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Executive Summary
INTRODUCTION
Species extinctions have occurred! since life has been on earth, but human activities are causing
the loss of biological diversity at an accelerating rate. The current rate of extinctions is among the
highest in the entire fossil record, and many scientists consoler it to have reached crisis proportions.
The 1973 Endangered Species Act (ESA) and its subsequent amendments are the latest in a long line of
federal legislation designed to protect wildlife. The ESA is the broaclest and most powerful law to
provide protection for endangered! species and their habitats. The economic and social costs of
complying with the ESA have been controversial in some cases. Because of those controversies, and
because the act is being considered for reauthorization, it has been receiving much attention recently.
That attention led to the request for this study to be concluctect by the National Research Council
(NRC).
The ESA defines three crucial categories: "enciangered" species, "threatened" species, and
"critical" habitats. ("Subspecies" of plants and animals and "distinct population segments" of
vertebrates can also qualify for protection as species uncler the ESA.) Endangered species and their
critical habitats receive extremely strong protection; it is illegal to take any endangered species of
animal (or plant in some circumstances) in the United States, its territorial waters, or the high seas. In
adclition to this direct prohibition, Section 7 of the act prohibits any federal action that will jeopardize
the future of any endangered species, including any threat to designated critical habitat. The act also
requires the secretaries of interior ant} commerce to use programs in their agencies in furtherance of the
act and requires other agencies to "utilize their authorities in furtherance of the purposes of [the act] by
carrying out programs for the conservation of endangered species ant! threatened species." The 1978
and later amendments to the ESA established a requirement for recovery plans to be prepared by the
U.S. Fish and Wildlife Service for inland species and by the National Marine Fisheries Service for
marine species, unless the secretary "finals that they will not promote the conservation of the species."
Those plans are required to include specific population goals, timetables, and estimated costs.
The strength of the ESA lies with its stringent mandates constraining the actions of private
parties and public agencies. Once a species is listed as threatener! or endangered, it becomes entitled to
shelter under the act's protective umbrella, a far-reaching array of provisions. Critical habitat must be
designated "to the maximum extent prudent and determinable" and recovery plans, designed to bring
the species to the point where it no longer needs the act's protections, are required if they will promote
the conservation of the species. Funds for habitat acquisition and cooperative state programs are
authorized. Federal agencies must ensure that their actions are not likely to jeopardize the survival of
listed species nor adversely mollify their critical habitats. Agencies are also required to use their
authorities to promote endangered species conservation.
In addition to the Section 7 prohibition of any federal action that jeopardizes an endangered
species or its critical habitat, Section 9 prohibits the taking of an endangered species of fish or wildlife)
(or, by regulation, of threatened species). Sections 7 and 9 are major sources of the act's power as
1 Section 9 provides somewhat lesser protection to plants, making it unlawful to "remove or reduce to
possession any such species from areas under Federal jurisdiction . . . or remove, cut, dig up, or damage or
destroy any such species on any other area in knowing violation of any law or regulation of any state . . .
..
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2
Science and the Endangered Species Act
well as numerous controversies. In particular, the prohibition against taking endangered species has
raised questions among private landowners: taking is fairly broadly clefined in the ESA and even more
broadly in some regulations. How broad the definition of taking in regulations should be is currently
undergoing review by the U.S. Supreme Court. The court's decision will be important in determining
the future of some of the controversies about the taking prohibition.
As human activities continue to affect species populations and their habitats, two major
questions arise concerning the ESA. First, the focus of this report: is the ESA soundly based in
science as an effective method of protecting enclangered species and their habitats? The second
question of great public importance, but not part of this committee's charge concerns the desired
public policy with respect to protecting endangerecl species and their habitats, i.e., what are the costs
and benefits, and to what extent is the public willing to incur the costs?
THE PRESENT STUDY
In November of 1991, Senator Mark Hatfield, Representative Thomas Foley, and
Representative Gerry Studds wrote to the chairman of the National Research Council requesting a study
of "several issues relater! to the Endangered Species Act." The request focused on scientific matters
related to the act. After receiving funding from the U.S. Fish and Wildlife Service in September 1992,
the NRC's Board on Environmental Studies and Toxicology convened the Committee on Scientific
Issues in the Endangered Species Act. The committee's membership includes expertise in ecology;
systematics; population genetics; wildlife management; risk and decision analysis; the legal, legislative,
and administrative history of the Endangered Species Act; economics; and the implementation of the
ESA from public and private perspectives. The committee's statement of task is based very closely on
the letter of request from the three members of Congress (see Appendix A).
