U.S. Fish & Wildlife Service

ESA Definitions

Below is a list of words and phrases commonly used among these pages focused on endangered species and the act that protects them. They are provided here to help broaden public awareness on how the Endangered Species Act works and how it the Endangered Species Program operates.



All activities or programs of any kind authorized, funded, or carried out, in whole or in part, by Federal agencies in the United States or upon the high seas. Examples include, but are not limited to: (a) actions intended to conserve listed species or their habitat; (b) the promulgation of regulations;(c) the granting of licenses, contracts, leases, easements, rights-of-way, permits, or grants-in-aid; or (d) actions directly or indirectly causing modifications to the land, water, or air [50 CFR §402.02].

action area

All areas to be affected directly or indirectly by the Federal action and not merely the immediate area involved in the action [50 CFR §402.02].

biological assessment

Information prepared by, or under the direction of, a Federal agency to determine whether a proposed action is likely to: (1) adversely affect listed species or designated critical habitat; (2) jeopardize the continued existence of species that are proposed for listing; or (3) adversely modify proposed critical habitat. Biological assessments must be prepared for "major construction activities." The outcome of this biological assessment determines whether formal consultation or a conference is necessary. The contents of a biological assessment are up to the discretion of the action agency, although the Code of Federal Regulations does provide a list of recommended contents (50 CFR §402.12(f)).

biological opinion

A biological opinion is the document that states the opinion of the Service as to whether or not the Federal action is likely to jeopardize the continued existence of listed species or result in the destruction or adverse modification of critical habitat. It also includes a summary of the information on which the opinion is based and a detailed discussion of the effects of the action on listed species or designated critical habitat.


these are animals that are caught incidentally when another species is targeted. For example, albatross, sea turtles and sharks can be caught on hooks intended to catch swordfish, tuna, or halibut.

critical habitat

When a species is proposed for listing as endangered or threatened under the Endangered Species Act, the Service must consider whether there are areas of habitat believed to be essential to the species' conservation. Those areas may be proposed for designation as "critical habitat."


removal of a species from the threatened and endangered species list. When a species’ population has recovered to the point that it is secure, threats no longer exist or are a low level, and its habitat is adequate and safe, we take the species off the threatened and endangered species list. Read about the species that live in Alaska and have been delisted: the Aleutian Canada Goose, the Arctic peregrine falcon, and the American peregrine falcon.

distinct population segment (DPS)

In 1996, the Service developed a policy that defined DPS. A DPS is a portion of a vertebrate population that is reproductively isolated from other portions of the population (discrete) and which is also significant to the evolutionary legacy of the species. A DPS can be listed under the ESA just like species and subspecies; however, only vertebrates (animals with backbones) qualify, not plants or invertebrates.


reclassification of a species from endangered to threatened or from threatened to removal from the threatened and endangered species list. Downlisting may be recommended after a 5-year review is conducted, but the Service must write a proposed rule and final rule before a species can be reclassified.


a species that is in danger of extinction through all or a significant portion of its range

incidental take

take that results from a Federal action but is not the purpose of the action


Under the ESA, jeopardy occurs when an action is reasonably expected, directly or indirectly, to diminish a species’ numbers, reproduction, or distribution so that the likelihood of survival and recovery in the wild is appreciably reduced.

likely to adversely affect

The appropriate finding in a biological assessment (or conclusion during informal consultation) if any adverse effect to listed species may occur as a direct or indirect result of the proposed action or its interrelated or interdependent actions, and the effect is not: discountable, insignificant, or beneficial (see definition of "is not likely to adversely affect"). In the event the overall effect of the proposed action is beneficial to the listed species, but is also likely to cause some adverse effects, then the proposed action "is likely to adversely affect" the listed species. If incidental take is anticipated to occur as a result of the proposed action, a "likely to adversely affect" determination should be made. A "likely to adversely affect" determination requires the initiation of formal section 7 consultation.

may affect

The appropriate conclusion made by a Federal action agency when a proposed action may pose any effects on listed species or designated critical habitat. When the Federal agency proposing the action determines that a "may affect" situation exists, then they must either initiate formal consultation or seek written concurrence from the Services that the action "is not likely to adversely affect" [see definition below] listed species.

Not likely to adversely affect - The appropriate conclusion when effects on listed species are expected to be discountable, insignificant, or completely beneficial. Discountable effects are those extremely unlikely to occur. Insignificant effects - relate to the size of the impact and should never reach the scale where take occurs. Beneficial effects - contemporaneous positive effects without any adverse effects to the species. Based on best judgment, a person would not: (1) be able to meaningfully measure, detect, or evaluate insignificant effects; or (2) expect discountable effects to occur.

Recovery for threatened and endangered species, recovery means that they are secure members of their ecosystem and that threats are to the individuals or their habitat are at a low enough level that the population will remain viable.

scientific and commercial data

to assure the quality of the biological, ecological, and other information used in the implementation of the Act, it is the policy of the Services to:

  1. evaluate all scientific and other information used to ensure that it is reliable, credible, and represents the best scientific and commercial data available;
  2. gather and impartially evaluate biological, ecological, and other information disputing official positions, decisions, and actions proposed or taken by the Services;
  3. document their evaluation of comprehensive, technical information regarding the status and habitat requirements for a species throughout its range, whether it supports or does not support a position being proposed as an official agency position;
  4. use primary and original sources of information as the basis for recommendations;
  5. retain these sources referenced in the official document as part of the administrative record supporting an action;
  6. collect, evaluate, and complete all reviews of biological, ecological, and other relevant information within the schedules established by the Act, appropriate regulations, and applicable policies; and
  7. require management-level review of documents developed and drafted by Service biologists to verify and assure the quality of the science used to establish official positions, decisions, and actions taken by the Services during their implementation of the Act.

service office

The Service has several offices in Alaska that work with partners on threatened and endangered species. Field offices are in Juneau, Kenai, Anchorage, and Fairbanks. The Regional Office is in Anchorage. See our contact page to reach the office nearest you.


To harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, or collect or attempt to engage in any such conduct [ESA §3(19)]. Harm is further defined by FWS to include significant habitat modification or degradation that results in death or injury to listed species by significantly impairing behavioral patterns such as breeding, feeding, or sheltering. Harass is defined by FWS as actions that create the likelihood of injury to listed species to such an extent as to significantly disrupt normal behavior patterns which include, but are not limited to, breeding, feeding or sheltering [50 CFR §17.3].


any species which is likely to become an endangered species (one in danger of extinction) within the foreseeable future throughout all or a significant portion of its range.