The variation in slope of concentration-effect relationships.

Implementing the variation within populations in ecological modeling of risk and effects of toxicants could contribute to a more sophisticated way of modeling. To give insight into the magnitude and spread in sensitivity within test populations the variation in sensitivity of individuals expressed as the slope of a concentration-effect curve (Sm) within more than 300 test populations from toxicity tests with 36 species and 116 substances is analyzed. The overall average variation from this study is 0.65 (Sm) and corresponds to an EC50/EC5 ratio of 2.9. The mode of action is often suggested as an explanation for the different slopes of concentration-effect curves. For metals, indeed, significant different median slope values are observed compared with organic substances. However, within the group of organic substances no distinction between the slope values of narcotic substances and substances with a specific mode of action could be made. Between species groups, significant differences in the amount of variation could be demonstrated. The median slopes for all toxicants within the data set of 96-h tests specified for fishes and mollusks are significantly steeper (median slopes of 0.38 and 0.37, respectively) than those for algae and crustaceans (1.2 and 0.72, respectively). Experimental factors that might explain these differences are discussed. The results from this study provide insight into the variation within a test population and can be used to estimate sensitivity parameters for risk and effect modeling.

Population consequences of cadmium toxicity in soil microarthropods.

Chronic toxicity experiments were performed using the collembolan Orchesella cincta (L.) and the oribatid mite Platynothrus peltifer (Koch), which were exposed to various levels of cadmium in the food (green algae). Experimental results were combined with life-history information to obtain realistic estimates of the intrinsic rate of population increase and its sensitivity to Cd stress. Chronic LC50 values for dietary exposure to Cd were estimated as 1.60 mumol/g for O. cincta and 3.18 mumol/g for P. peltifer. No-observed-effect concentrations (NOECs) for growth and reproduction were 0.042 mumol/g for O. cincta and 0.026 mumol/g for P. peltifer. The main effects of Cd were, however, different in each species. In O. cincta, Cd affected primarily female growth, without a direct effect on reproduction; in P. peltifer, the effect was primarily on reproduction. Uptake of Cd was higher in P. peltifer than in O. cincta, and caused a loss of zinc in the former species. As a consequence of their differing physiological responses to Cd, mites and collembolans also reacted differently in their population growth rates. The capacity for population increase of mites appeared to be rather sensitive to Cd, while collembolans were able, to some extent, to maintain their capacity for increase, in spite of toxic effects at the individual level. These results may contribute to developing ecotoxicological theory for the population consequences of toxic action in species with diverging life histories. Soil microarthropods may be suitable test organisms for evaluating the risk of chemicals to the soil ecosystem.