Experimental Populations Under the Endangered Species Act and Gray Wolves

Experimental Populations Under the Endangered Species Act and Gray Wolves

Experimental Populations Under the
July 28, 2023
Endangered Species Act and Gray Wolves
Erin H. Ward
Section 10(j) of the Endangered Species Act (ESA; 16 U.S.C. §§ 1531 et seq.) allows for the
Legislative Attorney
establishment of experimental populations of endangered or threatened species and delineates

how experimental populations are to be regulated. An experimental population is a population of
Benjamin M. Barczewski
an endangered or threatened species that is released into the wild outside of the current range of
Legislative Attorney
the species with the aim of contributing to the conservation of the species. This report discusses

the criteria and process for establishing experimental populations under the ESA, as well as how
such populations are regulated, with a focus on how this authority has been used with gray

wolves. The U.S. Fish and Wildlife Service (FWS) administers the ESA for gray wolves and is
responsible for implementing policies and regulations for the species.
To qualify as an experimental population, the specimens must be released into suitable habitat outside the current range of the
species. The population, when released, must be wholly geographically separate from existing populations. In addition, FWS
must determine that the experimental population will contribute to the conservation of the species in the wild. FWS considers
whether removing specimens from other wild populations to establish the new population may negatively affect existing
populations, whether the new population is likely to become well established and survive in the wild, and other factors. FWS
must also designate the experimental population as essential or nonessential to the conservation of the species. These
designations affect how consultation requirements apply to the population, and the FWS may designate critical habitat only
for essential experimental populations.
With certain exceptions, experimental populations are treated as threatened species under the ESA, regardless of the
designation of the listed species. Accordingly, an experimental population of a species that is listed as endangered—which is
entitled to certain automatic protections upon listing—would instead be treated as a threatened species, which receives such
protections only if they are extended to the population by regulation. This treatment allows FWS to tailor the protections and
exceptions that apply to the population, providing for flexible management while contributing to the conservation of the
species. For example, when introducing an experimental population of a predator species, FWS may allow some individual
animals of that species to be taken or harmed, without a permit, if specific criteria are met.
FWS has listed various populations of gray wolves under the ESA since it was enacted. It has also released experimental
populations of gray wolves on several occasions and has proposed to establish another experimental population in Colorado.
The repeated use of this authority with respect to gray wolves illustrates how FWS applies the statutory criteria and uses the
regulatory flexibilities included in the statute and how stakeholders respond to reintroducing a population into the wild.
Congress has long shown interest in the regulation of gray wolves under the ESA, and it has introduced or enacted legislation
affecting the listing status of gray wolf populations.
In the mid-1990s, FWS released two experimental populations of gray wolves in Idaho, Montana, and Wyoming. The
combined population formed by these wolves was eventually delisted through administrative and legislative action. In 1998,
FWS established an experimental population of Mexican gray wolves, a subspecies of the gray wolf, in Arizona and New
Mexico to recover the subspecies in the wild. FWS continues to support this population through a captive breeding program
to release additional Mexican gray wolves to support population growth and genetic diversity. Recently, in the wake of a
Colorado state referendum on introducing a gray wolf population in the state and a subsequent court decision that vacated a
2020 gray wolf delisting rule, FWS has proposed establishing an experimental population in Colorado. Each of these efforts
has encountered an array of stakeholder responses—from support for the conservation and ecosystem benefits gained by
reintroducing members of the species to concerns about effects of gray wolves on local livestock and game.
Congressional Research Service

link to page 4 link to page 5 link to page 5 link to page 6 link to page 6 link to page 6 link to page 7 link to page 8 link to page 8 link to page 8 link to page 8 link to page 9 link to page 10 link to page 10 link to page 10 link to page 13 link to page 15 link to page 18 link to page 14 link to page 17 link to page 19 Experimental Populations Under the Endangered Species Act and Gray Wolves

Contents
Introduction ..................................................................................................................................... 1
Criteria for Establishing an Experimental Population ............................................................... 2
Outside the Species’ Current Range .................................................................................... 2
Contribution to the Conservation of the Species ................................................................ 3
Wholly Separate Geographically ........................................................................................ 3
Essential and Nonessential Experimental Populations ....................................................... 3
Consequences of Designation ................................................................................................... 4
Process for Designating Experimental Populations and Promulgating 10(j) Rules
Under the Administrative Procedure Act ............................................................................... 5
Consulting with Interested Parties ...................................................................................... 5
Issuing Designation and 10(j) Rules ................................................................................... 5
Judicial Review of Designation and 10(j) Rules ................................................................. 6
Experimental Populations of Gray Wolves ...................................................................................... 7
Yellowstone Experimental Population and 10(j) Rule and the Northern Rocky
Mountain DPS ........................................................................................................................ 7
Mexican Gray Wolf Experimental Population and 10(j) Rule ................................................ 10
Additional Delisting, Litigation, and the Colorado Experimental Population ........................ 12
Considerations for Congress.......................................................................................................... 15

Figures
Figure 1. Map of Geographic Boundaries for the Mexican Gray Wolf 1998-2015 ........................ 11
Figure 2. Map of State of Colorado with General Area Proposed for Initial Release ................... 14

Contacts
Author Information ........................................................................................................................ 16

Congressional Research Service

Experimental Populations Under the Endangered Species Act and Gray Wolves

Introduction
Section 10(j) of the Endangered Species Act (ESA; 16 U.S.C. §§ 1531 et seq.) allows for the
establishment of experimental populations of endangered or threatened species and delineates
how experimental populations are to be regulated.1 An experimental population is a population of
an endangered or threatened species that is released into the wild outside of the current range of
the species that will contribute to the conservation of the species. With certain exceptions,
experimental populations are treated as threatened species under the ESA regardless of
designation of the listed species.2 This treatment allows the U.S. Fish and Wildlife Service (FWS)
and the National Marine Fisheries Service (NMFS) to tailor the protections and exceptions that
apply to the populations, providing for flexible management while contributing to the
conservation of the species.3
Section 10(j) was added to the ESA as part of the Endangered Species Act Amendments of 1982.4
Before the 1982 amendments, FWS or NMFS could translocate members of listed species into
unoccupied areas to assist in recovery, but the released population would be subject to the same
prohibitions and protections as the rest of the listed species.5 The amendments gave FWS and
NMFS more flexibility and discretion in determining which prohibitions would apply to an
experimental population, and they limited the extent to which the Section 7 consultation
requirements apply to experimental populations that are not essential to the conservation of the
species.6 The legislative history of the 1982 amendments shows that some Members of Congress
were concerned about political opposition to reintroduction efforts arising from some
stakeholders’ fears that a rigid application of ESA protections to experimental populations “would
halt development projects.”7 Congress provided greater regulatory flexibility in the 1982
amendments to address this concern.
This report addresses experimental populations of species listed under the ESA, with a specific
focus on experimental populations of gray wolves. FWS has released experimental populations of
gray wolves on several occasions and has proposed to establish another experimental population
in Colorado. Such efforts have encountered an array of stakeholder reactions—from support for
the conservation and ecosystem benefits of introducing an experimental population to concerns
about wolves harming local livestock. Some Members of Congress have shown interest in the
regulation of gray wolves under the ESA, and Congress has introduced or enacted legislation
affecting the listing status of gray wolf populations. With respect to experimental populations of
gray wolves, Congress may be interested in the process for authorizing an experimental

