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Distinct Population Segments, 4(d) Rules, and Experimental Populations

There are features built into the Endangered Species Act (ESA) and its implementing regulations that give the U.S. Fish and Wildlife Service (USFWS) flexibility in listing, protecting, managing, and recovering species that need the ESA's protections.

Distinct Population Segments

In addition to the listing and delisting of species and subspecies, the ESA allows the listing/delisting of Distinct Population Segments of vertebrate species (i.e., animals with backbones, mammals, birds, fish, reptiles, and amphibians). A Distinct Population Segment is a portion of a species' or subspecies' population or range. The Distinct Population Segment is described geographically instead of biologically, such as "all members of XYZ that occur north of 40 north latitude."

The use of Distinct Population Segments is a benefit to species conservation and a benefit to people whose activities may be affected by the ESA's prohibitions. Conservation efforts are more effective and less costly if they are started early and a Distinct Population Segment listing makes earlier listings possible. By listing a Distinct Population Segment, we apply the ESA's protections only to the deteriorating portion of a species' range. Threats can then be addressed in that specific (and smaller) area instead of waiting until the entire species has declined to the point that listing the entire species throughout its range is necessary.

Also, the USFWS uses Distinct Population Segment listings to customize application of the ESA across the range of listed vertebrate species. For listed species with improving populations, we can delist or reclassify (from endangered to threatened) a Distinct Population Segment. By doing this we remove or reduce the ESA's protections from part of the listed species' range (where it is doing well) while keeping full ESA protection for the Distinct Population Segment of that species that has not yet experienced recovery.

The USFWS's policy for designating Distinct Population Segments is sometimes called the Vertebrate Population Policy. This policy contains the criteria that must be met for a portion of a species' population to be designated as a Distinct Population Segment. Those criteria include the requirements that a Distinct Population Segment must be discrete and significant. This policy was published in the Federal Register (61 FR 4722-4725; February 7, 1996) and can be found on the Web at .

Examples of currently listed Distinct Population Segments:

the northern population of the copperbelly water snake

the interior population of the least tern

the northern population of the bog turtle

Section 4(d) Special Rules

Section 4(d) of the ESA allows the USFWS to establish special regulations for threatened (not endangered) species, subspecies, and Distinct Population Segments. These "4(d) rules" take the place of the normal protections of the ESA and may either increase or decrease the ESA's normal protections. The ESA specifies that 4(d) rules must be "necessary and advisable to provide for the conservation of such species."

One use of 4(d) rules is to relax the normal ESA restrictions to reduce conflicts between people and the protections provided to the threatened species by the ESA. A 4(d) rule can be used in such a situation if those conflicts would adversely affect recovery and if the reduced protection would not slow the species' recovery. This type of 4(d) rule is already in effect for gray wolves. Under authority of a 4(d) rule, Minnesota wolves that have preyed on domestic animals can be trapped and killed by designated government agents. This 4(d) rule was developed to avoid even larger numbers of wolves being killed by private citizens who might otherwise take wolf control into their own hands. (For more details on this example of a section 4(d) special rule, refer to Title 50 Code of Federal Regulations 17.40(d).)

Experimental Populations

Re-establishing a threatened or endangered species in areas of its former range is often necessary for recovery. However, residents and businesses frequently oppose such reintroductions because they fear the presence of the species will also bring severe restrictions on the use of private and public land in the area. To overcome this serious obstacle to species reintroductions, Congress added the concept of experimental populations to the ESA. Experimental population designations are sometimes referred to as section 10(j) rules.

An experimental population is a geographically described group of reintroduced plants or animals that is isolated from other existing populations of the species. Members of the experimental population are considered to be threatened under the ESA, and thus can have special regulations written for them under section 4(d). In addition, if the experimental population is determined to be "nonessential" to the survival of the species, for some activities the experimental population is treated like a species that is proposed for listing as threatened or endangered. In other words, the nonessential experimental population is not given the full protections of the ESA.

Among numerous examples of experimental populations are the Colorado pikeminnow, the southern sea otter, the gray wolf, and the black-footed ferret.


These three aspects of the ESA all can promote the recovery of declining species by fine-tuning the protections of the ESA. This fine-tuning minimizes adverse impacts on people and society while maximizing the likelihood of eventual recovery and delisting of the species. Thus, humans and rare species both benefit from their careful use.


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