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 D.C. No.

SERVICE; DAVE ALLEN, Regional  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
COUNSEL

Brent Plater, Center for Biological Diversity, San Francisco,

California; Stephanie M. Parent, Pacific Environmental Advocacy 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 Wash1218 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
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 threatened 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 species, on its own initiative, through notice-and-comment rulemaking. 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 available.” 16 U.S.C. § 1533(b)(1)(A). If the Service finds that a

petitioned action is warranted, it must promptly publish a proNORTHWEST ECOYSTEM ALLIANCE v. USFWS 1219
posed regulation to implement its finding. 16 U.S.C.

§ 1533(b)(3)(B)(ii). A decision by the Service to deny a petitioned 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 population 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. Policy Regarding the Recognition of Distinct Vertebrate Population 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 species to which it belongs,” and the “significance of the population 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 borders. Id. Significance, in turn, is analyzed under four nonexclusive 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

1220 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
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 Petition 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 squirrel is fairly common in California, where it is a regulated

game species, with an estimated population of eighteen million. 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 “protected 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

1The 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. 

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1221
Puget Sound, lives in a transitional ecological setting. Id. at

34,633. The population’s habitat of Oregon white oak woodlands is nestled between upland Douglas-fir forests and prairies. The habitat is wetter, flatter, and contains fewer mastproducing trees than the rest of the subspecies’ range. Consequently, the Puget Trough population is more dependent on

the Oregon white oak for sustenance than populations in ecologically more diverse habitats. “Although the western squirrel was once common on the partially wooded prairies

adjacent to the Puget Sound, the surviving Puget Trough population is now centered on Fort Lewis,” a military reservation.

Id. “During intensive surveys in 1998 and 1999, only 6 western 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 survey, and found eighteen previously unreported nests. The

reduced number of nests suggests a corresponding population

decline. 

The South Cascades population, which constitutes the largest remaining population of western gray squirrels in Washington, 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 measurements 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

1222 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
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 Washington population of the western gray squirrel as an endangered or threatened species. On October 29, 2002, the Service

published its initial finding that the petition presented substantial information to indicate that one or more distinct population segments of western gray squirrels may exist in

Washington. 90 day Finding for a Petition To List the Washington Population of the Western Gray Squirrel as Endangered 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 recommended 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 entitled 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. 

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1223
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 appellate 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 discretion, 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.” Independent Acceptance Co. v. California, 204 F.3d 1247, 1251 (9th

Cir. 2000) (citations omitted). We may not consider information 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 Overton 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 “considered 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). 

1224 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
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 statute 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 administrative 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 formulation, 

[i]f Congress has explicitly left a gap for an agency

to fill, there is an express delegation of authority to

2We 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 endangered 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 cannot 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. 

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1225
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 experience 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 consistency, formality, and relative expertness, and . . . the persuasiveness 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 deferential Skidmore standard. 

[2] Chevron deference applies “when it appears that Congress delegated authority to the agency generally to make

rules carrying the force of law, and that the agency interpretation 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-andcomment rulemaking, or by some other indication of a comparable 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 peti1226 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
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 statements is indistinguishable from notice-and-comment rulemaking 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 statements, 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 generally 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

3The 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. 

4Cf. EEOC v. Arabian American Oil Co., 499 U.S. 244, 256-58 (1991)

(no Chevron deference to agency guideline adopted without express congressional delegation of authority). 

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1227
from garden-variety policy statements that do not enjoy Chevron status.5

[3] Notwithstanding the robust process through which the

DPS Policy emerged, the Alliance submits that Chevron deference 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 population segments for the purposes of listing, delisting, and reclassifying 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 deferential Chevron standard. 

5The district court case that the Alliance cites involving the Habitat

Conservation Planning Handbook, National Wildlife Federation v. Babbitt, 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.

6The 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.) 

1228 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
[4] First, the Alliance challenges the DPS Policy’s requirement that a population be both discrete and significant in

order to qualify as a “distinct population segment.” The Alliance asserts that the words “distinct” and “discrete” are synonyms. 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 suggests. “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 incorporates 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 population 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 identical. 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)). 