The committee was asked to review the following issues and to evaluate how they relate to the
overall purposes of the Endangered Species Act:
· Definition of species. The committee was asked to review how the term species has been
used to implement the ESA, ant} what units would best serve the purposes of the act.
· Conservation conflicts between species. The committee was asked how frequent or severe
conflicting conservation needs are when more than one species in a geographic area are listed! as
endangered or threatened under the ESA, and to make recommendations to resolve these conflicts.
· Role of habitat conservation. The committee was asked to evaluate the role of habitat
protection in the conservation of .~necies and to review the relationship between habitat-protection and
other requirements of the act.
· Recovery planning. The committee was asked to review the role of recovery planning
under the act and to consider how recovery planning could better contribute to the purposes of the act.
· Risk. The committee was asked to review the role of risk in decisions made under the ESA
(such as what constitutes sufficient "endangerment" to require listing of a species, what constitutes
jeopardy, adverse modifications, reasonable and prudent alternatives, taking, conservation, and
recovery) It was also asked to review whether different decrees of risk ought to apply to different
. , , . .. ~ ~ ~ _ ~
~ ~ · · ~ 1 1 1 1 _ _ _ _ _1 _ _ _ _ · _ _ 1~ 1_ ~L1~ ~ `1~ .~1 ~:~ ~
types or decisions te.g., snoulcl an endangered species oe a~ "rearer risk rllall ~ [lilt;~ll~U ~1~ W
justify listing?) and to identify practical methods for assessing risk to achieve the purposes of the act
better while providing flexibility in appropriate circumstances to accommodate other objectives as well.
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Executive Summary
· Issues of honing. The committee was asked to review the timing of key decisions under the
ESA and to consider ways of improving such timing under the act to serve its purposes better while
minimizing unintended consequences.
The committee held meetings in Washington, D.C., and Irvine, California, where it received
briefings from federal officials, congressional staff, Senator Mark Hatfield, Secretary of the Interior
Bruce Babbitt, members of private conservation organizations and of private industry, and other
experts. It has also made use of many sources of information, including previous NRC reports;
documents and studies done by other agencies; and relevant published literature from scientific
3
journals, symposia, and books.
This report reviews scientific issues related to the ESA. The overall conclusion is that the ESA
is based on sound scientific principles. Many scientific advances have been made since the ESA was
passed in 1973, and they provide opportunities to improve the act's implementation, especially with
respect to identifying species, subspecies, and distinct population segments, with respect to estimating
risks of extinction, and economic and decision analyses. Although it is difficult to quantify the
effectiveness of the act in preventing species extinction, there is no doubt that it has prevented the
extinction of some species and slowed the declines of others. It is equally clear that the ESA by itself
cannot prevent the loss of many species and their habitats. Instead, the ESA is best viewed as one part
of a comprehensive set of ways of protecting species and their habitats. The committee was not asked
to comment on the social and political decisions concerning the ESA's goals and tradeoffs, and it has
not done so. Nonetheless, they are and should be an important part of the policy discussions about the
ESA.
EXTINCTIONS
Extinction is an essential part of evolution. In the past 20 years, we have learned a great deal
about the earth's physical and biological history. Over the past 500 million years, at least five mass
extinctions have occurred, with as much as 84% of the genera of marine invertebrates disappearing
from the fossil record. Those extinctions were associated with major physical events. Today, we are
again witnessing a major extinction. Unlike the earlier ones, which affected some kinds of organisms
and some kinds of habitats more severely than others, today's extinctions are affecting all major groups
of organisms in all nonmarine habitat types (the marine environment has not yet been affected as much
as terrestrial and freshwater environments).
We do not know how many species of organisms live on earth, but there are many ways of
estimating the rate of extinction in various habitats and in various kinds of organisms. The major cause
of the current extinctions is human activity, and most estimates suggest that human activity has
significantly increased the background extinction rater, perhaps by orders of magnitude. Such
activities include direct alteration of habitats by forestry, agriculture, fishing, and residential and
commercial development; indirect alteration of habitats by pollution of water, air, and the soil;
alteration of ecosystems by introductions of exotic organisms and the spread of diseases; removal or
2Although the number of documented extinctions might appear to be small compared with the number of
species alive, it is the rate of extinctions that is important. Even the mass extinctions of the past took many
thousands of years to occur; the current rate of extinctions appears to be comparable to the rates during those
events.
