The Cascade Mountains of Oregon may look like a pristine wilderness area, but frogs are dying off rapidly here as well, and human researchers at Oregon State university think they know why: UV radiation.
Ultraviolet radiation is particularly harmful to living organisms because it can damage DNA. Basically, the energy from UV rays can break the bonds that hold the molecules of DNA together, and the “loose ends” that result can join up again in strange configurations. Since DNA codes for the proteins that allow a cell to function, these mutations in the DNA can screw up the internal workings of cells and even kill them. In humans, UV radiation can cause cataracts and contributes to skin cancer. But cells are not completely at the mercy of the cruel sunlight: many organisms have mechanisms that can fix the damage that this radiation does. For example, some organisms--including amphibians--make an enzyme called photolyase that removes these damaged bits from the DNA before they can cause serious harm to the organism.
But not all organisms are created equal (at least in levels of photolyase): some species make more of this enzyme than others, and some amphibian species’ eggs have more than others. In particular, when researchers measured the levels of photolyase in Oregon frog eggs, they found that the species that were disappearing the fastest generally had the lowest levels of the enzyme in their eggs, while the Pacific treefrog, which is not declining, had the highest levels.
To determine if these two facts were related, the researchers gathered freshly-laid eggs from Cascade treefrogs, western toads, Pacific treefrogs, and northwestern salamanders. They put the eggs into three different types of boxes: one that was completely open to the sun, one with a clear Mylar cover that stopped the UV radiation from coming through, and one with a clear acetate cover that just covered the eggs and didn’t protect them from the radiation, as a control box. Then they returned the boxes to several different sites where the frogs and salamanders would normally lay their eggs and let the eggs develop in the boxes. The results were dramatic: more than 90% of the salamander eggs that were exposed to the UV radiation died, but only 45% of the protected ones did. More than 40% of the Cascade treefrog and western toad eggs that were exposed died, while only 10-20% of the shielded ones died. The Pacific treefrog eggs, who had the highest levels of photolyase, had the same survival rate in all three boxes.
Obviously, UV radiation is contributing to the rapid reduction of frog populations. But why all of the sudden? If Cascade treefrogs and western toads were more vulnerable to sunlight than the Pacific treefrog, why didn’t they just all die off or evolve all those millennia ago when they first appeared? Because there simply wasn’t as much radiation back then as there is now.
The Earth is surrounded by a layer of atmospheric gas called the ozone layer that absorbs the most of the UV rays before they get to the surface of the planet, much like the Mylar covers on the frog boxes, protecting us all from the worst effects of this radiation. But over the past several decades, human-made chemicals have been chewing holes in the ozone layer, so more and more UV radiation can get through. As a result, levels of photolyase that once were enough to protect the Cascade treefrog eggs are now too low. And unfortunately, there isn’t enough sunscreen in the world to fix that.