Conserving biodiversity (B) and ecosystem functioning (EF) is one of today’s key challenges. Meeting this challenge is only possible if the effects of environmental stressors such as anthropogenic chemicals on B and EF are well understood. To this end, I propose to test if ecological theory on coexistence can be used as an elegant approach to correctly predict the response of B and EF to chemicals. Based on this theory and associated models, I present hypotheses that I now propose to test in microcosm experiments containing freshwater phytoplankton. Testing these hypotheses will (1) critically enhance understanding of how species interactions determines chemical effects on B and EF; and (2) enable testing to what extent chemicals may directly impact on EF without affecting B, a topic less
well covered by contemporary research. Lastly, the influence of initial richness on the validity of the tested hypotheses will be evaluated, recognizing the buffering effect B may have on community and ecosystem robustness. In conclusion, I propose a project that will significantly contribute to the emerging field of community and ecosystem ecotoxicology and allow revisiting classic questions in ecology.