1 16 U.S.C. § 1539(j).
2 Id. § 1539(j)(2)(C). The ESA defines endangered species with limited exceptions as “any species which is in danger
of extinction throughout all or significant portion of its range” and threatened species as “any species which is likely to
become an endangered species within the foreseeable future throughout all or a significant portion of its range.” Id. §§
1532(6), (20).
3 See id.; see also Endangered and Threatened Wildlife and Plants; Experimental Populations, 49 Fed. Reg. 33,885,
33,886 (Aug. 27, 1984).
4 P.L. 97-304, § 6, 96 Stat. 1411 (1982).
5 49 Fed. Reg. at 33,886.
6 Id. Section 7 of the ESA, 16 U.S.C. § 1536, requires that federal agencies consult with the U.S. Fish and Wildlife
Service (FWS) or the National Marine Fisheries Service (NMFS) to ensure that their actions are not likely to jeopardize
the continued existence of any endangered or threatened species or result in the destruction or adverse modification of
designated critical habitat.
7 H.R. Rep. No. 97-567, at 8 (1982); see also 49 Fed. Reg. 33,886, 33,886 (Aug. 27, 1984); Wyo. Farm Bureau Fed’n
v. Babbitt, 199 F.3d 1224, 1231-32 (10th Cir. 2000).
Congressional Research Service

1

Experimental Populations Under the Endangered Species Act and Gray Wolves

population under the ESA, how designated experimental populations are regulated under the
ESA, and how this process has been applied to gray wolves. In particular, Congress may consider
the ongoing process to designate an experimental population of gray wolves in Colorado and
legislative options available to affect whether the experimental population is established, how it is
regulated, and whether to provide funding or oversight for managing the experimental population.
Criteria for Establishing an Experimental Population
Section 10(j) of the ESA provides the authority and lays out the criteria for establishing
experimental populations. Section 10(j) authorizes the Secretary of the Interior or of Commerce,
as delegated to FWS or NMFS, respectively, to release populations of endangered or threatened
species into the wild outside the current range of the species, provided FWS or NMFS determines
that the release will contribute to the conservation of the species.8 Such populations qualify as
experimental populations when they are wholly separate geographically from any non-
experimental populations of the species.9 FWS is responsible for administering the ESA with
respect to gray wolves, which are listed as endangered under the act. This report focuses on FWS
regulations for experimental populations.10
Outside the Species’ Current Range
FWS regulations implementing Section 10(j) of the ESA provide a regulatory framework for
designating experimental populations.11 Under those regulations, the experimental population
must be released into “suitable natural habitat.”12 The statute requires this suitable natural habitat
to be outside the species’ current range.
FWS regulations had established an additional requirement that experimental populations be
released within the species’ historical range under most circumstances. On July 3, 2023, however,
FWS published a final rule eliminating the requirement that an experimental population be
introduced in its historical range.13 FWS noted that climate change is causing species’ suitable
habitats to shift outside their historical ranges and that invasive species potentially reduce the
ability of habitat to support species within their historical ranges.14 As a result, FWS anticipated
that it may become necessary and appropriate to establish experimental populations outside the
historical range, as well as outside the current range, of the listed species.15 The revised
regulations also require FWS to consider potential adverse effects to the ecosystem when an
experimental population is to be released outside the species’ historical range.16

8 16 U.S.C. § 1539(j)(2)(A).
9 Id. § 1539(j)(1).
10 NMFS, within the Department of Commerce, administers the ESA for marine and anadromous species and has its
own regulations for experimental populations. 50 C.F.R. pt. 222, subpt. E. Though the two agencies’ regulations
generally contain identical or similar provisions, there are differences between them. For example, only FWS requires
consideration of the impact on the ecosystem when an experimental population is to be released outside the species’
probable historical range. Compare 50 C.F.R. § 17.81(a) (2022) with id. § 222.502(a) (2022).
11 50 C.F.R. pt. 17, subpt. H.
12 50 C.F.R. § 17.81(a).
13 FWS, Endangered and Threatened Wildlife and Plants; Designation of Experimental Populations, 88 Fed. Reg.
42,642 (July 3, 2023).
14 Id. at 42,643.
15 Id.
16 Id. at 42,651-52 (amending 50 C.F.R. § 17.81(b)).
Congressional Research Service

2

Experimental Populations Under the Endangered Species Act and Gray Wolves

Contribution to the Conservation of the Species
The ESA requires that an experimental population must contribute to the conservation of the
listed species.17 FWS’s regulations include factors for the agency to consider when evaluating this
criterion. Pursuant to its regulations, FWS must consider whether the experimental population is
likely to become established and survive for the foreseeable future.18 FWS must also examine the
anticipated effects that establishing an experimental population will have on the species’
recovery.19 FWS must further consider whether removing individuals from existing populations to
establish the experimental one will adversely affect those populations.20 Finally, FWS must
determine whether and to what extent existing or anticipated federal or state actions or private
activities may be affected by establishing the experimental population.21 Each of these factors
must be determined based on the best commercial and scientific data available.22
Wholly Separate Geographically
When determining whether a released experimental population is or is expected to be wholly
separate geographically from existing populations, FWS regulations focus on the population as a
whole rather than individuals within the population.23 The released population’s separation from
existing populations must be “reasonably predictable.”24 For example, fixed migration patterns or
natural or manmade barriers may create sufficiently predictable separation between the
experimental population and other populations.25 Individuals from an experimental population
that venture into an area of overlap with the non-experimental population are not recognized as
part of the experimental population while they are intermixed with a non-experimental
population.26 This distinction between populations and roaming individuals becomes particularly
relevant for species where individuals may venture far from their original or usual populations,
such as gray wolves.
Essential and Nonessential Experimental Populations
When FWS designates an experimental population, Section 10(j) of the ESA also requires that
FWS determine whether the experimental population is “essential to the continued existence” of
the species.27 Such experimental populations are designated as essential experimental
populations, and all others are designated as nonessential experimental populations.28 These
determinations are made based on the “best available information.”29 Pursuant to its regulations,
FWS considers an experimental population to be essential if losing the population would likely

