Stress induced fitness reduction is not determined by the most sensitive life-cycle trait.

1. It is widely assumed that stressors such as toxicants affect organisms by impairment of those life-cycle variables that are most sensitive to these toxicants. We tested this premise by contrasting a fitness assessment with the most sensitive life-cycle variable approach using cadmium and the nematode Plectus acuminatus as a case study. 2. Based on complete life-cycle experiments, a deterministic model was constructed relating changes in juvenile and adult variables, including a Weibull survival distribution, to fitness, which was defined as the intrinsic rate of population increase. 3. Based on a sensitivity analysis of the model it was indicated that impairment of the most sensitive trait, the reproductive period which was reduced by 45%, did not have any effect on fitness. However, a prolongation by cadmium of the juvenile period by 7.5%, the least sensitive trait, resulted in a fitness decrease of 5%. 4. Application of this approach to the evaluation of critical effect levels for cadmium implies that a hazard assessment based on the most sensitive life-cycle trait leads to erroneous predictions of the biological impact that toxicants cause.