Sublethal Sensitivity Index as an Ecotoxicity Parameter Measuring Energy Allocation under Toxicant Stress: Application to Cadmium in Soil Arthropods

Toxic substances may affect the life history of a species in a variety of ways. Different species maintain different priorities in coping with the physiological consequences of toxicant-induced stress. This is expressed by changes in energy allocation to different life-history characteristics which may have great consequences for the response at the population level. In this study the terrestrial invertebrates Platynothrus peltifer (Oribatida), Orchesella cincta, Folsomia candida (Collembola), and Porcellio scaber (lsopoda) were chosen to evaluate species-specific sensitivity of life histories. Effects on reproduction and weight increase under exposure to cadmium in the food were analyzed. The answer to the question of which species is the most sensitive depends on the parameter chosen. A comparison of sensitivity on the basis of sublethal effects showed P. peltifer to be the most sensitive species; on the basis of lethal effects however, the species O. cincta was the most sensitive. This discrepancy between effect parameters resulted in differences between the distance of the concentrations at which lethal and sublethal effects occur for different species. The ratio between the lethal effect concentration and the sublethal effect concentration is called the sublethal sensitivity index (SSI) and is proposed as a parameter expressing maintenance of sublethal functions under toxicant stress. The SSI seems to be a valuable parameter for evaluating the likelihood of population-level effects under toxicant stress. To extrapolate effects found in the laboratory to the field situation, more attention should be paid to the relationships between effects on life-history parameters and effects on population growth.