EPA/630/R-92/001 

February 1992 

FRAMEWORK FOR ECOLOGICAL RISK ASSESSMENT 

Risk Assessment Forum

U.S. Environmental Protection Agency

Washington, DC 20460
DISCLAIMER 

This document has been reviewed in accordance with U.S. Environmental Protection Agency 

policy and approved for publication. Mention of trade names or commercial products does not constitute 

endorsement or recommendation for use. 

ii 
CONTENTS 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............. vi

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ vii

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ ix

Contributors and Reviewers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............. x

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ xiv

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... 1

1.1. Purpose and Scope of the Framework Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 1

1.2. Intended Audience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 2

1.3. Definition and Applications of Ecological Risk Assessment ..................... 2

l. 4. Ecological Risk Assessment Framework .................................... 2

1.5. The Importance of Professional Judgment ................................... 6

1.6. Organization .......................................................... 6

2. PROBLEM FORMULATION ....................................................... 9

2.1. Discussion Between the Risk Assessor and Risk Manager (Planning) ............. 9

2.2. Stressor Characteristics, Ecosystem Potentially at Risk, and Ecological Effects ..... 9

2.2.1. Stressor Characteristics ................................................. 11

2.2.2. Ecosystem Potentially at Risk ............................................ 11

2.2.3. Ecological Effects ..................................................... 12

2.3. Endpoint Selection .................................................... 12

2.4. The Conceptual Model ................................................. 15

3. ANALYSIS PHASE ............................................................. 17

3.1. Characterization of Exposure ............................................ 17

3.1.1. Stressor Characterization: Distribution or Pattern of Change ............. 17

3.1.2. Ecosystem Characterization ....................................... 19

3.1.3. Exposure Analyses .............................................. 20

3.1.4. Exposure Profile ............................................... 21

iii 
3.2. Characterization of Ecological Effects ..................................... 21

3.2.1. Evaluation of Relevant Effects Data ................................ 21

3.2.2. Ecological Response Analyses .................................... 22

3.2.3. Stressor-Response Profile ........................................ 26

4. RISK CHARACTERIZATION ..................................................... 28

4.1. Risk Estimation 28

4.1.1. Integration of Stressor-Response and Exposure Profiles .................. 28

4.1.2. Uncertainly ..................................................... 30

4.2. Risk Description ...................................................... 31

4.2.1. Ecological Risk Summary .......................................... 32

4.2.2. Interpretation of Ecological Significance .............................. 33

4.3. Discussion Between the Risk Assessor and

Risk Manager (Results) ................................................ 34

5. KEY TERMS ................................................................... 37

6. REFERENCES .................................................................. 39

iv 
LIST OF BOXES

Physical and Chemical Stressors as a Focus of the Framework .............................. 1

Relationship of the Framework to a Paradigm for Human Health Risk Assessment .............. 3

Use of the Term”Exposure'' ......................................................... 5

Characterization of Ecological Effects Used Instead of Hazard Assessment .................... 6

Additional Issues Related to the Framework ............................................. 8

Example Stressor Characteristics ................................................... . . 11

Endpoint Terminology ............................................................ . . 12

Considerations in Selecting Assessment Endpoints ..................................... . . 13

Considerations in Selecting Measurement Endpoints ..................................... 14

Additional Issues in Problem Formulation .............................................. 16

Extrapolations and Other Analyses Relating Measurement and Assessment Endpoints ........... 23

Hill's Criteria for Evaluating Causal Associations (Hill, 1965) .............................. 25

Additional Issues Related to the Analysis Phase .......................................... 27

Additional Issues Related to the Risk Characterization Phase ............................... 36

LIST OF FIGURES

Figure 1. Framework for Ecological Risk Assessment ............................... 4

Figure 2. Problem Formulation ............................................... . . 10

Figure 3. Analysis ........................................................... 18

Figure 4. Risk Characterization ................................................ 29

v
ACKNOWLEDGMENTS 

This U.S. Environmental Protection Agency (EPA) report has been developed under the auspices 

of EPA's Risk Assessment Forum, a standing committee of EPA scientists charged with developing risk 

assessment guidance for Agency-wide use. An interoffice work group chaired by Susan Norton (Office of 

Health and Environmental Assessment), Donald Rodier (Office of Toxic Substances), and Suzanne 

Marcy (Office of Water) led this effort. Other members of the work group are Michael Brody, David 

Mauriello, Anne Sergeant, and Molly Whitworth. William van der Schalie and William Wood of the Risk 

Assessment Forum staff coordinated the project, which included peer review by scientists from EPA, 

other Federal agencies, and the private sector. 

vi 
FOREWORD 

Publication of this report, “Framework for Ecological Risk Assessment” (Framework Report), is 

a first step in a long-term program to develop risk assessment guidelines for ecological effects. EPA has 

been developing risk assessment guidelines primarily for human health effects for several years. 

In 1986, EPA issued five such guidelines, Including cancer, developmental toxicity, and exposure 

assessment (51 Federal Register 33992-34054, 24 September 1986). Although EPA had issued guidance 

for cancer risk assessment 10 years earlier (41 Federal Register 21402, 1976), the 1986 guidelines 

substantially enlarged the scope of EPA's formal guidance by covering additional health topics and by 

covering all areas in much greater depth. Each of the guidelines was a product of several years of 

discussion and review involving scientists and policymakers from EPA, other Federal agencies, 

universities, industry, public interest groups, and the general public. 

Preliminary work on comparable guidelines for ecological effects began in 1988. As part of this 

work, EPA studied existing assessments and identified issues to help develop a basis for articulating 

guiding principles for the assessment of ecological risks (U.S. EPA, 1991). At the same time, EPA's 

Science Advisory Board urged EPA to expand its consideration of ecological risk issues to include the 

broad array of chemical and nonchemical stressors for which research and regulation are authorized in 

the environmental laws administered by EPA (U.S. EPA, 1990b). As a result, EPA has embarked on a 

new program to develop guidelines for ecological risk assessment. Like the program for health effects 

guidance, this activity depends on the expertise of scientists and policymakers from a broad spectrum and 

draws principles, information, and methods from many sources. 

In May 1991, EPA invited experts in ecotoxicology and ecological effects to Rockville, 

Maryland, to attend a peer review workshop on the draft Framework Report (56 Federal Resister 20223, 

2 May 1991). The workshop draft proposed a framework for ecological risk assessment complemented 

by preliminary guidance on some of the ecological issues identified in the draft. On the basis of the 

Rockville workshop recommendations (U.S. EPA, 1991), the revised Framework Report is now limited 

to discussion of the basic framework (see figure 1), complemented by second-order diagrams that give 

structure and content to each of the major elements in the Framework Report (see figures 2 through 4). 

Consistent with peer review recommendations, substantive risk assessment guidance is being reserved for 

study and development in future guidelines. 

The Framework Report is the product of a variety of activities that culminated in the Rockville 

workshop. Beginning early in 1990, EPA work groups and the National Academy of Sciences' (NAS) 

Committee on Risk Assessment Methodology began to study the 1983 NAS risk assessment paradigm 

(NRC, 1983), which provides the organizing principles for EPA's health risk guidelines, as a possible 

foundation for ecological risk assessment. Early drafts of EPA's Framework Report received preliminary 

peer comment late in 1990. 

In February 1991, NAS sponsored a workshop in Warrenton, Virginia, to discuss whether any 

single paradigm could accommodate all the diverse kinds of ecological risk assessments. There was a 

consensus that a single paradigm is feasible but that the 1983 paradigm would require modification to 

fulfill this role. In April 1991, EPA sponsored a strategic planning workshop in Miami, Florida. The 

structure and elements of ecological risk assessment were further discussed. Some participants in both of 

these earlier meetings also attended the Rockville workshop. EPA then integrated information, concepts, 

and diagrams from these workshop reviews with EPA practices and needs to propose a 

vii 
working framework for interim use in EPA programs and for continued discussion as a basis for future 

risk assessment guidelines. 

Use of the framework described in this report is not a requirement within EPA, nor is it a 

regulation of any kind. Rather, it is an interim product that is expected to evolve with use and discussion. 

EPA is publishing the Framework Report before proposing risk assessment guidelines for public 

comment to generate discussion within EPA, among Government agencies, and with the public to 

develop concepts, principles, and methods for use in future guidelines. To facilitate such discussion, EPA 

is presenting the framework at scientific meetings and inviting the public to submit information relevant 

to use and development of the approaches outlined for ecological risk assessment in the report 

Dorothy E. Patton, Ph.D.

Chair

Risk Assessment Forum

viii 
PREFACE 

Increased interest in ecological issues such as global climate change, habitat loss, acid 

deposition, reduced biological diversity, and the ecological impacts of pesticides and toxic chemicals 

prompts this Framework Report. This report describes basic elements, or a framework, for evaluating 

scientific information on the adverse effects of physical and chemical stressors on the environment. The 

framework offers starting principles and a simple structure as guidance for current ecological risk 

assessments and as a foundation for future EPA proposals for risk assessment guidelines. 

The Framework Report is intended primarily for EPA risk assessors, EPA risk managers, and 

persons who perform work under EPA contract or sponsorship. The terminology and concepts described 

in the report may also assist other regulatory agencies, as well as members of the public who are 

interested in ecological issues. 

ix 
CONTRIBUTORS AND REVIEWERS 

This report was prepared by members of the EPA technical panel listed below, with assistance 

from the staff of EPA's Risk Assessment Forum. Technical review was provided by numerous 

individuals, including EPA scientists and participants in the May 1991 peer review workshop. Editorial 

assistance was provided by R.O.W. Sciences, Inc. 

Technical Panel 

Co-Chairs 

Suzanne Macy Marcy (through December 1990) 

Office of Water

Susan Braen Norton 

Office of Research and Development

Donald J. Rodier 

Office of Toxic Substances

Members 

Michael S. Brody

Office of Policy, Planning and Evaluation

David A. Mauriello 

Office of Toxic Substances

Anne Sergeant 

Office of Research and Development

Molly R. Whitworth 

Office of Policy, Planning and Evaluation

Risk Assessment Forum Staff 

William H. van der Schalie 

Office of Research and Development

William P. Wood 

Office of Research and Development

x 
Reviewers 

M. Craig Barber

U.S. Environmental Protection Agency

Environmental Research Laboratory

Athens, GA

Richard S. Bennett, Jr.

