Planning and Problem Formulation Phase: Identifying Potential Ecological Effects

According to U.S. EPA’s Guidelines for Ecological Risk Assessment (1998), problem formulation is a process for generating and evaluating preliminary hypotheses about why ecological effects have occurred—or may occur—from human activities. The process of problem formulation offers a systematic approach for organizing and evaluating available information on stressorsstressorsAny physical, chemical, or biological entity that can induce an adverse response (synonymous with agent). and possible effects. During this process, the following questions related to ecological effects might be considered.

• What are the type and extent of available ecological effects information (e.g., field surveys, laboratory tests, or structure-activity relationships)?
• Given the nature of the stressor(s) (if known), which effects are expected to be elicited by the stressor(s)?
• Under what circumstances will effects occur?

A plan for analyzing and characterizing ecological risks is determined based on the integration of available information on sources, stressors, effects, and ecosystem and receptor characteristics.

Analysis Phase: Characterizing Effects

During the analysis phase—which comprises exposure assessment and effects assessment—ecological effects are characterized. As described in U.S. EPA’s Guidelines for Ecological Risk Assessment (1998), to characterize ecological effects associated with a stressor, assessors do the following:

• Describe the effects elicited by the stressor.
• Conduct an ecological response analysisecological response analysisIn an ecological risk assessment, this is where the relationship between the stressor and the ecological effects elicited is quantified, and cause--and-effect relationships are evaluated. to evaluate how the magnitude of effects changes with varying stressor levels and the evidence that the stressor causes the effect. The ecological response analysis examines three primary elements:
  • Relationships between stressor levels and ecological effects (stressor-response analysis);
  • Plausibility that effects may occur or are occurring as a result of exposure to stressors (i.e., establishing cause-and-effect relationships or causality); and
  • Linkages between measurable ecological effects and assessment endpoints when the latter cannot be directly measured.

  Causality is the relationship between cause (one or more stressors) and effect (response to the stressor[s]). Without a sound basis for linking cause and effect, uncertainty in the conclusions of an ERA is likely to be high. Evidence of causality may be derived from observational evidence (e.g., bird kills associated with field application of a pesticide), experimental data (e.g., laboratory tests with the pesticides in question show bird kills at levels similar to those found in the field), or both (U.S. EPA, 1998).

  Bradford Hill criteria for causality provide a framework for assessing the evidence about causality. Aspects to aid in judging causality include: strength, consistency, and specificity of association; temporal relationship of the observed association; experimental evidence; biological plausibility; biological gradient (exposure-response relationship); coherence of evidence; and analogy based on similar effects (Hill, 1965; U.S. EPA, 1998; 2013).

• Prepare a stressor-response profilestressor-response profileThe 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. to summarize the data on the effects of a stressor and the relationships of the data to assessment endpointassessment endpointAn explicit expression of the environmental value to be protected, operationally defined as an ecological entity and its attributes.. The stressor-response profile relates the magnitude of an effect to the magnitude, duration, frequency, and timing of exposure.

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For narrowly defined assessments of a single stressor and effect, the calculated relationship may be expressed as a stressor-response curvestressor-response curveA graphic, quantitative representation of the relationship between a stressor and an ecological effect. or summarized as a single reference value (a point on the curve [e.g., LD₅₀]). In many cases, however, more complex interrelationships are present. Effect levels can be expressed in terms of lethality (e.g., LC₅₀ or LD₅₀) or effects other than lethality like growth and reproduction (e.g., EC₅₀ or ED₅₀). In conducting a toxicity test, an assessor might choose to establish no-effect stressor level (e.g., NOAEL) or lowest-effect stressor level (e.g., LOAEL) based on comparisons between experimental treatments and controls.

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Below is a simple example of a stressor-response curve that shows a range of responses for increasing stressor intensity. The points represent lethal dose (LD) levels for a given percent of the population. The shape of the stressor-response curve may be needed to determine the presence or absence of thresholdsthresholdsA concentration above which some effect (or response) will be produced and below which it will not. for significant effects or to evaluate incremental risks (U.S. EPA, 1998). The lower bound of a threshold would be based on consistent conservative assumptions and NOAEL toxicity values. The upper bound would be based on observed impacts or predictions that ecological impacts could occur. This upper bound would be developed using consistent assumptions, site-specific data, LOAEL toxicity values, or an impact evaluation (U.S. EPA, 1997; 1996).

Full size image of Increase Intensity of Stressor

EPA estimates the toxicity or hazard of a stressor by evaluating ecological effects tests that vary from short-term (acute) to long-term (chronic) laboratory studies and might also include field studies. In these tests, animals and plants are exposed to different amounts of a stressor, and their responses to these varying concentrations are measured. The results of these tests may be used to establish a dose-response or cause-and-effect relationship between the amount of stressor to which the organism is exposed and the effects on the organism.

