Two broad classes of models are used in ecological risk assessment: exposure models and effects models. Exposure models address the co-occurrence of organisms and chemicals, uptake rates, and accumulation in tissues. Effects models deal with exposure-response relationships or extrapolation between effects endpoints. Ecotoxicological models, as defined here, are effects models with output parameters that directly apply as assessment endpoints above the level of individual organism effects (e.g., population abundance, biodiversity, ecosystem productivity). A mathematical model is used to define precisely (1) the relationship between measures of exposure and effects and the assessment endpoint of interest, and (2) assumptions and uncertainties in the extrapolation between endpoints. Many ecological models incorporate mechanistic functions to describe natural processes, such as nutrient and energy flows, organism growth, life-stage transitions, dispersal, competition, predation, and interactions between organisms and the environment. Specific kinds of ecotoxicological models are described later in this article.
Although many ecological models can be used to assess toxic chemical effects by modifying input parameters to mimic the effects of chemicals on organism survival, growth, and reproduction, vital rates of populations, or other ecological attributes, true ecotoxicological models incorporate exposure-response relationships for the toxic chemicals of interest in a risk assessment. Such models may simulate the structure and dynamics of populations, food webs, whole ecosystems, or landscapes. More complex ecological models, such as the aquatic ecosystem model AQUATOX, have built-in functions to account for toxic effects, as well as chemical fate and transport.
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