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Washington, D.C. 1995
National Research Council
Species Act
Commission on Life Sciences
NATIONAL ACADEMY PRESS
Board on Environmental Studies and Toxicology
Committee on Scientific Issues in the Endangered Species Act
Science and the Endangered
i
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ii
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iii
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iv
NATIONAL ACADEMY PRESS 2101 Constitution Ave., N.W. Washington, D.C. 20418
NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose mem-
bers are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.
The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.
This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee con-
sisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.
The project was supported by the Department of the Interior under Contract No. 14-48-0009-92-010.
Library of Congress Cataloging-in-Publication Data
Science and the Endangered Species Act / Committee on Scientific Issues in the Endangered Species Act, Board on Environmental Studies
and Toxicology, Commission on Life Sciences.
p. cm
Includes bibliographical references and index.
ISBN 0-309-05291-2
1. Endangered species—United States. 2. Endangered species—Law and legislation—United States. 3. Habitat conservation—United States.
I. National Research Council (U.S.). Committee on Scientific Issues in the Endangered Species Act.
QH76.S38 1995
333.9516'0973—dc20 95-33322
Copyright 1995 by the National Academy of Sciences. All rights reserved.
Printed in the United States of America
First Printing, October 1995
Second Printing, February 1998
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v
COMMITTEE ON SCIENTIFIC ISSUES IN THE ENDANGERED SPECIES ACT
MICHAEL T. CLEGG (Chair), University of California, Riverside, Calif.
GARDNER M. BROWN, JR., University of Washington, Seattle, Wash.
WILLIAM Y. BROWN, RCG/Hagler Bailly Inc., Arlington, Va.
WILLIAM L. FINK, University of Michigan, Ann Arbor, Mich.
JOHN HARTE, University of California, Berkeley, Calif.
OLIVER A. HOUCK, Tulane University, New Orleans, La.
MICHAEL LYNCH, University of Oregon, Eugene, Oreg.
LYNN A. MAGUIRE, Duke University, Durham, N.C.
DENNIS D. MURPHY, Stanford University, Stanford, Calif.
PATRICK Y. O'BRIEN, Chevron Research & and Technology Company, Richmond, Calif.
STEWARD T. A. PICKETT, Institute of Ecosystem Studies, Millbrook, N.Y.
H. RONALD PULLIAM, University of Georgia, Athens, Ga., resigned 5/31/94
KATHERINE RALLS, Smithsonian Institution, Washington, D.C.
BERYL B. SIMPSON, University of Texas, Austin, Tex.
ROLLIN D. SPARROWE, Wildlife Management Institute, Washington, D.C.
DAVID W. STEADMAN, New York State Museum, Albany, N.Y.
JAMES M. SWEENEY, Champion International Corporation, Washington, D.C.
Staff
DAVID J. POLICANSKY, Project Director
PATRICIA PEACOCK, Staff Officer (until 02/03/95)
LEE R. PAULSON, Editor
ADRIÉNNE DAVIS, Senior Project Assistant
Sponsor
Department of the Interior
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vi
BOARD ON ENVIRONMENTAL STUDIES AND TOXICOLOGY
PAUL G. RISSER (Chair), Miami University, Oxford, Ohio
MICHAEL J. BEAN, Environmental Defense Fund, Washington, D.C.
EULA BINGHAM, University of Cincinnati, Cincinnati, Ohio
EDWIN H. CLARK II, Clean Sites, Inc., Alexandria, Va.
ALLAN H. CONNEY, Rutgers University, Piscataway, N.J.
ELLIS COWLING, North Carolina State University, Raleigh, N.Car.
JOHN L. EMMERSON, Eli Lilly & Company, Greenfield, Ind.
ROBERT C. FORNEY, Unionville, Pa.
ROBERT A. FROSCH, Harvard University, Cambridge, Mass.
KAI LEE, Williams College, Williamstown, Mass.
JANE LUBCHENCO, Oregon State University, Corvallis, Ore.
GORDON ORIANS, University of Washington, Seattle, Wash.
FRANK L. PARKER, Vanderbilt University, Nashville, Tenn.
GEOFFREY PLACE, Hilton Head, S.Car.
DAVID P. RALL, Washington, D.C.
LESLIE A. REAL, Indiana University, Bloomington, Ind.
KRISTIN SHRADER-FRECHETTE, University of South Florida, Tampa, Fla.
BURTON H. SINGER, Princeton University, Princeton, N.J.
MARGARET STRAND, Bayh, Connaughton & Malone, Washington, D.C.
GERALD VAN BELLE, University of Washington, Seattle, Wash.
BAILUS WALKER, JR., Howard University, Washington, D.C.
