Species Sensitivity Distributions in Ecotoxicology - Section 1 ppsx

Hans Toni RatteCentre for Substances and Risk Assessment National Institute of Public Health and the Environment RIVM Bilthoven, the Netherlands Dr.. and optimized the appearances of tab

Species Sensitivity Distributions

in Ecotoxicology

Series Editor

Michael C Newman

College of William and Mary Virginia Institute of Marine Science Gloucester Point, Virginia

Environmental and Ecological

Risk Assessment

Published Titles Coastal and Estuarine Risk Assessment

Edited byMichael C Newman, Morris H Roberts, Jr., and Robert C Hale

Risk Assessment with Time to Event Models

Edited byMark Crane, Michael C Newman, Peter F Chapman, and John Fenlon

Species Sensitivity Distributions in Ecotoxicology

Edited byLeo Posthuma, Glenn W Suter II, and Theo P Traas

LEWIS PUBLISHERS

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Edited by

Leo Posthuma Glenn W Suter II

Theo P Traas

Species Sensitivity

Distributions

inEcotoxicology

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Different species have different sensitivities to a chemical This variation can bedescribed with a statistical or empirical distribution function, and this yields a speciessensitivity distribution (SSD) The idea to use SSDs in risk assessment originatedalmost simultaneously in Europe and in the United States Scientists began to usethese distributions for the derivation of environmental quality criteria, challenged

by policy makers to make optimal use of single-species toxicity test data for icals This development coincided with the notion that risks cannot be completelyeliminated but should be reduced to an acceptable low level

chem-In 1990, the Organization for Economic Cooperation and Development (OECD)Hazard Assessment Advisory Body organized a workshop in Arlington, Virginia, todiscuss these and other approaches for extrapolation of laboratory aquatic toxicitydata to the real environment The extrapolation workshop, together with other work-shops on the application of quantitative structure–activity relationships (QSARs) toestimate ecotoxicity data (Utrecht, the Netherlands) and effects assessment of chem-

Guid-ance Document for Aquatic Effects Assessment, which was published in 1995 Thisguidance document is applied, for example, in the OECD existing chemicals program

As head of the OECD Environment, Health and Safety Division, which supportedthe transatlantic discussions on the use of SSDs in 1990, it is a great pleasure to seethat this specific approach in ecotoxicology has been taken up by scientists and isstill developing The fact that it has become so well used in environmental manage-ment should not keep us from being critical and demanding about the scientificrationale and validity of the methods used It is my firm belief that this bookcontributes to this goal and that it serves as an excellent stimulus to pursue thecontinued development of SSD-based risk assessment in ecotoxicology

Rob Visser

Head, Environment, Health and Safety Division Organization for Economic Cooperation and Development

AIMS OF THE BOOK

The aims of this book are many, but the most important ones are the following:

dis-tributions, is a practical method in ecological risk assessment and indecision-making processes It is used in the derivation of environmentalquality criteria and in ecological risk assessment of contaminated ecosys-tems The question is, whether the past adoption of the concept has been

a good decision, especially in view of the large investments in preventiveand curative actions resulting from decisions based, fully or in part, onapplication of the concept The editors, all working in governmentalinstitutes, felt a sense of urgency in the air to summarize the state of theart of the concept, its scientific underpinning, its current uses, and itspredictive accuracy, after approximately two decades of convergent evo-lution on two continents Eventually, a review of the state of the art shouldpromote better understanding of all issues relevant to the SSD conceptand its applications Therefore, the major aim is a better understanding

of the science of ecological risk assessment concerning the use of apractically adopted method

