Uncovering hidden sensitivity: interindividual growth variation in earthworms under fungicide exposure

Environmental risk assessments of chemicals typically rely on standardized ecotoxicological tests that overlook interindividual variability, despite its importance in ecological resilience and evolutionary success. Contaminants can disrupt individual differences by altering life history traits, amplifying fitness disparities, favoring certain phenotypes, and reducing genetic diversity, ultimately affecting population dynamics and adaptability. However, the extent to which pollutants influence interindividual variability and its population-level consequences remains poorly understood. To address this, our study examines the interindividual variability of growth trajectories in the earthworm Aporrectodea caliginosa in response to sublethal exposure to the two active substances of Swing Gold fungicide. Using a longitudinal design with 30 exposed and 30 unexposed individuals, we compared mean and interindividual growth rate variability. While the fungicide had a weak effect on the cohort's mean growth rate, we observed a 3-fold increase in interindividual variability in the exposed group. This increase highlighted a subset of highly sensitive individuals, whose growth was reduced by up to 10% as compared with the average response. Our results suggest that focusing solely on population mean effects could overlook impacts on sensitive individuals, who could serve as early indicators of environmental stress. Incorporating individual variability into ecotoxicological studies is challenging due to the labor-intensive nature of individual monitoring and the need for larger datasets. Nonetheless, these efforts are essential for refining higher-tier environmental risk assessment frameworks, improving safety factors for intraspecies variability, and defining regulatory thresholds. A better understanding of how contaminants affect interindividual variation will enhance the accuracy and ecological relevance of risk assessments, ultimately capturing the long-term implications for population and ecosystem dynamics.