Ecological risk assessment is patterned after human health risk assessment but is more complex. As a first step in the analysis, the ecological stressors are identified; then the ecosystem potentially impacted is determined. Ecological stressors can be categorized as chemical (e.g., toxic chemicals released into the atmosphere), physical (e.g., habitat destruction through logging), or biological (e.g., the introduction of an exotic species).
The Ecology and Welfare Subcommittee of the U.S. EPA Science Advisory Board has developed a method for ranking ecological problems (Science Advisory Board 1990). The subcommittee's approach is based on a matrix of ecological stressors and ecosystem types (Harwell and Kelly 1986). Risks are classified according to the following:
• Type of ecological response
• Intensity of the potential effect
• Time scale for recovery following stress removal
• Spatial scale (local or regional biosphere)
• Transport media (air, water, or terrestrial)
The recovery rate of an ecosystem to a stressor is a critical part of risk assessment. In an extreme case, an ecological stress leads to permanent changes in the community structure or species extinction. The subcommittee classifies ecosystem responses to stressors by changes in the following:
Biotic community structure (alteration in the food chain and species diversity) Ecosystem function (changes in the rate of production and nutrient cycling) Species population of aesthetic or economic value Potential for the ecosystem to act as a route of exposure to humans (bioaccumulation)
Determining potential risks and their likely effects is the first step in ecological assessment. Many stressors can be cumulative, finally resulting in large-scale problems. Both habitat degradation and atmospheric change are examples of ecological impact that gain attention.
Human activities affect many ecosystems by destroying the habitat. When a habitat is degraded, the survival of many interrelated species is threatened. The most drastic effect is species extinction. Habitat degradation is measured by losses in biodiversity, decreased population size and range, and decreased productivity and biomass accumulation.
Standard methods of assessing habitat degradation focus on those species of direct human interest: game fish and animals, songbirds, or valuable crops (Suter 1990).
Ecological degradation does not result from industrial activity alone. Rapid human growth creates larger residential areas and converts natural areas to agriculture. Both are major sources of habitat degradation.
A full impact assessment includes all scales of ecological impact. Impact can occur in local, regional, or global scales. Regional and local effects of pollution on atmosphere include acid rain and smog. Large-scale effects in clude global climate change caused by releases of greenhouse gases and increased ultraviolet (UV) radiation from ozone-depletion gases.
A relative scale is a useful method for characterizing the impact of emissions that deplete ozone or lead to global warming. For example, the heat-trapping ability of many gases can be compared to carbon dioxide, which is the main greenhouse gas. Similarly, the ozone-depleting effects of emissions can be compared to chlorofluorocarbons such as CFC-12. Using this common scale makes interpreting the results easier.
The specific ecological impact caused by pollution depends on its toxicity, degradation rate, and mobility in air, water, or land. Atmospheric, surface water, and groundwa-ter transport models help to predict the fate of chemical releases, but these models can be complex. Although crude, equilibrium partitioning models offer a simple approach for predicting the environmental fate of releases. Factors useful for predicting the environmental fate include:
• Bioconcentration factor (BCF)—the chemical concentration in fish divided by the chemical concentration in water
• Vapor pressure
• Water solubility
• Octanol/water partition coefficient—the equilibrium concentration in octanol divided by the equilibrium chemical concentration in the aqueous phase
• Soil/water partition coefficient—the chemical concentration in soil divided by the chemical concentration in the aqueous phase
Once pathways through the environment and final fate are determined, impact assessment focuses on the effects. For example, impact depends on the persistence of releases and whether these pollutants degrade into further hazardous by-products.
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