• Second, the many relevant publications by academic, regulatory, andindustrial scientists in North America and Europe have been scatteredthroughout the literature Few papers have been published in the easilyaccessible scientific journals; many are in the “gray literature.” Further-more, most texts explain the issues in various, context-dependent lan-guages, with local jargon added The secondary aim, necessary tounderstand the science, is to bring together open and gray literature, and

to make the sources available in clear language in this book

• Third, by compilation and study of the available material and by review

of past criticisms of the SSD concept and the solutions offered so far, afinal aim becomes apparent This aim is to suggest paths forward, tosuggest solutions for the most relevant criticisms voiced in the past, and

to break inertia in the evolution of the SSD concept itself This shouldeventually lead to clear views regarding the advantages and limitations ofthe method for different applications

THE EVOLUTION OF EDITORIAL RISK

The pursuit of these three aims began in 1998 At a conference in Bordeaux,organized by the European branch of the Society for Environmental Toxicology andChemistry (SETAC), various Europeans working with the SSD concept were inspired

by the local atmosphere to draft the raw outlines of a plan After approximately

15 years of evolution on two continents, the need was felt to evaluate the SSDconcept The thought simmered for some time It was brought to the Laboratory forEcotoxicology at the Dutch National Institute for Public Health and the Environment(RIVM) At RIVM, Herman Eijsackers sowed the seed, and he and Hans Cantoncared most for the undisturbed survival and growth of the young plant In the nextyear, it grew into a formal RIVM project RIVM employees were assigned to compileand evaluate the current state of the art, and to formulate ways forward This wasdeemed a necessary task for RIVM, since many sites in the Netherlands are exposed

at concentrations exceeding the Dutch Environmental Quality Criteria, and theproject was expected to help answer the question: “What are the quantitative eco-logical risks of mixtures of chemical compound concentrations in the environmentthat exceed the Environmental Quality Criteria?” The efforts were supported byscientific advisory bodies of the RIVM Soon, the RIVM project became an inter-national project, and the review plan reshaped into a book plan, with internationaleditorship and contributions

The addition of a North American editor to this effort continued a connectionthat began at a 1990 OECD workshop on ecotoxicological extrapolation models(OECD, 1992) The most significant result of that workshop was the realization that

a common approach was being used in the United States, the Netherlands, andDenmark to extrapolate from single species toxicity test results to biotic communi-ties Because there was no name for that class of models, the Working Group B

contrib-uted to the subsequent expansion of the use of SSDs from the setting of regulatorycriteria into the emerging field of ecological risk assessment More to the point, itestablished the contacts and common interests among users of SSDs in NorthAmerica and Europe that made this volume possible

ECOLOGICAL AND AUTHORSHIP RISKS

The contributors to this book are specialists on risks, especially risks from chemical

to this book in view of various realistic risks associated with it Nonetheless, theycontributed of their own free will

What risks did authors and editors face?

• First, they faced the risk that they would create a Gordian knot of riskconcepts, definitions, and research results, when their goal was to unravel

a knotty problem If you try to imagine how to describe a Gordian knot,

or a research plan to unravel it, you can guess how difficult that can be,especially when you want to do it in a scientific way Where are the rope

ends, and how do they causally connect? Those who contribute to a book

on such a knotty problem might never be understood by readers or even

by the other authors

• Second, there is the risk that the interpretation of the chosen risk definition(if any) would be strongly context dependent, yielding a hidden knotwithin a knot In a scientific context, one can communicate about risks

in a purely numerical context, without value judgments In the societalcontext of risk-based decision making, however, risk has an aspect ofvalue judgment The contributors were aware of this extra complication,

as they were recruited from those different contexts, so it was courageous

to join Thinkers and practitioners could have easily split, and two volumesrather than one volume could have resulted

• Third, there is the risk of interminable debate aroused by the publishedtext, as a consequence of the preceding risks The authors and editorscould have chosen to keep the results of their debates among themselves,since the above risks were effectuated in their internal discussions Theremight not have been a book at all