17 16 U.S.C. § 1539(j)(2)(A).
18 50 C.F.R. § 17.81(b)(2).
19 Id. § 17.81(b)(3).
20 Id. § 17.81(b)(1).
21 Id. § 17.81(b)(4).
22 Id. § 17.81(b).
23 Id.
24 Id. § 17.80(a).
25 Id.
26 Id.
27 16 U.S.C. § 1539(j)(2)(B).
28 50 C.F.R. § 17.80(b).
29 16 U.S.C. § 1539(j)(2)(B).
Congressional Research Service

3

Experimental Populations Under the Endangered Species Act and Gray Wolves

“appreciably reduce the likelihood” of the species surviving in the wild.30 To date, no
experimental population of any species has been designated as essential.
Consequences of Designation
In general, even if a species is listed as endangered, experimental populations of that species are
treated as threatened species under the statute.31 The prohibitions in Section 9 of the ESA—such
as on import, export, or “take” of species—apply to endangered species but do not automatically
apply to threatened species.32 These prohibitions may be extended to threatened species by
regulations, generally referred to as 4(d) rules.33 Section 9 prohibitions may similarly be extended
to experimental populations pursuant to their treatment as threatened species by regulations,
referred to as 10(j) rules.34
FWS must designate critical habitat for both endangered and threatened species to the maximum
extent prudent and determinable.35 Critical habitat generally consists of habitat that is essential to
the conservation of the species.36 Section 10(j) provides that critical habitat shall not be
designated for nonessential experimental populations—only for essential experimental
populations.37
The consultation requirements for federal agency actions in Section 7 of the ESA also apply
equally to endangered and threatened species.38 However, Section 10(j) provides that nonessential
experimental populations are not treated as threatened species for consultation purposes. Instead,
they are treated as species that are proposed to be listed unless the proposed federal agency action
will be within the National Park System or the National Wildlife Refuge System.39 For species
that are proposed to be listed, Section 7 requires federal agencies to confer with FWS, but they
may proceed with the proposed action in the meantime (i.e., they may irreversibly or irretrievably
commit resources to the action, which may foreclose alternatives that may have avoided jeopardy
to the species).40 Accordingly, if either the experimental population is essential or if the proposed
action will occur within the National Park System or National Wildlife Refuge System, then
federal agencies must consult with FWS to ensure that their proposed actions will not jeopardize
the continued existence of the experimental population or, for essential experimental populations,
adversely affect its designated critical habitat (if any). Otherwise, federal agencies need only
confer with respect to nonessential experimental populations.

30 50 C.F.R. § 17.80(b).
31 16 U.S.C. § 1539(j)(2)(C).
32 Id. § 1538(a). Take means “to harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, or collect, or to attempt to
engage in any such conduct.” 16 U.S.C. § 1532(19).
33 Id. § 1533(d).
34 50 C.F.R. § 17.82.
35 Id. § 1533(a)(3).
36 Id. § 1532(5).
37 Id. § 1539(j)(2)(C)(ii). No experimental populations have so far been designated as essential.
38 16 U.S.C. § 1536(a)(2). For a discussion of the ESA’s Section 7 consultation requirements see CRS Report R46677,
The Endangered Species Act: Overview and Implementation, by Pervaze A. Sheikh and Erin H. Ward.
39 Id. § 1539(j)(2)(C)(i). Section 7 consultation requirements do apply to essential experimental populations.
40 Id. § 1536(a)(4).
Congressional Research Service

4

Experimental Populations Under the Endangered Species Act and Gray Wolves

Process for Designating Experimental Populations and
Promulgating 10(j) Rules Under the Administrative Procedure Act
FWS establishes experimental populations and protections for those populations through the
federal rulemaking process. Federal agency rulemaking procedures are largely governed by the
Administrative Procedure Act (APA).41 FWS’s designations and 10(j) rules are thus governed
primarily by requirements established in the ESA, FWS’s implementing regulations, and the APA.
Consulting with Interested Parties
FWS regulations require the agency to consult with relevant state fish and wildlife agencies and
local governmental entities as well as with affected federal agencies and private landowners when
developing and implementing experimental population regulations.42 This consultation process
may involve holding meetings with interested members of the public.43 In its regulations, FWS
states that any experimental population regulation shall reflect an agreement between the agency
and the relevant stakeholders with which it consults “to the maximum extent practicable.”44
Issuing Designation and 10(j) Rules
To begin the rulemaking process, FWS must first publish a proposed rule providing notice of its
intent to designate an experimental population, to determine whether the population would be
designated as essential or nonessential, and to consider a 10(j) rule for the population. In
accordance with the APA, once FWS publishes the proposed rule, it must provide an opportunity
for the public to comment on the proposal.45 Typically, FWS accepts public comments for 60 days
when proposing to establish an experimental population.46 FWS also typically holds public
hearings in various locations that could be affected by the establishment of an experimental
population.47 After FWS receives and considers any public comments, it may choose to complete
the rulemaking process by publishing a final rule.48 At the time FWS issues the final rule, it must
provide a reasoned justification supporting its action.49 FWS’s justification for establishing the
experimental population and any 10(j) rules for that population must generally address significant

41 See 5 U.S.C. § 553; CRS In Focus IF10003, An Overview of Federal Regulations and the Rulemaking Process, by
Maeve P. Carey.
42 50 C.F.R. § 17.81(d).
43 Id.
44 Id.
45 See 5 U.S.C. § 553.
46 See, e.g., Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Population of the Gray
Wolf in Colorado, 88 Fed. Reg. 10,258 (proposed Feb. 17, 2023); Endangered and Threatened Wildlife and Plants;
Establishment of Nonessential Experimental Population Status for 15 Freshwater Mussels, 1 Freshwater Snail, and 5
Fishes in the Lower French Broad River and in the Lower Holston River, Tennessee, 71 Fed. Reg. 34,196 (Jun. 13,
2006); Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Population of the Mexican
Gray Wolf in Arizona and New Mexico, 63 Fed. Reg. 1,752 (Jan. 12, 1998).
47 See, e.g., id.
48 Id.
49 Id.
Congressional Research Service

