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Progress Report of the Ecological Committee on FIFRA Risk Assessment Methods: V. Terrestrial Effects Assessment

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The goal of the Terrestrial Effects Group of ECOFRAM is to provide insight into a practical and scientifically sound approach for translating a probabilistic assessment of dose to a probabilistic assessment of ecological risk at the level of individuals and populations. We have approached this goal by identifying five areas of concern in which to focus attention (see Figure). In general, we wish to define a distribution of risk that explicitly considers magnitudes of effect and the likelihood or probability of those effects. As a consequence, we wish to quantify variability and uncertainty. Equally important, we wish to identify those sources of variability and uncertainty that have the greatest impact on final conclusions. Even in the absence of intensive or extensive data, sensitivity analysis can identify those parameters whose variability or uncertainty have a large effect on final assessments of risk.

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The Ecological Committee on FIFRA Risk Assessment Methods (ECOFRAM) was formed in June 1997. The Committee's purpose is to develop tools and processes within the FIFRA framework for predicting the magnitude and probabilities of adverse effects to non- target aquatic and terrestrial species resulting from the introduction of pesticides into their environment. A Terrestrial Effects Subgroup was formed to identify and discuss probabilistic methods for terrestrial effects assessments and develop recommendations for future use by EPA. In addition, they will identify information that must be developed in order to validate the proposed methods and to ensure that the proposed assessment process, if adopted by EPA, supports environmental decisions that are scientifically defensible. A report of the Subgroup's deliberations to date will be presented.

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Lead Collaborators and Areas of Concern

Name and AssociationArea of Concern
Richard Bennett, Ecological Planning & Toxicology Test Sensitivity
Michael Hooper, Texas Tech University Intra-specific Extrapolations
Alain Baril, Canadian Wildlife Service Inter-specific Extrapolations
Thomas Lacher, Texas A&M University Population-level Extrapolations
Jennifer Shaw, Zeneca Ag Products Risk Characterization

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The suitability of tests is dependent on how exposure is characterized. Because the ECOFRAM is working on models to characterize exposure as dose received (i.e., mg/kg or mg/kg/day) rather than as bioavailability (e.g., ppm in food items), the suitability of toxicity tests must be evaluated for their potential to provide information on the toxicity of the chemical relative to dose.

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Probabilistic distributions of toxicity are only as reliable as the data on which they are based. A number of factors, intrinsic to the species and toxicity measurement process, contribute variability to estimates of toxicity. Identification of the sources of this variability make possible better data for use in probabilistic assessments, and assist in the extrapolation process that is necessary for under-represented species. We are examining the sources and repercussions of this variability.

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Considerable uncertainty may be associated with the process of extrapolating toxicity endpoints across species. The ECOFRAM is addressing a number of questions related to the sources of this uncertainty.

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An objective of ECOFRAM is to develop population-level projections of ecological effects. Such effects traditionally are modeled by treating all individuals as genetically, morphologically, and physiologically equal. Nonetheless, different age groups, sexes, body size classes, and even individuals can react differently to exposure to a toxicant. The ECOFRAM approach to probabilistic risk assessment explores several approaches for modeling risk at the population-level.

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Risk characterization is the final stage of risk assessment at which results of exposure and effects analyses are integrated to evaluate the likelihood of adverse ecological effects occurring following exposure to a stressor. The ecological significance of the adverse effects should be discussed, including consideration of the types and magnitudes of effects, their spatial and temporal patterns, and the likelihood of recovery.

In the FIFRA regulatory process, the quotient method has been used in risk assessment for pesticides. A quotient of single values for exposure and effects is given, and if the quotient is ≥ 1, an adverse effect is considered likely to occur. Limitations of this approach include:

  1. a lack of information on the magnitude or probability of adverse effects,
  2. as the quotient approaches 1, an increased dependence on expert judgment is required, and
  3. use of single points that represent the most sensitive or conservative data in the estimate; other available data are ignored usually.

On the other hand, a quotient is a simple and efficient means of identifying pesticides that are likely to be very safe in the environment. Therefore the method could remain a preliminary screening option. However, for decision making, risk managers may require more refined risk assessments that describe the probability and magnitude of adverse effects. A suite of methods (see Table) may be the most effective way to provide flexibility to manage a diversity of pesticide scenarios in which a refined risk assessment is necessary.

An essential element of the risk characterization stage will be to analyze and summarize uncertainties. These will include uncertainty associated with natural stochasticity, parameter error, and model error from exposure and effects analyses and the risk characterization. In addition to uncertainty, the risk characterization will provide a discussion of the ecological significance of effects with particular emphasis on the magnitude and spatio-temporal extent of population-level impacts.

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