Direct and indirect effects of sublethal toxicant exposure on population dynamics of freshwater rotifers: a modeling approach

Ecotoxicological assessments commonly consider the direct effects of anthropogenic toxicants on the survival, growth, and reproduction of organisms. However, toxicants also may have indirect effects on aquatic communities by altering species interactions such as predation and competition. Such indirect effects are often mediated by changes in animal behavior, and contribute to uncertainty in predicting the environmental effects of toxicants. In an attempt to better understand indirect effects, a modeling approach was used to simulate the population dynamics of the predatory rotifer Asplanchna girodi and six prey species of herbivorous rotifers in response to sublethal concentrations (110, 190 or 330 mg/l) of pentachlorophenol (PCP). Simulations were performed assuming three different toxicant effect scenarios: behavioral toxicity alone, reproductive toxicity alone, or both behavioral and reproductive toxicity. Population growth rates (G(r)), maximum population densities (D(max)), and the percent of carrying capacity at D(max) (%K) were calculated for A. girodi and each prey species for each effect scenario and toxicant concentration. Similar qualitative results were obtained regardless of which prey species was exploited by A. girodi. Toxicant effects on predator and prey population densities appeared to be the dominant cause of both direct and indirect toxicant effects, and behavioral toxicity was a redundant endpoint relative to reproductive toxicity. These results suggest that assessing toxicant effects on the population growth of individual species may provide sufficient data to predict toxicant effects on species interactions as well.