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Science and the Endangered Species Act
alteration of sources of foot! and shelter for organisms by human use of natural resources, and
unregulated harvesting, hunting, and fishing.
THE SPECIES CONCEPT
Species of organisms are iGunciamental objects of attention in all societies, and different cultures
have extensive literatures on the history of species concepts. The Endangered Species Act (ESA)
defines species to inclucle "any subspecies of fish or wildlife or plants, and any ctistinct population
segment of any species of vertebrate fish or wilcllife which interbreeds when mature." In the act, the
term species is used in a legal sense to refer to any of these entities. In addressing its use in the ESA,
one must remember, however, that species has vernacular, legal, and biological meanings.
Many societies have notions of kinds of organisms, usually organisms that are large and
conspicuous or of economic importance. The term species can be applied to many of those kinds and
can be accurate as a scientific and vernacular term, because the characteristics used to differentiate
species can be the same in both cases. Largely for this reason, the question of what a species is has not
been a major source of controversy in the implementation of the Endangered Species Act. Greater
difficulties have arisen in deciding about populations or groups of organisms that are genetically,
morphologically, or behaviorally distinct, but not distinct enough to merit the rank of species i.e.,
subspecies, varieties, ant! "clistinct population segments."
In particular, questions have arisen about how to recognize "distinct population segments." To
help in identifying them, the committee introduces the concept of an evolutionary unit (EU)3. An EU
is a group of organisms that represents a segment of biological diversity that shares a common
evolutionary lineage and contains the potential for a unique evolutionary future. Its uniqueness can be
sought in several attributes, including morphology, behavior, physiology, and biochemistry. Because
any specified group of organisms can be claimed to have a unique evolutionary future, a basic
characteristic of an EU is that it is distinct from other EUs. In most cases, an EU will also occupy a
nartic~lar ~en~ranhical area. Most currentIv recognized species and subspecies are EUs.
red -- =--o---r~ ~ ~ ~
~ . . . . · · ~ · · r 1 , 1 , ~ , _ _ $~ ~ ~ ~1: ~ a: ~ _ A: _ . _ ~ _ _ ~ /: _
Distinction implies an independent evolutionary future. estimates or alsuncllveness We.,
circumscription of EUs) are base(1 on genetic, molecular, behavioral, morphological, or ecological
characteristics. Any single method will often be inadequate to identify an EU (that is, to provide
compelling evidence of distinctiveness). The question of distinctiveness and the associated inference of
an independent evolutionary future usually requires the careful integration of several lines of evidence.
Committee Conclusion. The ESA is clear that species ant! subspecies of "fish or wildlife or
plants" defined in the act to inclucle all members of the plant and animal kingdoms are eligible for
protection. The ESA's emphasis on (listinct population segments i.e., taxa below the rank of
subspecies is soundly based on science.
Committee Recommendation. The committee concludes that the ESA's inclusion of species
and subspecies is soundly justified by current scientific knowle(lge and should be retained. Often,
competent systematists will be requires! to delineate subspecies, and sometimes species as well.
Committee Recommendation. To help provi(le scientific objectivity in identifying population
segments, the concept of the evolutionary unit (KU) should be acloptect. The EU is a segment of
3 Similar but not identical to the National Marine Fisheries Service's Evolutionary Significant Unit; see
Chapter 3.
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Executive Summary
5
biological diversity that contains a potential for a unique evolutionary future. To clarify the analyses,
identifying an EU should be separate from deciding whether it is in need of protection.
Committee Conclusion. The ESA explicitly covers species and subspecies of all plants and
animals. As currently written, however, it covers taxonomic units below the subspecies level (i.e.,
distinct population segments) only for vertebrate animals. There is no scientific reason (other than lack
of knowledge) to exclude any EUs of nonvertebrate animals and plants from coverage under the ESA.
Although the way organisms are divided into kingdoms has changed since the ESA was enacted in
1973, current scientific knowledge about how species concepts apply to these organisms does not lead
us to recommend that coverage be extended to prokaryotes and most single-celled eukaryotes, such as
yeasts.
Committee Conclusion. Application of the EU concept should not result in any substantial
change in the application of conservation laws. We hope it will move decisions of eligibility for
protection away from arguments only about taxonomic ranks and into a realm where more substantive
views about the degree to which populations are evolutionarily significant and new techniques can be
applied.
