Kurzfassung

The proportion of deformed tadpoles was no higher in ponds on agricultural land than those in more natural wetlands, a recent study in Germany has found. The researchers were expecting greater deformities in farmland ponds because they assumed that pesticide concentrations would be higher. They did find that tadpoles in agricultural areas matured earlier and had smaller body sizes but it is not clear what impact this would have on the population as a whole.
Populations of frogs and other...The proportion of deformed tadpoles was no higher in ponds on agricultural land than those in more natural wetlands, a recent study in Germany has found. The researchers were expecting greater deformities in farmland ponds because they assumed that pesticide concentrations would be higher. They did find that tadpoles in agricultural areas matured earlier and had smaller body sizes but it is not clear what impact this would have on the population as a whole.
Populations of frogs and other amphibians are falling dramatically around the world. Loss of habitat and new fungal diseases are thought to be among the key causes. Pesticides have also been suggested as a factor in their decline, but a clear link between agrochemicals and amphibian survival is yet to be found.
To explore this possible link, the researchers studied common frogs (Rana temporaria) in an agricultural region of western Germany. They also investigated whether the health of tadpoles could be used as a cost-effective indicator of the ecological effects of pesticides more widely, which avoids the expensive process of measuring pesticide concentrations in water.
They collected a total of 3 367 tadpoles from 11 ponds over five months. Five of the sites were in intensively-farmed cropland; the other six were nearby (less than 1 km away), but in more natural wetland settings.
Earlier lab studies have shown that, if frog spawn is exposed to pesticides, hatched tadpoles are more likely to develop deformities. Because they assumed that agricultural ponds would have higher pesticide concentrations, the researchers expected to see more deformities in the farmland tadpoles, compared to the natural land tadpoles.
In total, 3.83% of the tadpoles were deformed, with misshapen tails or bodies. Of the tadpoles collected in farmland 2.83% were deformed. Surprisingly, 4.98% of natural land tadpoles were deformed. The higher deformity rates were strongly linked to higher levels of ammonium in the water and warmer temperatures. The agricultural setting, however, did not appear to increase deformity rates.
The researchers comment that this was a small-scale study of just 11 ponds in the same part of Germany and they were not able to measure actual pesticide levels . They also accept the possibility that true number of malformed tadpoles was higher, as these are more likely to be caught by predators.
Previous research has concluded that a deformation rate of over 5% in a frog or toad population is unnatural. However, the authors of this study say that more evidence is needed on a link between pesticides and amphibian survival to identify a better threshold or indicator. In response to this study, the Association of German Engineers has changed the threshold from 5% to 5-10% in their guidelines for monitoring pesticides’ effects on biodiversity in GM agriculture.
The researchers did observe that older tadpoles from the farmland developed into juvenile frogs earlier than those from the natural land, and they were smaller. This was as expected as it is known that pesticides can affect development in this way. However, it is not known if these changes have any effects on the extinction risk of the population.
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