See Nat’l Ass’n of Home Builders v. Norton, 340 F.3d 835, 848 (9th
Cir. 2003) (noting similarity between the DPS Policy and § 1532(6)).
1230         NORTHWEST ECOYSTEM ALLIANCE v. USFWS
but that has no bearing on whether the population is actually
in danger of extinction for purposes of § 1532(6).

   [6] Third, the Alliance argues that the DPS Policy reflects
an impermissibly narrow understanding of the ESA’s purpose
and focuses excessively on conserving genetic resources. In
the Alliance’s view, the Service’s attention to genetic
resources is inappropriate in light of Congress’s finding,
expressed in the ESA’s preamble, that wildlife have “esthetic,
ecological, educational, historical, recreational, and scientific
value.” 16 U.S.C. § 1531(a)(3). The DPS Policy actually
describes the ESA’s purposes as follows: “The Services
understand the Act to support interrelated goals of conserving
genetic resources and maintaining natural systems and bio-
diversity over a representative portion of their historic occur-
rence. The draft policy was intended to recognize both these
intentions, but without focusing on either to the exclusion of
the other.” 61 Fed. Reg. at 4723. That statement is not incon-
sistent with the ESA’s expressly stated goal of providing “a
means whereby the ecosystems upon which endangered spe-
cies and threatened species depend may be conserved.” 16
U.S.C. § 1531(b). In addition, the significance requirement
(which is the only portion of the DPS Policy to which the
Alliance objects) may be satisfied not only by evidence of
genetic differences but also by a population’s persistence in
unusual ecological settings, its status as the only natural sur-
viving occurrence of a taxon, or evidence that its loss would
result in a significant gap in its taxon’s range. These are not
the hallmarks of a policy that focuses on the conservation of
genetic resources at the expense of all the other goals of the
ESA.8
   8
     Moreover, even if the DPS Policy privileges the conservation of
genetic resources, there is ample support for such an approach in the legis-
lative history. Although the term “distinct population segment” was not
added to the ESA until 1978, that language is consistent with Congress’
original intent in passing the ESA to preserve genetic resources. See H.R.
Rep. No. 93-412, at 5 (1973) (“From the most narrow point of view, it is
in the best interests of mankind to minimize the losses of genetic varia-
tions. The reason is simple: they are potential resources.”).
           NORTHWEST ECOYSTEM ALLIANCE v. USFWS             1231
   [7] Fourth, the Alliance argues that the Service, in formu-
lating the DPS Policy, improperly considered congressional
policy preferences expressed after the enactment of the ESA.
After the ESA was amended to encompass distinct population
segments, the General Accounting Office proposed that Con-
gress repeal the amendment to prevent a proliferation of
endangered species listings. Although Congress declined to
adopt the proposal, a Senate committee report cautioned that
it was “aware of the great potential for abuse” of the Service’s
DPS authority. S. Rep. No. 96-151, at 7 (1979). The report
further stated an expectation that the Service would “use the
ability to list populations sparingly and only when the biolog-
ical evidence indicates that such action is warranted.” Id.
(emphasis added). The DPS Policy expressed an intent to fol-
low that instruction. 61 Fed. Reg. at 4722. The Alliance con-
tends that the policy views of a subsequent Congress should
not be considered in interpreting a statutory term. The Alli-
ance’s position might be more persuasive if we were constru-
ing the statutory term on a blank slate, but that is not the
situation here. We are reviewing an agency determination
under the deferential Chevron standard. So long as the agency
action is not manifestly contrary to the statute, it is not
improper for the agency to consider the views of the elected
branches in interpreting an ambiguous statutory term. See
Chevron, 467 U.S. at 866 (“[F]ederal judges — who have no
constituency — have a duty to respect the legitimate policy
choices made by those who do.”). Here the committee
report’s language was not in obvious tension with the statu-
tory text and the Service did not err by relying on it in part.