• Fourth, risks are associated with working on the border between scienceand policy Scientists may develop methods that have policy implications,which may not be acceptable to policy makers or advocates for industry

or the environment Clearly, the assumption that SSDs are adequate els of the environment is such a case, and work on the book could havebeen stopped by the employers of the authors or editors

mod-• Fifth, publicizing controversial technical and conceptual issues may beunwelcome, because SSDs are firmly embedded in the regulatory practices

of the United States, the Netherlands, and other nations Regulators maynot want to be told that the scientific foundations of their actions are stillquestionable or subject to change

• Sixth, confusion and conflict could have been almost invited by the editors

by their wish to bring together two historical lines of SSD evolution (theNorth American and the European) in a single volume, each with its owncontext of adopted principles, terminology, and legislation

AUTHORSHIP RISKS IN PRACTICE

The editors have seen some of these risks in practice At the first public introduction

of the SSD concept in Europe, it was the initiator of the plan for this book who,metaphorically, suggested killing the first messenger In 1983, Bas Kooijman, fromthe Netherlands Organization for Applied Scientific Research (TNO), was asked bythe Dutch Ministry of the Environment to help resolve the ethical question: “Howmuch toxicity test data for how many species are needed to underpin adequate riskassessment based decisions?” As a result, an initial Dutch TNO report from 1985

subject of the derivation of hazardous concentrations for sensitive species Thisevolved further when Nico van Straalen from the Vrije Universiteit Amsterdam wasinvited to give a thought-provoking introductory plenary lecture at a 1995 meeting

of the Dutch Provisional Soil Protection Technical Committee (V-TCB) He began

his head on the guillotine, while the audience members were handed a rope to releasethe blade The lecture was completed in full health, although the pertinent audiencemember said in a whisper that he would have liked to pull the rope This illustratesthe risks of the science policy debate on the SSD concept in a nutshell

POST-WRITING RISKS

Despite these risks, the contributors have not been reluctant They produced 22chapters, and no authors left because of inability to describe their strand of the knot.The contributors also have been willing to project themselves into the role andcontext of their colleagues The 22 chapters are thus in one book, not two Althoughdebates have been many, we hope scientific growth has resulted

On publication of this book, only the post-writing risks remain There is a need

of risk management here The management of that risk is your task as reader, acting

in your own professional environment after reading the book To help you with this,

we have done our best to present the science and applications to you in manageableportions, despite the double Gordian knot We identified four sections:

I General Introduction and History of SSDs

II Scientific Principles and Characteristics of SSDs

III Applications of SSDs

A Derivation of Environmental Quality Criteria

B Ecological Risk Assessment

IV Evaluation and Outlook

By arranging the chapters within these sections, the different focuses of the chaptersare presented

We can help in managing the remaining risks only a bit further, by stating that

second from clearly defining or recognizing the context of those involved in thedebate, and third from clearly distinguishing the values obtained in risk calculationsfrom value judgments

All scientific fields can be seen as Gordian knots For the field of ecological riskassessment, we hope to have cut through some surface layers, and we hope to havefreed thereby some useful lengths of rope This book is the result of the risky effort

of many people, who all hope that the field of ecological risk assessment benefitsfrom their efforts

Leo Posthuma, Glenn W Suter II, and Theo P Traas

The editors wish to acknowledge the valuable contributions to this book by:

• Olivier Klepper, for starting the process that evolved into this book;

and who adapted their chapters based on comments of anonymous peerreviewers, section editors, and editors, so as to optimize scientific qualitywithin the chapters, and line of reasoning among chapters in the foursections and throughout the book;

reviewers, so that all chapters were read by reviewers representing twotypes, namely, those expected to be familiar with the environmental policysetting in the continent of the author and those almost completely unfa-miliar with that context; the latter helped remove unnecessary jargon;

enthusi-asm, resulting in main-line comments and detailed suggestions on allchapters, which greatly improved the contents of the book

The reviewers are:

