Using probability of extinction to evaluate the ecological significance of toxicant effects

A large component of uncertainty in ecological risk assessment (ERA) arises from the disparity between the time scale of toxicity measurements and the time scale of predictions of ERA models. It is difficult to make predictions about the persistence of populations from data from short-term toxicity tests. Reproductive toxicity tests provide data about how population growth rate (r) is reduced with toxicant exposure. Although reduction in r is believed to be one of the most important effects of toxicant exposure, its ecological significance has been difficult to quantify. For rotifers, r is typically reduced by 10 to 15% at no-observed-effect concentrations (NOEC). We investigated r reductions of 0 to 30% in time series models of the dynamics of natural rotifer populations to predict probability of quasiextinction. Computer simulations showed that small reductions in r can have large effects over long time periods. This means that purportedly safe toxicant concentrations like those at NOEC levels can reduce r so that population extinction becomes much more likely. Reductions in r greater than about 30% destine rotifer populations near certain extinction within 100 years. Simulations indicate that an r reduction of 5% (EC5) approximately doubles the probability of extinction and is probably the maximum tolerable for the long-term persistence of rotifer populations. When episodic catastrophic population reductions are coupled with r reduction, probabilities of extinction are increased substantially. Several models yielded similar results, so the conclusions do not seem strongly dependent on the form of the model. Incorporating population viability analysis into ecological risk assessments could improve our ability to define conditions required for the long-term sustainability of populations.