Opinion of the Court
phasis added). Only the “habitat” of the endangered species is eligible for designation as critical habitat. Even if an area otherwise meets the statutory definition of unoccupied critical habitat because the Secretary finds the area essential for the conservation of the species, Section 4(a)(3)(A)(i) does not authorize the Secretary to designate the area as critical habitat unless it is also habitat for the species.
The Center for Biological Diversity contends that the statutory definition of critical habitat is complete in itself and does not require any independent inquiry into the meaning of the term “habitat,” which the statute leaves undefined. Brief for Intervenor-Respondents 43–49. But the statutory definition of “critical habitat” tells us what makes habitat “critical,” not what makes it “habitat.” Under the statutory definition, critical habitat comprises areas occupied by the species “on which are found those physical or biological features (I) essential to the conservation of the species and (II) which may require special management considerations or protection,” as well as unoccupied areas that the Secretary determines to be “essential for the conservation of the species.” 16 U. S. C. §1532(5)(A). That is no baseline definition of habitat–it identifies only certain areas that are indispensable to the conservation of the endangered species. The definition allows the Secretary to identify the subset of habitat that is critical, but leaves the larger category of habitat undefined.
The Service does not now dispute that critical habitat must be habitat, see Brief for Federal Respondents 23, although it made no such concession below. Instead, the Service argues that habitat includes areas that, like Unit 1, would require some degree of modification to support a sustainable population of a given species. Id., at 27. Weyerhaeuser, for its part, urges that habitat cannot include areas where the species could not currently sur-