5

Experimental Populations Under the Endangered Species Act and Gray Wolves

comments and whether or not FWS has changed the final rule in response.50 Any changes in the
final rule that differ from the proposed rule must be a “logical outgrowth” of the proposal.51
FWS must find in its rule that the release of the species as an experimental population will further
the conservation of the species.52 Also, the rule must provide a means to identify the experimental
population, such as specifying the proposed location or anticipated migration. The rule must
identify any special management concerns such as management restrictions or protective
measures to isolate or contain the population from the rest of the species, and it must create a
process for periodic evaluation of how effective the experimental population is at conserving the
species.53 For an experimental population that FWS designates as essential, the agency may also
include critical habitat.54 FWS may also choose to implement a 10(j) rule or designate critical
habitat through a separate rulemaking process from the one designating the experimental
population.
Judicial Review of Designation and 10(j) Rules
Both the ESA and the APA may provide a basis for judicial review of FWS’s final rules,
depending on the plaintiff’s particular allegations.55 The ESA provides that any person may bring
a lawsuit in federal district court to, among other things, enjoin anyone, including government
entities, from violating any provision of the ESA or its regulations.56 The ESA, accordingly,
provides an avenue to federal court to challenge FWS actions allegedly in violation of Section
10(j) or regulations issued pursuant to 10(j). In reviewing alleged violations, courts apply the
arbitrary and capricious standard contained in the APA.57 Furthermore, challenges to FWS’s
administration of the ESA through the rulemaking process—whether it has given adequate public
notice or made a reasoned decision, for instance—must proceed pursuant to the APA itself, not
the ESA.58
Under the APA, a court must set aside an agency rule if it finds that the rule is “arbitrary,
capricious, an abuse of discretion, or otherwise not in accordance with law.”59 This standard is
known as “arbitrary and capricious review” or “hard look review.”60 For example, a reviewing
court might determine that an FWS rule is arbitrary and capricious if FWS has failed to rationally
connect the facts before FWS to its decision, if FWS has failed to consider an important aspect of
the problem, if FWS has relied on factors Congress did not intend for FWS to consider, or if the

50 See, e.g., Perez v. Mortg. Bankers Ass’n, 575 U.S. 92, 96 (2015) (citing Citizens to Preserve Overton Park Inc. v.
Volpe, 401 U.S. 402, 416 (1971)).
51 See Long Island Care at Home, Ltd. v. Coke, 551 U.S. 158, 174 (2007) (collecting cases). The “logical outgrowth”
standard limits substantive changes in the final rule that deviate sharply from the proposed rule. Nat. Black Media Coal.
v. FCC, 791 F.2d 1016, 1022 (D.C. Cir. 1986). The limitation ensures that stakeholders are given fair notice of what the
agency intends to do and have a meaningful opportunity to comment on the substance of the rule. Id.
52 50 C.F.R. § 17.81(b)-(c).
53 Id. § 17.81(c).
54 16 U.S.C. § 1539(j)(2)(C); 50 C.F.R. § 17.81(f). See also 16 U.S.C. § 1533(b).
55 See 5 U.S.C. §§ 702, 704.
56 16 U.S.C. § 1540(g)(1); Bennet v. Spear, 520 U.S. 154, 174 (1997).
57 Gerber v. Norton, 294, F.3d 173, 178 n.4 (D.C. Cir. 2002) (quoting Cabinet Mountains Wilderness v. Petersen, 685
F.2d 678, 685 (D.C. Cir. 1982)); W. Watersheds Project v. Kraayenbrink, 632 F.3d 472, 496 (9th Cir. 2011) (citing
Village of False Pass v. Clark, 733 F.2d 605, 609-10 (9th Cir. 1984)).
58 Bennet, 520 U.S. at 176.
59 Id. § 706(2)(a).
60 See Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 43 (1983); Patrick M. Garry, Judicial
Review and the “Hard Look” Doctrine, 7 NEV. L.J. 151, 152 (2008).
Congressional Research Service

6

Experimental Populations Under the Endangered Species Act and Gray Wolves

rule is not supported by the administrative record.61 When a court overturns an agency rule, the
court may either vacate the rule or leave the rule in place but remand it to the agency to address
whatever shortcoming the court found.62
While many claims for review of agency action proceed under the arbitrary and capricious review
standard set out in the APA, some claims turn on FWS’s interpretation of the ESA. The most
common standard of review for an agency interpretation of a federal statute it administers is
known as Chevron deference, after the Supreme Court case Chevron U.S.A. Inc. v. Natural
Resources Defense Council.63 Under Chevron, a reviewing court would generally defer to FWS’s
reasonable interpretation of any provisions of the ESA that are ambiguous.64 The Supreme Court,
however, has not applied Chevron in recent administrative law cases, including at least one case
under the ESA.65
Experimental Populations of Gray Wolves
FWS has released and designated three experimental populations of gray wolves—two in the
greater Yellowstone area, where the species is now delisted, and one population of the Mexican
gray wolf subspecies in New Mexico and Arizona.66 Each of the rules establishing these
experimental populations was litigated. Most recently, FWS issued a proposed rule to introduce a
new experimental population of gray wolves in Colorado.67 This section summarizes the
experimental population rules, their implementation, and resulting litigation.
Yellowstone Experimental Population and 10(j) Rule and the
Northern Rocky Mountain DPS
In 1978, FWS listed the gray wolf as endangered throughout the lower 48 states except for
Minnesota, where it was listed as threatened.68 In 1994, FWS finalized two rules establishing two
nonessential experimental populations of gray wolves in (1) the greater Yellowstone area and (2)

61 State Farm, 463 U.S. at 43.
62 Allied-Signal, Inc. v. U.S. Nuclear Regulatory Comm’n, 988 F.2d 146, 151 (D.C. Cir. 1993) (noting that the normal
rule when a regulation is found to be unlawful is to vacate and remand the rule, but remand without vacatur may be
appropriate where vacating the rule would be “disruptive” and there is a serious possibility that the agency will be able
to “substantiate” its decision if given the opportunity).
63 467 U.S. 837 (1984).
64 Id. at 842-43.
65 See Weyerhaeuser Co. v. U.S. Fish & Wildlife Serv., 139 S. Ct. 361, 368 (2018) (considering the meaning of the
ESA phrase unoccupied critical habitat without applying Chevron); see also, e.g., CRS Report R44954, Chevron
Deference: A Primer, by Benjamin M. Barczewski (tracking the Supreme Court’s declining interest in applying
Chevron); CRS In Focus IF12077, The Major Questions Doctrine, by Kate R. Bowers (noting that recently the
Supreme Court has not applied or referred to Chevron in reviewing agency actions); Kristin E. Hickman & Aaron L.
Nielson, The Future of Chevron Deference, 70 DUKE L.J. 1015, 1016 (2021) (noting the Supreme Court has been
reluctant to apply Chevron).
66 For more information on the history of the regulation of the gray wolf under the ESA, see CRS Report R46184, The
Gray Wolf Under the Endangered Species Act (ESA): A Case Study in Listing and Delisting Challenges, by Erin H.
Ward.
67 Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Population of the Gray Wolf in
Colorado, 88 Fed. Reg. 10,258 (proposed Feb. 17, 2023).
68 Reclassification of the Gray Wolf in the United States and Mexico, with Determination of Critical Habitat in
Michigan and Minnesota, 42 Fed. Reg. 9,607 (Mar. 9, 1978).
Congressional Research Service

