Amphibian populations are declining worldwide at alarming rates. Among the large variety of contributing stressors, chemical pollutants like pesticides have been identified as a major factor for this decline. Besides direct effects on aquatic and terrestrial amphibian stages, sublethal effects like impairments in reproduction can affect a population. Therefore, we investigated the reproductive capacity of common toads (Bufo bufo) in the pesticide-intensive viticultural landscape of Palatinate in Southwest Germany along a pesticide gradient. In a semi-field study, we captured reproductively active common toad pairs of five breeding ponds with different pesticide contamination level and kept them in a net cage until spawning. Toads from more contaminated ponds showed an increased fecundity (more eggs) but decreased fertilization rates (fewer hatching tadpoles) as well as lower survival rates and reduced size in Gosner stage 25, suggesting that the higher exposed populations suffer from long-term reproductive impairments. In combination with acute toxicity effects, the detected sublethal effects, which are mostly not addressed in the ecological risk assessment of pesticides, pose a serious threat on amphibian populations in agricultural landscapes.

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Background Although debates about the assessment of potential effects of pesticides on amphibians are ongoing, amphibians are not yet considered in the current EU environmental risk assessment of pesticides. Instead, the risk assessment of potential effects on aquatic amphibian life stages relies on use of data of surrogate species like the standard temperate fish species rainbow trout (Oncorhynchus mykiss). This assumption is mainly based on the comparison to amphibian species not native to Europe such as the aquatic African clawed frog (Xenopus laevis). It remains unclear whether these surrogate species cover semi-aquatic Central European amphibian sensitivities. Therefore, we assessed the acute sensitivity of aquatic stages of eight European amphibian species native in Germany (Bufo bufo, Bufotes viridis, Epidalea calamita, Hyla arborea, Pelobates fuscus, Pelophylax sp., Rana dalmatina, R. temporaria) towards commercial formulations of the fungicide folpet (Folpan® 500 SC, Adama) and the insecticide indoxacarb (Avaunt® EC, Cheminova). The determined acute sensitivities (median lethal concentration, LC50) were included in species sensitivity distributions and compared to experimentally determined LC50 values of X. laevis and literature values of O. mykiss. Results The results showed that native amphibian sensitivities differed between the tested pesticides with a factor of 5 and 11. Depending on the pesticide, X. laevis was five and nine times more tolerant than the most sensitive native amphibian species. Comparing literature values of O. mykiss to the experimentally determined sensitivities of the native amphibian species showed that the O. mykiss sensitivity was in the same range as for the tested amphibians for the formulation Folpan® 500 SC. The comparison of sensitivities towards the formulation Avaunt® EC showed an eight times lower sensitivity of O. mykiss than the most sensitive amphibian species. Conclusions A risk assessment using the 96-h LC50 values for fish covers the risk for the assessed aquatic stages of European amphibians after the application of the recommended uncertainty factor of 100 and thus may be adequate for lower tier risk assessment of the studied pesticides. If aquatic amphibian testing will be required for pesticide risk assessment nevertheless, acute tests with the model organism X. laevis and the application of an appropriate uncertainty factor might be a promising approach.