U.S. Environmental Protection Agency

Environmental Research Laboratory

Corvallis, OR

Steven Bradbury

U.S. Environmental Protection Agency

Environmental Research Laboratory

Duluth, MN

Janet Burris

U.S. Environmental Protection Agency

Office of Emergency and Remedial Response

Washington, DC

David W. Charters

U.S. Environmental Protection Agency

Office of Solid Waste and Emergency Response

Edison, NJ

Patricia A. Cirone

U.S. Environmental Protection Agency

Region 10

Seattle, WA

James R. Clark

U.S. Environmental Protection Agency

Environmental Research Laboratory

Gulf Breeze, FL

Robert Davis

U.S. Environmental Protection Agency

Region 3

Philadelphia, PA

Anne Fairbrother

U.S. Environmental Protection Agency

Environmental Research Laboratory

Corvallis, OR

Jay Garner

U.S. Environmental Protection Agency

Environmental Monitoring Systems Laboratory

Las Vegas, NV

Jack H. Gentile

U.S. Environmental Protection Agency

Environmental Research Laboratory

Narragansett, RI

Sarah Gerould

U.S. Geological Survey

Reston, VA

George R. Gibson, Jr.

U.S. Environmental Protection Agency

Office of Water

Washington, DC

Alden D. Hinckley

U.S. Environmental Protection Agency

Office of Policy, Planning and Evaluation

Washington, DC

Erich Hyatt

U.S. Environmental Protection Agency

Office of Research and Development

Washington, DC

Norman E. Kowal

U.S. Environmental Protection Agency

Systems Laboratory

Cincinnati, OH

Ronald B. Landy

U.S. Environmental Protection Agency

Office of Technology Transfer and Regulatory 

Support

Washington, DC

Foster L. Mayer

U.S. Environmental Protection Agency ~

Environmental Research Laboratory

Gulf Breeze, FL

xi 
Melissa McCullough

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Washington, DC

J. Gareth Pearson

U.S. Environmental Protection Agency

Environmental Monitoring Systems Laboratory

Las Vegas, NV

Ronald Preston

U.S. Environmental Protection Agency

Region 3

Philadelphia, PA

John Schneider

U.S. Environmental Protection Agency

Region 5

Chicago, IL

Harvey Simon

U.S. Environmental Protection Agency

Region 2

New York, NY

Michael W. Slimak

U.S. Environmental Protection Agency

Office of Environmental Processes and Effects

Research

Washington, DC

Q. Jerry Stober

U.S. Environmental Protection Agency

Region 4

Atlanta, GA

Greg R. Susanke

U.S. Environmental Protection Agency

Office of Pesticide Programs

Washington, DC

Leslie W. Touart

U.S. Environmental Protection Agency

Office of Pesticide Programs

Washington, DC

Michael E. Troyer

U.S. Environmental Protection Agency

Office of Technology Transfer and Regulatory 

Support

Washington, DC

Douglas J. Urban

U.S. Environmental Protection Agency

Office of Pesticide Programs

Washington, DC

Maurice G. Zeeman

U.S. Environmental Protection Agency

Office of Toxic Substances

Washington, DC

xii 
Peer Review Workshop Participants 

William J. Adams

ABC Laboratories

Columbia, MO

Lawrence W. Barnthouse

Oak Ridge National Laboratory

Oak Ridge, TN

John Bascietto

U.S. Department of Energy

Washington, DC

Raymond Beaumier

Ohio Environmental Protection Agency

Columbus, OH

Harold Bergman

University of Wyoming

Laramie, NY

Nigel Blakeley

Washington Department of Ecology

Olympia, WA

James Falco

Battelle Pacific Northwest Laboratory

Richland, WA

James A. Fava

Roy F. Weston, Inc.

West Chester, PA

Alyce Fritz

National Oceanic and Atmospheric 

Administration 

Seattle, WA

James W. Gillett

Cornell University

Ithaca, NY

Michael C. Harrass

Food and Drug Administration

Washington, DC

Mark Harwell

University of Miami

Miami, FL

Ronald J. Kendall

Clemson University

Pendleton, SC

Wayne G. Landis

Western Washington University

Bellingham, WA

Ralph Portier

Louisiana State University

Baton Rouge, LA

Kenneth Recknow

Duke University

Durham, NC

John H. Rodgers

University of Mississippi

University, MS

Peter van Voris

Battelle Pacific Northwest Laboratory

Richland, WA

James Weinberg

Woods Hole Oceanographic Institution

Woods Hole, MA

Randall S. Wentsel

U.S. Army Chemical Research, Development 

and Engineering Center

Aberdeen Proving Ground, MD

xiii 
xiv
EXECUTIVE SUMMARY 

This report, “Framework for Ecological Risk Assessment,” is the first step in a long-term effort 

to develop risk assessment guidelines for ecological effects. Its primary purpose is to offer a simple, 

flexible structure for conducting and evaluating ecological risk assessment within EPA. Although the 

Framework Report will serve as a foundation for development of future subject-specific guidelines, it is 

neither a procedural guide nor a regulatory requirement within EPA and is expected to evolve with 

experience. The Framework Report is intended to foster consistent approaches to ecological risk 

assessment within EPA, identify key issues, and define terms used in these assessments. 

Ecological risk assessments evaluate ecological effects caused by human activities such as 

draining of wetlands or release of chemicals. The term “stressor” is used here to describe any chemical, 

physical, or biological entity that can induce adverse effects on individuals, populations, communities, or 

ecosystems. Thus, the ecological risk assessment process must be flexible while providing a logical and 

scientific structure to accommodate a broad array of stressors. 

The framework is conceptually similar to the approach used for human health risk assessment, 

but it is distinctive in its emphasis in three areas. First, ecological risk assessment can consider effects 

beyond those on individuals of a single species and may examine a population, community, or ecosystem. 

Second, there is no single set of ecological values to be protected that can be generally applied. Rather, 

these values are selected from a number of possibilities based on both scientific and policy 

considerations. Finally, there is an increasing awareness of the need for ecological risk assessments to 

consider nonchemical as well as chemical stressors. 

The framework consists of three major phases: (1) problem formulation, (2) analysis, and (3) risk 

characterization. Problem formulation is a planning and scoping process that establishes the goals, 

breadth, and focus of the risk assessment. Its end product is a conceptual model that identifies the 

environmental values to be protected (the assessment endpoints), the data needed, and the analyses to be 

used. 

The analysis phase develops profiles of environmental exposure and the effects of the stressor. 

The exposure profile characterizes the ecosystems in which the stressor may occur as well as the biota 

that may be exposed. It also describes the magnitude and spatial and temporal patterns of exposure. The 

ecological effects profile summarizes data on the effects of the stressor and relates them to the 

assessment endpoints. 

Risk characterization integrates the exposure and effects profiles. Risks can be estimated using a 

variety of techniques including comparing individual exposure and effects values, comparing the 

distributions of exposure and effects, or using simulation models. Risk can be expressed as a qualitative 

or quantitative estimate, depending on available data. In this step, the assessor also: 

P describes the risks in terms of the assessment endpoint;

P discusses the ecological significance of the effects;

P summarizes overall confidence in the assessment; and

P discusses the results with the risk manager.

xv 
The framework also recognizes several activities that are integral to, but separate from, the risk 

assessment process as defined in this report. For example, discussions between the risk assessor and risk 

manager are important. At the initiation of the risk assessment, the risk manager can help ensure that the 

risk assessment will ultimately provide information that is relevant to making decisions on the issues 

under consideration, while the risk assessor can ensure that the risk assessment addresses all relevant 

ecological concerns. Similar discussions of the results of the risk assessment are important to provide the 

risk manager with a full and complete understanding of the assessment's conclusions, assumptions, and 

limitations. 

Other important companion activities to ecological risk assessment include data acquisition and 

verification and monitoring studies. New data are frequently required to conduct analyses that are 

performed during the risk assessment. Data from verification studies can be used to validate the 

predictions of a specific risk assessment as well as to evaluate the usefulness of the principles set forth in 

the Framework. Ecological effects or exposure monitoring can aid in the verification process and suggest 

additional data, methods, or analyses that could improve future risk assessments. 

xvi 
1. INTRODUCTION 

Public, private, and government sectors of society are increasingly aware of ecological issues 

including global climate change, habitat loss, acid deposition, a decrease in biological diversity, and the 

ecological impacts of xenobiotic compounds such as pesticides and toxic chemicals. To help assess these 

and other ecological problems, the U.S. Environmental Protection Agency (EPA) has developed this 

report, “Framework for Ecological Risk Assessment,” which describes the basic elements, or framework, 

of a process for evaluating scientific information on the adverse effects of stressors on the environment. 

The term “stressor” is defined here as any physical, chemical, or biological entity that can induce an 

adverse effect (see box1

). Adverse ecological effects encompass a wide range of disturbances ranging 

from mortality in an individual organism to a loss in ecosystem function. 

This introductory section describes the purpose, scope, and intended audience for this report; 

discusses the definition and application of 

ecological risk assessment; outlines the basic 

elements of the proposed framework; and 

describes the organization of this report. 

1.1. Purpose and Scope of the Framework 

Report 

An understanding of the finite purpose 

and scope of this Framework Report is important. 

EPA, other regulatory agencies, and other 

organizations need detailed, comprehensive 

guidance on methods for evaluating ecological 

risk. However, in discussing tentative plans for 

Physical and Chemical Stressors as a 

Focus of the Framework 

This report does not discuss accidentally or 

deliberately introduced species, genetically 

engineered organisms, or organisms used to control 

horticultural or agricultural pests. While the general 

principles described in the framework may be helpful 

in addressing risks associated with these organisms, 

the capacity of such organisms for reproduction and 

biological interaction introduces additional 

considerations that are not addressed in this 

document. 

developing such guidance with expert consultants (U.S. EPA, 1991; U.S. EPA, in press-a), EPA was 

advised to first develop a simple framework as a foundation or blueprint for later comprehensive 

guidance on ecological risk assessment. 

With this background, the framework (see section l.4) has two simple purposes, one immediate 

and one longer term. As a broad outline of the assessment process, the framework offers a basic structure 

and starting principles for EPA's ecological risk assessments. The process described by the framework 

provides wide latitude for planning and conducting individual risk assessments in many diverse 

situations, each based on the common principles discussed in the framework. The process also will help 

foster a consistent EPA approach for conducting and evaluating ecological risk assessments, identify key 

issues, and provide operational definitions for terms used in ecological risk assessments. 