   Fifth, the Alliance contends that the DPS Policy has been
applied inconsistently, pointing to National Association of
Home Builders as support for its claim. However, the western
pygmy-owl’s preliminary listing in that case took place before
the DPS Policy was issued in 1996 and thus cannot be used
to show inconsistency in the policy’s application. 340 F.3d at
839. Similarly, the bald eagle, grizzly bear, and gray wolf list-
ings that the Alliance cites all took place well before 1996
1232        NORTHWEST ECOYSTEM ALLIANCE v. USFWS
(the last of the three was in 1978) and hence shed no light on
the faithfulness of the Service’s adherence to the DPS Policy.9
The Alliance also offers recent examples of discrete, endan-
gered populations that were not granted protection by the Ser-
vice (because they failed the significance requirement) — but,
crucially, offers no examples of populations that were granted
protection under the DPS Policy despite being found insignifi-
cant to their taxon.

  [8] For all these reasons, the DPS Policy is a reasonable
construction of “distinct population segment.”

  C.    Whether the Service’s Denial of the Petition Was
        Arbitrary and Capricious

  Applying the DPS Policy, the Service found that western
gray squirrels in Washington constitute a discrete population,
but are not significant to the taxon. It therefore denied the
petition. The Alliance vigorously challenges the Service’s
determination on the “significance” prong.

   We note that the Service’s internal draft finding of May 16,
2003 recommended granting the petitioned action. The Alli-
ance complains that the Service’s final finding reached the
opposite conclusion without citing any new data. However,
the Service may change its mind after internal deliberation.
See Southwestern Ctr. for Biological Diversity v. Bureau of
Reclamation, 143 F.3d 515, 523 (9th Cir. 1998). The only
question before us is whether the Service, in reaching its ulti-
mate finding, “considered the relevant factors and articulated
a rational connection between the facts found and the choices
made.” Nat’l Ass’n of Home Builders, 340 F.3d at 841.
  9
   See Determination of Certain Bald Eagle Populations as Endangered or
Threatened, 43 Fed. Reg. 6230 (Feb. 14, 1978); Amendment Listing the
Grizzly Bear of the 48 Conterminous States as a Threatened Species, 40
Fed. Reg. 31,734 (July 28, 1975); Endangered Species, 32 Fed. Reg. 4001
(Mar. 11, 1967) (listing the gray wolf).
             NORTHWEST ECOYSTEM ALLIANCE v. USFWS                     1233
  The DPS Policy sets forth the following four factors to be
used to determine a population’s significance to its taxon:

          1. Persistence of the discrete population segment
       in an ecological setting unusual or unique for the
       taxon,

         2. Evidence that loss of the discrete population
       segment would result in a significant gap in the
       range of a taxon,

          3. Evidence that the discrete population repre-
       sents the only surviving natural occurrence of a
       taxon that may be more abundant elsewhere as an
       introduced population outside its historic range, or

          4. Evidence that the discrete population segment
       differs markedly from other populations of the spe-
       cies in its genetic characteristics.

61 Fed. Reg. at 4725. The Service found that the first, second,
and fourth factors warranted analysis and that none of those
factors established the Washington gray squirrels’ signifi-
cance to their taxon. The Alliance argues that the Service’s
findings on those factors were arbitrary and capricious, and
not supported by the scientific evidence in the record.

       1.   Ecological Setting

   [9] First, the Alliance challenges the Service’s finding that
the North Cascades and the Puget Trough do not constitute
unusual or unique ecological settings for the taxon.10 The
North Cascades habitat is notable for its lack of oak trees,
which are the main source of winter foods in most of the sub-
  10
    The Alliance does not dispute the Service’s finding that the South
Cascades habitat is similar to habitats in Oregon, and therefore not unique.
Final Finding, 68 Fed. Reg. at 34636.
1234       NORTHWEST ECOYSTEM ALLIANCE v. USFWS
species’ range. Gray squirrels in the North Cascades, as noted,
subsist primarily on the seeds of pine trees. The Alliance
believes that the lack of oak trees compels the finding that the
North Cascades habitat is a unique ecological setting. How-
ever, the Service offered reasonable grounds for its contrary
conclusion. While recognizing that oak trees are absent from
the North Cascades, the Service emphasized that “throughout
their range, western gray squirrels consume a variety of types
of tree seeds, including many conifer species.” Final Finding,
68 Fed. Reg. at 34,636. In other words, the North Cascades
habitat is not unique, because the squirrels there consume
conifer tree seeds just as they do in a variety of habitats. That
reasoning is not arbitrary or capricious.