HABITAT
Habitat the physical and biological setting in which organisms live and in which the other
components of the environment are encountered-is a basic requirement of all living organisms. It
embraces all components of a species' environment. The relationship, nationwide, between vanishing
habitats and vanishing species is well documented. The ecological relationship is simple and fairly
general: species diversity is positively correlated with habitat area. A corollary of this relationship is
~ 1 ~ ~ ~ _ _
. · ~ . . -. . · ~ . . - ~ ~ ~ · 1 1 1 · ~ `1~ -1 ~ __ -11 ~ _ 1 _~d
that it habitat IS substantially reduced in area or Degraded, species occurring in me WllL1 Will 0C 1055.
Therefore, habitat protection is a prerequisite for conservation of biological diversity and protection of
endangered and threatened species. The Endangered Snecies Act. in emphasizing habitat, reflects the
_ ~ ~ , ~ ~ .
current scientific understanding of the crucial biological role that habitat plays for species.
The question has been raised whether critical habitat should be determined at the time of listing
or whether it should be deferred to the time of recovery planning. Because of public concern over
economic consequences, the designation of critical habitat is often controversial and arduous, delaying
or preventing the protection it was intended to afford.
Committee Recommendation. Because habitat plays such an important biological role in
endangered species survival, some core amount of essential habitat should be designated for protection
at the time of listing a species as endangered as an emergency, stop-gap measure. As discussed below,
it should be identified without reference to economic impact. Economic review may need to remain
linked to critical habitat determination in the ESA, and determination of areas essential to the recovery
of a species, including areas not currently occupied by that species, can be especially complex. Hence
we suggest designation of survival habitat.
Survival habitat would be designated at the time of listing of an endangered species, unless
insufficient information were available or harm to the species would occur. For this purpose, survival
habitat would mean the habitat necessary to support either current populations of a species or
populations that are necessary to ensure short-term (25-50 years) survival, whichever is larger; survival
habitat would receive the full protection that the ESA accords to critical habitat. Because of its
emergency nature, no economic evaluation would be conducted before designating survival habitat.
The designation of survival habitat (and its protection under the ESA) would automatically expire with
the adoption of a recovery plan and the formal designation of critical habitat. Subsequent recovery
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6
r ·,.
Science and the Endangered Species Act
planning would include designation of critical habitat as currently defined in the ESA (including
economic evaluation) to include areas necessary for species recovery.
Because essential survival habitat is identified in our recommendation without reference to
economic impact, anti because it might not be sufficient to ensure long-term survival and recovery of
endangered species, the committee views it as an emergency, stop-gap measure until critical habitat can
be designated and a recovery plan can be completed, not as a substitute for those measures. Indefinite
delays in designating critical habitat and formulating recovery plans after designation of survival habitat
might cause harm to economic interests anti to the endangered species itself. Therefore,
implementation of this recommendation needs to include ways of preventing that delay from occurring.
Committee Recommendation. The committee endorses regionally based, negotiated
approaches to the development of habitat conservation plans. Guidance from FWS for the development
of such plans should include advice on the development of biological data, such as demographic and
genetic analyses, habitat requirements of the species involvecI, reserve design, and monitoring, and it
should also include advice on descriptions of management options and application of risk analyses in
consideration of alternatives.
RECOVERY
The ultimate goal of the ESA is to recover threatener! and endangered species. Recovery is
"the process by which the decline of a threatened or endangered! species is arrested or reversed, and
threats to its survival are neutralized, so that its long-term survival in nature can be ensured." Despite
increased attention from Congress, recovery plans are developed too slowly and recovery planning
remains handicapped by delays in its implementation, goals that are sometimes not scientifically
supported, and the uncertainty of its application to other federal activities.
No recovery plan, however good it might be, will help prevent extinction or promote recovery
if it is not implemented expeditiously. Indeed, the failure to implement a recovery plan quickly can
also increase the disruption of human activities, because of the resulting uncertainty among other
causes.
Committee Recommendation. To reduce uncertainty and permit the planning of activities not
directed at species recovery, all recovery planning should include an element of "recovery plan
guidance," particularly with regard to activities anticipated to be reviewed under sections 7, 9, and 10
of the ESA. FWS should convene a working group to develop explicit guidelines for the application of
data to the construction of recovery objectives and criteria. To the degree possible, the guidance
should identify activities that can be assumed to be consistent with the requirements of those sections,
activities that can be assumed to be inconsistent with them, and activities that require individual
evaluation. Topics would include a habitat-based approach to recovery; a logical, hierarchical
approach to analysis of ecological and genetic data on the species; guidance for demographic modeling,
stressing the inherent uncertainty of such modeling; outlining future research needs and how the
research will contribute to species and habitat management; and an effective monitoring scheme.
