Tadpoles exposed to higher levels of ultraviolet radiation are easier for predators to catch. Credit: A.N.T. Photo Library/NHPA

A small increase in exposure to ultraviolet light from the sun — similar to that caused by thinning of the ozone layer that guards Earth from radiation — can have a detrimental effect on tadpoles, according to research published today in the Proceedings of the Royal Society B 1.

The work shows that medium-wavelength ultraviolet radiation, known as ultraviolet-B (UVB), makes tadpoles easier for predators to catch, suggesting that depletion of the ozone layer caused by humans releasing chemicals into the atmosphere could be contributing to declines in the populations of the world's amphibian species.

Amphibians around the world have been in rapid decline since the late 1970s, with nearly 2,000 of more than 6,400 species listed as threatened with extinction, says the paper. A a range of potential causes from environmental contamination to a 'chytrid fungus' (Batrachochytrium dendrobatidis) are thought to be responsible, and ozone depletion has also been suspected as it fits with the timing.

Previous research has shown that UVB can harm amphibians2, but in recent years the link between decreasing populations and global increases in UVB exposure owing to ozone depletion has been disputed, says Lesley Alton, a conservation physiologist at the University of Queensland in Brisbane, Australia, who led the study.

"But what we really show here is that it has the potential to be relevant and we should not disregard it," she says. "If amphibians are being exposed to more UVB in their natural environment, it is certainly something that could be contributing to their population decline."

Working in the laboratory, Alton's team exposed striped marsh frog (Limnodynastes peronii) tadpoles to a UVB treatment. One group received a low dose, and another a dose that contained roughly 5% more UVB — a difference comparable to the increase in UVB exposure in the wild that has resulted from ozone depletion. The researchers then released the tadpoles into aquaria with a natural predator, the freshwater shrimp Macrobrachium australiense, and timed how long it took them to be caught.

Slow food movement

Survival times were cut by around a quarter in the tadpoles exposed to the higher UVB dose. The pattern was observed in high-dose tadpoles that had been conditioned to the presence of their predators, as well as in those that had received no clues.

"Perhaps the reason these tadpoles were more susceptible to the predators is because they got tired faster," says Alton. "We know UVB damages DNA and tadpoles do possess the ability to repair that damage. But it requires an energetic investment."

The study notes that reduced survival time in a predator environment was the only effect observed from the increased UVB exposure. UVB stress might be expected to cause changes in hatching time of embryos, hatching success, size, shape and 'burst swimming performance' (ability to sprint) for tadpoles, but these responses were not observed. The study suggests that it may be time to expand the typical suite of measured UVB responses.

Andrew Blaustein, who researches declining amphibian populations and UVB in frogs at Oregon State University in Corvallis, praises the "rigorous experimental approach" of the study and says that the results are "clear-cut". "This result is important and can have profound implications for amphibians in general. Obviously, avoiding predators is important, and if exposure to UVB influences that behaviour then amphibians may get eaten more readily," he says.

But Clive Cummins, a consultant on amphibian conservation and a former researcher at the Huntingdon, UK campus of the Centre for Ecology and Hydrology, says that he "remains to be convinced" by the study. The experiments were performed in "wholly artificial circumstances" in the laboratory, he says, and accurately measuring UVB exposure is an issue that "bedevils the whole field". "There may well be circumstances where UVB does have an effect but the effect observed in the study could not be explained and might be an anomaly," he says.

Ross Alford, a frog biologist from James Cook University in Townsville, Australia, says that the results increase laboratory knowledge of predation in at least one species, but he urges caution in taking it too far. "Every time someone finds something that can increase the mortality rates of frogs under certain circumstances, there is a tendency to say that here may be a cause of amphibian declines. It is possible, certainly, but there are probably hundreds of as-yet-undocumented sources of mortality in frogs," he says.

Alton is confident that the amount of UVB used in the lab was accurately quantified, and she would like to replicate the experiment in the wild. She agrees that care should be taken in extrapolating the results to other species, but says that the work "certainly suggests" that there is the potential for it.