Prof Dr Wim Admiraal

Department of Aquatic Ecology

The Cadmus Group, Inc

Oak Ridge, Tennessee, USA

Dr Jacques J.M Bedaux

Institute of Ecological Science

Vrije Universiteit

Amsterdam, the Netherlands

Prof Dr Hans Blanck

Botanical InstituteGöteborg UniversityGöteborg, Sweden

Dr Kym Rouse Campbell

The Cadmus Group, Inc

Oak Ridge, Tennessee, USA

Environmental Sciences Division

Oak Ridge National Laboratory

Oak Ridge, Tennessee, USA

Office of Research and Development

U.S Environmental Protection Agency

Cincinnati, Ohio, USA

Dr John H Gentile

Center for Marine

and Environmental Analysis

University of Miami

Miami, Florida, USA

Dr Jeff Giddings

The Cadmus Group, Inc

Marion, Massachusetts, USA

Dr Lenwood Hall, Jr.

University of Maryland

Queenstown, Maryland, USA

Dr Patrick Hofstetter

Harvard School of Public Health

Cincinnati, Ohio, USA

Dr Udo Hommen

Private Consultant for Ecological Modelling and StatisticsAlsdorf, Germany

Dr Steve Hopkin

School of Animal and Microbial Sciences

University of ReadingReading, United Kingdom

Prof Dr Olivier Jolliet

Laboratory of Ecosystem ManagementEcole Polytechnique Fédérale

de LausanneLausanne, Switzerland

Dr Lorraine Maltby

Department of Animal and Plant SciencesUniversity of SheffieldSheffield, United Kingdom

Dr Dwayne Moore

The Cadmus Group, Inc

Ottawa, Ontario, Canada

Prof Dr David F Parkhurst

School of Public and Environmental AffairsIndiana University

Bloomington, Indiana, USA

Dr Hans Toni Ratte

Centre for Substances

and Risk Assessment

National Institute of Public Health and

the Environment (RIVM)

Bilthoven, the Netherlands

Dr Eric P Smith

Department of Statistics

Virginia Polytechnic Institute

and State University

Blacksburg, Virginia, USA

Dr Timothy A Springer

Wildlife International, Ltd

Easton, Maryland, USA

Mr Charles E Stephan

U.S Environmental Protection Agency

Duluth, Minnesota, USA

Dr Helen M Thompson

Environmental Research TeamCentral Science LaboratoryYork, United Kingdom

Dr Nelly Van der Hoeven

ECOSTATStatistical Consultancy in Ecology, Ecotoxicology and Agricultural Research

Leiden, the Netherlands

Dr William H Van der Schalie

National Center for Environmental Assessment

U.S Environmental Protection AgencyWashington, D.C., USA

Dr Bert Van Hattum

Institute of Ecological ScienceVrije Universiteit

Amsterdam, the Netherlands

Prof Dr Nico M van Straalen

Institute of Ecological ScienceVrije Universiteit

Amsterdam, the Netherlands

Dr Donald J Versteeg

The Procter & Gamble CompanyMiami Valley LaboratoriesCincinnati, Ohio, USA

Supe-• Dick de Zwart (at RIVM), the electronics polyglot of the book team,who shaped all electronic formats into one, thereby removing the non-scientific transatlantic heterogeneity in file formats, and who shaped

and optimized the appearances of tables and figures and the singlereference list;

• Miranda Mesman and Dick de Zwart for assistance in proofreading oftechnically edited chapters;

• Martin Middelburg at the Studio of RIVM for formatting of variouschapter figures;

• The directors of the Dutch National Institute of Public Health and the Environment (RIVM), especially of the Division of Risks, Envi- ronment and Health, who provided the atmosphere in which scientificideas on risks of various agents for humans and environment can flourishwith both open scientific discussions and an eye on practical use, and whoprovided funding and all technical means to achieve the goals of this bookproject;

• The former and current acting Head of the Laboratory for Ecotoxicology,

Herman Eijsackers and Hans Canton, and the Head and Deputy Head

van Leeuwen, who stimulated and gave ample room for planning andexecuting the work for the book project;