___________________________ 1

The boxes used throughout this document serve several purposes. Some boxes provide additional 

background and rationale for terms, whereas other boxes expand on concepts, described in the text. The 

boxes at the end of each chapter highlight issues that are integral components of the risk assessment 

process but require more research, analysis, and debate. Further discussion of these issues is reserved for 

later guidelines. 

1
In addition, the framework offers basic principles around which long-term guidelines for 

ecological risk assessment can be organized. With this in mind, this report does not provide substantive 

guidance on factors that are integral to the risk assessment process such as analytical methods, techniques 

for analyzing and interpreting data, or guidance on factors influencing policy. Rather, on the basis of 

EPA experience and the recommendations of peer reviewers, EPA has reserved discussion of these 

important aspects of any risk assessment for future guidelines, which will be based on the process 

described in this report. 

1.2. Intended Audience 

The framework is primarily intended for EPA risk assessors, EPA risk managers, and other 

persons who either perform work under EPA contract or sponsorship or are subject to EPA regulations. 

The terminology and concepts described here also may be of assistance to other Federal, State, and local 

agencies as well as to members of the general public who are interested in ecological issues. 

1.3. Definition and Applications of Ecological Risk Assessment 

Ecological risk assessment is defined as a process that evaluates the likelihood that adverse 

ecological effects may occur or are occurring as a result of exposure to one or more stressors. A risk does 

not exist unless (1) the stressor has the inherent ability to cause one or more adverse effects and (2) it cooccurs with or contacts an ecological component (i.e., organisms, populations, communities, or 

ecosystems) long enough and at a sufficient intensity to elicit the identified adverse effect. Ecological 

risk assessment may evaluate one or many stressors and ecological components. 

Ecological risk may be expressed in a variety of ways. While some ecological risk assessments 

may provide true probabilistic estimates of both the adverse effect and exposure elements, others may be 

deterministic or even qualitative in nature. In these cases, the likelihood of adverse effects is expressed 

through a semiquantitative or qualitative comparison of effects and exposure. 

Ecological risk assessments can help identify environmental problems, establish priorities, and 

provide a scientific basis for regulatory actions. The process can identify existing risks or forecast the 

risks of stressors not yet present in the environment. However, while ecological risk assessments can play 

an important role in identifying and resolving environmental problems, risk assessments are not a 

solution for addressing all environmental problems, nor are they always a prerequisite for environmental 

management. Many environmental matters such as the protection of habitats and endangered species are 

compelling enough that there may not be enough time or data to do a risk assessment. In such cases, 

professional judgment and the mandates of a particular statute will be the driving forces in making 

decisions. 

1.4. Ecological Risk Assessment Framework 

The distinctive nature of the framework results primarily from three differences in emphasis 

relative to previous risk assessment approaches. First, ecological risk assessment can consider effects 

beyond those on individuals of a single species and may examine population, community, or ecosystem 

impacts. Second, there is no one set of assessment endpoints (environmental values to be protected) that 

can be generally applied. Rather, assessment endpoints are selected from a very large number of 

possibilities based on both scientific and policy considerations. Finally, a comprehensive 

2
approach to ecological risk assessment may go beyond the traditional emphasis on chemical effects to 

consider the possible effects of non chemical stressors. 

The ecological risk assessment 

framework is shown in figure 1. The risk 

assessment process is based on two major 

elements: characterization of exposure and 

characterization of ecological effects. 

Although these two elements are most 

prominent during the analysis phase, aspects 

of both exposure and effects also are 

considered during problem formulation, as 

illustrated by the arrows in the diagram. The 

arrows also flow to risk characterization, 

where the exposure and effects elements are 

integrated to estimate risk. The framework is 

conceptually similar to the National Research 

Council (NRC) paradigm for human health 

risk assessments (NRC, 1983). 

The first phase of the framework is 

problem formulation. Problem formulation 

includes a preliminary characterization of 

exposure and effects, as well as examination 

of scientific data and data needs, policy and 

regulatory issues, and site-specific factors to 

define the feasibility, scope, and objectives 

for the ecological risk assessment. The level 

of detail and the information that will be 

needed to complete the assessment also are 

determined. This systematic planning phase 

Relationship of the Framework to a Paradigm 

for Human Health Risk Assessment 

In 1983, NRC published a paradigm that has been 

used in the development of EPA's human health risk 

assessment guidelines. The paradigm has four 

phases: hazard identification, dose- response 

assessment, exposure assessment, and risk 

characterization (NRC, 1983). Although the 

framework's problem formulation phase is not 

explicitly identified in the NRC paradigm, 

comparable planning issues are addressed in practice 

at the beginning of all EPA risk assessments. In the 

framework's analysis phase, characterization of 

exposure is analogous to exposure assessment, while 

characterization of ecological effects includes 

aspects of both hazard identification and doseresponse assessment. (The framework uses the term 

“stressor response” rather than “dose response” 

because many Agency programs must address 

stressors other than chemicals, and dose has been 

used only for chemicals.) Risk characterization is a 

similar process in both the framework and the NRC 

paradigm. 

is proposed because ecological risk assessments often address the risks of stressors to many species as 

well as risks to communities and ecosystems. In addition, there may be many ways a stressor can elicit 

adverse effects (e.g., direct effects on mortality and growth and indirect effects such as decreased food 

supply). Problem formulation provides an early identification of key factors to be considered, which in 

turn will produce a more scientifically sound risk assessment. 

3

The second phase of the framework is termed analysis and consists of two activities, 

characterization of exposure and characterization of ecological effects. The purpose of characterization 

of exposure is to predict or measure the spatial and temporal distribution of a stressor and its cooccurrence or contact with the ecological 

components of concern, while the purpose of 

characterization of ecological effects is to identify 

and quantify the adverse effects elicited by a 

stressor and, to the extent possible, to evaluate 

cause-and-effect relationships. 

The third phase of the framework is risk 

characterization. Risk characterization uses the 

results of the exposure and ecological effects 

analyses to evaluate the likelihood of adverse 

ecological effects associated with exposure to a 

stressor. It includes a summary of the assumptions 

used, the scientific uncertainties, and the strengths 

and weaknesses of the analyses. In addition, the 

Use of the Term “Exposure” 

Some reviewers of earlier drafts of this interim 

framework proposed that the term “exposure”,-­

which, as used in human health risk assessment, 

generally refers to chemical stressors,--not be used 

for the nonchemical stressors that can affect a 

variety of ecological components. Other terms, 

including “characterization of stress”, have been 

suggested. At this time, EPA prefers exposure, 

partly because characterization of stress does not 

convey the important concept of the co-occurrence 

and interaction of the stressor with an ecological 

component as well as exposure does. 

ecological significance of the risks is discussed with consideration of the types and magnitudes of the 

effects, their spatial and temporal patterns, and the likelihood of recovery. The purpose is to provide a 

complete picture of the analysis and results. 

In addition to showing the three phases of 

the framework, figure 1 illustrates the need for 

discussions between the risk assessor and risk 

manager. At the initiation of the risk assessment, 

the risk manager can help ensure that the risk 

assessment will ultimately provide information that 

is relevant to making decisions on the issues under 

consideration, while the risk assessor can ensure 

that the risk assessment addresses all relevant 

ecological concerns. Similar discussions of the 

results of the risk assessment are important to 

provide the risk manager with a full and complete 

understanding of the assessment's conclusions, 

assumptions, and limitations. 

Figure 1 also indicates a role for 

verification and monitoring in the framework. 

Characterization of Ecological Effects 

Used Instead of Hazard Assessment 

The framework uses characterization of 

ecological effects rather than hazard assessment 

for two reasons. First, the term “hazard” can be 

ambiguous, because it has been used in the past 

to mean either evaluating the intrinsic effects of 

a stressor (U.S. EPA, 1979) or defining a margin 

of safety or quotient by comparing a 

toxicological endpoint of interest with an 

estimate of exposure concentration (SETAC, 

1987). Second, many reviewers believed that 

hazard is more relevant to chemical than to 

nonchemical stressors 

Verification can include validation of the ecological risk assessment process as well as confirmation of 

specific predictions made during a risk assessment. Monitoring can aid in the verification process and 

may identify additional topics for risk assessment. Verification and monitoring can help determine the 

overall effectiveness of the framework approach, provide necessary feedback concerning the need for 

future modifications of the framework, 

5
help evaluate the effectiveness and practicality of policy decisions, and point out the need for new or 

improved scientific techniques (U.S. EPA, in press-a). 

The interaction between data acquisition and ecological risk assessment is also shown in figure 1. 

In this report, a distinction is made between data acquisition (which is outside of the risk assessment 

process) and data analysis (which is an integral part of an ecological risk assessment). In the problem 

formulation and analysis phases, the risk assessor may identify the need for additional data to complete 

an analysis. At this point, the risk assessment stops until the necessary data are acquired. When a need 

for additional data is recognized in risk characterization, new information generally is used in the 

analysis or problem formulation phases. The distinction between data acquisition and analysis generally 

is maintained in all of EPA's risk assessment guidelines; guidance on data acquisition procedures are 

provided in documents prepared for specific EPA programs. 

The interactions between data acquisition and ecological risk assessment often result in an 

iterative process. For example, data used during the analysis phase may be collected in tiers of increasing 

complexity and cost. A decision to advance from one tier to the next is based on decision triggers set at 

certain levels of effect or exposure. Iterations of the entire risk assessment process also may occur. For 

example, a screening-level risk assessment may be performed using readily available data and 

conservative assumptions; depending on the results, more data then may be collected to support a more 

rigorous assessment. 

1.5. The Importance of Professional Judgment 

Ecological risk assessments, like human health risk assessments, are based on scientific data that 

are frequently difficult and complex, conflicting or ambiguous, or incomplete. Analyses of such data for 

risk assessment purposes depends on professional judgment based on scientific expertise. Professional 

judgment is necessary to: 

P design and conceptualize the risk assessment; 

P evaluate and select methods and models; 

P determine the relevance of available data to the risk assessment; 

P develop assumptions based on logic and scientific principles to fill data gaps; and 

P interpret the ecological significance of predicted or observed effects. 

Because professional judgment is so important, specialized knowledge and experience in the 

various phases of ecological risk assessment is required. Thus, an interactive multidisciplinary team that 

includes biologists and ecologists is a prerequisite for a successful ecological risk assessment. 

