Opinion for the Court filed by Chief Judge MIKVA, except section 11(A)(1), which is filed per curiam. Opinion concurring in section 11(A)(1) filed by Chief Judge MIKVA. Opinion concurring in section 11(A)(1) filed by Circuit Judge WILLIAMS.
Opinion dissenting in part filed by Circuit Judge SENTELLE.
MIKVA, Chief Judge:
Appellants, a group of non-profit citizens’ groups, lumber companies, and lumber trade associations, oppose two regulations promulgated by the Fish and Wildlife Service (“FWS” or “agency”) under the Endangered Species Act (“ESA”), 16 U.S.C. §§ 1531-44. They appeal a memorandum opinion and order by the district court upholding the regulations by summary judgment and denying appellants’ own motion for summary judgment. We find that the challenged regulations are reasonable interpretations of the ESA. We also reject appellants’ claim that one of the regulations is void for vagueness. We therefore affirm the district court’s judgment.
I. BACKGROUND
The Endangered Species Act of 1973 is a multifaceted and comprehensive law directed toward halting the extinction of species. It is implemented primarily by the Fish and Wildlife Service, an agency of the Department of the Interior. The ESA employs a number of techniques to preserve endangered and threatened species, including land acquisition by the government, the implementation of conservation programs by federal agencies, and the prohibition of various federal and private actions that harm listed species.
Among the more important sections of the ESA is 16 U.S.C. § 1538(a)(1), which forbids any person from committing any of a broad array of activities deemed dangerous to the continued survival of endangered fish and wildlife species. This appeal focuses largely on the prohibition against “taking” an endangered species.
[W]ith respect to any endangered species of fish or wildlife listed pursuant to section 1533 of this title it is unlawful for any person subject to the jurisdiction of the United States to ... take any such species within the United States or the territorial sea of the United States[.]
16 U.S.C. § 1538(a)(1)(B) (emphasis added).
The ESA defines “take” as follows: “[T]o 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) (emphasis added). Much of the controversy surrounding this definition has concerned the meaning of “harm” and the degree to which this term encompasses damage to habitats. One of the FWS regulations challenged by appellants states:
Harm in the definition of “take” in the Act means an act which actually kills or injures wildlife. Such act may include significant habitat modification or degradation where it actually kills or injures wildlife by significantly impairing essential behavioral patterns, including breeding, feeding, or sheltering.
50 C.F.R. § 17.3 (as amended in 1981). Appellants contend that this regulation’s inclusion of habitat modification within the meaning of “take” violates the ESA. They also assert that even if the regulation is not ultra vires of the ESA, it is not clear precisely what sort of habitat modification the regulation forbids. They therefore argue that this Court should either declare the regulation void for vagueness or adopt a limiting construction of the regulation, holding that “harm” occurs only where there is an intentionally-caused actual physical injury to a specific member of a listed wildlife species.
The other regulation under review extends the protections for endangered species to threatened species as well. The ESA defines an “endangered species” 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 ESA also protects [3]*3species that are in less immediate peril but are listed as “threatened species.” This term refers to “any species which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range.” 16 U.S.C. § 1532(20).
On its face, 16 U.S.C. § 1538(a)(1) applies its prohibitions, including the prohibition against takings, only to endangered species. However, the ESA allows the FWS to apply these prohibitions to threatened species, as well.
Whenever any species is listed as a threatened species ... the Secretary shall issue such regulations as he deems necessary and advisable to provide for the conservation of such species. The Secretary may by regulation prohibit with respect to any threatened species any act prohibited under section 1538(a)(1) of this title, in the case of fish or wildlife....
16 U.S.C. § 1533(d).
Appellants challenge a FWS regulation which, at one fell swoop, brings all threatened fish and wildlife species into the protective net of 16 U.S.C. § 1538(a)(1).
Except as provided in subpart A of this part, or in a permit issued under this part, all of the provisions in [50 C.F.R. § 17.21, the regulation implementing the section 1538(a)(1) prohibitions] shall apply to threatened wildlife....
50 C.F.R. § 17.31(a) (1978). Appellants argue that this regulation violates the ESA, because § 1533(d) requires the FWS to extend the prohibitions to threatened species on a species-by-species basis and to do so only after making a specific finding that each such extension was “necessary and advisable.”