Several habitat-related features of the ESA stiffer without scientific basis, in particular,
standards applicable to the protection of plants ant! to the determination of jeopardy and modification of
critical habitat, and different standards of protection on public and private lands. For example, Section
9 fails to protect endangered plants from habitat mollification to the same degree that it protects
animals, especially on private lancts.
Committee Conclusion. The biological differences between animals and plants underlying
their taxonomic separation offer no scientific reason for lesser protection of plants. The biological and
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Executive Summary
physical requirements of species inclucling endangered and threatened species do not vary according
to the ownership of the habitats that they occupy. Therefore, there is no biological reason to have
different standards for determination of "jeopardy," "survival." or "recovery" on public and on Private
lands (there could of course be other kinds of reasons).
7
-- r A- --- ~--~ - -- r
Committee Conclusion. Public agencies and incliviclual public servants on public lands behave
clifferently from private landowners, both corporations and individuals, on private lands, because their
rewards and incentives are different. Therefore, requirements applied equally on private and public
lands will not necessarily provide the same degree of protection, although the biological standards or
criteria on which the regulations are based are the same. It follows, then, that different mechanisms
may be needed for avoiding endangerment and achieving recovery on public and private lancis.
Committee Conclusion. The act ant! its regulations distinguish between species "survival" and
"recovery" for purposes of determining jeopardy to species ant! adverse modification of their critical
habitats. Survival and recovery are points on a continuum. Clearly, if a species does not survive, it
cannot recover. It is less obvious, but still true, that any action that jeopardizes recovery also decreases
the probability of long-term survival.
Committee Recommendation. To permit a rational evaluation of survival and recovery goals,
estimates should be provident of probabilities of achieving various goals over various periocis. The
periods should be expressed both in years and in generation times of the organism of concern.
Evaluation of long-term and irreversible impacts should be conducted in terms of long-term recovery of
the species. Although it will often be difficult to make these estimates, even the attempt to make them
will have value by requiring an objective analysis and by requiring assumptions to be specified.
CONSERVATION CONFLICTS BETWEEN SPECIES
Because plants and animals are linked to other organisms in ecosystems in a variety of ways, it
is inevitable that conflicts will arise when attempts are made to protect individual species of plants or
animals. One of the charges presented to the committee concerned conservation conflicts between
species.
Committee Conclusion. We have found few well-documented cases where management
practices focusing on particular species protected under the Endangered Species Act result in direct
conflict with the needs of another.
It is possible that this low number stems from lack of knowledge of the ecological networks of
which threatened and endangered species are part; from the fact that comparatively few species are
currently listed and that recovery plans have been formulated for even fewer; and from the inadvertent
protection for other listed species uncler some current recovery plans. We expect that our knowledge
of such conflicts and the potential for their occurrence will increase as ecologies of listed species
become better known, more recovery plans are formulated, and habitat for conserving endangered
species becomes more constricted.
Committee Conclusion. Under current policies, the greatest potential for conflicts in
protecting species and for management of individual species will arise in situations in which habitat
reductions especially extreme reductions themselves are the causes of endangerment and the habitats
of listed species are largely overlapping.
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8
Science and the Endangered Species Act
Committee Conclusion. The most effective way to avoid conflicts resulting from management
plans for individual species is to maintain large enough protected areas to allow the existence of
mosaics of habitats and dynamic processes of change within these areas. In addition to, and as part of,
this strategy, multispecies plans should be devised to ensure the maintenance of habitat mosaics and
ecological networks. Habitat (in the broadest sense) thus plays a crucial role in protecting individual
target species and, ultimately, in reducing the need for listing additional species. When insufficient
habitat is available to resolve such conflicts, other factors must be evaluated to resolve the conflicts,
such as the consequences of various management options on each species, the ecological importance of
the species, and the distribution of the species.
