ABSTRACT
Stressor distribution refers to the distribution of the particular stressor under evaluation across the landscape, or in time. In the case of a chemical stressor in a retrospective risk assessment, chemical distributions would be measured across the site or landscape. Depending on the type and scale of the risk assessment being conducted, a wide range of empirical data might be required. The chapter examines empirical methods that can be used in evaluating population level ecological impacts, both as stand-alone assessment techniques and for use in models. It describes methods available for collecting empirical data and evaluating that data for evidence of population-level impacts. Population genetic markers are used to characterize genetic variation for a number of purposes. Selection of specific approaches and markers depends on the goals of the study as well as technological constraints of measurement and availability of markers for the species of concern.
