Concerning population-level response a recent study by van Kirk and Hill (2007) suggests that when there are density-dependence relationships, the response to a contaminant may be lower than the one obtained at individual level up to a certain point when toxic effects outweigh compensatory mechanism at this level, after the effects increase sharply. For these reasons, the authors propose a stochastic modelling approach based on individual effects for analysing population response.
At ecosystem level we can find indirect effects (see Fleeger et al., 2003 for a recent survey). These effects even on tolerant species occur by other ecological mechanisms rather than toxic effects, e.g. direct influences of contaminants on predators can lead to cascading indirect effects on resistant species in other trophic levels by altering competitive interactions and therefore modifying substantially its abundance and dynamical behaviour. Such effects are called indirect (or secondary) contaminant effects (Flegger et al., 2003) and sometimes can be as or more significant that the direct (toxic) effects of a contaminant. Ecological models have become effective tools in evaluating indirect effects (Bartell, 1996; Pastorok et al., 2003). In addition, ecological models may be applied to forecast future potential risks or to estimate risks when field experiments cannot be performed, i.e. the release of a new chemical into the environment. They are useful tools for testing alternative hypothesis or to reconstruct past situations where evidence of toxic exposure cannot be demonstrated.
On top of that, the number, diversity and complexity of chemicals produced and released to the environment is overwhelming, and despite this there is only information regarding environmental fate and/or impact on ecosystem and human health for a small fraction (Swoboda-Colberg, 1995). ). In fact, of the more than 100,000 synthetic organic chemicals in use (Howard and Muir 2006), the number of chemicals of environmental concern is unknown, and there are environmental fate and transport and ecotoxicological data for less than 1% of the total anthropogenic chemical classes.