Recently, at the direction of the Biden White House, the U.S. Fish and Wildlife Service and National Marine Fisheries Service proposed to rescind a Trump-era rule that defines the term “habitat” for the purpose of informing the designation of “critical habitat” by those two agencies under the federal Endangered Species Act (ESA). While the federal wildlife agencies offer certain rationales for the proposed rescission, there is no question that the primary motivation is to provide discretion to federal wildlife agency regions, offices, and line staff to make ad hoc determinations regarding the meaning of the term “habitat.” This shift away from an explicit definition of habitat that provides for transparency and consistency in agency decisions is contrary to sound public policy, will erode faith in government decision-making, and is unlikely to lead to improved conservation outcomes for species protected under the Act.

The principal protection afforded to species listed as threatened or endangered under the ESA derives from the prohibitions in section 9 of the Act, most notably the prohibition on “take” of protected species. But the ESA also includes more limited protections to areas designated as “critical habitat.” While the Act does not define habitat, it does define “critical habitat” as areas occupied by the species at the time it was listed “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,” and areas not occupied by the species at the time it was listed “upon a determination by the Secretary that such areas are essential for the conservation of the species.”


In the absence of a definition of the term habitat prior to December 2020, there was substantial controversy and confusion regarding the scope of the federal government’s authority to designate critical habitat.  This came to a head in a case ultimately decided by the U.S. Supreme Court in 2018 regarding the designation of critical habitat for the dusky gopher frog in the American southeast.  In its decision, the Court clarified that “’critical habitat’ is the subset of ‘habitat’ that is ‘critical’ to the conservation of an endangered species.” In other words, as a threshold matter, an area must be habitat before the federal government can consider whether to designate it as critical habitat. 


That begged the question, what is habitat for species that are protected under the ESA and led the government down the path to the definition promulgated just a year ago.  For the purposes of designating of critical habitat, the federal wildlife agencies then defined habitat as “the abiotic and biotic setting that currently or periodically contains the resources and conditions necessary to support one or more life processes of a species.”


The agencies considered adopting a definition drawn from the literature in the field of ecology, but ultimately decided that in that field there was no settled definition of the concept. That may seem nonsensical on its face. One might fairly presume that researchers and practitioners in wildlife and fish biology work from a common definition of the foundational term habitat, but it’s not so. In fact, an article on the subject 25 years ago by Hall and colleagues surveyed the literature on habitat and found that the term is used in a manner that is vague and imprecise in most cases, and where imperiled species are involved references to habitat can be dangerously unclear or incorrect.


Despite this and the routine misuse of the term “habitat” in agency determinations and implementation of conservation actions under the Act, there are certain elements of the definition of habitat on which there is broad agreement in the scientific community.


  • Habitat is a species-specific concept. Each species has its own habitat, which may overlap in space and time with habitats of other species.

  • Habitat is composed of both (i) a combination of abiotic (physical) and biotic (living) components and (ii) ecological processes.

  • Habitat must be capable of supporting a species during one or more of its life stages. It must be habitable, though it need not be occupied at all times and may in fact be unoccupied for extended periods of time.

Each of those three elements of habitat are incorporated into the present definition that ostensibly guides the federal wildlife agencies.


In the proposed rule seeking to rescind but not replace the definition of habitat, the federal wildlife agencies offer the first and principal rationale that the definition could constrain the ability of the federal wildlife agencies to designate landscape areas as critical habitat — specifically, where such landscape areas do not currently or intermittently contain the resources and conditions necessary to support one or more life processes of a species. 


The agencies reason that certain geographic areas should be considered habitat because even though they do not currently accommodate the resources and conditions necessary to support one or more life processes of a species, they could do so at some point in the future, either as a consequence of natural processes or human intervention. The problem with that reasoning is that it is directly at odds with the third element identified above — an area must be habitable to be habitat. The concept of habitat loses its meaning if it extends to any area that may at some point be habitat as a consequence of natural processes or human intervention.


An extreme example is the high Arctic, where pack ice is melting at a frightening rate. It is habitat for polar bears and ringed seals, of course, but is it habitat for Hawaiian hoary bats or unarmored three-spine sticklebacks?  Not if one looks to the literature on ecology (for example, Odum 1971, Morrison et al 1992, Kearney 2006, Krausman and Morrison 2016). Or, for that matter, the definition of habitat promulgated in the final rule. According to the federal wildlife agencies under the current administration, the fatal flaw in definitions of habitat proposed by experts in ecology is that the warming Arctic may yet become habitat for temperate zone species that are presently relegated to habitats in southern latitudes. The federal wildlife agencies reason that they must have latitude to determine that any area is habitat for a species so long as it could at some point in the future support one or more life stages of that species, either as a result of processes beyond immediate human control or due to direct human intervention.


A second rationale for rescission of the standing rule set forth in the proposed rule is that “areas not currently in an optimal state to support the species could nonetheless be considered ‘habitat.’” Skip the fact that habitats are never in an “optimal state,” the rule offers a straw man argument since the existing definition does not indicate that quality is a factor in determining whether a landscape area can be considered habitat. Habitat quality is important to consider in all conservation planning circumstances. Just as habitat in an “optimal state” is habitat, so too is habitat in other, lesser conditions.


In the proposed rule the federal wildlife agencies make a series of claims that taken together suggest there is risk in any definition they might promulgate, arguing it is likely to be “overly vague” and could “create conflict or inconsistencies” and is “inherently confusing.”  In truth, the opposite is the reality. The absence of a regulatory definition leaves the agencies’ regional and field offices and line staff to make ad hoc determinations regarding the meaning of the term “habitat.” As a result, uncertainty both for the impacted species and regulated communities increases without necessarily leading to improved conservation outcomes for those species. For that reason, it is difficult not to view the agencies’ arguments as disingenuous.


The reality is that under the previous administration the federal wildlife agencies saw value in defining habitat for the purpose of designating critical habitat, because it would provide guidance to decision-makers and resource managers, promote the uniform application of the law, and reduce regulatory uncertainties. In contrast, under the current administration the federal wildlife agencies see value in the absence of a definition of habitat for the purpose of designating critical habitat, because it would provide discretion to agency regions, offices, and staff. That would potentially allow extraordinarily broad geographic areas to be characterized as habitat for listed species, thereby expanding regulatory authority. While some may see greater value in the latter set of goals than the former, the new rule undermines sound public policy both because it will reduce transparency in government decision-making and because it will result in inconsistent application of the law. It is not possible to make a credible argument that the current proposal is more in line with science and the scientific literature regarding habitat than the existing rule.



Literature cited


Kearney, M. 2006. Habitat, environment, and niche: what are we modelling. Oikos 115:186-191.

Krausman, P.R. and M.L. Morrison. 2016. Journal of Wildlife Management 80:1143-1144.

Morrison, M.L., B.G. Marcot, and R.W. Mannan. 1992. Wildlife-habitat relationships: concepts and applications. Island Press, Washington D.C. 

Odum, E.P. 1971. Fundamentals of ecology. Saunders, Philadelphia.


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