Synergistic, antagonistic and additive effects of multiple stressors: predation threat, parasitism and pesticide exposure in Daphnia magna

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Summary: Predation and parasitism are important factors in the ecology and evolution of natural populations and may, along with other environmental factors, interact with the impact of anthropogenic pollutants. Our study aimed at identifying potential interactions between three stressors (predation threat, parasitism and pesticide exposure) and at exploring the predictability of their joint effects by using the model of independent action. We assessed in a full‐factorial design the impacts of these stressors on key life‐history traits and population growth rate of the water flea Daphnia magna. When applied as single stressors, predation threat and parasite challenge induced varying stressor‐specific adaptive responses. The pesticide carbaryl was applied at a generally sublethal concentration, which caused low mortality only in first‐brood offspring. Pesticide exposure interacted synergistically with parasite challenge regarding survival, which suggests immunomodulatory activity of the pesticide. Predation threat by phantom midge larvae showed antagonistic interactions for amount of first‐brood offspring with both parasite challenge and carbaryl exposure. All stressors additively affected age and size at maturity, which added up to a considerable delay in the onset of reproduction in the three‐stressor combination. The intrinsic rate of natural increase, r, reflected the non‐additive and additive effects on single endpoints and showed significant synergistic interactions for all two‐stressor combinations. The combination of all stressors resulted in a dramatic reduction of r compared to the stressor‐free control. The model of independent action proved useful in quantitatively predicting effects of additively acting stressors, and in visualizing the occurrence and magnitude of non‐additive effects in accordance with results of analysis of variances. Synthesis and applications. Cumulative additive effects and non‐additive interactions of natural antagonists and pollutants are shown to result in considerable impacts on ecologically relevant parameters. As a starting point for an environmentally more realistic risk assessment of chemicals, it may be a valuable strategy to screen for non‐additive effects among many stress factors simultaneously in simplified experimental designs by using the model of independent action.

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