Mechanistic Models to Explore Combined Effects of Toxic Chemicals and Natural Stressing Factors: Case Study on Springtails

This study compared two methods, based on re-analyzed data from a partly published Life Table Response Experiment (LTRE), to help determine the optimal approach for designing ecotoxicological assessments. The 36-day LTRE data recorded the toxic effects of cadmium (Cd) and imidacloprid, alone and in combination, on the reproduction and survivorship of aphids (Acyrthosiphon pisum Harris). We used this data to construct an age-classified matrix model (six age classes, each six days long) to estimate aphid population growth rate (λ) under each treatment. For each treatment, an elasticity analysis and a demographic decomposition analysis were performed, and results were compared. Despite different results expected from the two toxicants, the elasticity values were very similar. The elasticity of λ with respect to survival was highest in the first age class, and that with respect to fertility was highest in the second age class. The demographic decomposition analysis examined how changes in life-history traits contributed to differences in λ between control and treated populations (∆λ). This indicated that the most important contributors to ∆λ were the differences in survival (resulting from both demographic sensitivity and toxicity) in the 1st and the 2nd age classes of aphids and differences in fertility in the 3rd and the 4th age classes. Additionally, the toxicants acted differently. Cadmium reduced ∆λ