   [10] The Alliance also attacks the Service’s finding on the
Puget Trough habitat. Unlike the North Cascades, the Puget
Trough is notable for its concentration of oak trees, which
makes the Puget Trough’s vegetation more homogenous than
elsewhere in the subspecies’ range. In contrast, “[e]lsewhere
in the subspecies’ range, Oregon white oaks occur in commu-
nities having a wider range of mast-producing tree species,
including a variety of oak and pine species.” Id. at 34,636.
While recognizing the concentration of oak in the Puget
Trough habitat, the Service concluded that the difference
between the Puget Trough and other habitats “are not so
great” as to constitute a “unique or unusual ecological setting
for the western gray squirrel.” Id. Although the Service could
have explained its reasoning in more detail, it is clear in con-
text that the Service had in mind the widespread persistence
of the Oregon white oak throughout the subspecies’ range.
Because it is undisputed that Oregon white oaks are not
unique to the Puget Trough habitat, the Service’s conclusion
was not arbitrary or capricious.

    2.   Significant Gap in the Range

  The Alliance also disputes the Service’s finding that a
hypothetical loss of the entire Washington population would
           NORTHWEST ECOYSTEM ALLIANCE v. USFWS            1235
not cause a significant gap in the range of the taxon. For pur-
poses of the “gap in the range” analysis, the term “significant”
has “its ‘commonly understood meaning,’ which is ‘impor-
tant.’ ” Nat’l Ass’n of Home Builders, 340 F.3d at 846 (cita-
tions omitted).

   The Service’s discussion of this factor is not a paragon of
clarity. Nonetheless, the Service’s reasoning can be discerned
with careful reading. See id. (the court may uphold agency
decisions “of less than ideal clarity if the agency’s path may
reasonably be discerned” (citation omitted)). At the outset, the
Service noted that Washington gray squirrels constitute an
isolated, peripheral population at the northern portion of the
subspecies’ range. Final Finding, 68 Fed. Reg. at 34,636. As
a general matter, peripheral populations often face ecological
circumstances not found elsewhere in the taxon’s range, and
may consequently develop distinctive morphological, behav-
ioral, or genetic characteristics through adaptation to local
conditions. The Service then considered whether the Wash-
ington population had developed such distinctive qualities.

   First, the Service considered evidence suggesting that
Washington gray squirrels may be more shy and secretive —
as they are rarely seen and often flee from human observers
— than their counterparts in Oregon and California. Id. at
34,637. The Service found the evidence to be indeterminate.
The Service noted that the evidence was anecdotal, and that
there are no comparative studies on elusive behavior across
the range of the species. The Service also cited evidence that
secretive behavior was not unique to Washington squirrels,
because similar behavior had also been observed among Ore-
gon squirrels. The Service further stated that even if such
behavior does indeed characterize Washington squirrels, there
was no evidence that it was caused by adaptation to a periph-
eral habitat. In the Service’s view, behavioral differences, if
any, could simply be attributable to the larger populations
south of the Columbia River and their adaptability and prox-
imity to urban areas. Id.
1236       NORTHWEST ECOYSTEM ALLIANCE v. USFWS
   The Service next considered evidence on morphology and
home range size. A study of gray squirrels in Klickitat
County, part of the South Cascades population, found them to
be significantly larger in body dimension than elsewhere in
the subspecies’ range. Id. at 34,637. The same study also
found that the gray squirrels in Klickitat County had substan-
tially larger home range size when compared with elsewhere
in the subspecies’ range. The Service, however, declined to
credit the results of the study. The Service noted that the study
was based on a small sample size — which the record showed
to be fewer than fifty squirrels — in a small area of Klickitat
county. The Service further theorized that variations in mea-
surement methods, rather than actual differences, might
account for the observed results in home range size. Even if
the results were accurate, the Service could discern no basis
for attributing them to the peripheral location of Washington
gray squirrels.

