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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 1 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 consider 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 broadest 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 conducted by the National Research Council (NRC). The ESA defines three crucial categories: ''endangered" species, "threatened" species, and "critical" habitats. ("Subspecies" of plants and animals and "distinct population segments" of vertebrates can also qualify for protection as species under 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 addition 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 use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 2 habitat. The act also requires the secretaries of interior and 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 and 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 (FWS) for inland species and by the National Marine Fisheries Service for marine species, unless the secretary "finds 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 threatened 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 modify 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 is likely to jeopardize an endangered species or adversely modify or destroy its critical habitat, Section 9 prohibits the taking of an endangered species of fish or wildlife1 (or, by regulation, of threatened species). Sections 7 and 9 are major sources of the act's power as well as numerous controversies. In particular, the prohibition against taking endangered species has raised concerns among private landowners because of its application to habitat: taking is fairly broadly defined in the ESA and even more broadly in some regulations. How broad the definition of taking in regulations recently was reviewed 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 use the print version of this publication as the authoritative version for attribution. 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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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 3 protecting endangered 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 endangered 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 related 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, and what taxonomic 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 species 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 use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 4 recovery). It was also asked to review whether different degrees of risk ought to apply to different types of decisions (e.g., should an endangered species be at greater risk than a threatened species to 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. • Issues of timing. 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 and materials from federal officials, congressional staff, 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 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 with respect to 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. use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 5 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 rate,2 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 alteration of sources of food and shelter for organisms by human use of natural resources, and unregulated harvesting, hunting, and fishing. THE SPECIES CONCEPT Species of organisms are fundamental objects of attention in all societies, and different cultures have extensive literatures on the history of species concepts. The Endangered Species Act defines species to include "any subspecies of fish or wildlife or plants, and any distinct population segment of any species of vertebrate fish or wildlife 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, and distinct population segments. use the print version of this publication as the authoritative version for attribution. 2 Although 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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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 6 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 particular geographical area. Most currently recognized species and subspecies are EUs. Distinction implies an independent evolutionary future. Estimates of distinctiveness (i.e., circumscription of EUs) are based 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 and subspecies of "fish or wildlife or plants"— defined in the act to include all members of the plant and animal kingdoms—are eligible for protection. The ESA's inclusion of distinct 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 knowledge and should be retained. Often, competent systematists will be required to delineate subspecies, and sometimes species as well. Committee Recommendation. To help provide scientific objectivity in identifying population segments, the concept of the evolutionary unit (EU) should be adopted. The EU is a segment of 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 use the print version of this publication as the authoritative version for attribution. 3 Similar but not identical to the National Marine Fisheries Service's Evolutionary Significant Unit; see Chapter 3.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 7 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 that if habitat is substantially reduced in area or degraded, species occurring in the wild will be lost. Therefore, habitat protection is a prerequisite for conservation of biological diversity and protection of endangered and threatened species. The Endangered Species 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, use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 8 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 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, and 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 and 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 involved, 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 threatened 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 imple use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 9 mentation, 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 differ without scientific basis, in particular, standards applicable to the protection of plants and 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 modification to the same degree that it protects animals, especially on private lands. 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 physical requirements of species—including 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). Committee Conclusion. Public agencies and individual public servants on public lands behave differently from private landowners, both corporations and individuals, on private lands, because their rewards and incentives are different. Therefore, requirements applied equally on private use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 10 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 lands. Committee Conclusion. The act and its regulations distinguish between species "survival" and "recovery" for purposes of determining jeopardy to species and 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 provided of probabilities of achieving various goals over various periods. 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 under 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, use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 11 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. 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 use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 12 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 effective size than its true size. Populations with long-term mean sizes greater than approximately 1,000 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 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, use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 13 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 did not 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 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 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 har use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 14 boring listed species. The act prohibits consideration of human objectives unrelated to species protection in decisions regarding listing, take, and jeopardy, 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 must be evaluated. Again, because these tradeoff decisions are often difficult and controversial, it is important to use well-structured and explicit methods for making them. ESA decisions are inevitably based on limited information, and so agencies are obliged to act in the face of 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 examined explicitly, this asymmetric error structure can bias decisions under 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 modify 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 development 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 denied 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 use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 15 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 handicapped by delays 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 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. 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 use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 16 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 people 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 developments 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 Endangered 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. • 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 use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 17 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 validity 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 development. 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 and 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 provided for by the ESA are habitat conservation plans and natural community conservation planning. But those are only a beginning. Many other approaches have been discussed in various fora. 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 lead to successful prevention of many species extinctions, conservation of biological diversity, and reduced economic and social uncertainty and disruption. But sound science is an essential starting point. Combined with innovative and workable policies, it can help to solve these and related problems. use the print version of this publication as the authoritative version for attribution.