Applied Ecosystem Services, LLC

The Environmental Issues Doctor

Photo of What is a Species?


Estimated reading time: 9 minutes

For enforcement of the Endangered Species Act federal regulators (the Fish & Wildlife Service and NOAA Fisheries/National Marine Fisheries Service) consider local populations and management units (stocks) to be species, regardless of biological theory.

NOAA Fisheries calls these small groups “Evolutionarily Significant Units” (ESU); the US Fish & Wildlife Service calls them “Discrete Population Segments” (DPS).

These political definitions affect how decisions are made regarding ESA-listed biota in the areas where businesses have their operations. Understanding how these policies differ from biological reality helps you avoid, or resolve, concerns about your operations.

In this post I discuss how federal regulators define species under the Endangered Species Act, the ESA. You need to understand this if your operation requires ESA compliance. Knowing what is a species for ESA compliance allows you to document that your operation is not likely to have adverse impacts on a listed species.

For most biologists a species is a unique group of organisms capable of reproducing more of themselves. The two federal agencies tasked with ensuring compliance with the ESA are the US Fish and Wildlife Service (FWS) and NOAA Fisheries (formerly the National Marine Fisheries Service, NMFS).

In 1995 these two agencies worked together and decided the biological definition of species was inadequate. Instead, they decided that separate management units are each a species under the ESA. To justify these definitions the two agencies agreed on a policy to identify these management units, or local populations.

This policy uses three elements to be considered in decisions about their status as endangered or threatened under the ESA:

  1. Discreteness.
  2. Significance.
  3. Endangered or threatened.

The first two elements—discreteness and significance—can be discussed in scientific terms. The third element asks (quote) is the population segment, when treated as if it were a species, endangered or threatened (end quote). This is non-scientific, circular reasoning and should have been dropped from further consideration by the resource agencies. But, it wasn’t.

The policy’s fundamental assumption that a species can be managed as separate groups for protection and recovery is not been scientifically justified. Individuals or population segments can be grouped using appearance, behavior, or genetics. Each characteristic could be more or less important in assigning individuals to a group, depending upon the species. When we see an individual animal by itself we cannot confidently define its membership in a DVP or ESU. For example, some grey wolves from eastern Oregon have migrated to California and Arizona.

To illustrate discreteness and significance I’m using NOAA Fisheries’ Pacific salmon ESUs as an example.

Element 1: discreteness

This element can be satisfied by meeting either of two criteria:

  1. being markedly separated from other populations of the same taxon, or
  2. it is delimited by international boundaries.

Notice the subjectivity in this definition. What separation distance is ‘markedly’? And, why does an international boundary delimit a population? Is a herd of moose whose members cross back and forth between Canada and the US a non-species as far as the ESA is concerned because the heard is not restricted to the US?

Chinook salmon in the Columbia and Snake Rivers are all the same species, but adults return from the Pacific Ocean to reproduce in the stream where they were born from April to November.

There are three peaks in this seven-month period based on when the fish migrate past Bonneville Dam, 146 river miles from the ocean. Fish passing the dam in April and May are called spring chinook, those passing the dam during June and July are summer chinook, and those passing the dame in August through November are fall chinook.

NOAA Fisheries considers each run (a management unit) to be a separate species for regulation under the ESA. So, if a salmon arrives at the dam at the end of May and immediately climbs the fish ladder it’s a spring chinook. But if it remains below the dam until early June it’s a summer chinook. An arbitrary distinction that has no scientific basis. And both taste just as good.

To illustrate why how an ESA species is defined, consider the legal and management confusion of trying to work with subspecies, hybrids, DVPs, or ESUs. Arguably, there is no biological difference among these terms. If a female spring chinook passes Bonneville Dam in May but mates with a male summer chinook are the new chinook salmon who come out of that same redd (nest) spring or summer chinook? They could be either depending upon when they return as adults. Scientifically it is better to manage all chinook salmon as a single species with specific policies or actions dependent on time and place.

Element 2: significance

If a population segment is distinct, then its significance needs to be evaluated to determine whether or not it is needed to conserve genetic diversity.

Species evolve because populations are reproductively isolated by space or time.

Populations of plant and animal species have ranges of varying size. In general, throughout the species’ distribution range the physical and biological environment varies. Some areas are optimal for survival, some areas are less well suited, and other areas are marginal. In general, populations in the optimal areas have the greatest adaptability to survive environmental change. That is, they contain individuals with the genetic makeup to successfully exploit a wide range of habitats and environmental conditions.