Staff
JAMES J. REISA, Director
DAVID J. POLICANSKY, Associate Director and Program Director for Natural Resources and Applied Ecology
CAROL A. MACZKA, Program Director for Toxicology and Risk Assessment
LEE R. PAULSON, Program Director for Information Systems and Statistics
RAYMOND A. WASSEL, Program Director for Environmental Sciences and Engineering
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vii
COMMISSION ON LIFE SCIENCES
THOMAS D. POLLARD (Chair), Johns Hopkins Medical School, Baltimore, Md.
BRUCE N. AMES, University of California, Berkeley, Calif.
JOHN C. BAILAR III, McGill University, Montreal, Quebec
MICHAEL BISHOP, Hooper Research Foundation, University of California Medical Center, San Francisco,
Calif.
JOHN E. BURRIS, Marine Biological Laboratory, Woods Hole, Mass.
MICHAEL T. CLEGG, University of California, Riverside, Calif.
GLENN A. CROSBY, Washington State University, Pullman, Wash.
MARIAN E. KOSHLAND, University of California, Berkeley, Calif.
RICHARD E. LENSKI, Michigan State University, East Lansing, Mich.
EMIL A. PFITZER, Hoffmann-La Roche Inc., Nutley, N.J.
MALCOLM C. PIKE, University of Southern California School of Medicine, Los Angeles, Calif.
HENRY C. PITOT III, University of Wisconsin, Madison, Wisc.
JONATHAN M. SAMET, The Johns Hopkins University School of Medicine, Baltimore, Md.
HAROLD M. SCHMECK JR., Armonk, N.Y.
CARLA J. SHATZ, University of California, Berkeley, Calif.
SUSAN S. TAYLOR, University of California at San Diego, La Jolla, Calif.
P. ROY VAGELOS, Merck & Company, Whitehouse Station, N. J.
JOHN L. VANDEBERG, Southwestern Foundation for Biomedical Research, San Antonio, Tex.
PAUL GILMAN, Executive Director
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viii
T he N ational Academy of Sciences i s a private, nonprofit, self-perpetuating society of distinguished
scholars engaged in scientific and engineering research, dedicated to the furtherance of science and
technology and to their use for the general welfare. Upon the authority of the charter granted to it by the
Congress in 1863, the Acade my has a mandate that requires it to advise the federal government on
scientific and technical matters. Dr. Bruce Alberts is president of the National Academy of Sciences.
The National Academy of Engineering was established in 1964, under the charter of the National Academy
of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in
the selection of its members, sharing with the National Academy of Sciences the responsibility for advising
the federal government. The National Academy of Engineering also sponsors engineering programs aimed at
meeting national needs, encourages education and research, and recognizes the superior achievements of
engineers. Dr. Harold Liebowitz is president of the National Academy of Engineering.
The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the
services of eminent members of appropriate professions in the examination of policy matters pertaining to
the health of the public. The Institute acts under the responsibility given to the National Academy of
Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative,
to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute
of Medicine.
The National Research Council was organized by the National Academy of Sciences in 1916 to associate
the broad community of science and technology with the Academy’s purposes of furthering knowledge and
advising the federal government. Functioning in accordance with general policies determined by the
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Academy, the Council has become the principal operating agency of both the National Academy of
Sciences and the National Academy of Engineering in providing services to the government, the public, and
the scientific and engineering communities. The Council is administered jointly by both Academies and the
Institute of Medicine. Dr. Bruce Alberts and Dr. Harold Liebowitz are chairman and vice chairman,
respectively, of the National Research Council.
www.national-academies.org
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PREFACE ix
Preface
The Endangered Species Act (ESA) is an important legislative tool for the protection of threatened and
endangered species in the United States. The ESA asserts a legal claim on behalf of those species in the United
States to habitat that sometimes conflicts with competing management goals for both private and public lands. It
is inevitable that these conflicts play out in the political arena. Our committee was asked to provide advice on
scientific aspects of the ESA and to consider whether the act is "protecting endangered species and their
habitats." We have endeavored to restrict our advice to the areas where science can better inform the public
policy debate. The distinction between science and public policy is often fuzzy, because the possession of
scientific knowledge and the implementation of that knowledge are so closely linked. Our goal in this report has
been to explore and illuminate the knowledge side of the equation.
Since the original passage of the ESA in 1973, scientific knowledge has been anything but static. Our
understanding of biological species, in terms of their genetic and phylogenetic integrity, has greatly expanded
since 1973. A rich array of new experimental tools has been acquired from both genetics and computational
biology during the past two decades, and these have helped to drive a revolution in the traditional sciences of
taxonomy and systematics. At the same time, new theoretical constructs have been elaborated that have given
greater depth to definitions of species.