7

Experimental Populations Under the Endangered Species Act and Gray Wolves

central Idaho and southwestern Montana.69 In 1995 and 1996, FWS released a total of 66
Canadian gray wolves in Yellowstone, southwestern Montana, and central Idaho.70 The initial
management goal was for each experimental population to raise two pups for two consecutive
years after three to five years of reintroduction efforts.71 If all went well, FWS estimated that
between the two experimental populations and a naturally occurring population in Montana, wolf
recovery could be expected to reach 10 breeding pairs in each of the three areas by 2002.72 By the
end of 2000, the wolf population met its initial management goals of 30 breeding pairs and more
than 300 individuals across Montana, Idaho, and Wyoming.73
Concurrently with the introduction of two experimental populations, FWS also issued regulations
pursuant to ESA Section 10(j) specifying protections for the two populations.74 As nonessential
experimental populations, the wolves were treated as threatened (or as a candidate species for the
purposes of Section 7 consultation in areas outside of Yellowstone National Park).75
The 10(j) rules for the Yellowstone and Idaho/Montana experimental populations of gray wolves
illustrate how such rules may be tailored and amended to allow for flexible management. The
10(j) rules for the two populations permitted unintentional, non-negligent, and accidental take of
a member of the experimental populations “provided that the take was incidental to otherwise
lawful activities ... or was in defense of human life.”76 Any such take had to be reported to FWS
within 24 hours.77 The regulations also permitted FWS or designated public entities to take
wolves that posed a threat to livestock or property.78 Private landowners were permitted to harass
wolves found on private property so long as any such harassment was reported to FWS within
seven days. Private landowners were also permitted to injure or kill wolves that were in the act of
wounding or killing livestock on private land. The landowner was required to report any such
take to FWS within 24 hours and provide evidence of wounded or dead livestock to corroborate
the nature of the wolf attack.
Once six or more breeding pairs were established in the experimental area, livestock owners
could apply for permits from FWS to injure or kill wolves that attacked livestock on public
grazing lands. Permits for take on public grazing land would be issued only after wolf relocation
efforts failed.79 Under the 10(j) regulations, FWS could relocate wolves that FWS determined
were responsible for excessive predation on big game (e.g., elk, moose, bison) that affected a

69 Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Experimental Population of Gray
Wolves in Yellowstone National Park in Wyoming, Idaho, and Montana, 59 Fed. Reg. 60,252, 60,253-54 (Nov. 22,
1994); Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Experimental Population of
Gray Wolves in Central Idaho and Southwestern Montana, 59 Fed. Reg. 60,266, 60,267-69 (Nov. 22, 1994).
70 J. DWIGHT HINES, THE CO-LIVING OF HUMANS AND WOLVES IN THE GREATER YELLOWSTONE ECOSYSTEM 194 (2017).
71 59 Fed. Reg. at 60,255.
72 Id.
73 Endangered and Threatened Wildlife and Plants; Revision of Special Regulation for the Central Idaho and
Yellowstone Area Nonessential Experimental Populations of Gray Wolves in the Northern Rocky Mountains, 73 Fed.
Reg. 4,720, 4,721 (Feb. 27, 2008); see also Ed Bangs, et al., Gray Wolf Restoration in the Northwestern United States,
18 ENDANGERED SPECIES UPDATE 147, 149 (2001).
74 59 Fed. Reg. at 60,253-54; 59 Fed. Reg. at 60,270-71.
75 See id.; 16 U.S.C. § 1539(j). As mentioned above, Section 4(d) of the ESA permits FWS to establish by rule which
prohibitions contained in Section 9 will apply to a species listed as threatened. See 16 U.S.C. § 1533(d).
76 59 Fed. Reg. at 60,270.
77 Id.
78 Id.
79 Id.
Congressional Research Service

8

Experimental Populations Under the Endangered Species Act and Gray Wolves

state or tribe’s big game management goals.80 Wolves, however, could not be intentionally killed
to resolve predation conflicts with big game.81
FWS also provided for state or tribal management of the experimental populations outside of
Yellowstone National Park through cooperative agreements, which would include FWS oversight
and technical assistance.82 If a state or tribe declined to enter into an agreement, FWS managed
the wolf population in the relevant area.83
In 2005, FWS revised its 10(j) rules for the experimental populations by marginally expanding
the circumstances when wolves could be injured or killed.84 FWS again amended the 10(j) rules
in 2008.85 The 2008 amendment permitted wolf removal when a state or tribe determined that
wolf predation was one of the major causes keeping a big game herd from reaching state or tribal
management goals.86
The reintroduction of wolves to Yellowstone and the surrounding area generated significant
litigation over FWS’s authority to establish the two experimental populations in the selected
geographic areas. Section 10(j) of the ESA requires that experimental populations must be
“wholly separate geographically from non-experimental populations of the same species.”87
When it established the experimental populations, FWS interpreted that provision to require
separation based on the areas occupied by existing gray wolf populations, even if individual gray
wolves—lone dispersers from a pack—might be found elsewhere. In the rules, FWS stated that it
would treat any individual gray wolves found in the experimental population area as part of that
population. Farm bureaus, researchers, and conservation organizations challenged FWS’s
interpretation of Section 10(j), and the federal district court in Wyoming agreed with the
challengers.88 On appeal, however, the U.S. Court of Appeals for the Tenth Circuit (Tenth Circuit)
disagreed and reversed the district court’s ruling.89 The Tenth Circuit rejected the argument that
Section 10(j) required experimental populations to be separate from every naturally occurring
individual animal.90 Observing that wild animals—particularly wolves—move, the court
concluded that protecting specimens based on where they are rather than where they came from
was a reasonable interpretation of “wholly geographically separate.”91 Under the court’s opinion,
FWS may locate experimental populations in areas occupied by lone individuals of the same
species so long as they are “wholly separate geographically” from populations of that species.92

80 Id.
81 Id.
82 Id.
83 Id.
84 Endangered and Threatened Wildlife and Plants; Regulation for Nonessential Experimental Populations of the
Western Distinct Population Segment of the Gray Wolf, 70 Fed. Reg. 1,286, 1,299 (Feb. 7, 2005).
85 Endangered and Threatened Wildlife and Plants; Revision of Special Regulation for the Central Idaho and
Yellowstone Area Nonessential Experimental Populations of Gray Wolves in the Northern Rocky Mountains, 73 Fed.
Reg. 4,720 (Feb. 27, 2008).
86 73 Fed. Reg. at 4,722-23. Prior versions of the regulations required wolves to be the “primary cause” of keeping a
big game herd from reaching its management goals. Id. at 4,721.
87 16 U.S.C. § 1539(j)(1).
88 Wyo. Farm Bureau Fed’n v. Babbitt, 987 F. Supp. 1349, 1371-76 (D. Wyo. 1997).
89 Wyo. Farm Bureau Fed’n v. Babbitt, 199 F.3d 1224, 1234 (10th Cir. 2000) (deferring to FWS’s interpretation of the
ESA at step two of Chevron).
90 Id. at 1234, 1237.
91 Id. at 1235-36.
92 Id.
Congressional Research Service