1.6. Organization 

The next three sections of this report are arranged to follow the framework sequentially; Section 

2 describes problem formulation; this section is particularly important for assessors to c6nsider when 

specific assessment endpoints are not determined a priori by statute or other authority. Sections 3 and 4 

6
discuss analysis and risk characterization, respectively. Section 5 defines the terms used in this report, 

and section 6 provides literature references. The lists of ecological risk assessment issues at the end of 

sections 1 through 4 highlight areas for further discussion and research. EPA believes that these issues 

will require special attention in developing ecological risk assessment guidelines. 

7
Additional Issues Related to the Framework 

• Use of the framework for evaluation risks with biological stressors. 

• Use of the term exposure (versus characterization of stress) for both chemicals and 

nonchemical stressors. 

• Use of the term characterization of ecological effects rather than hazard assessment. 

8
2. PROBLEM FORMULATION 

Problem formulation is the first phase of ecological risk assessment and establishes the goals, 

breadth, and focus of the assessment. It is a systematic planning step that identifies the major factors to 

be considered in a particular assessment, and it is linked to the regulatory and policy context of the 

assessment. 

Entry into the ecological risk assessment process may be triggered by either an observed 

ecological effect, such as visible damage to trees m a forest, or by the identification of a stressor or 

activity of concern, such as the planned filling of a marsh or the manufacture of a new chemical. The 

problem formulation process (figure 2) then begins with the Initial stages of characterizing exposure and 

ecological effects, including evaluating the stressor characteristics, the ecosystem potentially at risk, and 

the ecological effects expected or observed. Next, the assessment and measurement endpoints are 

identified. (Measurement endpoints are ecological characteristics that can be related to the assessment 

endpoint.) The outcome of problem formulation is a conceptual model that describes how a given stressor 

might affect the ecological components in the environment. The conceptual model also describes the 

relationships among the assessment and measurement endpoints, the data required, and the 

methodologies that will be used to analyze the data. The conceptual model serves as input to the analysis 

phase of the assessment. 

2.1. Discussion Between the Risk Assessor and Risk Manager (Planning) 

To be meaningful and effective, ecological risk assessments must be relevant to regulatory needs 

and public concerns as well as scientifically valid. Although risk assessment and risk management are 

distinct processes, establishing a two--way dialogue between risk assessors and risk managers during the 

problem formulation phase can be a constructive means of achieving both societal and scientific goals. 

By bringing the management perspective to the discussion, risk managers charged with protecting 

societal values can ensure that the risk assessment will provide relevant information to making decisions 

on the issue under consideration. By bringing scientific knowledge to the discussion, the ecological risk 

assessor ensures that the assessment addresses all important ecological concerns. Both perspectives are 

necessary to appropriately utilize resources to produce scientifically sound risk assessments that are 

relevant to management decisions and public concerns. 

2.2. Stressor Characteristics, Ecosystem Potentially at Risk, and Ecological Effects 

The initial steps in problem formulation are the identification and preliminary characterization of 

stressors, the ecosystem potentially at risk, and ecological effects. Performing this analysis is an 

interactive process that contributes to both the selection of assessment and measurement endpoints and 

the development of a conceptual model. 

9

2.2.1. Stressor Characteristics 

The determination of stressor characteristics begins with the identification of potential chemical 

or physical stressors. Chemical stressors include a variety of inorganic and organic substances. Some 

chemicals may result in secondary stressors, as in the case of stratospheric ozone depletion caused by 

chlorofluorocarbon that could result in increased exposures to ultraviolet radiation. Physical stressors 

include extremes of natural conditions (e.g., temperature and hydrologic changes) and habitat alteration 

or destruction. Stressors that may result from management practices, such as harvesting of fishery or 

forest resources, also may be considered. Example stressor characteristics are summarized in the box 

below. Gathering information on the characteristics of a stressor helps define the ecosystems potentially 

at risk from the stressor as well as the ecological effects that may result. 

2.2.2. Ecosystem Potentially at Risk 

The ecosystem within which effects occur 

provides the ecological context for the assessment. 

Knowledge of the ecosystem potentially at risk can 

help identify ecological components that may be 

affected and stressor-ecosystem interactions relevant 

to developing exposure scenarios. The approach to 

identifying the ecosystem potentially at risk from a 

stressor depends in part on how the risk assessment 

was initiated. If a stressor first was identified, 

information on the spatial and temporal distribution 

patterns of the stressor can be helpful in identifying 

ecosystems potentially at risk. Similarly, if the risk 

assessment is Initiated by observing effects, these 

effects can directly indicate ecosystems or 

ecological components that may be considered in the 

assessment. 

A wide range of ecosystem properties may 

be considered during problem formulation. These 

properties include aspects of the abiotic 

environment (such as climatic conditions and soil or 

sediment properties), ecosystem structure (including 

the types and abundances of different species and 

their trophic level relationships), and ecosystem 

function (such as the ecosystem energy source, 

Example Stressor Characteristics 

Type 

Chemical or physical 

Intensity 

Concentration or magnitude 

Duration 

Short or long term 

Frequency 

Single event, episodic, or continuous 

Timing 

Occurrence relative to biological cycles 

Scale

Spatial heterogeneity and extent

pathways of energy utilization, and nutrient processing) (U.S. EPA, in press-b). In addition, knowledge of 

the types and patterns of historical disturbances may be helpful in predicting ecological responses to 

stressors. 

The need to evaluate spatial and temporal distribution and variation is inherent in many of these 

example characteristics. Such information is especially useful for determining potential exposure, that is, 

where there is co-occurrence of or contact between the stressor and ecological components. 

11
2.2.3. Ecological Effects 

Ecological effects data may come from a variety of sources. Relevant sources of information 

include field observations (e.g., fish or bird kills, changes in aquatic community structure), field tests 

(e.g., microcosm or mesocosm tests), laboratory tests (e.g., single species or microcosm tests), and 

chemical structure-activity relationships. Available information on ecological effects can help focus the 

assessment on specific stressors and on ecological components that should be evaluated. 

Many factors can influence the utility of available ecological effects data for problem 

formulation. For example, the applicability of laboratory-based tests may be affected by any 

extrapolations required to specific field situations, while the interpretation of field observations may be 

influenced by factors such as natural variability or the possible presence of stressors other than the ones 

that are the primary focus of the risk assessment 

2.3. Endpoint Selection 

Information compiled in the first 

stage of problem formulation is used to help 

select ecologically based endpoints that are 

relevant to decisions made about protecting 

the environment. An endpoint is a 

characteristic of an ecological component 

(e.g., increased mortality in fish) that may be 

affected by exposure to a stressor (Suter, 

1990a). Two types of endpoints are 

distinguished in this report. Assessment 

endpoints are explicit expressions of the 

actual environmental value that is to be 

protected. Measurement endpoints are 

measurable responses to a stressor that are 

related to the valued characteristics chosen as 

the assessment endpoints (Suter, 1990a). 

Endpoint Terminology 

Several reviewers have suggested using the term 

“indicator” in place of “measurement endpoint”. 

At this time, measurement endpoint is preferred 

because it has a specific meaning (a characteristic 

of an ecological system that can be related to an 

assessment endpoint), whereas indicator can have 

several different meanings. For example, 

indicator has been used at EPA to mean (1) 

measures of administrative accomplishments 

(e.g., number of permits issued), (2) measures of 

exposure (e.g., chemical levels in sediments), or 

(3) measures of ecosystem integrity. These 

indicators cannot always be related to an 

assessment endpoint. 

Assessment endpoints are the ultimate focus in risk characterization and link the measurement 

endpoints to the risk management process (e.g., policy goals). When an assessment endpoint can be 

directly measured, the measurement and assessment endpoints are the same. In most cases, however, the 

assessment endpoint cannot be directly measured, so a measurement endpoint (or a suite of measurement 

endpoints) is selected that can be related, either qualitatively or quantitatively, to the assessment 

endpoint. For example, a decline in a sport fish population (the assessment endpoint) may be evaluated 

using laboratory studies on the mortality of surrogate species, such as the fathead minnow (the 

measurement endpoint). Sound professional judgment is necessary for proper assessment and 

measurement endpoint selection, and it is important that both the selection rationale and the linkages 

between measurement endpoints, assessment endpoints, and policy goals be clearly stated. 

12
Assessment and measurement endpoints may involve ecological components from any level of 

biological organization, ranging from individual organisms to the ecosystem itself. In general, the use of 

a suite of assessment and measurement endpoints at different organizational levels can build greater 

confidence in the conclusions of the risk assessment and ensure that all important endpoints are 

evaluated. In some situations, measurement endpoints at one level of organization may be related to an 

assessment endpoint at a higher level. For example, measurement endpoints at the individual level (e.g., 

mortality, reproduction, and growth) could be used in a model to predict effects on an assessment 

endpoint at the population level (e.g., viability of a trout population in a stream). 

General considerations for selecting assessment and measurement endpoints are detailed in the 

following boxes. More detailed discussions of endpoints and selection criteria can be found in Suter 

(1989, 1990a), Kelly and Harwell (1990), U.S. Department of the Interior (1987), and U.S. EPA (1990a). 

Considerations in Selecting Assessment Endpoints 

Ecological Relevance 

Ecologically relevant endpoints reflect important characteristics of the system and are functionally 

related to other endpoints. Selection of ecologically relevant endpoints requires some understanding 

of the structure and function of the ecosystem potentially at risk. For example, an assessment endpoint 

could focus on changes in a species known to have a controlling influence on the abundance and 

distribution of many other species in its community. Changes at higher levels of organization may be 

significant because of their potential for causing major effects at lower organizational levels. 

Policy Goals and Societal Values 

Good communication between the risk assessor and risk manager is important to ensure that 

ecologically relevant assessment endpoints reflect policy goals and societal values. Societal concerns 

can range from protection of endangered or commercially or recreationally important species to 

preservation of ecosystem attributes for functional reasons (e.g., flood water retention by wetlands) or 

aesthetic reasons (e.g., visibility in the Grand Canyon). 

Susceptibility to the Stressor 

Ideally, an assessment endpoint would be likely to be both affected by exposure to a stressor and 

sensitive to the specific type of effects caused by the stressor. For example, if a chemical is known to 

bioaccumulate and is suspected of causing eggshell thinning; an appropriate assessment endpoint 

might be raptor population viability. 

13
Considerations in Selecting Measurement Endpoints 

Relevance to an Assessment Endpoint 

When an assessment endpoint cannot be directly measured, measurement endpoints are identified that 

are correlated with or can be used to infer or predict changes in the assessment endpoint. 