   The Service ultimately determined that, while a hypotheti-
cal loss of the Washington population would “represent a seri-
ous reduction in the species[’] range,” it would not be of
biological and ecological significance to the taxon as a whole.
Id. Put differently, the Service believed that any gap caused
by the loss of the Washington population would not be signif-
icant because the population lacks biologically distinctive
traits.

   The Alliance complains that the Service dismissed evi-
dence purely on the basis of scientific uncertainty. The ESA
instructs the Service to make its determinations “solely on the
basis of the best scientific and commercial data available,” 16
U.S.C. § 1533(b)(1)(A), and the Service may not ignore evi-
dence simply because it falls short of absolute scientific cer-
tainty. See Defenders of Wildlife v. Babbit, 958 F. Supp. 670,
679-80 (D.D.C. 1997) (Service applied wrong legal standard
in dismissing scientific evidence because it was not “conclu-
sive”); Center for Biological Diversity v. Lohn, 296 F. Supp.
2d 1223, 1236-40 (W.D. Wash. 2003) (NMFS was capricious
           NORTHWEST ECOYSTEM ALLIANCE v. USFWS             1237
and arbitrary in relying on outdated taxonomic classifications
which the best available science showed to be incorrect). We
are unpersuaded that the Service was justified in rejecting the
secretive behavior evidence and Klickitat County study solely
because they were anecdotal. Of course a rigorous, large-scale
study of Washington gray squirrels’ behavior and morphology
would be preferable, but in the absence of such a study, credi-
ble anecdotal evidence represents the “best scientific . . . data
available” and cannot be ignored. Nevertheless, we hold that
the Service did not arbitrarily and capriciously fail to find a
significant gap on the basis of such data. Similarly shy and
secretive behavior has been documented among Oregon squir-
rels, and the Klickitat County study was limited geographi-
cally to a subset of one of the three Washington habitats of the
gray squirrel. While crediting the anecdotal evidence, the Ser-
vice could have concluded that the extirpation of squirrels
whose behavior is not unique and whose larger size is known
to exist only in a single county would not create a significant
gap in the taxon.

   The Alliance further challenges the Service’s determination
that a “serious reduction” in the subspecies’ range does not
amount to a “significant gap” due to the absence of biologi-
cally distinctive traits in the Washington population. See Final
Finding, 68 Fed. Reg. at 34,637. The Alliance analogizes the
“significant gap” factor in the DPS Policy to 16 U.S.C.
§ 1532(6), which defines an “endangered species” as a species
that is “in danger of extinction throughout all or a significant
portion of its range.” We have recognized that a species can
be considered extinct throughout a significant portion of its
range “if there are major geographical areas in which it is no
longer viable but once was.” Defenders of Wildlife v. Norton,
258 F.3d 1136, 1145 (9th Cir. 2001). It does not follow that
a serious reduction in the western gray squirrel’s geographic
range similarly suffices to satisfy the “significant gap” factor.
Unlike § 1532(6), the “significance” inquiry under the DPS
Policy is not limited to geographic factors. On its face, the
DPS Policy considers ecological, historical, and genetic fac-
1238         NORTHWEST ECOYSTEM ALLIANCE v. USFWS
tors in addition to geography. Nothing in the DPS Policy or
in the ESA limits the Service’s significant gap inquiry to geo-
graphic factors.