ESTIMATING RISK
The concept of risk is central to the implementation of the ESA. The main risks involved in the
implementation of the Endangered Species Act are the risk of extinction (related to the probability of
both biological and nonbiological events) and the risks associated with unnecessary expenditures or
curtailment of land use in the face of substantial uncertainties about the accuracy of estimated risks of
extinction and about future events. Since the passage of the ESA, there have been enough
developments in conservation biology, population genetics, and ecological theory that substantially
more scientific input can now be used in the listing and recovery-planning processes. Numerous
models have been developed for estimating the risk of extinction for small populations. Although most
of these models have shortcomings, they do provide valuable insights into the potential impacts of
various management (or other) activities and of recovery plans. In particular, they are valuable for
comparing the likely effects of alternative management options and of alternative adverse effects on the
species.
Despite the major advances that have been made in models for predicting mean extinction
times, the existing methods still have substantial limitations. Often, risk factors are not well known.
Most of the models deal with only one risk factor at a time and fail to incorporate the interactive effects
of multiple risk factors on reducing the time to extinction. This might result in a tendency for such
models to underestimate the risk of extinction. Efforts to integrate various sources of random variation
(genetic, demographic, and environmental) into spatially explicit frameworks are badly needed.
Most extinction models primarily address the mean time to extinction. Because decisions
associated with endangered species usually are couched in fairly short time frames less than 100
years models that predict the cumulative probability of extinction through various time horizons would
have greater practical utility than current models.
Committee Conclusion. With only a few exceptions, biologically explicit, quantitative models
for risk assessment have played only a minor role in decisions associated with the ESA. They should
play a more central role, especially as guides to research and as tools for comparing the probable
effects of various environmental and management scenarios.
Committee Conclusion. Results from population-genetic theory provide the basis for one
fairly rigorous conclusion. Small population sizes usually lead to the loss of genetic variation,
especially if the populations remain small for long periods. If the members of the population do not
mate with each other at random (the case for most natural populations), then the effect of small size on
loss of genetic variation is made more severe; the population is said to have a smaller elective size than
its true size. Populations with long-term mean sizes greater than approximately i,000 breeding adults
can be viewed as genetically secure; any further increase in size would be unlikely to increase the
amount of adaptive variation in a population. If the effective population size is substantially smaller
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Executive Summary
than actual population size, this conclusion can translate into a goal for survival for many species of
maintaining populations with more than a thousand mature individuals per generation, perhaps several
thousand in some cases. An appropriate, specific estimate of the number of individuals needed for
long-term survival of any particular population must be based on knowledge of the population's
breeding structure and ecology. If information on that species is lacking, information about a related
species might be useful.
MAKING ESA DECISIONS IN THE FACE OF UNCERTAINTY
To ensure that ESA decisions protect endangered species as they are intended to in a
scientifically defensible way requires objective methods for assessing risk of extinction and for
assigning species to categories of protection according to that risk. Standards for assigning species to
categories should be quantitative wherever possible and, when this is not possible, qualitative
procedures should at least be systematic and clearly defined. Major advances in both theory and
methods of estimating risk of extinction allow us to base listing and recovery decisions on scientific
principles. In the past, many ESA decisions have failed to meet the guidelines suggested by current
scientific thinking, listing species as endangered only when populations had dropped to the point where
extinction was imminent and proposing recovery goals that left the species still at high risk of
extinction.
Committee Conclusion. We can find no scientific basis for setting different levels of risk for
different taxonomic groups, such as plants or animals, or for public versus private actions that may
affect listed species. However, it is critical to understand that because public and private entities may
behave differently, different management policies may be required for public and private lands in order
to achieve the same biological risks for listed species in the two settings. No implementation of the
ESA can be fully successful without recognizing these differences.
Committee Recommendation. To the degree that they can be be quantified, the levels of risk
associated with endangered status should be higher than those for threatened status. Once a species no
longer qualifies for threatened status, it should be considered recovered and delisted. Levels of risk to
trigger ESA decisions should be framed as a probability of extinction during a specified period (i.e.,
x% probability of extinction over the next y years). Although some crises may call for short time
9
horizons (on the order of tens of years), ordinarily it will be necessary to view extinction over longer
periods (on the order of hundreds of years) so that short-term solutions do not create long-term
problems. The selection of particular degrees of risk associated with particular periods as the standards
for listing species as endangered or threatened reflects both scientific knowledge and societal values.
Although the objectives of the ESA are not intrinsically conflicting, the act must be
implemented with limited budgets, and so conflicts can arise in determining how to allocate funds
among listed species, all of which qualify for the act's protection. Scientific considerations, such as
whether a species or its habitat possesses unusually distinctive attributes or whether protection of a
taxon would confer protection on other candidate taxa and their habitats, should be used to help set
priorities for action. Decisions to set priorities for implementation of the act are often difficult and
controversial, and the procedures for making them should be explicit and well documented. Structured
methods, such as decision analysis, can improve both the substance of these decisions and the
justifications offered for them.