Species are composed of systems of populations (metapopulations) that have both temporal and spatial
dimensions. The temporal history of individual species and of the migrating continental land masses that contain
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PREFACE x
terrestrial habitats is known in much greater detail today than in 1973. The earth is dynamic and contemporary
biological diversity is the unique realization of this long history of change. The time scales involved in biological
change are long relative to human generations and, as a consequence, it is easy for us to see the biological world
as static. Nothing could be further from the truth. Modern biology reveals that species are reservoirs of unique
genetic adaptations to multifaceted physical and biological environments. The accumulation of these diverse
adaptations is the result of a shared evolutionary history that typically involves hundreds of thousands of years of
genetic continuity. The extinction of a species constitutes the irreversible loss of a suite of unique genetic
adaptations that have been acquired (much like interest) over a long history of investment.
Rates of extinction are uneven over geological time. Several episodes of major extinction are now
recognized, including the Permian-Triassic event (245 million years ago), when approximately 65% of terrestrial
species became extinct, and the Cretaceous-Tertiary event (65 million years ago), when approximately 90% of
terrestrial and marine reptiles became extinct. When viewed on a global scale, the present era constitutes yet
another major episode of biological extinction. In contrast to the past, however, the present cause of extinction is
a single biological species that has become so successful and so exploitive that it threatens to destroy the very
capital that is necessary for its own long-term survival. That single species—humankind—is capable of rational
analysis and planning, so that it can influence its own long-term destiny.
The earth's non-human biota is crucial to humans' long-term survival. We depend on the photosynthetic
capability of green plants for the oxygen that we breathe and for virtually all of our food and energy
requirements. The ability of green plants to grow is in turn dependent on a fixed supply of nitrogen (nitrates and
nitrites) that are largely the product of a specialized group of microorganisms (Rhizobia). Many of our modern
drugs have been derived from biotic sources. The list of human dependencies on the complex web of biological
species is virtually endless.
Habitat, the spatial dimension of species, is absolutely crucial to species survival. Habitat is the theater in
which the network of interactions between the physical and biological worlds play out. The landscape theory of
habitat emphasizes the heterogeneity, complexity, and dynamic character of the physical and biological
environment. The metapopulations of species are distributed on this shifting mosaic. If these are the scientific
realities, then how do we match science to wise habitat conservation?
The authors of the ESA recognized that species conservation must include strong provisions for habitat
conservation. These provisions included a trigger (threatened or endangered status of a species) that caused
certain legal prohibitions (jeopardy and taking restrictions). The law provides for
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PREFACE xi
the recovery of species through the designation of critical habitat and through the elaboration and
implementation of recovery plans. During the 20-year evolution of the ESA, additional provisions have been
added, including additional mechanisms for habitat conservation, and others aimed at the resolution of conflicts
engendered by ESA prohibitions. The committee was not charged with reviewing how the ESA is implemented
by various federal agencies and did not directly address this question. We do, however, have several
recommendations that would help improve the administration of the ESA if they were adopted (see Chapters 4
and 10, for example).
In general our committee finds that there has been a good match between science and the ESA. There are,
of course, points where the agreement between science and the ESA is poorer. These include lack of timely
designation of endangered or threatened status and similarly timely removal from these categories when recovery
goals have been achieved. Survival habitat should be identified and designated for protection if necessary when
species are listed as endangered. We have been able to align the "distinct population segment" language of the
ESA with our contemporary understanding of evolutionary units. We hope that such alignment helps to achieve
Congress's intent that distinct population segments be listed only sparingly and on a sound scientific basis and
thus reduces the danger that the ESA itself could be jeopardized by carrying that language to an absurd extreme.
The analytical tools to evaluate species health have been greatly developed in recent years. The emergence
of extinction theory from population genetics and ecology, the combination of demography and genetics in
population viability analysis and the extension of risk analyses into the realm of biological conservation promise
to lead us to wiser allocations of effort in the future. The field of ecosystem management has also emerged as a
significant field of applied biology, in part as a response to the need for a more global view of conservation
imperatives. The rich growth of these areas of science has also illuminated areas where our knowledge is still
inadequate. In response to the charges given our committee, we attempt to identify areas of critical scientific
uncertainty.
To paraphrase the great 20th century ecologist G. E. Hutchinson, species are the actors in the ecosystem
theater. To sustain a viable future for our descendants, we must find ways to preserve both species and
ecosystems. The ESA is a critically important part of our efforts to conserve species and thereby conserve
ecosystems. By virtue of the habitat restrictions that accompany endangered status, species that happen to share
habitat with an endangered species gain a measure of protection. The 20-year history of the ESA has validated its
focus on species endangerment. Species are objective entities that are easily recognized. Their health and needs
can be assessed and sound scientific management plans can be implemented.
