What is an appropriate level of protection? An example considering selenium exposures by aquatic birds.

Evaluating population-level risks to ecological receptors or developing toxicity thresholds intended to be protective of a population requires a population model to truly understand possible chemical-related impacts to the population of interest. For various reasons (e.g., lack of training in the application of population models to ecotoxicological questions), we often use laboratory-based (more common) or field-based (less common) toxicity data to develop a toxicity threshold that is assumed to be protective of population-level effects. Under this latter approach, an appropriate level of protection against exposure should focus on protecting the viability and productivity of populations of organisms, that is, maintaining approximately the same density of individuals over time. The EC values can be used to set technically defensible levels of protection, with the appropriate effect level being determined on the basis of data- and site-specific considerations and dose-response relationships that are amenable for use as inputs in population models. Even without the use of predictive population models, the ECO10 or EC20 are commonly used values in risk assessment or criteria development with the assumption of adequate protection of populations. In the Se example presented here, there is strong evidence that egg hatchability is the critical toxicity endpoint for birds based on dietary organic Se exposures and that mallards are a sensitive bird species. These factors support that the dietary Se EC10 derived by Ohlendorf (2003) is sufficiently low to not have any measurable effects on aquatic birds in the field. Further, effect levels below the EC10 are likely to be statistically indistinguishable from the controls in most situations (as it was for Se in this example), and, for Se and other naturally occurring elements, it is not unusual for lower EC values to approach or fall below background levels at a site. A determination as to whether higher EC values would also provide an adequate level of protection could be made by appropriate population modeling.