Meeting the objectives of the act can sometimes conflict with other human objectives, such as
development of private or public property harboring listed species. The act prohibits consideration of
human objectives unrelated to species protection in decisions regarding listing, "take,' and "jeopardy,''
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Science and the Endangered Species Act
but directs that these other objectives be taken into account in decisions about critical habitat and
implementation of recovery plans. Tradeoffs between species protection and economic or other
benefits or costs tnust be evaluated. Again, because these tradeoff decisions are often difficult and
controversial, it is important to use well-structured anti explicit methods for making them.
ESA decisions are inevitably based on limited information, and so agencies are obliged to act in
the face uncertainty about species status and the impacts of proposed activities. Decisions in the face of
uncertainty carry the prospect of being wrong in various ways and with varying, and often
asymmetrical, consequences. For example, managers concerned! with delisting a formerly endangered
species must be wary of two types of errors: delisting when the species is actually still in peril, and
failing to delist when the species has truly recovered to the target level. Each type of error has both
biological and nonbiological consequences. The first error has adverse biological consequences for the
endangered species it would be irreversible if the species became extinct and, perhaps, positive
socioeconomic consequences for sectors whose activities may have been constrained by recovery
guidelines. The second error has neutral to positive consequences for the species but potential negative
socioeconomic consequences. It is not possible to minimize the risks of both types of errors
simultaneously. A decision rule that guards against the first will allow too many of the second and vice
versa. To set acceptable rates for each type of error, both the likelihood and the magnitude of
biological and nonbiological benefits and costs must be weighed in a decision-analytic framework.
These decisions are too complicated and too consequential to be entrusted to unaided intuition.
If not examiner} explicitly, this asymmetric error structure can bias decisions uncler the act to
the detriment of endangered species, especially if they are based on analyses that do not take the
asymmetric risk function into account. Although the wording of the ESA suggests that the "burden of
proof" to show no effect is on those proposing to moclify habitat or harm a listed species, the way that
hypothesis tests are phrased and error rates are set can put the burden on those attempting to show that
a species should be listed] or that a clevelopment proposal should be denied or modified.
Committee Recommendation. Because the structure of hypothesis testing related to listing
and jeopardy decisions can make it more likely for an endangered species to be clenied needed
protection than for a nonendangered species to be protected unnecessarily, decisions under the act
should be structured to take explicit account of all the types of errors that could be made and their
consequences, both biological and nonbiological. The phrasing of the null hypothesis and setting of
error rates should reflect societal, as well as scientific, judgments about what level of risk is acceptable
for which types of errors.
TIMING
The committee's comments on the timing of key decisions under the ESA are incorporated in
discussions of various other topics. In particular, timing is considered in discussions of recovery
planning (where the committee concludes that recovery plans are developed too slowly and recovery
planning remains handicappe(i by (relays in implementation) and identification of survival habitat
(whose designation is recommended to overcome the effects of delays in designation of critical habitat).
BEYOND THE ENDANGERED SPECIES ACT
The Endangered Species Act's goal is the prevention of species extinction, and its legal
apparatus to protect endangered species is strong. It does not appear to have been intended as an
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Executive Summary
overall policy act for the preservation of all of the nation's ecosystems and biota. It is, as the
committee understands it, intended as a safety net.
Committee Conclusion. Although it is impossible to quantify the ESA's biological
effects i.e., how well it has prevented species from becoming extinct the committee concludes that
fewer species have become extinct than would have without the ESA. In other words, the ESA has
successfully prevented some species from becoming extinct. Retention of the ESA would help to
prevent species extinction. Some changes, as outlined in this report, would probably make the act
more effective and predictable, and provide a more objective basis for its implementation.
11
Committee Conclusion. It is also clear that some species have become or are almost certain to
become extinct despite the protection of the ESA. In other words, the ESA cannot by itself prevent all
species extinctions, even if it is modified. Therefore, the committee concludes that additional
approaches to the management of natural resources will need to be developed and implemented as
complements to the ESA to prevent the continued, accelerating loss of species. Indeed, many federal,
state, and local governments and private organizations are developing such approaches.
