The Sierra Nevada’s disappearing frogs
Dr. Roland Knapp, at work in the field … (Photo: Anders Halverson)
By Katie Vane
Doctor Roland Knapp, a research biologist at the Sierra Nevada Aquatic Research Lab (SNARL), opened Tuesday night’s Green Church screening of the 2009 PBS Documentary “Frogs: The Thin Green Line” on a humorous note. “Thanks,” he said, “for putting frogs above a Lakers game.” The film that followed was a moving reminder of how much is at stake in the loss of these tiny creatures, these “super survivors” now disappearing in huge numbers across the globe.
The film focuses on frog populations in Australia and Panama, including the striking golden frog, and on the mountain yellow-legged frog, which is indigenous to the Sierra Nevada. Once found in abundance, the mountain yellow-legged frog has now disappeared from 92% of its native grounds, leaving behind what Knapp describes as an “eerie silence.”
These frogs, the southern mountain and Sierra Nevada yellow-legged frog, have lived in the mountains of California and Nevada for millions of years. They have survived the advance and retreat of glaciers, adapting to thrive in particularly cold, harsh climates of elevations between 4,500 and 12,000 feet. Yet in just 70 short years, human impact has driven mountain yellow-legged frogs to the brink of extinction.
Knapp attributes the frog population’s decline primarily to the introduction of non-native trout to the high Sierra lakes by the California Department of Fish and Game. These trout have become a predator the frogs have few defenses against. A fish removal experiment in the 1990s brought frog populations back to previous density. Shortly thereafter, however, these frogs suffered another, more insidious blow.
They fell prey to the amphibian disease chytridiomycosis, or chytrid, which has decimated amphibian populations around the world. Scientists have only just begun to crack the origins of the disease, which is transmitted via a fungus, Batrachochytrium dendrobatidis, or Bd, that attacks frog skin, depriving it of oxygen. Researchers such as Dr. Knapp are working to figure out what created the disease, and how it spreads.
“Frogs: The Thin Green Line” offers a sobering image as a reminder of the consequences of our inability to fully understand or combat chytrid: mountain yellow-legged frogs, limbs spread, cream-colored bellies up, floating dead in mountain pools.
What then can we do to stop this plague on frogs? Dr. Knapp said he thinks that “the solution is evolution.” He believes scientists must now focus on a way to keep frogs and fungus together until frogs evolve a more balanced relationship with the disease. Since 2006 he has been conducting an experiment that transported small populations of frogs that had persisted in spite of chytrid to five locations where other frogs had become extinct.
One in five of these transported populations has survived in its new location and is doing well, in spite of the disease. Now Dr. Knapp and his colleagues just need to figure out why that single population has survived.
But even frog populations not decimated by the introduction of non-native predators, or chytrid, must survive other modern threats like pesticides and increasing UV radiation, a result of global warming. The frog’s permeable skin, through which it drinks and breathes, makes it particularly susceptible to harmful changes in environment. In this way, frogs act like canaries in the coal mine; their death tells us that we should beware. And, because frogs also uniquely occupy multiple levels of the food web — as eggs, tadpoles, and finally mature frogs — their loss will have multiple impacts on any ecosystem they occupy.
At a time when horrific images of the BP oil spill flood the media, this quietly elegiac PBS documentary offers a reminder of how many amphibian populations are disappearing unnoticed every day. “Frogs: The Thin Green Line” is a reminder that it’s time to listen to these special creatures, before all we’re left to listen to is silence.