   [11] The Alliance counters that previous administrative
applications of the DPS Policy have found “significant gaps”
in the range of taxa solely on account of geographic factors.
The Alliance asserts that the Service’s current refusal to do
the same for Washington gray squirrels is arbitrary and capri-
cious. See National Cable & Telecomm. Ass’n v. Brand X
Internet Servs., 545 U.S. 967, 125 S. Ct. 2688, 2699 (2005)
(“Unexplained inconsistency is, at most, a reason for holding
an interpretation to be an arbitrary and capricious change
from agency practice”). The Alliance cites four prior adminis-
trative decisions. Three actually considered biological factors
as part of their “significant gap” analyses.11 Only one of the
four relied solely on geography to find that the population’s
extirpation would cause a significant gap.12 In practice, the
   11
      See Final Endangered Status for a Distinct Population Segment of
Smalltooth Sawfish (Pristis pectinata) in the United States, 68 Fed. Reg.
15,674, 15,675-76 (Apr. 1, 2003) (finding that loss of U.S. smalltooth
sawfish population would create a significant gap partly because the popu-
lation “comprises an important component of the sawfishes’ remaining
global biological diversity”); Final Rule to List the Columbia Basin Dis-
tinct Population Segment of the Pygmy Rabbit (Brachylagus idahoensis)
as Endangered, 68 Fed. Reg. 10,388, 10,397-98 (Mar. 5, 2003) (finding
that loss of Columbia Basin pygmy rabbits would constitute a significant
gap partly because the population is experiencing increased “directional
selection” and “exhibiting genetic consequences of long-term isolation”);
Final Endangered Status for a Distinct Population Segment of Anadro-
mous Atlantic Salmon (Salmo salar) in the Gulf of Maine, 65 Fed. Reg.
69,459, 69,460 (Nov. 17, 2000) (finding that loss of the Gulf of Maine
population of Atlantic salmon would “negatively affect the genetic
resources of Atlantic salmon as a whole because it would contribute to fur-
ther range reduction”).
   12
      Determination of Threatened Status for the Northern Population of the
Copperbelly Water Snake, 62 Fed. Reg. 4183, 4184 (January 29, 1997)
(“The loss of the peripheral, isolated, northern population [of copperbelly
water snakes] is considered as significant as characterized under policy, as
it would result in a significant reduction in the range of the taxon.”) .
             NORTHWEST ECOYSTEM ALLIANCE v. USFWS                   1239
Service has regarded the “significant gap” inquiry as a flexi-
ble one, and has considered various non-geographic factors on
different occasions. Here, the Service has articulated a reason-
able basis — the Washington population’s lack of biologically
and ecologically distinguishing features — for its conclusion
that the loss of the population would not cause a significant
gap. We cannot say that the Service’s analysis, which is in
fact substantially more detailed than those cited by the Alli-
ance, is an arbitrary and capricious departure from prior prac-
tice.

       3.   Marked Genetic Differences

   The third factor the Service considered was whether the
Washington population’s genetic characteristics differ “mark-
edly” from the remainder of the subspecies’ range. “Under the
DPS Policy, ‘markedly’ is given its common meaning, which
in this context is ‘appreciably.’ ” Nat’l Ass’n of Home Build-
ers, 340 F.3d at 851. The parties do not dispute that some
genetic differences exist between Washington gray squirrels
and the rest of the taxon in Oregon and California. The ques-
tion is whether their genetic profiles differ “markedly.”

   The Service based its decision on a peer-reviewed study of
genetic differences among gray squirrel populations in Wash-
ington, Oregon, and California. Final Finding, 68 Fed. Reg. at
34,638. The study was carried out by Kenneth I. Warheit, a
senior research scientist with the Washington Department of
Fish and Wildlife. Dr. Warheit conducted microsatellite DNA
analysis on samples from 3 California gray squirrels, 24 Ore-
gon squirrels, and 101 Washington squirrels.13 He analyzed
the samples’ alleles, which are series of two or more different
genes that occupy the same position on a chromosome. He
  13
    “Microsatellites are short (no longer than six base pairs (nitrogenous
bases that are part of the DNA molecule, such as cytosine and guanine))
tandemly repeated segments interspersed throughout the chromosome.”
Final Finding, 68 Fed. Reg. at 34,638.
1240        NORTHWEST ECOYSTEM ALLIANCE v. USFWS
found that each population in California and Oregon showed
at least three “private alleles” that are present only in that
population. In contrast, no Washington population had private
alleles. In other words, “all alleles present in each of the
Washington population are also present in at least one of the
Oregon or California populations.” Id. At the same time, he
found reduced genetic diversity in the Washington popula-
tions compared with Oregon and California, and “considera-
bly more genetic differentiation between Washington and
Oregon or California[ ] than there is between Oregon and Cal-
ifornia populations.” Id. He then conducted mitochondrial
control region sequence analysis on a subset of 67 squirrels.14
He determined that no haplotype — which is the set of one
allele of each gene, and which comprises the genetic constitu-
tion of an individual or taxon — was shared across the
Columbia river. Mitochondrial DNA analysis also demon-
strated that the Washington population had substantially less
genetic diversity. Dr. Warheit found only 3 haplotypes from
40 Washington squirrels, compared with 14 haplotypes from
27 California and Oregon squirrels.