Some of the impacts or ecological effects that are measured in ecotoxicity tests include:

• mortality,
• reduction in growth,
• reproductive impairment,
• changes in numbers of species,
• bioaccumulation of residues in non-target organisms, and
• disruption of community and ecosystem-level functions.

The table below provides examples of toxicity endpoints that might be used in a screening assessment to calculate risk for aquatic and terrestrial receptors.

¹Ecological Effects Test Guidelines (Series 850) are available at Series 850 - Ecological Effects Test Guidelines.

Literature reviews, laboratory toxicity tests, and field studies are measure-of-effect tools to help link contaminant concentrations to effects on ecological receptors (see table). These tools provide “dose-response” information: that is, how much toxicant is associated with how much of an adverse effect (U.S. EPA, 1991). Adverse ecological effects are changes that are considered undesirable because they alter valued structural or functional characteristics of ecosystems or their components. An evaluation of adversity may consider the type, intensity, and scale of the effect as well as the potential for recovery (U.S. EPA, 1998).

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Risk Characterization: Quantifying Effects in Ecological Risks

The risk characterization quantifies the links between exposure to stressorsstressorsAny physical, chemical, or biological entity that can induce an adverse response (synonymous with agent). and effects to ecological receptors and characterizes the types, extent, and severity of those risks. Completing risk characterization allows risk assessors to reach conclusions regarding the occurrence of exposure and the adversity of existing or anticipated effects (U.S. EPA, 1998).

During risk characterization, exposure and stressor-response profilesstressor-response profilesThe 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. are integrated through the risk estimation process. The final product is a risk description in which the results of the integration are presented, including an interpretation of ecological adversity and descriptions of uncertainty and lines of evidence. The acceptability of adverse effects is determined by risk managers. In characterizing adversity, primary (direct) and secondary (indirect) effects are considered, and sometimes the impact of the secondary effects can outweigh that of the primary effect (U.S. EPA, 1998).

A common approach to quantifying ecological risks to receptors is to compare exposure concentrations of a stressor in an environmental medium by a toxicity reference value (TRV)—a concentration which if exceeded is expected to result in adverse ecological effects. Where exposure-response functions are not available or developed, the quotient method of comparing an estimated exposure concentration to a threshold for response can be used (U.S. EPA, 1997). Typically, the ratio (or quotient) is expressed as an exposure concentration divided by an effects concentration. Quotients are commonly used for chemical stressors for which reference or benchmark toxicity values are widely available (U.S. EPA, 1998).

References

• Hill, AB. (1965). The environment and disease: Association or causation? Proc R Soc Med 58: 295-300.
• U.S. EPA. (1991). ECO update, volume 1, number 2 (8 pp, 202 K, About PDF) [EPA Report]. Office of Solid Waste and Emergency Response.
• U.S. EPA. (1994a). ECO update, volume 2, number 1 (12 pp, 264 K, About PDF) [EPA Report]. Office of Solid Waste and Emergency Response.
• U.S. EPA. (1994b). ECO update, volume 2, number 2 (4 pp, 58 K, About PDF) [EPA Report]. Office of Solid Waste and Emergency Response.
• U.S. EPA. (1994c). ECO update, volume 2, number 3 (14 pp, 152 K, About PDF) [EPA Report]. Office of Solid Waste and Emergency Response.
• U.S. EPA. (1996). ECO update, volume 3, number 2 (12 pp, 192 K, About PDF) [EPA Report]. Office of Solid Waste and Emergency Response.
• U.S. EPA. (1997). Ecological risk assessment guidance for superfund: Process for designing and conducting ecological risk assessments - Interim final [EPA Report]. (EPA/540/R-97/006). Office of Solid Waste and Emergency Response.
• U.S. EPA. (1998). Guidelines for ecological risk assessment [EPA Report]. (EPA/630/R-95/002F). Washington, DC: U.S. Environmental Protection Agency, Risk Assessment Forum.
• U.S. EPA. (2005). Guidance for developing ecological soil screening levels [EPA Report]. (OSWER Directive 9285.7-55). Office of Solid Waste and Emergency Response.
• U.S. EPA. (2011a). Policy assessment for the review of the secondary national ambient air quality standards for oxides of nitrogen and oxides of sulfur (345 pp, 14.2 MB, About PDF) [EPA Report]. (EPA-452/R-11-004a).
• U.S. EPA. (2011b). Review of the national ambient air quality standards for lead: risk and exposure assessment planning document (60 pp, 938 K, About PDF) [EPA Report]. (EPA-452/P-11-003). 
• U.S. EPA. (2013). Integrated science assessment for lead: Final report [EPA Report]. (EPA/600/R-10/075F). Washington, DC.

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Tools

Resources that provide information related to effects in ecological risk assessments are provided below.

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