the Clean Water Act. Their purpose is to assess injury (adverse effects) and damages (eco-
nomic costs) to natural resources caused by the release of harmful materials. Several
approaches to ERAs have been developed. Here we focus on the U.S. EPA framework.
Under this framework, the ERA ‘‘evaluates the likelihood that adverse ecological effects
may occur or are occurring as a result of exposure to one or more stressors.’’ Under this
definition, likelihood can be expressed in deterministic, probabilistic, or even qualitative
terms.
The U.S. EPA framework has the same risk assessment steps as described above for
human health risk assessment, with slightly different terminology:
Problem formulation (hazard identification) determines stressors, populations at risk,
ecological effects of concern, and measurement endpoints.
Analysis: includes two concurrent steps:
Characterization of exposure (exposure assessment)
Characterization of ecological effects (toxicity assessment)
Risk characterization
The exposure and ecological effects characteriza tion are considered parallel steps as
part of an analysis phase. The U.S. EPA framework also makes explicit the interactions
between risk assessment and risk management on one hand and data acquisition on the
other. A dialogue between the risk assessor and the risk manager is expected to ensure that
the risk assessment will satisfy both scientific and societal goals.
Ecological risk assessment has many more sources of uncertainty than human health
risk assessment. The release of hazardous chemicals to the environment can affect numer-
ous species. Even if a particular sensitive species were selected, often less is known about
its sensitivity. The response of cold-blooded species may depend on environmental tem-
perature, whereas laboratory toxicity studies are typically made at a single temperature.
Problem Formulation Phase In the problem formulation step, an identification and pre-
liminary characterization are made of the stressor, the population affected, and the ecolo-
gical effects. This information is used to select assessment and measurement endpoints,
and a conceptual site model predicting how the stressor affects the ecosystem and the end-
points.
A stressor is a physical, chemical, or biological factor that can cause adverse effects
on individuals, populations, communities, or ecosystems. Examples of physical stressors
are erosion, flooding, or thermal pollution. Chemical stressors may include pesticides,
heavy metals, and so on. Biological stressors include exotic species, pathogens, or
pests. Characteristics of stressors that should be determin ed include their intensity, dura-
tion, frequency, timi ng relative to biological cycles, and spatial distribution.
The ecosystem or ecological components potentially at risk may include the broadest
definition of ecosystem to include all living and nonliving things in an area, or may focus
selectively on a particular species , the indicator species. The indicator species may be
chosen because of their sensitivity to the stressor, or because they are endangered or threa-
tened, or because they represent sign ificant economic or recreational populations.
The ecological effects may be selected based on the observations that prompted the risk
assessment in the first place, such as fish kills or population depletion. Alternatively, they
may be selected based on known relationships between stressors and their effects on the
848 APPLICATIONS OF TOXICOLOGY