A dangerous combination for frogs Even in non-lethal doses nitrate can be fatal for amphibian populations

Even if they are not fatal on their own, various environmental factors can interact to contribute to global amphibian extinction. In a laboratory study, a research group at Technische Universität Braunschweig has shown that, in combination with temperature stress, nitrate impairs the adaptability of amphibians even in sublethal doses. The findings of the researchers of the Conservation Physiology and Ecotoxicology  group of the Zoological Institute have now been published in the Journal of Experimental Biology.

The group around biologist Dr. Katharina Ruthsatz of TU Braunschweig conducted several experiments focussing on a native amphibian species, the European common frog (Rana temporaria), which has experienced a severe population decline in recent years. They chose temperature stress and nitrate pollution as example stressors, as these stressors have a significant effect on amphibian waters in Germany, since the start of the amphibian’s breeding season coincides with the start of agricultural land cultivation. In the project funded by the German Research Foundation (DFG) they investigated how temperature and nitrate stress affect the larvae of the common frog.

Ability of larvae to adapt endangered

Ectothermic animals such as amphibians have the ability to adapt to higher temperatures by accelerating their development rate, enabling their larvae to leave the water faster and survive as small frogs. However, this adaptability is associated with high energetic costs. These are also associated with the exposure to environmental pollutants such as nitrate, as detoxification processes also require considerable energy. The results of the study show that additional nitrate reduces the adaptability of amphibian larvae. They are less flexible in their development rate, which can lead to them remaining longer in polluted waters. They are also less able to stabilise their energy consumption when temperatures rise and show reduced heat tolerance. These effects were not only observed at very high nitrate concentrations (100 mg/L), but also at moderate levels (50 mg/L).

“This is particularly worrying,” says Dr Ruthsatz.  “In view of increasing heatwaves in the context of climate change, this could have serious consequences. Of course, this has been a study under lab conditions so far, but we are currently working on researching these effects under natural conditions. EU monitoring already shows that that many surface waters contain more than 50 mg/l nitrate and that the pollution has not decreased in recent years. Our results emphasise the importance of avoiding nitrate inputs into surface waters – not only for amphibians, as similar effects can also be expected for other living organisms.”