Conducting an Ecological Risk Assessment
Quick Tip: For more detailed materials and tools related to ecological risk assessments, see EPA’s Ecological Risk Assessment Toolbox (EPA EcoBox). EcoBox is a compendium of tools that provides links to guidance documents, databases, models, reference materials, and other related resources. For the latest handbook on conducting an ecological risk assessment, see EPA’s Guidelines for Ecological Risk Assessment.
Ecological risk assessments provide information to risk managers about potential adverse effects of different risk management decisions. Examples include:
- Nationwide rulemaking.
- Setting environmental limits for chemicals.
- Superfund site-remediation.
- Pesticide application approval.
- Prioritizing environmental stressors for regulatory attention.
Ecological risk assessments also inform an environmental damage assessment in which loss of ecological services (e.g., flood protection, fishable waters) will be valued monetarily. The risk assessment’s purpose determines who participates in planning. All ecological risk assessment plans must:
- determine the scope and boundaries of the assessment,
- select ecological entities for the focus of the assessment, and
- ensure that the product of the assessment will support environmental decision making.
For problem or stressor-oriented risk assessment, effects on both human health and the environment can be anticipated. Planning can allow coordination among plans for both types of assessments. Some types of data and some assessment methods can be relevant to both.
Who will be involved in planning? Planning involves collaboration among decision makers and risk managers, risk assessors, scientific experts, and other interested parties.
- Who are the decision makers or risk managers? Who has the responsibility or authority to act or to require action to mitigate an identified risk? Decision makers often are staff in EPA or other federal agencies or state environmental offices who have legal authority to protect a resource.
- Who are the risk assessors and scientific experts? For many ecological risk assessments, the risk assessor must be familiar with multiple fields of expertise including ecology, population biology, ecotoxicology, statistics, or chemistry and other specialties. Staff with expertise in these and other specific fields might be needed.
- Who are other interested parties or stakeholders? They may include federal, state, tribal and municipal governments, industrial leaders, environmental interest groups, small-business owners, landowners, and other segments of society concerned about an environmental issue at hand or attempting to influence risk management decisions.
Planning also identifies the roles of the people involved in the assessment. The key decision maker and other risk managers generally define goals, scope, funding, and timing. They may consult with the risk assessors to answer several questions:
- What are the risk management goals and decisions needed?
- What is the spatial scope of the decision needed (e.g., local, statewide, national)?
- How will the ecological risk assessment inform the decision?
- What are the policy considerations (e.g., law, environmental justice)?
- What level of uncertainty is acceptable?
- What is the management timeline? Will monitoring be needed to evaluate the efficacy of a decision?
The products of planning an ecological risk assessment include definition of the management goals, management options to be evaluated, and the overall scope and complexity of the risk assessment.
- Management goals are expressed at a high level in the planning phase, such as “restore and maintain self-sustaining native fish populations and their habitat.” Exactly how that goal will be realized operationally is considered in Problem Formulation.
- Explicit management options (and criteria for selecting amongst them) determine the scope, focus, and conduct of a risk assessment.
- Planners must specify the scope and complexity of an assessment. Often, a plan specifies an iterative assessment or a tieredapproach. This can save time and resources by first screening risks to focus on of those of most concern.
Finally, planning generally includes documentation of agreements reached by the assessment team. These documents may be officially mandated (as required by law) or more informal. Documentation of planning supports Problem Formulation and ensures clear communication among the risk assessment team.
In the next phase of ecological risk assessment, Problem Formulation, risk assessors work with risk managers to weigh many additional factors and to specify the exact assessment endpoints, methods, and data that will be evaluated.
Planning and Problem Formulation can overlap substantially. Both articulate the purpose for the assessment, define the problem, and make a plan for analyzing and characterizing risk.
The objective of the problem formulation phase is to:
- refine the objectives for the risk assessment,
- determine which ecological entities are at risk, and
- determine which characteristics are important to protect.
These assessment endpoints identify both the entity and its characteristics for evaluation.
An ecological entity can be defined at one or more levels of organization:
- A species (e.g., endangered piping plover, key service provider such as honeybees).
- A functional group of species (e.g., fish eaters like pike or mink).
- A community (e.g., benthic invertebrates, soil microbial communities).
- An ecosystem (e.g., wetland, lake, watershed, island).
- A specific valued habitat (e.g., wet meadows, national wildlife refuge).
Once the entity of concern has been identified, the next step is to determine which specific attributes of the entity are important to protect. It is rarely clear which ecosystem components are most critical to ecosystem function or most valuable to the public. Prioritizing the protection of various ecological characteristics is challenging. Three principal criteria clarify this choice:
- Ecological relevance.
- Susceptibility to known or potential stressors.
- Relevance to management goals.
Determining ecological relevance requires professional judgment based on site-specific information, preliminary surveys, and related information. To understand the ecological relevance of a hazard, one must consider:
- Nature and intensity of effects.
- Spatial and temporal scales of effects.
- Potential for recovery.
- Level(s) of organization potentially affected.
- The impacted entity's role in the ecosystem.
When choosing the assessment endpoints, risk assessors must balance scientific rigor and ecological value to risk managers and the public. Considerations include:
- Endangered species or ecosystems.
- Commercially or recreationally important species.
- Ecosystem functions or services (e.g., food supply, flood control, nutrient cycling).
- Aesthetic values (e.g., clean air in national parks).
- Charismatic species of concern to the public (e.g., eagles, whales).
Once assessment endpoints are chosen, a conceptual model (a map, flow chart, or schematic) is developed to provide a visual representation of hypothesized relationships between ecological entities and the stressors to which they may be exposed. The model includes a written description with information about:
- Sources and types of stressors.
