Wildlife biologist Harold Werner and I were walking the river's edge when he knelt by a small pool.
"Tadpoles," he pointed out. My heart skipped a beat. "Pseudacris regilla," he said. "Oh," I said, disappointed.
He had seen tadpoles of the Pacific tree frog, but we had been walking and talking about a different, missing, amphibian. Once common, the Foothills yellow-legged frog (Rana boylei) has not been seen in Sequoia and Kings Canyon National Parks for over 20 years. They are not the only ones in trouble. Recent studies show that numbers of the Mountain yellow-legged frog (R. muscosa), a close relative, are rapidly declining; it may soon suffer the same fate as its lower-altitude cousin.
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| Foothills Yellow-legged Frog |
Harold Werner works hard at monitoring and protecting the wildlife at Sequoia and Kings Canyon. "I consider the loss of species, particularly amphibians, the most serious problem we face," he says.
This problem may be a particularly tough one to address. Harold explained to me that amphibians are disappearing worldwide. It is a strange and disturbing fate for the vertebrates that first colonized land over 300 million years ago, and saw the coming and going of the dinosaurs.
It is not as if frogs lack experience in surviving. They live in a wide variety of environments all over the world, and their metamorphosis from tadpole to adult provides an advantage in the age-old race for survival. The aquatic, gill-breathing tadpoles are bottom feeders, while the lung-breathing, amphibious (water-and-land dwelling) adults feed on insects. Thus parent and offspring don't compete for the same food source.
In today's world, however, being an amphibian may have disadvantages. Their eggs have no protective shell, and their thin, moist skin is permeable; water - and other chemicals - are easily absorbed. It may be that these traits make them more vulnerable to dangers such as pesticides and ultraviolet light, horrors harder to avoid than the stomping of a Tyrannosaurus rex.
What is happening to amphibians? Dr. Gary Fellers wants to know. A former Park Service biologist recently transferred to the National Biological Survey, he has started a three-year study on the amphibians of the Sierra Nevada. Last summer a team searched many areas where frogs were once abundant. "We found no Foothills yellow-legged frogs south of Yosemite," he says.
This is of great concern. How can entire populations disappear, especially from protected areas such as parks? In not so many years past, employees at park headquarters in Ash Mountain could find Foothills yellow-legged frogs in a five-minute lunchtime walk. There are still some Mountain yellow-legged frogs in Sequoia and Kings Canyon, but they are now gone from half the sites where they were found three decades ago.
For the mountain populations, there is no question what part of their problem is. The lakes in the Sierra Nevada were dug out by glaciers about 13,000 years ago. They were fishless until the arrival of Europeans, who planted trout for sport. Trout eat the tadpoles of the yellow-legged frog. In most lakes today, where there are fish there are no frogs.
But now frogs are even disappearing from locations where there are no fish. Part of the problem may be that frogs have been isolated into smaller populations which are more susceptible to extinction, and lakes that have lost frog populations are less likely to be recolonized. While these factors may affect the Mountain yellow-legged frog, they do not explain the disappearance of the Foothills variety or the disappearance of amphibians worldwide.
Scientists are looking at many possible causes. Recent research in the Pacific Northwest suggests that increased ultraviolet light may be one factor. Chemicals released into the air have destroyed some of the protective ozone layer high in the atmosphere that used to shield earth from high levels of the sun's ultraviolet radiation.
Scientists are also looking at the effects of multiple pesticides and herbicides, global climate change, habitat destruction, and other factors.
The Tablelands in Sequoia National Park, a lake-studded expanse of rolling granite highcountry above Lodgepole, used to have thousands of Mountain yellow-legged frogs; now there are none. As an experiment, Gary Fellers would like to bring them back. He would reintroduce eggs, tadpoles, and subadults, then monitor the frogs as they develop, trying to identify the factors causing their deaths.
We should hope that he and other scientists can identify the culprit, so that something can be done before it's too late. We would miss the cheerful and varied voices of frogs. These fascinating creatures also have been valuable in scientific research, showing us how hormones act in humans and how fertilized eggs develop. And frogs eat insects - lots of insects.
Perhaps most importantly, the factors affecting them may be harming other species - including us. Scientists believe that amphibians were the first vertebrates to move from the water to the brave new world of land. It would be ironic if they are now showing the way in a less hopeful direction.