9

Experimental Populations Under the Endangered Species Act and Gray Wolves

In 2009, FWS designated the Northern Rocky Mountain (NRM) population of gray wolves as a
distinct population segment (DPS),93 including both experimental populations that had been
introduced in 1995 and 1996, and delisted that population with the exception of wolves found in
Wyoming.94 FWS did not delist the Wyoming population because FWS determined that
Wyoming’s wolf management plan, to be implemented after delisting, was inadequate.95 Under
the 2009 delisting, Wyoming’s population of gray wolves continued to be treated as an
experimental population subject to the original 1994 10(j) regulations.96 In 2010, a federal district
court in Montana vacated the delisting, finding that the rule violated the ESA because it provided
protections to only a portion of a DPS.97 In 2011, however, Congress reinstated the delisting
through legislation.98
In 2012, FWS delisted the gray wolf in Wyoming, finding that Wyoming’s post-delisting
management plan met FWS standards.99 Environmental groups challenged the delisting, but the
U.S. Court of Appeals for the District of Columbia Circuit upheld the rule.100 As a result, no
portion of the NRM DPS of gray wolves is listed as endangered or threatened, including wolves
in Wyoming, and the experimental population regulations no longer apply to the NRM
population.
Mexican Gray Wolf Experimental Population and 10(j) Rule
The Mexican gray wolf, a subspecies of the gray wolf, was listed as endangered in 1976.101
Although there were occasional sightings of the wolf in Mexico, by the time it was listed as
endangered, the Mexican gray wolf had been “effectively eliminated” in the U.S. Southwest.102 In
the 1970s and 1980s, the United States and Mexico developed a bilateral captive breeding
program with the goal of saving the wolf from absolute extinction and ultimately reintroducing
individuals from the program into the wild.103
In 1998, FWS issued a final rule establishing a nonessential experimental population of Mexican
gray wolves in the Blue Range Wolf Recovery Area (BRWRA), which is largely centered on the

93 In general, to qualify as a DPS, a population of a species must be discrete from the remainder of the species and
significant to that species. See Policy Regarding the Recognition of Distinct Vertebrate Population Segments Under the
Endangered Species Act, 61 Fed. Reg. 4722 (Feb. 7, 1996).
94 Endangered and Threatened Wildlife and Plants; Final Rule To Identify the Northern Rocky Mountain Population of
Gray Wolf as a Distinct Population Segment and To Revise the List of Endangered and Threatened Wildlife, 74 Fed.
Reg. 15,123 (Apr. 2, 2009).
95 Id. at 15,125.
96 Id.
97 Defs. of Wildlife v. Salazar, 729 F. Supp. 2d 1207, 1228 (D. Mont. 2010).
98 Department of Defense and Full-Year Appropriations Act, 2011, P.L. 112-10, § 1713, 125 Stat. 150 (2011). Parties
challenged this legislation as unconstitutional for violating the separation of powers doctrine. All. for the Wild Rockies
v. Salazar, 800 F. Supp. 2d 1123 (D. Mont. 2011). The courts upheld the legislation, holding that Congress
substantively amended the ESA and did not direct the federal courts to make specific findings about the rule’s validity
under the ESA. Id., aff’d, All. for the Wild Rockies v. Salazar, 672 F.3d 1170, 1175 (9th Cir. 2012).
99 Endangered and Threatened Wildlife and Plants; Removal of the Gray Wolf in Wyoming From the Federal List of
Endangered and Threatened Wildlife and Removal of the Wyoming Wolf Population’s Status as an Experimental
Population, 77 Fed. Reg. 55,530 (Sept. 10, 2012).
100 Defs. of Wildlife v. Zinke, 849 F.3d 1077, 1093 (D.C. Cir. 2017).
101 Determination That Two Species of Butterflies Are Threatened Species and Two Species of Mammals Are
Endangered Species, 41 Fed. Reg. 17,736 (Apr. 28, 1976).
102 U.S. FWS, Conserving the Mexican Wolf, https://www.fws.gov/program/conserving-mexican-wolf/species (last
visited Mar. 14, 2023).
103 See 63 Fed. Reg. at 1,753.
Congressional Research Service

10

Experimental Populations Under the Endangered Species Act and Gray Wolves

Gila and Apache National Forests in Arizona and New Mexico.104 FWS released 14 family groups
over five years with the goal of reaching 100 wild wolves.105 Although FWS identified a larger
Mexican Wolf Experimental Population Area that spanned from Interstate 40 in the north to
Interstate 10 in the south and from the western border of Arizona to the eastern border of New
Mexico, the rule provided that wolves found outside the BRWRA must be returned to the
BRWRA.106 Releases of wolves bred in captivity were also limited to the primary recovery
zones.107
Figure 1. Map of Geographic Boundaries for the Mexican Gray Wolf 1998-2015

Source: Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Population of the
Mexican Gray Wolf in Arizona and New Mexico, 63 Fed. Reg. 1,752 (Jan. 12, 1998).
At the same time that FWS established the experimental population, it also issued a 10(j) rule that
established some protections for the population but with exceptions allowing take under certain
circumstances.108 The Mexican gray wolf experimental population 10(j) rule largely mirrored the
10(j) rule for the NRM experimental populations of gray wolves.109 Relevant differences included
a provision allowing FWS to kill, capture, or subject to genetic testing “any feral wolf-like

104 Id. at 1,752
105 Id. at 1,754.
106 Id.
107 Id.
108 Id. at 1763.
109 Compare id. with Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Experimental
Population of Gray Wolves in Yellowstone National Park in Wyoming, Idaho, and Montana, 59 Fed. Reg. 60,252,
60,253-54 (Nov. 22, 1994); Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential
Experimental Population of Gray Wolves in Central Idaho and Southwestern Montana, 59 Fed. Reg. 60,266, 60,270-71
(Nov. 22, 1994).
Congressional Research Service

11

Experimental Populations Under the Endangered Species Act and Gray Wolves

animal, feral wolf hybrid, or any feral dog” found within the experimental population area, as
well as limitations on predator control techniques used by the U.S. Department of Agriculture.110
FWS amended its 10(j) rule in 2015.111 The 2015 amendment expanded the Mexican Wolf
Experimental Population Area south to the U.S.-Mexico border, did away with the BRWRA
designation, and expanded the areas where wolves could be reintroduced and naturally
disperse.112 The 2015 rule also slightly expanded the circumstances when wolves could be
injured, killed, or harassed.113
As with the reintroduction of wolves to the Yellowstone area, reintroduction of the Mexican gray
wolf to the U.S. Southwest generated significant litigation. Shortly after FWS issued the 1998
rule, a group of ranchers sued to block the reintroduction of the Mexican gray wolf.114 The
ranchers claimed that FWS failed to comply with the National Environmental Policy Act when it
issued the 1998 rule because, they alleged, FWS underestimated the number of domestic
livestock likely to be lost to wolf depredation.115 A federal court in New Mexico rejected the
ranchers’ arguments, finding that FWS had “reasonable grounds” for its decision and that the
administrative record it assembled supported its actions.116 That rule remains in effect.
When FWS delisted the gray wolf throughout the lower 48 states in 2020, it stated that the
delisting rule did not affect the Mexican gray wolf subspecies.117 Accordingly, the Mexican gray
wolf continues to be governed as a nonessential experimental population and subject to FWS’s
10(j) rule.
Additional Delisting, Litigation, and the Colorado Experimental
Population
In November 2020, FWS issued a final rule delisting the gray wolf (excluding the Mexican gray
wolf) in the areas of the lower 48 states and Mexico where it had not already been delisted.118
FWS’s delisting decision generated another round of litigation challenging the delisting. In
February 2022, a federal district court in California vacated the rule delisting the gray wolf,
effectively relisting the gray wolf as endangered throughout the lower 48 states except for the
NRM DPS (which was delisted in 2011 and 2012) and Minnesota (which had previously been