Consideration of indirect Effects 

Indirect effects occur when a stressor acts on elements of he ecosystem that are required by the 

ecological component of concern. For example, if the assessment endpoint is the population viability 

of trout, measurement endpoints could evaluate possible stressor effects on trout prey species or 

habitat requirements. 

Sensitivity an Response Time 

Rapidly responding measurement endpoints may be useful in providing early warnings of ecological 

effects, and measurement endpoints also may be selected because they are sensitive surrogates of the 

assessment endpoint in many cases, measurement endpoints at lower levels of biological organization 

may be more sensitive than those at higher levels. However, because of compensatory mechanisms 

and other factors, a change in a measurement endpoint at a lower organizational level (e.g., a 

biochemical alteration) may not necessarily be reflected in changes at a higher level (e.g., population 

effects). 

Signal-to-Noise Ratio 

If a measurement endpoint is highly variable, the possibility of detecting stressor-related effects may 

be greatly reduced even if the endpoint is sensitive to the stressor. 

Consistency With Assessment Endpoint Exposure Scenarios 

The ecological component of the measurement endpoint should be exposed by similar routes and at 

similar or greater stressor levels as the ecological component of the assessment endpoint. 

Diagnostic Ability 

Measurement endpoints that are unique or specific responses to a stressor may be very useful in 

diagnosing the presence or effects of a stressor. For example, measurement of acetylcholinesterase 

inhibition may be useful for demonstrating responses to certain types of pesticides. 

Practicality Issues 

Ideal measurement endpoints are cost effective and easily measured. The availability of a large 

database for a measurement endpoint desirable to facilitate comparisons and develop models. 

14
2.4. The Conceptual Model 

The major focus of the conceptual model (figure 2) is the development of a series of working 

hypotheses regarding how the stressor might affect ecological components of the natural environment 

(NRC, 1986). The conceptual model also includes descriptions of the ecosystem potentially at risk and 

the relationship between measurement and assessment endpoints. 

During conceptual model development, a preliminary analysis of the ecosystem, stressor 

characteristics, and ecological effects is used to define possible exposure scenarios. Exposure scenarios 

consist of a qualitative description of how the various ecological components co-occur with or contact 

the stressor. Each scenario is defined in terms of the stressor, the type of biological system and principal 

ecological components, how the stressor will contact or interact with the system, and the spatial and 

temporal scales. 

For chemical stressors, the exposure scenario usually involves consideration of sources, 

environmental transport, partitioning of the chemical among various environmental media, 

chemical/biological transformation or speciation processes, and identification of potential routes of 

exposure (e.g., ingestion). For nonchemical stressors such as water level or temperature changes or 

physical disturbance, the exposure scenario describes the ecological components exposed and the general 

temporal and spatial patterns of their co-occurrence with the stressor. For example, for habitat alterations, 

the exposure scenario may describe the extent and distributional pattern of disturbance, the populations 

residing within or using the disturbed areas, and the spatial relationship of the disturbed area to 

undisturbed areas. 

Although many hypotheses may be generated during problem formulation, only those that are 

considered most likely to contribute to risk are selected for further evaluation in the analysis phase. For 

these hypotheses, the conceptual model describes the approach that will be used for the analysis phase 

and the types of data and analytical tools that will be needed. It is important that hypotheses that are not 

carried forward in the assessment because of data gaps be acknowledged when uncertainty is addressed 

in risk characterization. Professional judgment is needed to select the most appropriate risk hypotheses, 

and it is important to document the selection rationale. 

15
Additional Issues in Problem Formulation 

• Role of risk management concerns in establishing assessment endpoints. 

Although it is important to consider risk management concerns when assessment endpoints are 

selected, there is still uncertainty as to how these inputs should influence the goals of the risk 

assessment, the ecological components to be protected, and the level of protection required. 

• Identifying specific assessment and measurement endpoints for different stressors and 

ecosystems. 

16
3. ANALYSIS PHASE 

The analysis phase of ecological risk assessment (figure 3) consists of the technical evaluation of 

data on the potential effects and exposure of the stressor. The analysis phase is based on the conceptual 

model developed during problem formulation. Although this phase consists of characterization of 

ecological effects and characterization of exposure, the dotted line in figure 3 illustrates that the two are 

performed interactively. An interaction between the two elements will ensure that the ecological effects 

characterized are compatible with the biota and exposure pathways identified in the exposure 

characterization. The output of ecological effects characterization and exposure characterization are 

summary profiles that are used in the risk characterization phase (section 4). Discussion of uncertainty 

analysis, which is an important part of the analysis phase, may be found in section 4.1.2. 

Characterization of exposure and ecological effects often requires the application of statistical 

methods. While the discussion of specific statistical methods is beyond the scope of this document, 

selection of an appropriate statistical method involves both method assumptions (e.g., independence of 

errors, normality, equality of variances) and data set characteristics (e.g., distribution, presence of 

outliers or influential data). It should be noted that statistical significance does not always reflect 

biological significance, and profound biological changes may not be detected by statistical tests. 

Professional judgment often is required to evaluate the relationship between statistical and biological 

significance. 

3.1. Characterization of' Exposure 

Characterization of exposure (half of the analysis phase shown in figure 3) evaluates the 

interaction of the stressor with the ecological component. Exposure can be expressed as co-occurrence or 

contact depending on the stressor and the ecological component involved. An exposure profile is 

developed that quantifies the magnitude and spatial and temporal distributions of exposure for the 

scenarios developed during problem formulation and serves as input to the risk characterization. 

3.1.1. Stressor Characterization: Distribution or Pattern of Change 

Stressor characterization involves determining the stressor's distribution or pattern of change. 

Many techniques can be applied to assist in this stressor characterization process. For chemical stressors, 

a combination of modeling and monitoring data often is used. Available monitoring data may include 

measures of releases into the environment and media concentrations over space and time. Fate and 

transport models often are used that rely on physical and chemical characteristics of the chemical 

coupled with the characteristics of the ecosystem. For nonchemical stressors such as physical alterations 

or harvesting, the pattern of change may depend on resource management or land-use practices. 

Depending on the scale of the disturbance, the data for stressor characterization can be provided by a 

variety of techniques, including ground reconnaissance, aerial photographs, or satellite imagery. 

17

During stressor characterization, one considers not only the primary stressor but also secondary 

stressors that can arise as a result of various processes. For example, removal of riparian (stream-side) 

vegetation not only alters habitat structure directly, but can have additional ramifications such as 

increased siltation and temperature rise. For chemicals, secondary stressors can be produced by a range 

of environmental fate processes. 

The timing of the stressor's interaction with the biological system is another important 

consideration. If the stressor is episodic in nature, different species and life stages may be affected. In 

addition, the ultimate distribution of a stressor is rarely homogeneous; it is important to quantify such 

heterogeneity whenever possible. 

3.1.2. Ecosystem Characterization 

During ecosystem characterization, the ecological context of the assessment is further analyzed. 

In particular, the spatial and temporal distributions of the ecological component are characterized, and 

the ecosystem attributes that influence the distribution and nature of the stressor are considered. 

Characteristics of the ecosystem can greatly modify the ultimate nature and distribution of the 

stressor. Chemical stressors can be modified through biotransformation by microbial communities or 

through other environmental fate processes, such as photolysis, hydrolysis, and sorption. The 

bioavailability of chemical stressors also can be affected by the environment, which in turn influences the 

exposure of ecological components. 

Physical stressors can be modified by the ecosystem as well. For example, siltation in streams 

depends not only on sediment volume, but on flow regime and physical stream characteristics. Similarly, 

nearby wetlands and levees influence water behavior during flood events. 

The spatial and temporal distributions of ecological components also are considered in 

ecosystem characterization. Characteristics of ecological components that influence their exposure to the 

stressor are evaluated, including habitat needs, food preferences, reproductive cycles, and seasonal 

activities such as migration and selective use of resources. Spatial and temporal variations in the 

distribution of the ecological component (e.g., sediment invertebrate distribution) may complicate 

evaluations of exposure. When available, species-specific information about activity patterns, abundance, 

and life histories can be very useful in evaluating spatial and temporal distributions. 

Another important consideration is how exposure to a stressor may alter natural behavior, 

thereby affecting further exposure. In some cases, this may lead to enhanced exposure (e.g., increased 

preening by birds after aerial pesticide spraying), while in other situations initial exposure may lead to 

avoidance of contaminated locations or food sources (e.g., avoidance of certain waste effluents or 

physically altered spawning beds by some fish species). 

19
3.1.3 Exposure Analyses 

The next step is to combine the spatial and temporal distributions of both the ecological 

component and the stressor to evaluate exposure. In the case of physical alterations of communities and 

ecosystems, exposure can be expressed broadly as co-occurrence. Exposure analyses of individuals often 

focus on actual contact with the stressor, because organisms may not contact all of the stressors present 

in an area. For chemical stressors, the analyses may focus further on the amount of chemical that is 

bioavailable, that is, available for uptake by the organism. Some chemical exposure analyses also follow 

the chemical within the organism's body and estimate the amount that reaches the target organ. The focus 

of the analyses will depend on the stressors being evaluated and the assessment and measurement 

endpoints. 

The temporal and spatial scales used to evaluate the stressor need to be compatible with the 

characteristics of the ecological component of interest. A temporal scale may encompass the lifespan of a 

species, a particular life stage, or a particular cycle, for example, the long-term succession of a forest 

community. A spatial scale may encompass a forest, a lake, a watershed, or an entire region Stressor 

timing relative to organism life stage and activity patterns can greatly influence the occurrence of adverse 

effects. Even short-term events may be significant if they coincide with critical life stages. Periods of 

reproductive activity may be especially important, because early life stages often are more sensitive to 

stressors, and adults also may be more vulnerable at this time. 

The most common approach to exposure analysis is to measure concentrations or amounts of a 

stressor and combine them with assumptions about co-occurrence, contact, or uptake. For example, 

exposure of aquatic organisms to chemicals often is expressed simply as concentration in the water 

column; aquatic organisms are assumed to contact the chemical. Similarly, exposures of organisms to 

habitat alteration often is expressed as hectares of habitat altered; organisms that utilize the habitat are 

assumed to co-occur with the alteration. Stressor measurements can also be combined with quantitative 

parameters describing the frequency and magnitude of contact. For example, concentrations of chemicals 

in food items can be combined with ingestion rates to estimate dietary exposure of organisms. 

In some situations, the stressor can he measured at the actual point of contact while exposure 

occurs. An example is the use of food collected from the mouths of nestling birds to evaluate exposure to 

pesticides through contaminated food (Kendall, 1991). Although such point-of-contact measurements can 

be difficult to obtain, they reduce the need for assumptions about the frequency and magnitude of 

contact. 