   After reviewing Dr. Warheit’s study, the Service concluded
that “[w]hile there is clearly some genetic information that
shows that the Washington populations are different from
other populations . . . at this time we do not believe them to
be markedly so.” Id. at 34,639. The Service explained that
evidence of genetic differentiation is “counterbalanced” by
the fact that the Washington population has no private alleles,
“that some haplotypes in Washington are more closely related
to haplotypes in Oregon than other haplotypes in Washing-
ton,” and “the fact that the Washington populations . . . show
reduced genetic diversity.” Id. The Service further explained
that the results of the study may have been confounded by
  14
    “Mitochondria are structures in the cell, but outside of the nucleus,
which contains DNA inherited only from the mother.” Final Finding, 68
Fed. Reg. at 34638. In contrast, microsatellite DNA is inherited from both
parents.
           NORTHWEST ECOYSTEM ALLIANCE v. USFWS            1241
“the effects of small population size and the consequent
inbreeding and genetic drift” in Washington. Id. The Service
also noted that the study used a small sample size for the Cali-
fornia population.

   [12] The Alliance contends that the presence of unique
haplotypes in the Washington population is sufficient to
establish marked genetic differentiation, notwithstanding the
fact that the population has no private alleles. However, we
must defer to the agency’s interpretation of complex scientific
data. United States v. Alpine Land and Reservoir Co., 887
F.2d 207, 213 (9th Cir. 1989) (“Deference to an agency’s
technical expertise and experience is particularly warranted
with respect to questions involving engineering and scientific
matters.”). We decline the Alliance’s invitation to impose our
own view on whether alleles or haplotypes constitute the best
markers of genetic differentiation.

   Next, the Alliance maintains that reduced genetic diversity
in Washington is itself evidence of the population’s genetic
distinctiveness. Nothing in the DPS Policy or in the ESA
compels the Service to focus on relative genetic diversity,
rather than on the presence or absence of unique alleles, as the
best indicators of genetic differentiation. As an internal Ser-
vice memorandum explained, because “there was no evidence
from the analysis that [Washington] populations possessed
unique microsatellite alleles . . . most of the differentiation
[between Washington and the rest of the range] comes from
reduced numbers of alleles” in Washington. Whether the Ser-
vice was correct to focus on the alleles is beside the point;
interpretation of complex genetic data falls within the domain
of the Service’s scientific discretion, to which we must defer
so long as the Service has articulated a rational basis for its
conclusion.

   Lastly, the Alliance argues that whether some haplotypes in
Washington are closer to haplotypes in Oregon than to others
in Washington is irrelevant for the question of overall genetic
1242      NORTHWEST ECOYSTEM ALLIANCE v. USFWS
differences between Washington and Oregon. This argument
ignores the inference that if all of the Washington haplotypes
were closer to each other than to haplotypes in California and
Oregon, the overall genetic profile of the Washington popula-
tion would be more distinct.

   [13] Because the Service has articulated reasoned connec-
tions between the record and its conclusion, its genetic analy-
sis was not arbitrary or capricious.

IV.    CONCLUSION

   [14] As set forth above, the DPS Policy is entitled to Chev-
ron deference. Under the Chevron standard, the DPS Policy
was a reasonable construction of the ESA. The Service’s deci-
sion denying the petition was not arbitrary or capricious.

  AFFIRMED.