On the other hand, populations in marginal or extreme habitats tend to become more narrowly adapted to their local environmental conditions. This results in those populations having a reduced ability to survive changes in their environment.

For each animal species, populations may have differences in appearance and genetics which reflect their location within their range. Usually, animals from populations in optimal habitats could be relocated to marginal habitats with relatively high chances for continued survival and increase in number.

Migration is how animal species exploit new habitats or relieve overpopulation pressures. Within a limited number of generations, these transplants could assume the physical and genetic adaptations to the environment shown by the previous populations in that habitat. Conversely, the probability of successfully moving individuals from one marginal habitat to a different marginal habitat would be low because of environment-specific adaptations that are highly specialized.

The four criteria of the significance element are:

  1. Persistence of the discrete population segment in an ecological setting that is unusual or unique for the taxon. In other words, a suboptimal habitat.

This criterion can be applied to exotic, invasive species. For example, Africanized honey bees or zebra mussels. But, if the population is reproductively isolated from other populations of the same species it could evolve into a new species. This is what happened over time in the Galapagos Islands in the Pacific Ocean off the coast of Ecuador.

  1. Evidence that loss of the discrete population segment would result in a significant gap in the range of a taxon.

This criterion lacks scientific meaning. It doesn’t define significant. Individuals of other populations could migrate to the newly-available habitats and repopulate the area thereby closing that gap.

  1. Evidence that the discrete population segment represents the only surviving natural occurrence of a taxon that may be more abundant as an introduced population outside its historic range.

This criterion also appears to be a stretch of administrative logic without a scientific foundation. What is a natural occurrence of a species? Are there unnatural population occurrences of a species? I’ve no idea.

  1. Evidence that the discrete population segment differs markedly from other populations of the species in its genetic characteristics.

By definition, genetic diversity means differences within a species, either within a population or among different populations. And, is markedly more quantitative and scientific than significant? This subjective language results in lawyers and judges making scientific decisions because the science is not clearly explained.

To illustrate failure of these criteria to define ESA species I’ll use Columbia River salmonid adult run timings.

There are spring, summer, and fall chinook salmon; summer and winter steelhead trout; early and late coho salmon. Each are ESUs under the ESA, yet this trait is easily manipulated by managers.

For decades the use of hatchery brood stock to replace fewer naturally reproduced population numbers has too often inadvertently selected for the earliest returning adults. Repeated over many generations, this has shifted run timings (and possibly the genetic composition) of Columbia River stocks. Are these changes significant?. This depends on the question being asked. These change may be maladaptive to a naturally reproducing populations. Therefore, apparent behavioral differences can be very flexible and respond to both natural and human influences.

Salmonids in the basin are managed as distinct stocks. Yet fertilized eggs from one hatchery have been used to supply eggs to other hatcheries throughout the basin. Since some adults do return to their natal hatcheries, it suggests that these stocks are not distinct units for listing under the ESA.

And then there are the strays and other hatchery-origin fish that have interbred with populations of natural spawners, sometimes for many generations. If the hatchery fish had their ancestry from another part of the basin, but can produce viable offspring with local native fish, it raises concerns about the utility of the evolutionarily significant unit population segment concept in carrying out the purposes of the ESA. The discreteness which is implied by using different “stock” designations is an artifact of human classification. It does not accurately reflect differences in the fish which must be preserved to conserve genetic diversity in the species themselves.

The FWS and NMFS policy should not have been adopted. Of the three elements to be considered, discreteness depends on the existence of taxonomic levels finer than species; there is no scientific support for this idea. Significance (as defined by the four criteria cannot be quantitatively determined. And the idea of artificially raising an arbitrary subspecies or hybrid to the specific taxonomic level is circular, capricious, and scientifically indefensible.

When you are required to demonstrate that a proposed or active activity does not adversely effect an ESA-listed species you can do so by collecting sampling data and species’ information from the scientific literature and analyzing them with consideration of your planned or existing activities. I know from experiences involving Columbia River salmonids, bald eagles, western pond turtles, and other species that this can be done and I encourage you to present either NOAA Fisheries or the Fish and Wildlife Service with technically sound and legally defensible reports that would justify them in issuing a not likely to adversely impact biological opinion. Contact me when you need to effectively resolve this issue.

This work was originally published on the Applied Ecosystem Services, LLC web site at

It is offered under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International license. In short, you may copy and redistribute the material in any medium or format as long as you credit Dr. Richard Shepard as the author. You may not use the material for commercial purposes, and you may not distribute modified versions.

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