Defining populations: a key step in identifying spatial and temporal scales.

The scale at which ecological risk assessors frame their analyses can be driven by a number of considerations. In many cases, the selection of the scale is among the most important decisions within the 'problem-formulation' phase of the ecological risk characterization. The scale of a risk characterization is often defined based on ownership boundaries, ecological, or habitat boundaries and/or physical and hydrogeological boundaries (e.g., watersheds). With increasing value being placed on consideration of spatial scales in risk characterization, the need to define a scale that balances analytical and ecological realities is imperative. Defining the spatial scale based on the scale of a population offers the risk assessor the opportunity to improve the ecological relevance of the risk characterization. Arriving at a single operational definition of a population would be difficult and not appropriate because of the variability in sites, issues, and management goals; however, we can begin to review operational definitions and establish analytical frameworks by which to select the appropriate, site-specific scale within the problem-formulation phase. Defining a scale based on a combination of ecological and site ownership considerations and the inclusion of habitat quality in the assessment of scale can improve the power and relevance of the problem formulation and risk characterization. In this analysis, we assess methods for incorporating spatial considerations into risk analyses, beginning with developing an understanding of the context of a site within the larger landscape. Clearly defining the scale of an assessment including the operational definition of populations is an important step in the problem formulation. Within a site or landscape, the scale may be refined further based on the percentage of the land within the scale of the analysis that provides habitat for a given species.