· Ecosystem management. Despite diverse definitions of ecosystem management and despite
scientific uncertainties, it is clear that managing ecosystems and landscapes as an addition to the
protection of individual species can lead to improved natural-resource management and can help reduce
species extinctions. Properly implemented, it can also help to reduce uncertainty and thus reduce
economic disruptions.
· Reconstruction or rehabilitation of ecosystems. Restoration ecology is a growing discipline.
Many ecosystems functions have been improved or restored by such activities, and reconstruction or
rehabilitation of ecosystem functioning holds much promise for the protection of endangered species. It
is not usually possible to return an ecosystem to some prior pristine condition, however. Many
ecosystems have been so altered that it is difficult to decide what prior condition we might want to
return to. The trajectory taken by the ecosystem to get to its current condition is not retraceable in the
way that a highway is, because many events occur in an ecosystem's history that are not precisely
reversible. Genetic variability is lost; evolution occurs; exotic species are introduced; human
populations in the region increase. and PeonIe develop dependence on a variety of modern
technologies, cultures, and economic systems; and other natural and anthropogenic environmental
changes affect the range of biophysical and socioeconomic possibilities for future states of the system.
In brief, the past provides opportunities for the future but also constrains it. Thus, attempts to
rehabilitate ecosystem functioning should keep these constraints in mind, so that inappropriately high
expectations are not generated.
· Mixed management plans. Often, resource managers manage areas either for protection of
biota or for human use. It is increasingly difficult to keep people and the effects of their activities
separate from wildlife sanctuaries. Although such sanctuaries (e.g., national parks, wilderness areas,
wildlife refuges, marine sanctuaries) are indispensable for protecting endangered species, greater
attention needs to be paid to developing mixed-use areas. These would be urban recreation areas or
residential and commercial clevelopments adjacent to untrammeled areas designed to improve
opportunities for wildlife while maintaining opportunities for human activities. Although the value of
this approach is becoming increasingly recognized, its development is still in the early stages.
· Cooperative management. Various experiences with cooperative management the sharing
of planning and decision making by various government and nongovernment groups-have had some
success. To some degree, habitat conservation plans represent an example of this approach, but it is
likely that cooperative management will be necessary in cases where the strict requirements of the
Endangerecl Species Act have not yet been applied. It is important to include the major interested
parties without having so many interests involved that consensus is difficult to reach.
"7 , . ~
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12
Science and the Endangered Species Act
· Revised economic accounting. Too often, economic calculations underlying public and
private decision making are incomplete. Often, they cover too short a time span, and they often
exclude nonmarket values. A short-term loss might turn into a long-term gain: for example, losing an
economic activity today might provide opportunities for greater economic activities of different types at
some time in the future. Again, the valiclity of expanding economic accounting to cover longer periods
and to include nonmarket values is becoming more widely recognized but it is still in the early stages of
clevelopment.
SCIENCE, POLICY, AND THE ESA
This committee was asked to review the scientific aspects of the ESA and it has done so. It has
not uncovered any major scientific issue that seriously hinders the implementation of the act, although
its review has suggested several scientific improvements. Many of the conflicts and disagreements
about the ESA do not appear to be based on scientific issues. Instead, they appear to result because the
act in the committee's opinion designed as a safety net or act of last resort is called into play when
other policies and management strategies or their failures, or human activities in general, have led to
the endangerment of species ant} populations. In some cases, policies and programs have been based
on sound science, but other factors have prevented them from working. The committee does not see
any likelihood that those endangerments will soon cease to occur or that the ESA can or should be
expected to prevent them from occurring. It therefore concludes that any coherent, successful program
to prevent species extinctions and to protect the nation's biological diversity is going to require more
enlightened commitments on the part of all major parties to achieve success.
To conserve natural habitats, approaches must be developed that rely on cooperation and
innovative procedures; examples provicled for by the ESA are habitat conservation plans and natural
community consecration planning. But those are only a beginning. Many other approaches have been
discussed in various fore. They include cooperative management (sharing decision-making authority
among several governmental and nongovernmental groups), transfer of development credits, mitigation
banks, tax incentives, and conservation easements.
An analysis of these and other policy and management options is beyond this committee's
charge, but sound science alone will not leaci to successful prevention of many species extinctions,
conservation of biological diversity, and reduced economic and social uncertainty ant} disruption. But
sound science is an essential starting point. Combined with innovative and workable policies, it can
help to solve these and related problems.
Representative terms from entire chapter:
species act