• Colleagues who participated in the discussion at the Interactive PosterSession on SSDs, held at the 20th North American Annual Meeting ofthe Society for Environmental Toxicology and Chemistry (SETAC) inPhiladelphia, PA, USA, in 1999;

and SETAC office personnel, who provided the opportunity to organize

an Interactive Poster Session on SSDs at the 20th North American AnnualMeeting of SETAC in Philadelphia, PA, USA, in 1999;

• The editors gratefully acknowledge the support of their life partners,

Connie Posthuma, Linda Suter, and Evelyn Heugens

Development of this book was supported in part by the Dutch National Institute

of the strategic RIVM project “Ecological Risk Assessment,” RIVM project numberS/607501

About the Editors

Leo Posthuma is currently Research Staff Member

in the Laboratory for Ecotoxicology at the DutchNational Institute of Public Health and the Environ-ment (RIVM), where he is involved in the develop-ment, testing, and validation of methods for eco-logical risk assessment He studied Biology andreceived a Ph.D in Ecology and Ecotoxicologyfrom the Vrije Universiteit, Amsterdam, the Neth-erlands He has authored and co-authored more than

75 open literature publications, reports, and bookchapters, and has acted as book co-editor Hisresearch experience has included phytopathologicalstudies and studies on the evolutionary ecology andpopulation genetics of contaminant adaptation ofexposed soil arthropod populations, on community tolerance evolution, on the bio-availability of toxic compounds for terrestrial organisms, on joint effects of com-pound mixtures, and on stability and resilience of soil ecosystems

Glenn W Suter II is currently Science Advisor inthe U.S Environmental Protection Agency’sNational Center for Environmental Assess-ment–Cincinnati, and was formerly a SeniorResearch Staff Member in the Environmental Sci-ences Division, Oak Ridge National Laboratory,U.S.A He holds a Ph.D in Ecology from the Uni-versity of California, Davis, and has 26 years ofprofessional experience including 20 years of expe-rience in ecological risk assessment He is the editorand principal author of two texts in the field ofecological risk assessment, and has edited two otherbooks and authored more than a hundred open lit-erature publications He is Associate Editor for Eco-

Society for Environmental Toxicology and Chemistry (SETAC) He has served onthe International Institute of Applied Systems Analysis Task Force on Risk andPolicy Analysis, the Board of Directors of the SETAC, an Expert Panel for the

Tox-icology and Chemistry, Environmental Health Perspectives, and Ecological tors. His research experience includes development and application of methods for

Indica-ecological risk assessment, development of soil microcosm and fish toxicity tests,and environmental monitoring He is a Fellow of the American Association for theAdvancement of Science.

Theo P Traas is currently Research Staff ber in the Centre for Substances and Risk Assess-ment at the Dutch National Institute of PublicHealth and the Environment (RIVM) He studiedBiology at the Vrije Universiteit, Amsterdam, theNetherlands His main task is the derivation ofenvironmental risk limits, using species sensitiv-ity distributions and probabilistic food chain mod-els He is involved in the development, testing,and validation of models for ecological riskassessment He has authored and co-authoredmore than 35 open literature publications, reports,and book chapters

Mem-Editors and Principal Authors

Leo Posthuma

RIVM (Dutch National Institute of Public Health and the Environment)

Laboratory for Ecotoxicology

Bilthoven, the Netherlands

Glenn W Suter II

U.S Environmental Protection Agency

National Center for Environmental Assessment

Cincinnati, Ohio, USA

Theo P Traas

RIVM (Dutch National Institute of Public Health and the Environment)

Centre for Substances and Risk Assessment

Bilthoven, the Netherlands

Section Editors

Section I

Theo P Traas (RIVM, Bilthoven, the Netherlands)

Herman J P Eijsackers (Alterra Green World Research, Wageningen,

the Netherlands)

