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.