The district court rejected appellants’ challenges to both 50 C.F.R. § 17.3 and § 17.-31(a) and granted summary judgment to the government. 806 F.Supp. 279. Because all of the issues in this appeal are issues of law, we will review the district court’s judgment de novo.
II. ANALYSIS
A. The “harm” regulation
1. Compliance with the Endangered Species Act
Appellants argue that the “harm” regulation, 50 C.F.R. § 17.3, violates the ESA, because the statute excludes habitat modification from the types of forbidden actions that qualify as “takings” of species. They assert that the ESA’s language and structure, as well as its legislative history, clearly demonstrate that Congress did not intend to prohibit habitat modification when it defined “take” to include “harm” to an endangered species. 16 U.S.C. § 1532(19). They claim that the meaning of harm should therefore be limited to direct physical injury to an identifiable member of a listed wildlife species.
We hold, per curiam, that the “harm” regulation.does not violate the ESA by including actions that modify habitat among prohibited “takings.”
2. The “void for vagueness” claim
Appellants also maintain that, on its face, the “harm” regulation is void for vagueness. They point out that the Ninth Circuit has interpreted the regulation broadly so that prohibited “harm” includes habitat modification even without proof that death or physical injury to a specific member of a listed species has occurred. Palila v. Hawaii Dept. of Land and Natural Resources, 852 F.2d 1106, 1107-09 (1988). The appellants could also have mentioned that the FWS itself offered a similarly broad interpretation of the regulation when it introduced the current definition of “harm” in 1981:
Some of the comments in favor of the redefinition ... viewed the action as limiting “harm” to direct physical injury to an individual member of the wildlife species. This was not the intent of the Service and the final redefinition addresses that perception. The purpose of the redefinition was to preclude claims of a Section 9 taking for habitat modification alone without any attendant death or injury of the protected wildlife. Death or injury, however, [4]*4may be caused by impairment of essential behavioral patterns which can have significant and permanent effects on a listed species.
46 Fed.Reg. 54,748, 54,748 (1981).
Appellants argue that so long as the regulation is subject to such broad constructions, it is impermissibly vague. They claim it will be left to the whims and predictions of biologists to determine when a habitat modification is “significant” and when such a modification “significantly impair[s] essential behavioral patterns.” 50 C.F.R. § 17.3 (emphasis added). Property owners, say appellants, will thus be subject to criminal sanctions under the ESA based on “some biologist’s subjective view.”
We are urged to address this problem by construing the regulation to state that “harm” occurs only where there is proof of an intentionally-caused physical injury to a specific member of a listed wildlife species. If we determine that the regulation does not require such proof, appellants contend that we should declare the entire regulation void for vagueness.
It is true, as appellants assert, that regulations with criminal sanctions must “define the criminal offense with sufficient definiteness that ordinary people can understand what conduct is prohibited and in a manner that does not encourage arbitrary and discriminatory enforcement.” Kolender v. Lawson, 461 U.S. 352, 357, 103 S.Ct. 1855, 1858, 75 L.Ed.2d 903 (1983). This principle, however, does not lead to the conclusion that any person can have a regulation wiped off the books (or prompt a limiting judicial construction of the regulation) merely by showing that it will be impermissibly vague in the context of some hypothetical application.
The seminal case concerning pre-enforcement facial challenges- on the grounds of vagueness is Village of Hoffman Estates v. Flipside, Hoffman Estates, Inc., 455 U.S. 489, 102 S.Ct. 1186, 71 L.Ed.2d 362 (1982). Flipside states that when a court examines such a challenge, “assuming the enactment implicates no constitutionally protected conduct, [the court] should uphold the challenge only if the enactment is impermissibly vague in all of its applications.” Id. at 495, 102 S.Ct. at 1191 (emphasis added). Although the Supreme Court does not state precisely what it means by “constitutionally protected conduct,” it is clear that it is referring primarily to the First Amendment expressive freedoms, which have long received special protection in vagueness cases. See, e.g. Smith, Sheriff v. Goguen, 415 U.S. 566, 573, 94 S.Ct. 1242, 1247, 39 L.Ed.2d 605 (1974). It is equally clear that the Court is not referring to economic activity, which modern vagueness cases have invariably afforded less protection. See, e.g., Flipside, 455 U.S. at 498, 102 S.Ct. at 1193 (“economic regulation is subject to a less strict vagueness test”).