Patterns of exposure can be described using models that combine abiotic ecosystem attributes, 

stressor properties, and ecological component characteristics. Model selection is based on the model's 

suitability for the ecosystem or component of interest, the availability of the requisite data, and the study 

objectives. Model choices range from simple, screening-level procedures that require a minimum of data 

to more sophisticated methods that describe processes in more detail but require a considerable amount 

of data. 

Another approach to evaluating exposure uses chemical, biochemical, or physiological evidence 

(e.g., biomarkers) of a previous exposure. This approach has been used primarily for assessing chemical 

exposures and is particularly useful when a residue or biomarker is diagnostic of exposure to a particular 

chemical. These types of measurements are most useful for exposure 

20
characterization when they can be quantitatively linked to the amount of stressor originally contacted by 

the organism. Pharmacokinetic models are sometimes used to provide this linkage. 

3.1.4. Exposure Profile 

Using information obtained from the exposure analysis, the exposure profile quantifies the 

magnitude and spatial and temporal patterns of exposure for the scenarios developed during problem 

formulation and serves as input to risk characterization. The exposure profile is only effective when its 

results are compatible with the stressor-response profile. For example, appraisals of potential acute 

effects of chemical exposure may be averaged over short time periods to account for short-term pulsed 

stressor events. It is important that characterizations for chronic stressors account for both long-term lowlevel exposure and possible shorter term higher level contact that may elicit similar adverse chronic 

effects. 

Exposure profiles can be expressed using a variety of units. For chemical stressors operating at 

the organism level, the usual metric is expressed in dose units (e.g., mg body weight/day). For higher 

levels of organization (e.g., an entire ecosystem), exposure may be expressed in units of 

concentration/unit area/time. For physical disturbance, the exposure profile may be expressed in other 

terms (e.g., percentage of habitat removed or the extent of flooding/year). 

An uncertainty assessment is an integral part of the characterization of exposure. In the majority 

of assessments, data will not be available for all aspects of the characterization of exposure, and those 

data that are available may be of questionable or unknown quality. Typically, the assessor will have to 

rely on a number of assumptions with varying degrees of uncertainty associated with each. These 

assumptions will be based on a combination of professional judgment, inferences based on analogy with 

similar chemicals and conditions and estimation techniques, all of which contribute to the overall 

uncertainty. It is important that the assessor characterize each of the various sources of uncertainty and 

carry them forward to the risk characterization so that they may be combined with a similar analysis 

conducted as part of the characterization of ecological effects. 

3.2. Characterization of Ecological Effects 

The relationship between the stressor and the assessment and measurement endpoints identified 

during problem formulation is analyzed in characterization of ecological effects (figure 3). The 

evaluation begins with the evaluation of effects data that are relevant to the stressor. During ecological 

response analysis, the relationship between the stressor and the ecological effects elicited is quantified, 

and cause--and-effect relationships are evaluated. In addition, extrapolations from measurement 

endpoints to assessment endpoints are conducted during this phase. The product is a stressor-response 

profile that quantifies and summarizes the relationship of the stressor to the assessment endpoint. The 

stressor-response profile is then used as input to risk characterization. 

3.2.1. Evaluation of Relevant Effects Data 

The type of effects data that are evaluated depends largely on the nature of the stressor and the 

ecological component under evaluation. Effects elicited by a stressor may range from mortality and 

reproductive Impairment in individuals and populations to disruptions in community and ecosystem 

function such as primary productivity. The evaluation process relies on professional judgment, 

21
especially when few data are available or when choices among several sources of data are required. If 

available data are inadequate, new data may be needed before the assessment can be completed. 

Data are evaluated by considering their relevance to the measurement and assessment endpoints 

selected during problem formulation. The analysis techniques that will be used also are considered; data 

that minimize the need for extrapolation are desirable. Data quality (e.g., sufficiency of replications, 

adherence to good laboratory practices) is another important consideration. Finally, characteristics of the 

ecosystem potentially at risk will influence what data will be used. Ideally, the test system reflects the 

physical attributes of the ecosystem and will include the ecological components and life stages examined 

in the risk assessment. 

Data from both field observations and experiments in controlled settings can be used to evaluate 

ecological effects. In some cases, such as for chemicals that nave yet to be manufactured, test data for the 

specific stressor are not available. Quantitative structure-activity relationships (QSARs) are useful in 

these situations (Auer et al., 1990; Clements et al., 1988; McKim et al., 1987). 

Controlled laboratory and field tests (e.g., mesocosms) can provide strong causal evidence 

linking a stressor with a response and can also help discriminate between multiple stressors. Data from 

laboratory studies tend to be less variable than those from field studies, but because environmental 

factors are controlled, responses may differ from those in the natural environment 

Observational field studies (e.g., comparison with reference sites) provide environmental realism 

that laboratory studies lack, although the presence of multiple stressors and other confounding factors 

(e.g., habitat quality) in the natural environment can make it difficult to attribute observed effects to 

specific stressors. Confidence in causal relationships can be improved by carefully selecting comparable 

reference sites or by evaluating changes along a stressor gradient where differences in other 

environmental factors are minimized. It is important to consider potential confounding factors during the 

analysis. 

3.2.2. Ecological Response Analyses 

The data used in characterization of ecological effects are analyzed to quantity the stressorresponse relationship and to evaluate the evidence for causality. A variety of techniques may be used, 

including statistical methods and mathematical modeling. In some cases, additional analyses to relate the 

measurement endpoint to the assessment endpoint may be necessary. 

Stressor-Response Analyses 

The stressor-response analysis describes the relationship between the magnitude, frequency, or 

duration of the stressor in an observational or experimental setting and the magnitude of response. The 

stressor-response analysis may focus on different aspects of the stressor-response relationship, depending 

on the assessment objectives, the conceptual model, and the type of data used for the analysis. Stressorresponse analyses, such as those used for toxicity tests, often portray the magnitude of the stressor with 

respect to the magnitude of response. Other important aspects to consider include the temporal (e.g., 

frequency, duration, and timing) and spatial distributions of the stressor in the experimental or 

observational setting. For physical stressors, specific attributes of the environment after disturbance (e.g., 

reduced forest stand age) can be related to the response (e.g., decreased use by spotted owls) (Thomas et 

al., 1990). 

22
Analyses Relating Measurement and Assessment Endpoints 

Ideally, the stressor-response evaluation 

quantifies the relationship between the stressor 

and the assessment endpoint. When the 

assessment endpoint can be measured, this 

analysis is straightforward. When it cannot be 

measured, the relationship between the stressor 

and measurement endpoint is established first, 

then additional extrapolations, analyses, and 

assumptions are used to predict or infer changes 

in the assessment endpoint. The need for 

analyses relating measurement and assessment 

endpoints also may be identified during risk 

characterization, alter an initial evaluation of 

risk. 

Measurement endpoints are related to 

assessment endpoints using the logical structure 

presented in the conceptual model. In some 

cases, quantitative methods and models are 

available, but often the relationship can be 

described only qualitatively. Because of the lack 

of standard methods for many of these analyses, 

professional judgment is an essential 

component of the evaluation. It is important to 

clearly explain the rationale for any analyses 

and assumptions. 

Extrapolations commonly used include 

those between species, between responses, from 

laboratory to field, and from field to field. 

Differences in responses among taxa depend on 

many factors, including physiology, 

metabolism, resource utilization, and life 

history strategy. The relationship between 

responses also depends on many factors, 

including the mechanism of action and internal 

distribution of the stressor within the organism. 

When extrapolating between different 

laboratory and field settings, important 

considerations include differences in the 

physical environment and organism behavior 

that will alter exposure, interactions with other 

stressors, and interactions with other ecological 

components. 

Extrapolations and Other Analyses Relating 

Measurement and Assessment Endpoints 

Extrapolation Between Taxa 

example: from bluegill sunfish mortality to 

rainbow trout mortality 

Extrapolation Between Responses 

quail NOEL (no observed effect level) 

example: from bobwhite quail LC50 to bobwhite 

Extrapolation From Laboratory to Field 

example: from mouse mortality under laboratory 

conditions to mouse mortality in the field 

Extrapolation From Field to Field 

example: from reduced invertebrate community 

diversity in one stream to another stream 

Analysis of Indirect Effects 

example: relating removal of long-leaf pine to 

reduced populations of red-cockaded woodpecker 

Analysis of Higher Organizational Levels 

example: relating reduced individual fecundity to 

reduced population size 

Analysis of Spatial and Temporal Scales 

example: evaluation of the loss of a specific 

wetland used by migratory birds in relation to the 

larger scale habitat requirements of the species 

Analysis of Recovery 

example: relating short-term mortality to longterm depauperation 

23
In addition to these extrapolations, an evaluation of indirect effects, other levels of organization, 

other temporal and spatial scales, and recovery potential may be necessary. Whether these analyses are 

required in a particular risk assessment will depend on the assessment endpoints identified during 

problem formulation. 

Important factors to consider when evaluating indirect effects include interspecies interactions 

(e.g., competition, disease), trophic-level relationships (e.g., predation), and resource utilization Effects 

on higher (or lower) organizational levels depend on the severity of the effect, the number and life stage 

of organisms affected, the role of those organisms in the community or ecosystem, and ecological 

compensatory mechanisms. 

The implications of adverse effects at spatial scales beyond the immediate area of concern may 

be evaluated by considering ecological characteristics such as community structure and energy and 

nutrient dynamics. In addition, information from the characterization of exposure on the stressor's spatial 

distribution may be useful. Extrapolations between different temporal scales (e.g., from short-term 

impacts to long-term effects) may consider the stressors' distribution through time (intensity, duration, 

and frequency) relative to ecological dynamics (e.g., seasonal cycles, life cycle patterns). 

In some cases, evaluation of long-term impacts will require consideration of ecological recovery. 

Ecological recovery is difficult to predict and depends on the existence of a nearby source of organisms, 

life history and dispersal strategies of the ecological components, and the chemical-physical 

environmental quality following exposure to the stressor (Cairns, 1990; Poff and Ward, 1990; Kelly and 

Harwell, 1990). In addition, there is some evidence to suggest that the types and frequency of natural 

disturbances can influence the ability of communities to recover (Schlosser, 1990). 