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PREFACE xii
Despite this, the task of managing each of the vast multitude of species on a case-by-case basis is beyond human
capabilities. This is further compounded by the fact that many species remain undescribed. A challenge for the
future is to find more integrated mechanisms to sustain both species and ecosystems that do not depend on case-
by-case management.
It was my great good fortune to work with a knowledgeable, effective, and collegial committee. The various
chapters of this report are the product of much hard work and spirited debate. I want to express my deep
gratitude to the committee—including H. Ronald Pulliam, who resigned from the committee when he assumed
the directorship of the National Biological Service in May 1994—for their wisdom, patience, and cheerful
acceptance of the tasks imposed by this project. On behalf of the committee, I thank Project Assistant Adriénne
Davis for attending to our many needs. Staff Officer Patricia Peacock was a source of much practical experience
in conservation policy and she was a diligent editor and critic. Project Director David Policansky contributed his
vast experience in science policy, especially in the realm of conservation policy, to this project. David
Policansky and Pat Peacock also wrote, rewrote and edited many sections of this report. They contributed greatly
to the finished product. Finally, thanks to the many representatives of public agencies—especially the Fish and
Wildlife Service—and private groups who made written and oral presentations to our committee. They added an
essential dimension to our understanding of the complex issues that surround the ESA.
MICHAEL T. CLEGG
CHAIRMAN
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CONTENTS xiii
Contents
Executive Summary 1
Introduction 1
The Present Study 3
Extinctions 4
The Species Concept 5
Habitat 7
Recovery, 8
Conservation Conflicts Between Species 10
Estimating Risk 11
Making ESA Decisions in the Face of Uncertainty 12
Timing 14
Beyond the Endangered Species Act 14
Science, Policy, and the ESA 16
1 Introduction 18
History 18
The Present Study 21
References 23
2 Species Extinctions 24
Extinctions Over Geological Time 24
Prehistoric Human Impact on Continental Ecosystems 27
Prehistoric Human Impact on Island Ecosystems 30
Relating the Past to the Present 32
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CONTENTS xiv
Conclusions and Recommendations 38
References 40
3 Species Definitions And The Endangered Species Act 46
Historical Use of the Term Species in Implementation of the Endangered Species Act 47
History of Species Concepts Before and After the Endangered Species Act 51
A Concept of Species for the Purposes of the ESA 56
Conclusions and Recommendations 67
References 68
4 The Role of Habitat Conservation and Recovery Planning 71
The Importance of Habitat 71
The Role of Habitat Conservation Under the ESA 73
Critical Habitat and Federal Activities 75
Private Activities and Habitat Conservation Planning 78
Recovery Planning 80
Natural Community Conservation Planning Program and Coastal Sage Scrub Community of 84
Southern California
Habitat-Related Standards 89
Conclusions and Recommendations 91
References 93
5 Modern Perspectives of Habitat 94
Landscapes and Populations 97
Sources and Sinks 98
Metapopulations 99
Spatially Explicit Models 100
A Spatial Perspective and Population Viability Analysis 103
Conclusions 105
References 106
6 Conservation Conflicts Between Species 111
Interactions of Species in Nature 111
Northern Goshawk and Mexican Spotted Owl 113
Winter-Run Chinook Salmon and Delta Smelt 113
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Bachman's Sparrow and Red-Cockaded Woodpecker 116
Marine Mammals and Salmonids 117
Conclusions 120
Recommendations 121
References 121
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CONTENTS xv
7 Estimating Risk 124
Estimating the Risk of Extinction 125
Sources of Risk 125
Limitations of Our Ability to Estimate Risk 141
Conclusions and Recommendations 142
References 143
8 Making ESA Decisions in the Face of Uncertainty 148
Decisions Required Under the ESA 148
The Need for New Approaches to Decision Making 149
Providing Objective Risk Standards 152
Using Structured Approaches to Decision Making 157
Conclusions and Recommendations 174
References 176
9 Areas of Scientific Uncertainty 179
Ecosystem-Based Protection 179
Inadequate Knowledge of Species and Their Roles in Ecosystems 180
Estimation of the Risk of Extinction 181
Lack of Basic Information 182
The Protection of Genetic Diversity 184
Feasible Management Strategies 184
Valuing Rarity 186
References 190
10 Beyond the Endangered Species Act 193
Is the ESA Working? 193
Reducing Extinction 194
Recovery Success 195
Protection of Ecosystems 198
The Future: Beyond the Endangered Species Act 200
Science, Policy, and the ESA 202
References 203
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Appendixes
A Letter from Congress Requesting ESA Study 207
B Endangered Species Act 211
C Biographical Information on Committee and Staff 259
Index 263
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CONTENTS
xvi
