Comparing ecotoxicological effect concentrations of chemicals established in multi-species vs. single-species toxicity test systems

Most ecological effect assessment methodologies use effect concentrations derived from single-species testing (ECx,single-species-test) as the basis to estimate ‘safe’ environmental concentrations (such as environmental quality criteria). Here, we examined to what extent such ECx,single-species-test are representative for population-level effect concentrations in a community setting (ECx,multi-species-test). Data from USEPA's ECOTOX database revealed the existence of considerable scatter around the relationship between ECx,single-species-test (endpoint: mortality) and ECx,multi-species-test (endpoint: population abundance). However, we demonstrate that this scatter is reduced when ECx,single-species-test and ECx,multi-species-test are determined simultaneously and by the same research group. Indeed, if these conditions are fulfilled, the quotient of both ECx values for invertebrates approaches 1 for chemicals that directly target invertebrates. Unfortunately, comparable data for other classes of chemicals and/or taxonomic groups were not found. However, theoretical ecosystem model simulations, which confirmed the results based on the above-mentioned analysis of the ECOTOX database, indicated that for phytoplankton, EC10,single-species-test>EC10,multi-species-test, for chemicals that directly target invertebrates. For chemicals that directly target phytoplankton, the ecosystem model simulations suggest that ECx,single-species-test>ECx,multi-species-test for both phytoplankton and invertebrates. Hence, our observation based on the analysis of existing experimental data that the ECx,single-species-test is similar to the ECx,multi-species-test may be biased by the fact that only data were available for invertebrates and for chemicals targeting invertebrates. Experimental research is required to test the predictions made by the model simulations for phytoplankton as well as for chemicals directly targeting phytoplankton.