110 Id.
111 Endangered and Threatened Wildlife and Plants; Revision to the Regulations for the Nonessential Experimental
Population of the Mexican Wolf, 80 Fed. Reg. 2512 (Jan. 16, 2015).
112 Id. at 2519.
113 Id. at 2525.
114 N.M. Cattle Growers Ass’n v. FWS, No. Civ. 98-367M/JHG, 1999 WL 34797509, at *1 (D.N.M. Oct. 28, 1999).
115 Id. at *19.
116 Id. at *28.
117 Endangered and Threatened Wildlife and Plants; Removing the Gray Wolf (Canis lupus) from the List of
Endangered and Threatened Wildlife, 85 Fed. Reg. 69,778, 69,778 (Nov. 3, 2020). As described in the next section, the
delisting rule itself has been subject to litigation. Because of the exception stated in the delisting rule, the Mexican gray
wolf remains listed regardless of the outcome of that litigation.
118 Endangered and Threatened Wildlife and Plants; Removing the Gray Wolf (Canis lupus) from the List of
Endangered and Threatened Wildlife, 85 Fed. Reg. 69,778 (Nov. 3, 2020).
Congressional Research Service

12

Experimental Populations Under the Endangered Species Act and Gray Wolves

listed as threatened).119 An appeal of the district court’s decision is currently pending before the
U.S. Court of Appeals for the Ninth Circuit (Ninth Circuit).120
Also in November 2020, the voters of Colorado approved a ballot measure that directed Colorado
Parks and Wildlife to reintroduce gray wolves to the western part of the state by 2023.121 Because
the California district court subsequently vacated FWS’s delisting of the gray wolf, Colorado’s
reintroduction plans must now proceed with federal oversight pursuant to the ESA. Colorado
accordingly petitioned FWS to establish an experimental population of gray wolves in
conjunction with its gray wolf reintroduction effort.122 In February 2023, FWS issued a proposed
rule that would establish a nonessential experimental population of gray wolves in Colorado.123
The comment period for the proposed rule closed on April 18, 2023.124
FWS proposes to introduce wolves in the northwest corner of Colorado, but FWS has designated
the entire state of Colorado as the experimental population area because wolves are highly
mobile.125 Under the proposed rule, FWS would evaluate the reintroduction program every year
for a minimum of five years to determine whether recovery goals are being met and if any
modification is needed to reintroduction protocols.126

119 Defs. of Wildlife v. FWS, 584 F. Supp. 3d 812, 834 (N.D. Cal. 2022). For more information about the litigation
surrounding the delisting of the gray wolf see CRS Legal Sidebar LSB10697, U.S. District Court Vacates Gray Wolf
Delisting Rule, by Erin H. Ward. Because the Mexican gray wolf was excluded from the 2020 delisting, it also remains
listed, but its status was not affected by this litigation.
120 WildEarth Guardians v. U.S. Dep’t of Interior, No 22-15626 (9th Cir.).
121 Bruce Finley, Voter Approval of Colorado Wolf Reintroduction Means “Paws on the Ground” by Late 2023, THE
DENVER POST (Nov. 5, 2020), https://www.denverpost.com/2020/11/05/colorado-proposition-114-election-results-gray-
wolves-final-results/.
122 Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Population of the Gray Wolf in
Colorado, 88 Fed. Reg. 10,258 (proposed Feb. 17, 2023).
123 Id.
124 Id.
125 Id. at 10,264.
126 Id. at 10,279.
Congressional Research Service

13

Experimental Populations Under the Endangered Species Act and Gray Wolves

Figure 2. Map of State of Colorado with General Area Proposed for Initial Release

Source: Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Population of the Gray
Wolf in Colorado, 88 Fed. Reg. 10,258, 10,265 (Feb. 17, 2023).
Notes: Crosshatch: proposed area for initial release.
The proposed rule also includes a proposed 10(j) rule for managing the experimental
population.127 The 10(j) regulations would permit Colorado or an Indian tribe within the state to
assume lead authority for wolf conservation and management if the state or tribe has a wolf
management plan that is consistent with the proposed rule.128 Colorado has already issued a draft
restoration and management plan for the proposed experimental population.129
The proposed 10(j) rule would also allow take of members of the experimental population in
certain circumstances that largely mirror the 10(j) rule issued for the Yellowstone experimental
populations.130 For example, landowners would be allowed to kill or injure wolves when they are
in the act of attacking livestock on their private land.131 Any such take would have to be reported
to FWS within 24 hours.132 FWS would also have the authority to issue “shoot on-sight” permits
to take wolves on public or private land in cases where there was at least one prior wolf
depredation on that land, FWS had determined that problem wolves are routinely present, and
FWS had already authorized its agents to lethally remove wolves from that land.133
Due to the ongoing litigation, the future listing status of the gray wolf is uncertain. If the Ninth
Circuit reverses the district court’s decision, the gray wolf could again be delisted, and states
would have greater authority to manage the gray wolf. In that case, while Colorado would be able
to conduct its reintroduction program without federal oversight, Colorado’s reintroduction

127 Id. at 10,271.
128 Id.
129 COLO. PARKS AND WILDLIFE COMM’N, COLORADO WOLF RESTORATION AND MANAGEMENT PLAN (2023).
130 88 Fed. Reg. at 10,271-72.
131 Id. at 10,271.
132 Id.
133 Id. at 10,272.
Congressional Research Service