24
Evaluation of Causal Evidence 

Another important aspect of the ecological response analysis is to evaluate the strength of the 

causal association between the stressor and the measurement and assessment endpoints. This information 

supports and complements the stressor-response assessment and is of particular importance when the 

stressor-response relationship is based on field observations. Although proof of causality is not a 

requirement for risk assessment, an evaluation of causal evidence augments the risk assessment. Many of 

the concepts applied in human epidemiology can be useful for evaluating causality in observational field 

studies. For example, Hill (1965) suggested nine evaluation criteria for causal associations. An example 

of ecological causality analysis was provided by Woodman and Cowling (1987), who evaluated the 

causal association between air pollutants and injury to forests. 

Hill's Criteria for Evaluating Causal Associations (Hill, 1965) 

1. Strength: A high magnitude of effect is associated with exposure to the stressor. 

2. Consistency: The association is repeatedly observed under different circumstances. 

3. Specificity: The effect is diagnostic of a stressor. 

4. Temporality: The stressor precedes the effect in time. 

5. Presence of a biological gradient: A positive correlation between the stressor and response. 

6. A plausible mechanism of action. 

7. Coherence: The hypothesis does not conflict with knowledge of natural history and biology. 

8. Experimental evidence. 

9. Analogy: Similar stressors cause similar responses. 

Not all of these criteria must be satisfied, but each incrementally reinforces the argument for causality. 

Negative evidence does not rule out a causal association but may indicate incomplete knowledge of 

the relationship (Rothman, 1986). 

25
3.2.3. Stressor-Response Profile 

The results of the characterization of ecological effects are summarized in a stressor-response 

profile that describes the stressor-response relationship, any extrapolations and additional analyses 

conducted, and evidence of causality (e.g., field effects data). 

Ideally, the stressor-response relationship will relate the magnitude, duration, frequency, and 

timing of exposure in the study setting to the magnitude of effects. For practical reasons, the results of 

stressor-response curves are often summarized as one reference point, for instance, a 48-hour LC50. 

Although useful, such values provide no information about the slope or shape of the stressor-response 

curve. When the entire curve is used, or when points on the curve are identified, the difference in 

magnitude of effect at different exposure levels can be reflected in risk characterization. 

It is important to clearly describe and quantitatively estimate the assumptions and uncertainties 

involved in the evaluation, where possible. Examples include natural variability in ecological 

characteristics and responses and uncertainties in the test system and extrapolations. The description and 

analysis of uncertainty in characterization of ecological effects are combined with uncertainty analyses 

for the other ecological risk assessment elements during risk characterization. 

26
Additional Issues Related to the Analysis Phase 

• Quantifying cumulative impacts and stress-response relationships for multiple stressors. 

• Improving the prediction of ecosystem recovery. 

• Improving the quantification of indirect effects. 

• Describing stressor-response relationships for physical perturbations. 

• Distinguishing ecosystem changes due to natural processes from those caused by man. 

27
4. RISK CHARACTERIZATION 

Risk characterization (figure 4) is the final phase of risk assessment. During this phase, the 

likelihood of adverse effects occurring as a result of exposure to a stressor are evaluated. Risk 

characterization contains two major steps: risk estimation and risk description. The stressor-response 

profile and the exposure profile from the analysis phase serve as input to risk estimation. The 

uncertainties identified during all phases of the risk assessment also are analyzed and summarized. The 

estimated risks are discussed by considering the types and magnitude of effects anticipated, the spatial 

and temporal extent of the effects, and recovery potential. Supporting information in the form of a 

weight-of-evidence discussion also is presented during this step. The results of the risk assessment, 

including the relevance of the identified risks to the original goals of the risk assessment, then are 

discussed with the risk manager. 

4.1. Risk Estimation 

Risk estimation consists of comparing the exposure and stressor-response profiles as well as 

estimating and summarizing the associated uncertainties. 

4.1.1. Integration of Stressor-Response and Exposure Profiles 

Three general approaches are discussed to illustrate the integration of the stressor-response and 

exposure profiles: (1) comparing single effect and exposure values; (2) comparing distributions of effects 

and exposure; and (3) conducting simulation modeling. Because these are areas of active research, 

particularly in the assessment of community- and landscape-level perturbations, additional integration 

approaches are likely to be available in the future. The final choice as to which approach will be selected 

depends on the original purpose of the assessment as well as time and data constraints. 

Comparing Single Effect and Exposure Values 

Many risk assessments compare single effect values with predicted or measured levels of the 

stressor. The effect values from the stressor-response profile may be used as is, or more commonly, 

uncertainty or safety factors may be used to adjust the value. The ratio or quotient of the exposure value 

to the effect value provides the risk estimate if the quotient is one or more, an adverse effect is 

considered likely to occur. This approach, known as the Quotient Method (Barnthouse et al., 1986), has 

been used extensively to evaluate the risks of chemical stressors (Nabholz 1991; Urban and Cook, 1986). 

Although the Quotient Method is commonly used and accepted, it is the least probabilistic of the 

approaches described here. Also, correct usage of the Quotient Method is highly dependent on 

professional judgment, particularly in instances when the quotient approaches one. Greater insight into 

the magnitude of the effects expected at various levels of exposure can be obtained by evaluating the full 

stressor-response curve instead of a single point and by considering the frequency, timing, and duration 

of the exposure. 

28

Comparing Distributions of Effects and Exposure 

This approach uses distributions of effects and exposure (as opposed to single values) and thus 

makes probabilistic risk estimates easier to develop. Risk is quantified by the degree of overlap between 

the two distributions; the more overlap, the greater the risk. An example of this approach, Analysis of 

Extrapolation Error, is given in Barnthouse et al. (1986). To construct valid distributions, it is important 

that sufficient data amenable to statistical treatment are available. 

Conducting Simulation Modeling 

Simulation models that can integrate both the stressor-response profile and exposure profile are 

useful for obtaining probabilistic estimates of risk. Two categories of simulation models are used for 

ecological risk assessment: single-species population models are used to predict direct effects on a single 

population of concern using measurement endpoints at the individual level, while multi-species models 

include aquatic food web models and terrestrial plant succession models and are useful for evaluating 

both direct and indirect effects. 

When selecting a model, it is important to determine the appropriateness of the model for a 

particular application. For example, if indirect effects are of concern, a model of community-level 

interactions will be needed. Direct effects to a particular population of concern may be better addressed 

with population models. The validation status and use history of a model also are important 

considerations in model selection. Although simulation models are not commonly used for ecological 

risk assessment at the present time, this is an area of active research, and the use of simulation models is 

likely to increase. 

In addition to providing estimates of risks, simulation models also can be useful in discussing the 

results of the risk characterization to the risk manager. This dialogue is particularly effective when the 

relationship between risks to certain measurement endpoints and the assessment endpoint are not readily 

apparent (e.g., certain indirect effects and large-scale ecosystem-level disturbances). 

4.1.2. Uncertainty 

The uncertainty analysis identifies, and, to the extent possible, quantifies the uncertainty in 

problem formulation, analysis, and risk characterization. The uncertainties from each of these phases of 

the process are carried through as part of the total uncertainty in the risk assessment. The output from the 

uncertainty analysis is an evaluation of the impact of the uncertainties on the overall assessment and, 

when feasible, a description of the ways in which uncertainty could be reduced. 

A complete discussion of uncertainty is beyond the scope of this report, and the reader is referred 

to the works of Finkel (1990), Holling (1978), and Suter (1990b). However, a brief discussion of the 

major sources of uncertainty in ecological risk assessment is appropriate. For illustrative purposes, four 

major areas of uncertainty are presented below. These are not discrete categories, and overlap does exist 

among them. Any specific risk assessment may have uncertainties in one or all of these categories. 

30
Conceptual Model Formulation 

As noted earlier, the conceptual model is the product of the problem formulation phase, which, in 

turn, provides the foundation for the analysis phase and the development of the exposure and stressorresponse profiles. If incorrect assumptions are made during conceptual model development regarding the 

potential effects of a stressor, the environments impacted, or the species residing within those systems, 

then the final risk assessment will be flawed. These types of uncertainties are perhaps the most difficult 

to identify, quantify, and reduce. 

Information and Data 

Another important contributor of uncertainty is the incompleteness of the data or information 

upon which the risk assessment is based. In some instances, the risk assessment may be halted 

temporarily until additional information is obtained. In other cases, certain basic information such as life 

history data may be unobtainable with the resources available to the risk assessment. In yet other cases, 

fundamental understanding of some natural processes with an ecosystem may be lacking. In instances 

where additional information cannot be obtained, the role of professional judgment and judicial use of 

assumptions are critical for the completion of the assessment 

Stochasticity (Natural Variability) 

Natural variability is a basic characteristic of stressors and ecological components as well as the 

factors that influence their distribution (e.g., weather patterns, nutrient availability). As noted by Suter 

(1990b), of all the contributions to uncertainty, Stochasticity is the only one that can be acknowledged 

and described but not reduced. Natural variability is amenable to quantitative analyses, including Monte 

Carlo simulation and statistical uncertainty analysis (O'Neill and Gardner, 1979; O'Neill et al., 1982). 

Error 

Errors can be introduced through experimental design or the procedures used for measurement 

and sampling. Such errors can be reduced by adherence to good laboratory practices and adherence to 

established experimental protocols. Errors also can be introduced during simulation model development. 

Uncertainty in the development and use of models can be reduced through sensitivity analyses, 

comparison with similar models, and field validation. 

In summary, uncertainty analyses provide the risk manager with an insight into the strengths and 

weaknesses of an assessment. The uncertainty analysis also can serve as a basis for making rational 

decisions regarding alternative actions as well as for obtaining additional information to reduce 

uncertainty in the risk estimates. 

4.2. Risk Description 

Risk description has two primary elements. The first is the ecological risk summary, which 

summarizes the results of the risk estimation and uncertainty analysis and assesses confidence in the risk 

estimates through a discussion of the weight of evidence. The second element is interpretation of 

ecological significance, which describes the magnitude of the identified risks to the assessment endpoint. 

31
4.2.1. Ecological Risk Summary 

The ecological risk summary summarizes the results of the risk estimation and discusses the 

uncertainties associated with problem formulation, analysis, and risk characterization. Next, the 

confidence in the risk estimates is expressed through a weight-of-evidence discussion. The ecological 

risk summary may conclude with an identification of additional analyses or data that might reduce the 

uncertainty in the risk estimates. These three aspects of the ecological risk summary are discussed in the 

following sections. 