Section II

Tom Aldenberg (RIVM, Bilthoven, the Netherlands)

Dik van de Meent (RIVM, Bilthoven, the Netherlands)

Glenn W Suter II (U.S EPA, Cincinnati, Ohio, USA)

Section III

Robert Luttik (RIVM, Bilthoven, the Netherlands)

Dick de Zwart (RIVM, Bilthoven, the Netherlands)

Section IV

Leo Posthuma (RIVM, Bilthoven, the Netherlands)

Glenn W Suter II (U.S EPA, Cincinnati, Ohio, USA)

Alterra Green World Research, Department of Water and the Environment,

Wageningen, the Netherlands

Theo C M Brock

Paul J van den Brink

RIKZ (National Institute for Coastal and Marine Management), Middelburg,

RIVM (National Institute of Public Health and the Environment), Centre for

Sub-stances and Risk Assessment, Bilthoven, the Netherlands

Trudie Crommentuijn*

Cornelis J van Leeuwen

Robert Luttik

Hans Mensink

Dick T.H.M Sijm

Theo P Traas

Annemarie P van Wezel

RIVM (National Institute of Public Health and the Environment), Laboratory for

Ecotoxicology, Bilthoven, the Netherlands

Dik van de Meent

Leo Posthuma

Aart Sterkenburg

Dick de Zwart

RIVM (National Institute of Public Health and the Environment), Laboratory for

Water and Drinking Water Research, Bilthoven, the Netherlands

Tom Aldenberg

University of Amsterdam, Institute for Biodiversity and Ecosystem Dynamics,

Amsterdam, the Netherlands

Mark A J Huijbregts*

Vrije Universiteit, Institute of Ecological Science, Amsterdam, the Netherlands

Nico M van Straalen

Wageningen University, Toxicology Group, Wageningen, the Netherlands

U.S Environmental Protection Agency, National Center for Environmental

Assessment, Cincinnati, Ohio, USA

Glenn W Suter II

* Current affiliation: Ministry of Housing, Physical Planning and the Environment, The Hague, the

Netherlands

* Current affiliation: University of Nijmegen, Faculty of Science, Mathematics and Informatics,

Depart-ment of EnvironDepart-mental Studies, Nijmegen, the Netherlands

U.S Environmental Protection Agency, Midcontinent Ecology Division,

Duluth, Minnesota, USA

of SSDs

Chapter 1

General Introduction to Species Sensitivity Distributions

Leo Posthuma, Theo P Traas, and Glenn W Suter II

Chapter 2

North American History of Species Sensitivity Distributions

Glenn W Suter II

Chapter 3

European History of Species Sensitivity Distributions

Nico M van Straalen and Cornelis J van Leeuwen

Extrapolation Factors for Tiny Toxicity Data Sets from Species Sensitivity

Distributions with Known Standard Deviation

Tom Aldenberg and Robert Luttik

Chapter 7

Species Sensitivity Distributions in Ecological Risk Assessment:

