Hierarchical Responses of Soil Invertebrates (Earthworms) to Toxic Metal Stress

Abstract

The concept of a hierarchical cascade of biological responses to stress occurring across different levels of biological organization is an underlying principle of both theoretical and regulatory ecology/ecotoxicology. This study investigates the reality of this cascade for earthworms exposed to toxic metal stress. Gene expression was the most sensitive endpoint (EC50 = 616 microg Zn g(-1)) followed by the integrity of coelomocyte lysosomal membrane (EC50 = 645 microg Zn g(-1)). This confirms that, in accordance with the cascade concept, suborganism level endpoints respond at lower metal concentrations than higher organization endpoints. The relative sensitivity of the higher organization parameters was not as predicted by the cascade. Organic material removal was more sensitive (EC50 = 997 microg Zn g(-1)) than L. rubellus reproduction (EC50 = 3236 microg Zn g(-1)), L. rubellus population size (EC50 = 5000-11500 microg Zn g(-1)), and earthworm community diversity (EC50 = 1737 microg Zn g(-1)). This can be attributed to (1) the relative insensitivity of L. rubellus to metals and (2) general toxic effects of metals on the earthworm energy budget (and thus feeding). On the basis of these results, it can be concluded that predictive assessments of the consequences of environmental stressors needs to include approaches that respect the relative sensitivities of different taxa, while retrospective appraisals should exploit the sensitivity of low organization level responses.