Appellants, who are not currently the subject of an enforcement action under 50 C.F.R. § 17.3, nonetheless contend that the regulation will inhibit their ability to develop their land, especially by harvesting timber. In other words, the conduct implicated by this case is economic activity. (To the degree that appellants contend that the regulation results in a “taking” of their property in the Fifth Amendment sense, their remedy would be compensation, not a voiding of the regulation.) In accordance with Flipside, we therefore will not find 50 C.F.R. § 17.3 void for vagueness unless the regulation is imper-missibly vague in all of its applications.
In fact, the regulation contains features that prevent it from being invariably vague as applied. The definition of “harm” explicitly limits prohibited habitat modification to that which “actually kills or injures wildlife.” 50 C.F.R. § 17.3. Moreover, in order to establish a civil or criminal violation under the “take” provision of the ESA or a regulation implementing that provision, the government must establish that the charged party knowingly violated the statute or regulation. 16 U.S.C. § 1540(a) and (b). The Supreme Court has recognized that “a scienter requirement may mitigate a law’s vagueness, especially with respect to the adequacy of notice to the complainant that his conduct is proscribed.” Flipside, 455 U.S. at 499, 102 S.Ct. at 1193.
In light of these limitations, there are obviously types of activity, including habitat modification, that 50 C.F.R. § 17.3 clearly prohib[5]*5its without a hint of vagueness. For example, it obviously forbids the very sort of conduct that appellants argue it should be limited to — habitat modification that causes ascertainable physical injury or death to an individual member of a listed species. Furthermore, § 17.3 unquestionably prohibits major acts of habitat degradation that destroy a species’ ability to breed, feed, or shelter. For instance, a person aware of the regulation would undoubtedly be held accountable for clear-cutting an entire forested area known to be populated by spotted owls.
Because the regulation is not vague.in all of its applications, we may not declare it void on its face. We do not dismiss the possibility that some enforcement contexts may arise in which the challenged regulation would be considered impermissibly vague. Under Flipside, however, the fact that there might be some type of activity whose legality is blurry under the regulation does not render the entire regulation facially invalid. Specific vagueness concerns about the regulation can be addressed when and if they are properly raised in the framework of a concrete challenge to a particular application of the regulation.
Appropriate judicial restraint obligates us to wait for specific applications of the regulation to arise, for, as Flipside observed, the government may in the meantime take further steps “that will sufficiently narrow potentially vague or arbitrary interpretations” of the regulation. 455 U.S. at 504, 102 S.Ct. at 1196. “Although it is possible that specific future applications ... may engender concrete problems of constitutional dimension, it will be time enough to consider any such problems when they arise.” Joseph E. Seagram & Sons, Inc. v. Hostetler, 384 U.S. 35, 52, 86 S.Ct. 1254, 1265, 16 L.Ed.2d 336 (1966), overruled on different grounds by Healy v. Beer Institute, Inc., 491 U.S. 324, 109 S.Ct. 2491, 105 L.Ed.2d 275 (1989).
B. The blanket extension of protections to all threatened species
Appellants also challenge 50 C.F.R. § 17.31(a), by which the FWS extended the 16 U.S.C. § 1538(a)(1) prohibitions as to endangered species to all threatened species as well. The FWS issued this regulation under the authority granted to it by the ESA at 16 U.S.C. § 1533(d). For the sake of convenience, we quote that provision again:
Whenever any species is listed as a threatened species ... the Secretary shall issue such regulations as he deems necessary and advisable to provide for the conservation of such species. The Secretary may by regulation prohibit with respect to any threatened species any act prohibited under section 1538(a)(1) of this title, in.the case of fish or wildlife....
16 U.S.C. § 1533(d).
The challenged regulation provides in relevant part:
(a) Except as provided in subpart A of this part, or in a permit issued under this subpart, all of the provisions in § 17.21 [which implements the prohibitions contained in 16 U.S.C. § 1538(a)(1) ] shall apply to threatened wildlife, except § 17.-21(c)(5).
* * * ¡'fi * *
(c) Whenever a special rule in §§ 17.40 to 17.48 applies to a threatened species, none of the provisions of [paragraph (a) ] of this section will apply. The special rule will contain all the applicable prohibitions and exceptions.
