The large number and different kinds of ecological effects that are ofpotential concern distinguish, in part, ERA from more traditional human health risk assessment. The diversity ofeffects reflects the comprehensive nature ofecology and the environmental sciences. Ecologists have recognized several levels of organization as being useful in describing the natural world. These traditional levels include individual organisms, populations, communities, and ecosystems. Within recent decades, these levels have expanded to include macromolecules and landscapes. ERAs commonly identify more than one kind of ecological effect of concern in problem formulation. The ecological effects of concern identified during problem formulation should be ecologically important, sensitive to the stressor(s), and relevant to risk management.
Endpoints in ERA can include several effects at different levels of organization. An ERA might address alterations in basic physiological processes (e.g., photosynthesis, respiration) and corresponding lethal or sublethal (i.e., reduced growth) effects on individual organisms. In rare instances, impacts on individual organisms might be selected as endpoints. In these cases, the individuals are likely to represent small populations of endangered species.
ERAs routinely emphasize impacts on populations. The dynamics of populations has been a subject of basic ecological study for more than a century and it is no surprise that population-level endpoints have become a norm in ERA. In practice, the effects ofinterest emphasize decreases in the population sizes ofone or more species of interest. It is not uncommon for species officially designated as threatened or endangered to be the focal points for ERA. Population-level endpoints include, for example, reductions in population size, impaired reproduction, alterations in genetics, and the likelihood oflocal extinction. Note that for socially or economically undesirable species (e.g., pests, invasives, toxic blue-green algae), the endpoint would be the probability of a population increase. Population models have been developed extensively and these models are being used increasingly to estimate ecological risks.
Ecologists recognize that individual populations do not persist in an ecological vacuum. The number of species, their absolute and relative abundances, and their correlation of occurrence in space and time define community structure. A variety of concepts and methods for describing community structure have been developed by ecologists. Subsequently, alterations in community structure have been introduced as endpoints in ERA. Such alterations have taken the form of reductions in species diversity or changes in community similarity in response to the stressor of interest. Indices of biotic integrity and measures of community similarity have been introduced into the assessment of ecological risk.
Ecosystem ecology addresses important feedback mechanisms between biotic and abiotic processes that determine ecosystem structure and function. The effects of stressors on fundamental ecosystem processes (e.g., primary production, total system respiration, decomposition, nutrient cycling) are becoming increasingly important as endpoints in ERA. The ecosystem concept also emphasizes the scale dependence and asymmetry of ecological interactions. Even in highly complex systems, not all components and processes are of equal importance. Delineation of critical scales and feedbacks can help define the relevant spatial-temporal scales in designing ERAs.
More recent recognition of stressors that operate at larger scales (e.g., acid precipitation, climate change) has led to the consideration of landscape-level impacts in ERA. Landscape endpoints in risk assessment include alterations in the spatial distribution and extent of different habitat types within landscapes. Changes in the size, shape, and proximity of similar habitat areas (patches) can be measured and modeled.
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