Fitness Consequences of Toxic Stress Evaluated Within the Context of Phenotypic Plasticity

A method is presented to evaluate the impact of environmental contaminants on organisms based on the relationship between toxicant-induced plastic responses in life cycle traits and fitness, which is defined as the intrinsic rate of population increase. Using the bacterivorous nematode Plectus acuminatus and the toxicant pentachlorophenol as a case study, life cycle experiments were conducted to assess critical effect concentrations (EC) for various traits. It was found that pentachlorophenol significantly reduced both juvenile survival and the reproductive period, while daily reproduction significantly increased. The juvenile survival was less sensitive to pentachlorophenol compared to the reproductive period (EC20 = 4.3 μmol/L and 1.3 μmol/L, respectively). Using a demographic model based on life cycle data, physiological trade-off curves were constructed, indicating that fitness reductions were determined by plasticity in daily reproduction and juvenile survival. To further assess the relationship between fitness impairment and the EC20 values, a sensitivity analysis of the model was performed, revealing that a 20% decrease in the reproductive period did not influence fitness, whereas a 20% reduction in juvenile survival resulted in a 5% fitness decrease. The results imply that (1) critical effect levels for sensitive life cycle traits are not sufficient for assessing the potential impact of toxicants on fitness, and (2) insight into the relationship between plasticity of life cycle traits and fitness provides a proper basis for the ecological risk assessment of toxicants on populations.