50 C.F.R. § 17.31.
In short, the FWS has, .with this regulation, established a regime in which the prohibitions established for endangered species are extended automatically to all threatened species by a blanket rule and then withdrawn as appropriate, by special rule for particular species and by permit in particular situations. Although appellants’ dissatisfaction is focused entirely on this treatment of the “take” prohibition, the regulation applies all of the § 1538(a)(1) prohibitions to threatened species in this manner.
Appellants contend that the FWS has created the system upside down, in violation of the ESA. They maintain that § 1533(d) requires the Secretary to extend the prohibitions to threatened species on a species-by-species basis. They also assert that the statute requires the agency to explain in each [6]*6instance why it is “necessary and advisable” to apply the prohibitions to a threatened species.
We are not persuaded. As was the case with the “harm” regulation, there is no clear indication that § 17.31(a) violates the intent of the ESA. The statute does not unambiguously compel the agency to expand regulatory protection for threatened species only by promulgating regulations that are specific to individual species. In light of the substantial deference we thus owe the agency under the principles of Chevron USA, Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837, 104 S.Ct. 2778, 81 L.Ed.2d 694 (1984), we uphold the challenged regulation as a reasonable interpretation of the statute.
Appellants argue that the plain language of the ESA establishes that Congress intended regulations extending the § 1538(a)(1) prohibitions to threatened species to apply only to individual species. They focus on the first sentence of § 1533(d), pointing out its clear use of the singular. (“Whenever any species is listed as a threatened species-”) They maintain that in light of the first sentence’s unambiguous reference to regulations for a single threatened species, the phrase “any threatened species” in the second sentence must also refer to the singular. Appellants finally note that the Senate Report bolsters their interpretation of the language of § 1533(d) as authorizing regulations tailored to individual species.
[The section] requires the Secretary, once he has listed a species of fish or wildlife as a threatened species, to issue regulations to protect that species. Among other protective measures available, he -may make any or all of the acts and conduct defined as “prohibited acts” ... as to “endangered species” also prohibited acts as to the particular threatened species.
S.Rep. No. 93-307, 93d Cong., 1st Sess. 8 (1973), U.S.Code Cong. & Admin.News 1973, pp. 2989, 2996. (emphasis added).
It is, however, less than clear that the language of § 1533(d) is actually intended to require regulations extending the § 1538(a)(1) prohibitions- to apply only to individual species. The reference to “any threatened species” in the second sentence of § 1538(d) could just as easily mean “any or all threatened species” as “any one threatened species.” And appellants’ claim that the clear meaning of the first sentence should control our interpretation of the second is not necessarily valid, for, as appellees argue, the two sentences may represent separate grants of authority. It is possible that it is the second sentence alone that grants the agency authority to extend the § 1538(a)(1) prohibitions to threatened species. See infra pp. 7-8.
Furthermore, in response to appellants’ use of the Senate Report, appellees counter-pose the use of plural language in the discussion of § 1533(d) in the House Report.
The Secretary is authorized to issue appropriate regulations to protect endangered or threatened species; he may also make specifically applicable any of the prohibitions with regard to threatened species that have been listed in section 9(a) as are prohibited with regard to endangered species. Once an animal is on the threatened list, the Secretary has almost an infinite number of options available to him with regard to the permitted activities for those species. He may, for example, permit taking, but not importation of such species, or he may choose to forbid both taking and importation but allow the transportation of such species.
H.R.Rep. No. 93-412, 93d Cong., 1st Sess. 12 (1973). The possible conflict between the two reports, as well as the apparent inconsistency within the above-quoted paragraph itself as to singular and plural, shows the perils of attempting to use ambiguous legislative history to clarify ambiguous words within statutes.
In any ease, even assuming that the reference to “any threatened species” in the second sentence of § 1533(d) is singular, the statute still would not clearly forbid the FWS from proceeding in the manner it did. Appellants impart inappropriate significance to the use of the singular versus the plural. The very first provision of the United States Code states, “In determining the meaning of any Act of Congress, unless the context indicates otherwise ... words importing the sin-[7]*7guiar include and apply to several persons, parties, or things [and] words importing the plural include the singular.” 1 U.S.C. § 1.