14

Experimental Populations Under the Endangered Species Act and Gray Wolves

program would not receive the same level of federal assistance as it would if the gray wolf
remains listed as an endangered species.
Considerations for Congress
Congress included Section 10(j) in the ESA to give FWS flexibility to manage experimental
populations, particularly when experimental populations may affect local development. Section
10(j) allows FWS to select which ESA prohibitions will apply to the experimental population,
limits ESA consultation requirements in certain circumstances, and bars FWS from designating
critical habitat for nonessential experimental populations. A decision to create an experimental
population may impose comparatively fewer restrictions on stakeholders than the presence of
endangered species. Nonetheless, introduction of an experimental population may still raise
stakeholder concerns, especially when large predators such as gray wolves are at issue.
As noted above, litigation over the listing and delisting of the gray wolf, including experimental
populations, has drawn the interest of Congress on several occasions, leading to congressional
action that could affect the status of experimental populations. In 2011, in response to a federal
court vacating FWS’s delisting of the NRM gray wolf DPS, Congress legislatively reinstated the
delisting through an appropriations rider.134 A similar bill, known as the Trust the Science Act, has
been introduced in the 118th Congress to reinstate the gray wolf delisting rule that was vacated by
a federal district court in 2022.135 The bill would require FWS to reissue its rule delisting the gray
wolf in the lower 48 states and Mexico 60 days after the bill is enacted.136 Were the bill enacted, it
would return management of the gray wolf, including experimental populations of gray wolves,
to the states. Delisting the gray wolf would also have the effect of precluding FWS from creating
new experimental populations pursuant to Section 10(j). Delisting may also affect FWS’s ability
to provide technical, financial, and personnel assistance to efforts to reintroduce and manage gray
wolf experimental populations. Congress may wish to consider such effects when reviewing
delisting legislation.
The proposed Colorado experimental population of gray wolves illustrates the various
perspectives that experimental populations can raise among stakeholders.137 The presence of gray
wolves, either naturally or as a result of the introduction of an experimental population, often
creates tension with ranchers, sportsmen, and other stakeholders. For example, while there are
conflicting accounts of the scale of wolf predation on livestock and game species,138 some
stakeholders in Colorado are concerned that the introduction of gray wolves will increase wolf
predation on livestock and game species that will ultimately affect their livelihood or recreational

134 Department of Defense and Full-Year Appropriations Act, 2011, P.L. 112-10, § 1713, 125 Stat. 150 (2011).
135 Trust the Science Act, H.R. 764, 118th Cong. (2023).
136 Id. § 2.
137 See Michael Doyle, Feds Plan for Colorado Gray Wolves Provokes Debate, GREENWIRE, Apr. 17, 2023,
https://subscriber.politicopro.com/article/eenews/2023/04/17/feds-plan-for-colorado-gray-wolves-provokes-debate-
00092327.
138 See, e.g., Wyo. Game and Fish Dep’t, Comment Letter on Establishment of a Nonessential Experimental Population
of the Gray Wolf in Colorado (Apr. 18, 2023) (noting wolf predation on livestock is likely higher than FWS data
indicates); Colo. Wool Growers Ass’n, Comment Letter on Proposed Colorado Gray Wolf Draft Environmental Impact
Statement and 10(j) Rulemaking 10 (Apr. 17, 2023) (arguing that FWS underestimates livestock loss due to wolf
predation). FWS expects its proposed 10(j) rule to significantly reduce wolf predation on livestock. U.S. FISH AND
WILDLIFE SERVICE, DRAFT ENVIRONMENTAL IMPACT STATEMENT COLORADO GRAY WOLF 10(J) RULEMAKING 6-4 (Feb.
2023).
Congressional Research Service

15

Experimental Populations Under the Endangered Species Act and Gray Wolves

interests (e.g., hunting, hiking, and camping).139 Section 10(j) allows FWS to address some of
these concerns by establishing a rule that allows both lethal and nonlethal take of wolves
(including harassing, hazing, and relocating) to protect livestock or game species. For naturally
occurring endangered wolves, those activities are prohibited without an individual permit from
FWS.140
The flexibility that Section 10(j) allows has also raised concerns for some stakeholders. For
example, some environmental groups have urged FWS to limit or prohibit lethal take of members
of the proposed experimental population in Colorado in order to restore natural predator-prey
relationships with existing wild ungulate populations.141 Some stakeholders have also argued that
permitting lethal take of wolves in the experimental population undermines the purpose of the
ESA to conserve endangered species because it may slow or prevent the attainment of a
sustainable wolf population.142 Finally, other stakeholders have asserted that wolf predation on
livestock is rare, and as a result FWS should restrict or prohibit take under its 10(j) rule for
wolves.143
Congress may consider whether to leave such decisions to the discretion of the agency or to
provide more direction as to how experimental populations in general or gray wolf populations in
particular should be managed. Congress may also consider legislation directly targeting
stakeholder concerns outside the 10(j) context, such as through depredation programs for
livestock owners in Colorado or generally. More fundamentally, Congress could also direct FWS
to, or prohibit FWS from, establishing experimental populations of particular species.

Author Information

Erin H. Ward
Benjamin M. Barczewski
Legislative Attorney
Legislative Attorney

139 See, e.g., Board of County Commissioners of Rio Blanco County, Colo., Comment Letter on Establishment of a
Nonessential Experimental Population of the Gray Wolf in Colorado (Apr. 18, 2023) (noting concerns over wolf
predation on livestock); Colo. Wildlife Conservation Project, Comment Letter on Reintroduced Gray Wolves to be
Established as a Nonessential Experimental Population in Colorado under ESA Section 10(j) 4 (Apr. 18, 2023) (noting
concerns related to impacts of wolf predation on game species).
140 Compare 88 Fed. Reg. at 10,271-72, with 16 U.S.C. § 1538(a)(1).
141 See Defs. of Wildlife, Comment Letter on Establishment of a Nonessential Experimental Population of the Gray
Wolf in Colorado (Apr. 18, 2023); Ctr. for Biological Diversity, Comment Letter on Proposed rule and draft
environmental impact statement on establishment of a nonessential experimental population of the gray wolf in
Colorado, 88 Fed. Reg. 10,258 (Feb. 17, 2023) (Apr. 18, 2023).
142 See Ctr. for Biological Diversity, Comment Letter on Proposed rule and draft environmental impact statement on
establishment of a nonessential experimental population of the gray wolf in Colorado, 88 Fed. Reg. 10,258 (Feb. 17,
2023) (Apr. 18, 2023).
143 The Humane Soc’y of the U.S., Comment Letter on Establishment of a Nonessential Experimental Population of the
Gray Wolf in Colorado 4 (FWS-R6-ES-2022-0100) (Apr. 19, 2023) (arguing that data shows that wolves have a
negligible impact on livestock and lethal removal can make impacts on livestock from wolves worse).
Congressional Research Service

16

Experimental Populations Under the Endangered Species Act and Gray Wolves

Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan
shared staff to congressional committees and Members of Congress. It operates solely at the behest of and
under the direction of Congress. Information in a CRS Report should not be relied upon for purposes other
than public understanding of information that has been provided by CRS to Members of Congress in
connection with CRS’s institutional role. CRS Reports, as a work of the United States Government, are not
subject to copyright protection in the United States. Any CRS Report may be reproduced and distributed in
its entirety without permission from CRS. However, as a CRS Report may include copyrighted images or
material from a third party, you may need to obtain the permission of the copyright holder if you wish to
copy or otherwise use copyrighted material.

Congressional Research Service
R47581 · VERSION 4 · UPDATED
17