Sunlight’s UV radiation not the amphibian killer

Declines in amphibian populations around the globe is not because of increasing ultraviolet radiation, scientists claim.

According to Wendy Palen, lead author and a Simon Fraser University ecologist and Daniel Schindler, UW professor of aquatic and fishery sciences, UV radiation is harmful in particular conditions.

Naturally occurring murky water and females who choose to lay their eggs in the shade keep embryos of one of the nation”s most UV-sensitive amphibian species out of harm”s way most of the time, new research shows.

Less than 2 percent of the embryos of the long-toed salamander received lethal doses of UV across 22 breeding sites across nearly 8 square miles (20 square kilometers) in Washington state”s Olympic National Park.

For a second amphibian, the Cascades frog – known to be among the least UV-sensitive Pacific Northwest species – the researchers found no instances where eggs received lethal doses.

Palen said: “These findings don’t contest hundreds of studies demonstrating the harmful effects of UV radiation for many organisms, including humans. Rather, it points out the need to understand where and when it is harmful.”

Papers published in the late 1990s and early 2000s raised the alarm that UV exposure was triggering amphibian declines, with many of the findings based on Pacific Northwest amphibians.

Palen says previous research wasn’t wrong: some species proved extremely sensitive to UV radiation – with especially high mortality for eggs and larvae – as shown in physiological studies done mostly in highly controlled laboratory experiments or at just one or two natural ponds or sites.

But conditions in labs or a few isolated sites are not what the animals typically encounter in the wild and they do not behave in labs as they do in their natural habitat, the new study of a large number of breeding sites, 22 altogether, revealed.

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“When simple tests of species physiology are interpreted outside of the animal’s natural environment, we often come to the wrong conclusions,” Palen said.

For one thing there are lots of “natural sunscreens” in the water. They are in the form of dissolved organic matter – remnants of leaves and other matter from wetlands and terrestrial areas that are dissolved in the water, much like tea dissolved in a mug of water. The more dissolved organic matter, the less UV exposure.

And places where the water is more crystal clear, the females from the susceptible salamander behaved differently.

Schindler said: “There hasn’t been a lot of work on whether organisms are capable of sensing UV intensity, but these salamanders certainly do. They change their behavior, with the females laying their eggs in the shade when the clarity of the water puts their eggs at risk.”

If for some reason UV radiation were to become much more intense, it could reach a point where amphibians can”t behave in ways that protect them, Palen says.

But the restrictions on the use of ozone-depleting chemicals, under what’s called the Montreal Protocol, appear to be helping restore the ozone layer, which filters the amount of UV radiation reaching Earth.

Palen added: “By critically evaluating what appear to be threats to ecosystems, we can refine our research and conservation priorities and move onto those that will make a difference in helping amphibians survive.”

The study area includes one of the richest amphibian habitats in northwest Washington’s Olympic National Park. The work was conducted in the Seven Lakes Basin of the Sol Duc drainage in subalpine terrain, that is, on mountain sides just at the point trees struggle to grow.

The study has been published in the Proceedings of the National Academy of Sciences May 25.

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