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1229
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 biodiversity over a representative portion of their historic occurrence. 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 inconsistent with the ESA’s expressly stated goal of providing “a

means whereby the ecosystems upon which endangered species 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 surviving 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

8Moreover, even if the DPS Policy privileges the conservation of

genetic resources, there is ample support for such an approach in the legislative 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 variations. The reason is simple: they are potential resources.”). 

1230 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
[7] Fourth, the Alliance argues that the Service, in formulating 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 Congress 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 biological evidence indicates that such action is warranted.” Id.

(emphasis added). The DPS Policy expressed an intent to follow that instruction. 61 Fed. Reg. at 4722. The Alliance contends that the policy views of a subsequent Congress should

not be considered in interpreting a statutory term. The Alliance’s position might be more persuasive if we were construing 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 statutory 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 listings that the Alliance cites all took place well before 1996

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1231
(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, endangered populations that were not granted protection by the Service (because they failed the significance requirement) — but,

crucially, offers no examples of populations that were granted

protection under the DPS Policy despite being found insignificant 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 Alliance 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 ultimate 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). 

1232 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
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 represents 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 species 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’ significance 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 sub10The 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. 

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1233
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. However, 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 communities 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 context 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

1234 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
not cause a significant gap in the range of the taxon. For purposes of the “gap in the range” analysis, the term “significant”

has “its ‘commonly understood meaning,’ which is ‘important.’ ” Nat’l Ass’n of Home Builders, 340 F.3d at 846 (citations 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, behavioral, or genetic characteristics through adaptation to local

conditions. The Service then considered whether the Washington 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 Oregon 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 peripheral 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 proximity to urban areas. Id. 

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1235
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 substantially 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 measurement 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 hypothetical loss of the Washington population would “represent a serious 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 significant because the population lacks biologically distinctive

traits. 

The Alliance complains that the Service dismissed evidence 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 evidence simply because it falls short of absolute scientific certainty. 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 “conclusive”); Center for Biological Diversity v. Lohn, 296 F. Supp.

2d 1223, 1236-40 (W.D. Wash. 2003) (NMFS was capricious

1236 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
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, credible 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 squirrels, and the Klickitat County study was limited geographically to a subset of one of the three Washington habitats of the

gray squirrel. While crediting the anecdotal evidence, the Service 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 biologically 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 facNORTHWEST ECOYSTEM ALLIANCE v. USFWS 1237
tors in addition to geography. Nothing in the DPS Policy or

in the ESA limits the Service’s significant gap inquiry to geographic 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 capricious. 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 administrative 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

11See 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 population “comprises an important component of the sawfishes’ remaining

global biological diversity”); Final Rule to List the Columbia Basin Distinct 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 Anadromous 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 further range reduction”). 

12Determination 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.”) . 

1238 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
Service has regarded the “significant gap” inquiry as a flexible one, and has considered various non-geographic factors on

different occasions. Here, the Service has articulated a reasonable 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 Alliance, is an arbitrary and capricious departure from prior practice. 

3. Marked Genetic Differences

The third factor the Service considered was whether the

Washington population’s genetic characteristics differ “markedly” 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 Builders, 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 question 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 Washington, 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 Oregon 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. 

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1239
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 populations compared with Oregon and California, and “considerably more genetic differentiation between Washington and

Oregon or California[ ] than there is between Oregon and California 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 constitution of an individual or taxon — was shared across the

Columbia river. Mitochondrial DNA analysis also demonstrated 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 Washington,” 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. 

1240 NORTHWEST ECOYSTEM ALLIANCE v. USFWS
“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 California 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 Service 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 Service 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

NORTHWEST ECOYSTEM ALLIANCE v. USFWS 1241
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 population would be more distinct. 

[13] Because the Service has articulated reasoned connections between the record and its conclusion, its genetic analysis was not arbitrary or capricious. 

IV. CONCLUSION 

[14] As set forth above, the DPS Policy is entitled to Chevron deference. Under the Chevron standard, the DPS Policy

was a reasonable construction of the ESA. The Service’s decision denying the petition was not arbitrary or capricious. 

AFFIRMED.

1242 NORTHWEST ECOYSTEM ALLIANCE v. USFWS