Summary of Risk Estimation and Uncertainty 

Ideally, the conclusions of the risk estimation are described as some type of quantitative 

statement (e.g., there is a 20 percent chance of 50 percent mortality). However, in most instances, 

likelihood is expressed in a qualitative statement (e.g., there is a high likelihood of mortality occurring). 

The uncertainties identified during the risk assessment are summarized either quantitatively or 

qualitatively, and the relative contribution of the various uncertainties to the risk estimates are discussed 

whenever possible. 

Weight of Evidence 

The weight--of-evidence discussion provides the risk manager with insight about the confidence 

of the conclusions reached in the risk assessment by comparing the positive and negative aspects of the 

data, including uncertainties identified throughout the process. The considerations listed below are useful 

in a weight-of-evidence discussion: 

P The sufficiency and quality of the data. A risk assessment conducted with studies that 

completely characterize both the effects and exposure of the stressor has more credibility 

and support than an assessment that contains data gaps. It is important to state if the data 

at hand were sufficient to support the findings of the assessment. In addition, data 

validity (e.g., adherence to protocols, having sufficient replications) is an important facet 

of the weight-of-evidence analysis. 

P Corroborative information. Here the assessor incorporates supplementary information 

that is relevant to the conclusions reached in the assessment. Examples include reported 

incidences of effects elicited by the stressor (or similar stressor) and studies 

demonstrating agreement between model predictions and observed effects. 

P Evidence of causality. The degree of correlation between the presence of a stressor and 

some adverse effect is an important consideration for many ecological risk assessments. 

This correlation is particularly true when an assessor is attempting to establish a link 

between certain observed field effects and the cause of those effects. Further discussions 

of the evaluation of causal relationships may be found in the section on characterization 

of ecological effects (section 3.2.2.). 

32
Identification of Additional Analyses 

The need for certain analyses may not be identified until after the risk estimation step. For 

example, the need to analyze the risks to a fish population (an assessment endpoint) due to an indirect 

effect such as zooplankton mortality (a measurement endpoint) may not be established until after the risk 

to zooplankton has been characterize In such cases, another iteration through analysis or even problem 

formulation may be necessary. 

4.2.2. Interpretation of Ecological Significance 

The interpretation of ecological significance places risk estimates in the context of the types and 

extent of anticipated effects. It provides a critical link between the estimation of risks and the 

communication of assessment results. The interpretation step relies on professional judgment and may 

emphasize different aspects depending on the assessment. Several aspects of ecological significance that 

may be considered include the nature and magnitude of the effects, the spatial and temporal patterns of 

the effects, and the potential for recovery once a stressor is removed. 

Nature and Magnitude of the Effects 

The relative significance of different effects may require further interpretation, especially when 

changes in several assessment or measurement endpoints are observed or predicted. For example, if a risk 

assessment is concerned with the effects of stressors on several ecosystems in an area (such as a forest, 

stream, and wetland), it is important to discuss the types of effects associated with each ecosystem and 

where the greatest impact is likely to occur. 

The magnitude of an effect will depend on its ecological context. For example, a reduction in the 

reproductive rate may have little effect on a population that reproduces rapidly, but it may dramatically 

reduce the numbers of a population that reproduces slowly. Population-dependent and - independent 

factors in the ecosystem also may influence the expression of the effect 

Finally, it is important to consider the effects in the context of both magnitude and the likelihood 

of the effect occurring. In some cases, the likelihood of exposure to a stressor may be low, but the effect 

resulting from the exposure would be devastating. For example, large oil spills may not be common, but 

they can cause severe and extensive effects in ecologically sensitive areas. 

Spatial and Temporal Patterns of the Effects 

The spatial and temporal distributions of the effect provide another perspective important to 

interpreting ecological significance. The extent of the area where the stressor is likely to occur is a 

primary consideration when evaluating the spatial pattern of effects. Clearly, a stressor distributed over a 

larger area has a greater potential to affect more organisms than one confined to a small area. However, a 

stressor that adversely affects small areas can have devastating effects if those areas provide critical 

resources for certain species. In addition, adverse effects to a resource that is small in scale (e.g., acidic 

bogs) may have a small spatial effect but may represent a significant degradation of the resource because 

of its overall scarcity. 

33
The duration of any effect is dependent on the persistence of the stressor as well as how often the 

stressor is likely to occur in the environment. It is important to remember that even short-term effects can 

be devastating if such exposure occurs during critical life stages of organisms. 

Recovery Potential 

A discussion of the recovery potential may be an integral part of risk description, although the 

need for such an evaluation will depend on the objective of the assessment and the assessment endpoints. 

An evaluation of the recovery potential may require additional analyses, as discussed in section 3.1., and 

will depend on the nature, duration, and extent of the stressor. 

Depending on the assessment objectives, all of the above factors may be used to place the risks 

into the broader ecological context. This discussion may consider the ramifications of the effects on other 

ecological components that were not specifically addressed in the assessment. For example, an 

assessment that focused on the decline of alligator populations may include a discussion of the broader 

ecological role of the alligator, such as the construction of wallows that act as water reservoirs during 

droughts. In this way, the potential effects on the community that depends on the alligator wallows can 

be brought Out in risk characterization. 

4.3. Discussion Between the Risk Assessor and Risk Manager (Results) 

Risk characterization concludes the risk assessment process and provides the basis for 

discussions between the risk assessor and risk manager that pave the way for regulatory decisionmaking. The purpose of these discussions is to ensure that the results of the risk assessment are clearly 

and fully presented and to provide an opportunity for the risk manager to ask for any necessary 

clarification. Proper presentation of the risk assessment is essential to reduce the chance of over- or 

under- interpretation of the results. To permit the risk manager to evaluate the full range of possibilities 

contained in the risk assessment, it is important that the risk assessor provide the following types of 

information: 

P the goal of the risk assessment; 

P the connection between the measurement and assessment endpoints; 

P the magnitude and extent of the effect, including spatial and temporal considerations and, 

if possible, recovery potential; 

P the assumptions used and the uncertainties encountered during the risk assessment; 

P a summary profile of the degrees of risk as well as a weight-of-evidence analysis; and 

P the incremental risk from stressors other than those already under consideration (if 

possible). 

The results of the risk assessment serve as input to the risk management process, where they are used 

along with other inputs defined in EPA statutes, such as social and economic concerns, to evaluate risk 

management options. 

34
In addition, based on the discussions between the risk assessor and risk manager, follow-on 

activities to te risk assessment may be identified, including monitoring, studies to verify the predictions 

of the risk assessment, or the collection of additional data to reduce the uncertainties in the risk 

assessment. While a detailed discussion of the risk management process is beyond the scope of this 

report, consideration of the basic principles of ecological risk assessment described here will contribute 

to a final product that is both credible and germane to the needs of the risk manager. 

35
Additional Issues Related to the Risk Characterization Phase 

• Predicting the time required for an ecological component to recover from a stressor. 

• Combining chemical and nonchemical stressors in risk characterization. 

• Incorporating critical effect levels into risk characterization. 

• Better quantification of uncertainty. 

• Developing alternative techniques for expressing uncertainty in risk characterization. 

36
5. KEY TERMS 

assessment endpoint--An explicit expression of the environmental value that is to be protected. 

characterization of ecological effects–A portion of the analysis phase of ecological risk assessment that 

evaluates the ability of a stressor to cause adverse effects under a particular set of circumstances. 

characterization of exposure--A portion of the analysis phase of ecological risk assessment that evaluates 

the interaction of the stressor with one or more ecological components. Exposure can be 

expressed as co-occurrence, or contact depending on the stressor and ecological component 

involved. 

community--An assemblage of populations of different species within a specified location in space and 

time. 

conceptual model--The conceptual model describes a series of working hypotheses of how the stressor 

might affect ecological components. The conceptual model also describes the ecosystem 

potentially at risk, the relationship between measurement and assessment endpoints, and 

exposure scenarios. 

direct effect--An effect where the stressor acts on the ecological component of interest itself, not through 

effects on other components of the ecosystem (compare with definition for indirect effect). 

ecological component--Any part of an ecological system, including individuals, populations, 

communities, and the ecosystem itself. 

ecological risk assessment--The process that evaluates tie likelihood that adverse ecological effects may 

occur or are occurring as a result of exposure to one or more stressors. 

ecosystem--The biotic community and abiotic environment within a specified location in space and time. 

exposure--Co-occurrence of or contact between a stressor and an ecological component. 

exposure profile--The product of characterization of exposure in the analysis phase of ecological risk 

assessment. The exposure profile summarizes the magnitude and spatial and temporal patterns of 

exposure for the scenarios described in the conceptual model. 

exposure scenario--A set of assumptions concerning how a exposure may take place, including 

assumptions about the exposure setting, stressor characteristics, and activities that may lead to 

exposure. 

indirect effect--An effect where the stressor acts on supporting components of the ecosystem, which in 

turn have an effect on the ecological component of interest. 

37
measurement endpoint--A measurable ecological characteristic that is related to the valued characteristic 

chosen as the assessment endpoint. Measurement endpoints are often expressed as the statistical 

or arithmetic summaries of the observations that comprise the measurement 

median lethal concentration--A statistically or graphically estimated concentration that is expected to he 

lethal to 50 percent of a group of organisms under specified conditions (ASTM, 1990). 

no observed effect level (NOEL)--The highest level of a stressor evaluated in a test that does not cause 

statistically significant differences from the controls. 

population--An aggregate of individuals of a species within a specified location in space and time. 

recovery--The partial or full return of a population or community to a condition that existed before the 

introduction of the stressor. 

risk characterization--A phase of ecological risk assessment that integrates the results of the exposure and 

ecological effects analyses to evaluate the likelihood of adverse ecological effects associated 

with exposure to a stressor. The ecological significance of the adverse effects is discussed, 

including consideration of the types and magnitudes of the effects, their spatial and temporal 

patterns, and the likelihood of recovery. 

stressor--Any physical, chemical, or biological entity that can induce an adverse response. 

stressor-response profile--The product of characterization of ecological effects in the analysis phase of 

ecological risk assessment The stressor-response profile summarizes the data on the effects of a 

stressor and the relationship of the data to the assessment endpoint 

trophic levels--A functional classification of taxa within a community that is based on feeding 

relationships (e.g., aquatic and terrestrial green plants comprise the first trophic level and 

herbivores comprise the second). 

xenobiotic--A chemical or other stressor that does not occur naturally in the environment. Xenobiotics 

occur as a result of anthropogenic activities such as the application of pesticides and the 

discharge of industrial chemicals to air, land, or water. 

38
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