Distributional Assumptions, Alternate Bootstrap Techniques, and Estimation

of Adequate Number of Species

Michael C Newman, David R Ownby, Laurent C A Mézin,

David C Powell, Tyler R L Christensen, Scott B Lerberg,

Britt-Anne Anderson, and Tiruponithura V Padma

Chapter 8

Observed Regularities in Species Sensitivity Distributions

for Aquatic Species

Dick de Zwart

Chapter 9

The Value of the Species Sensitivity Distribution Concept for Predicting Field

Effects: (Non-)confirmation of the Concept Using Semifield Experiments

Paul J van den Brink, Theo C M Brock, and Leo Posthuma

A Derivation of Environmental Quality Criteria

Chapter 10

Effects Assessment of Fabric Softeners: The DHTDMAC Case

Cornelis J van Leeuwen and Joanna S Jaworska

Chapter 11

Use of Species Sensitivity Distributions in the Derivation of Water Quality

Criteria for Aquatic Life by the U.S Environmental Protection Agency

Charles E Stephan

Chapter 12

Environmental Risk Limits in the Netherlands

Dick T H M Sijm, Annemarie P van Wezel, and Trudie Crommentuijn

Ecotoxicological Soil Quality Criteria in Denmark

Janeck J Scott-Fordsmand and John Jensen

B Ecological Risk Assessment

Chapter 15

Probabilistic Risk Assessment Using Species Sensitivity Distributions

Keith R Solomon and Peter Takacs

Chapter 16

The Potentially Affected Fraction as a Measure of Ecological Risk

Theo P Traas, Dik van de Meent, Leo Posthuma, Timo Hamers,

Belinda J Kater, Dick de Zwart, and Tom Aldenberg

Chapter 17

Methodology for Aquatic Ecological Risk Assessment

William J Warren-Hicks, Benjamin R Parkhurst, and Jonathan B Butcher

Chapter 18

Toxicity-Based Assessment of Water Quality

Dick de Zwart and Aart Sterkenburg

Ecotoxicological Impacts in Life Cycle Assessment

Mark A J Huijbregts, Dik van de Meent, Mark Goedkoop,

and Renilde Spriensma

Conceptual and Technical Outlook on Species Sensitivity Distributions

Leo Posthuma, Theo P Traas, Dick de Zwart, and Glenn W Suter II

Section I

General Introduction and History of SSDs

This section describes the context and history of the development of species tivity distributions (SSDs) for use in ecotoxicology The general introduction showsthat SSDs are used for two purposes: the derivation of environmental quality criteriaand ecological risk assessment for contaminated ecosystems It is followed byhistorical overviews of the partly independent and convergent evolution of the SSDconcept on two continents (North America and Europe) The section illustrates theevents that have occurred at the interface of science and regulation, homologies anddivergence in SSD-based methods, and the need to unite the existing theories andapplications

sensi-General Introduction

to Species Sensitivity Distributions

Leo Posthuma, Theo P Traas, and Glenn W Suter II

1.5 Aims of the Book

Abstract — The species sensitivity distribution (SSD) concept was proposed two decades ago as an ecotoxicological tool that is useful for the derivation of environmental quality criteria and ecological risk assessment Methodologies have evolved and are applied in various risk management frameworks Both support and criticisms have been voiced, spread over diverse sources in reports and scientific literature This chapter introduces the issues and their interrelationships treated in this book The aims of the book on SSDs are to present (1) the historical context, (2) the basic scientific principles, characteristics, and assumptions, (3) the current practical applications, and (4) an evaluation and outlook regarding the SSD concept and its uses.

1.1 INTRODUCTION

The possible threat of toxic compounds to ecosystems has elicited a request bysociety to science, to derive “safe” ambient concentrations for protection of ecosys-tems and methods to assess ecological risks Although this societal request is difficult

to answer for many reasons, one major difficulty is the estimation of effects ondiverse species and ecosystems This book focuses on the variation in speciessensitivities to toxicant exposure, and on a specific method to address this variation.1

Different ecologists and ecotoxicologists independently designed ecotoxicologicalassessment systems based on the variance in response among species (Klapow andLewis, 1979; Mount, 1982; Blanck, 1984; McLaughlin and Taylor, 1985; U.S EPA,1985a; Kooijman, 1987) Interspecies variation in sensitivity to environmental pol-lutants is apparently not only a core problem, but also a basis for finding solutions.This book focuses on the history, theories, and current practices of the ecotox-icological extrapolation models known as species sensitivity distributions (SSDs).SSDs represent the variation in sensitivity of species to a contaminant by a statistical

or empirical distribution function of responses for a sample of species The emphasis

on the issue of “extrapolation” from the single species to the community level that

is captured in the SSD model should not mean neglect of environmental factors.That is, there are other relevant factors modulating the predicted risk of contaminants

in ecosystems in addition to sensitivity differences, such as variation in biologicalavailability of the compounds and the occurrence of ecological interactions There-fore, it is often necessary to make additional extrapolations, to improve predictionaccuracy of the SSD The contributors to this book aim to present an overview andevaluation of the use of SSDs in current ecotoxicology, taking into account theimportance of the other sources of variation

