FITNESS CONSEQUENCES OF TOXIC STRESS EVALUATED WITHIN THE CONTEXT OF PHENOTYPIC PLASTICITY

A method is presented to evaluate the impact of environmental contamin ants on organisms based on the relationship between toxicant-induced p lastic responses in life cycle traits and fitness, which is defined as the intrinsic rate of population increase, Using the bacterivorous ne matode Plectus acuminatus and the toxicant pentachlorophenol as a case study, Life cycle experiments were conducted to assess critical effec t concentrations (EC) for various traits. It was found that pentachlor ophenol significantly reduced both juvenile survival and the reproduct ive period, while daily reproduction significantly increased. The juve nile survival was less sensitive to pentachlorophenol compared to the reproductive period (EC20 = 4.3 mu mol/L and 1.3 mu mol/L, respectivel y). 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 survi val. To further assess the relationship between fitness impairment and the EC20 values, a sensitivity analysis of the model was performed, r evealing that a 20% decrease in the reproductive period did not influe nce fitness, whereas a 20% reduction in juvenile survival resulted in a 5% fitness decrease. The results imply that (1) critical effect leve rs for sensitive life cycle trails are not sufficient for assessing th e potential impact of toxicants on fitness, and (2) insight into the r elationship between plasticity of life cycle traits and fitness provid es a proper basis for the ecological risk assessment of toxicants on p opulations.