- The ecological entities (called “receptors”) and attributes of concern (together, the assessment endpoint).
- Potential exposure pathways from the source(s) to the receptors.
- Possible effects of the stressors on the assessment endpoint (risk hypotheses).
Problem formulation integrates available information on sources, stressors, receptors, as described in EPA’s EcoBox table “Assessing the Available Information: Sources, Stressors, Exposures, and Receptors.”
Problem formulation concludes with an “analysis plan” that delineates the assessment design, data needs, and anticipated uncertainties. The plan specifies:
- Exposure pathways and ecological relationships for evaluation.
- Measures and methods for assessing exposure and effects.
- Quantitative and qualitative descriptors of risk.
The objective of the analysis phase is to evaluate ecological responses to stressors under exposure conditions of interest. Risk assessors assess exposure by determining which receptors (e.g., plants, animals) are or are likely to be exposed and to what degree they would be exposed. They also evaluate stressor-response relationships or ecological effects. They analyze what is known about the relationship between the magnitude of a stressor and likelihood or magnitude of effects in the entities of concern.
The exposure assessment describes the course a stressor takes from the source to the receptor (e.g., runoff from fields into a lake) and how exposure occurs (e.g., food ingestion or whole-body exposure in water). For chemical stressors, several considerations are important:
- For chemicals that bioaccumulate, that is chemicals are taken up by a plant or animal faster than they can be eliminated or metabolized, all exposure pathways that occur at the same time can be relevant.
- A few chemicals also biomagnify, that is they are found at higher concentrations in the tissues of top predators than in their prey.
- For chemicals in the environment to bioaccumulate, they must be bioavailable, that is in a form that an organism can absorb (e.g., dissolved in water, contained in fat).
The exposure assessment also describes the contact between the stressor and the receptor at its location or during susceptible times. For example, it might determine what proportion of the wildlife species’ habitat is contaminated at a level that might be harmful. For wildlife, considerations include territory or home-range size. Different lifestages of a species (e.g., egg, juvenile) might be more or less susceptible to a stressor. The risk assessor considers whether the stressor presence coincides with sensitive lifestages.
The exposure profile provides a complete picture of how, when, and where exposure occurs or has occurred. It is developed by evaluating
- Sources and releases of the stressor.
- Distribution of the stressor in the environment.
- Extent and pattern of contact of receptor(s) with the stressor(s).
The stressor-response profile, or ecological effects analysis, evaluates evidence that exposure of a type of receptor to the stressor of specified intensities cause effects of concern. For chemicals, water concentrations associated with fish death or failure to reproduce might be identified in laboratory experiments or from field data (e.g., experimental lakes). Stressors that reduce normal growth rates also can reduce yield (e.g., fisheries) and increase death rates from grazers or predators.
The stressor-response profile also links the effect(s) to the assessment endpoint(s). For example, reduction in adult body weight by a few percent might be deemed harmless because it is within the normal variation of body weights of the population. Field studies might indicate that that more substantial adult body weight reductions could result in increased mortality and population declines.
The objective of the risk characterization phase is to use the results of analysis to estimate the risk posed to the ecological assessment endpoints. The risk assessor:
- describes the risk;
- indicates the overall degree of confidence in the risk estimates;
- summarizes uncertainties;
- cites evidence supporting exposure, stressor-response, and risk estimates; and
- interprets the adversity of ecological effects.
Adverse ecological effects are undesirable because they alter the structure, function, or services provided by the assessment endpoints. Ecosystem services include the benefits humans receive from nature, such as flood control, soil fertility, or food production.
When estimating ecological risk, assessors consider factors that include:
- Is the risk short-term and infrequent, after which the populations can be expected to recover, or is the risk chronic or frequent with little chance for recovery?
- What is the possible severity of effects on populations locally or regionally?
- Is the risk to one or a few species for which other species might maintain ecosystem services (e.g., soil decomposition, planktonic foods for fisheries)? Or might a key functional group (e.g., grazers, predators) of species decline?
Some approaches used to answer these questions and to estimate risks both quantitatively and qualitatively include:
- Field observational studies (surveys that compare stressor levels with observed effects in areas with and without specified stressors).
- Categorical rankings of different types of stressors compared to one another (e.g., systems for weighting chemicals based on potential ecological effects and exposure).
- Process models that rely partially or entirely on theoretical approximations of exposure (e.g., chemical fate and transport) and effects (e.g., fishery population models).
- Comparisons of exposure and effects data (e.g., daily average exposure concentrations compared with a lowest-observed-effects concentration for fish survival).
A good ecological risk characterization will:
- restate the scope of the assessment;
- express results clearly;
- articulate major assumptions and uncertainties;
- identify reasonable alternative interpretations; and
- separate scientific conclusions from policy judgments.
EPA's risk characterization policy calls for conducting risk characterizations in a manner consistent with the following principles:
- Transparency - The characterization should fully and explicitly disclose the risk assessment methods, default assumptions, logic, rationale, extrapolations, uncertainties, and overall strength of each step in the assessment.
- Clarity - The products from the risk assessment should be readily understood by readers inside and outside of the risk assessment process. Documents should be concise, free of jargon, and should use understandable tables, graphs, and equations as needed.
- Consistency - The risk assessment should be conducted and presented in a manner that is consistent with EPA policy and consistent with other risk characterizations of similar scope prepared across programs within the EPA.
- Reasonableness - The risk assessment should employ sound judgment, use methods and assumptions consistent with the current state-of-the-science, and be conveyed in a manner that is complete, balanced, and informative.
In order to achieve a sound risk characterization, these same principles must be applied in all of the assessment steps leading up to risk characterization.
Quick Tip: Learn more about risk characterization from the Risk Characterization Handbook.