1.2 VARIABILITY AND SPECIES SENSITIVITY

Living organisms constitute a vast diversity of taxonomy, life history, physiology,morphology, behavior, and geographical distribution For ecotoxicology, these bio-logical differences mean that different species respond differently to a compound at

a given concentration (i.e., different species have different sensitivities) Theacknowledgment that species sensitivities to toxic compounds differ (withoutattempting to explain the cause) and description of that variation with a statisticaldistribution function yields SSDs

The basic assumption of the SSD concept is that the sensitivities of a set ofspecies can be described by some distribution, usually a parametric distributionfunction such as the triangular, normal, or logistic distribution (Chapters 4 and 5).Nonparametric methods are used as well (Chapter 7) The available ecotoxicologicaldata are seen as a sample from this distribution and are used to estimate the param-eters of the SSD The variance in sensitivity among the test species and the meanare used to calculate a concentration expected to be safe for most species of interest,which can be used to set an environmental quality criterion (EQC) A more recentapplication is the use of SSDs in ecological risk assessment (ERA)

Since SSDs were originally proposed to derive EQCs in the late 1970s and 1980s in the United States and Europe, respectively, their importance in ecotoxicityevaluations has steadily grown Intensive discussions have taken place on principles,statistics, assumptions, data limitations, and applications (e.g., Hopkin, 1993; Forbesand Forbes, 1993; Smith and Cairns, 1993; Chapman et al., 1998) The history ofSSD approaches for North America and Europe is the subject of Chapters 2 and 3.These chapters explain the purposes for which SSDs were originally developed andtheir expanding use in various regulatory and management contexts The readershould also be aware that the use of SSDs has spread beyond its two continents of

mid-origin to South Africa (Roux et al., 1996), Australia and New Zealand (ANZECC,2000a,b), and elsewhere In these new contexts, the concept is expanding bothconceptually and technically.

1.3 SSD BASICS

A SSD is a statistical distribution describing the variation among a set of species intoxicity of a certain compound or mixture The species set may be composed of aspecies from a specific taxon, a selected species assemblage, or a natural community.Since we do not know the true distribution of toxicity endpoints, the SSD is estimatedfrom a sample of toxicity data and visualized as a cumulative distribution function

(PDF) The CDF curve follows the distribution of the sensitivity data obtained fromecotoxicological testing, plotting effect concentrations derived from acute or chronic

(NOECs), respectively The number of data to construct SSDs varies widely, between

no data at all (for many compounds) to more than 50 or 100 sensitivity values (for

a few compounds) It is evident that the number of data is highly important for thederivation of the SSD, and for conclusions based on them

The arrows in the graphs indicate that the SSD concept can be used in a “forward”

as well as “inverse” way (Van Straalen and Denneman, 1989; Chapter 4) For theinverse use, such as the derivation of environmental quality criteria, a cutoff per-

FIGURE 1.1 The basic appearance of SSDs, expressed as a CDF The dots are input data The line is a fitted SSD Forward use (arrows from X → Y) yields the PAF as defined in Chapter 4, or similar estimates of risk as defined by other authors (see also Chapters 5, 15, and 17) Inverse use (arrows from Y → X) yields an EQC at a certain cutoff value, here the hazardous concentration for 5% of the species, HC5 (e.g., Van Straalen and Denneman, 1989).

Risk Assessment

PAF

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Log Concentration ( µg.l -1 ) EQC  HC 5

L(E)C50 or NOEC Species Sensitivity Distribution