Furthermore, regardless of the use of singular and plural words, § 1533(d) simply does not speak directly to the question of whether the FWS must promulgate protections speeies-by-species or may extend such protection in a single rulemaking. We will rarely impose a particular procedural strategy on an agency when the relevant statute does not even explicitly address how the agency should proceed. “In [the] process of filling any gap left, implicitly or explicitly, by Congress, the courts must respect the interpretation of the agency to which Congress has delegated the responsibility for administering the statutory program.” INS v. Cardoza-Fonseca, 480 U.S. 421, 448, 107 S.Ct. 1207, 1221, 94 L.Ed.2d 434 (1987) (citations omitted).
Appellants attempt to bolster their argument by claiming that the design and structure of the ESA unavoidably require the FWS to extend the § 1533(d) prohibitions to threatened species on a species-by-species basis. They contend that Congress created the distinct categories of endangered and threatened species specifically so that each category would receive the precise degree of protection it required. By presumptively according the very same protections to threatened species as to endangered species, they argue, the FWS is violating this statutory design.
This argument also fails, because regardless of the ESA’s overall design, § 1533(d) arguably grants the FWS the discretion to extend maximum protection to all threatened species at once if, guided by its expertise in the field of wildlife protection, it finds it expeditious to do so. In any case, even assuming that Congress intended there to be an indelible difference between the treatment of endangered species and threatened species, appellants fail to acknowledge the very real differences that exist even under the challenged regulation. As noted above, supra, p. 5, the regulation contemplates the issuance of “special rules” for individual threatened species. 50 C.F.R. § 17.31(c). The FWS has actually issued special rules for a substantial number of the fish and wildlife species listed as threatened. See 50 C.F.R. §§ 17.40-48.
Moreover, FWS regulations provide for the issuance of permits, under certain circumstances, to individual applicants who request permission to commit otherwise prohibited activities with respect to threatened wildlife. 50 C.F.R. § 17.32. Similar permits are available with regard to endangered species, 50 C.F.R. §§ 17.22-.23, but the FWS has made clear that they are more readily available for threatened species.
The new Subpart D would establish a set of blanket prohibitions for threatened wildlife. These are the same prohibitions that the Act provides for endangered wildlife. ... The new Subpart D would then provide for permits for any threatened wildlife. These permits would be more liberal than permits available under Sub-part C ... for endangered wildlife, in that they would be available for more purposes, and the strict procedural rules for endangered wildlife permits would not apply.
40 Fed.Reg. 28,713 (1975).
The FWS has thus maintained a two-tier approach to species protection. Consequently, even if the ESA does in fact mandate that the agency preserve different approaches to the conservation of endangered and threatened species; the regulatory scheme satisfies the statute.
Lastly, appellants claim that 50 C.F.R. § 17.31(a) violates the ESA because § 1533(d) requires the FWS to make individualized formal findings that the prohibitions are “necessary and advisable” for the conservation each threatened species to which they are extended. Not only did the agency fail to make such findings for each individual species; it did not even make such a finding in regard to the category of threatened species as a whole when it issued the blanket extension of prohibitions.
As appellees argue, however, there is a reasonable reading of § 1533(d) that would not require the FWS to issue formal “necessary and advisable” findings when extending the prohibitions to threatened species. According to this interpretation, the two sen[8]*8tences of § 1533(d) represent separate grants of authority. The second sentence gives the FWS discretion to apply any or all of the § 1538(a)(1) prohibitions to threatened species without obligating it to support such actions with findings of necessity. Only the first sentence of § 1538(d) contains the “necessary and advisable” language and mandates formal individualized findings. This sentence requires the FWS to issue whatever other regulations are “necessary and advisable,” including regulations that impose protective measures beyond those contained in § 1538(a)(1).
In sum, we find it far from clear that 16 U.S.C. § 1533(d) requires the FWS to extend protections to threatened species on a spe-eies-by-species basis or that it mandates that the agency issue formal findings of necessity to support each such extension. In light of the statute’s ambiguity, the challenged FWS regulation is a reasonable and permissible construction of the ESA. We therefore uphold 50 C.F.R. § 17.31(a).
III. CONCLUSION
Neither the regulation defining harm nor the regulation extending blanket protection to threatened species is an unreasonable interpretation of the Endangered Species Act. Moreover, the “harm” regulation is not im-permissibly vague on its face. We therefore affirm the judgment of the district court and uphold both regulations.
It is so ordered.