When it comes to knowing the wilds around us, it’s often a shame we don’t live a little longer. Tack a few centuries onto our lives and give us strong minds able to memorize the minutiae, and we’d have plenty of changes to gape at. For instance, if we’d ambled beside a high sierra stream or lake in the summer of 1850 and then returned to the same place this year, we would surely wonder at the absence of certain mottled, hopping creatures with yellowy tints to their under-legs. About 150 years ago there would have been plenty of them perched on stones in the morning sun while droves of their young swam in the shallows, and none of us could have approached an alpine lake without sending scatters of wary frogs splashing back into the water.
The mountain yellow-legged frog is now facing extinction, but it was probably once the most abundant vertebrate in the Sierra Nevada. Vertebrates are creatures with backbones and such common critters as Douglas squirrels, Steller’s jays, chickadees, and homo sapiens all fit the description; but until the last century mountain yellow-legged frogs outnumbered us all.
Mature yellow-legged frogs are often two or three inches long. Their backs are speckled with lichen-colored spots and their underbellies range in color from creamy to bright yellow. As tadpoles they can become exceptionally large, sometimes growing to the length of adult frogs. They have a distinct call, but it’s hard to hear since they usually utter it underwater.
If you see one, count yourself lucky. Researchers estimate that mountain yellow-legged frogs have perished from more than 90 percent of their previous territory. Before so many started to die, the frogs could be found from 5,000 feet of elevation in the western foothills to as high as 12,000 feet on the west side of the Sierra crest. Their home waters stretched from the Kern River in the south to Mt. Lassen in the north, and other populations of the frogs were scattered through mountains in Southern California and Nevada. Today, the frogs have almost completely vanished from elevations below 9,000 feet and it’s uncommon to find them in even the most remote areas of their former sub-alpine and alpine habitat. Fewer than 100 adults remain in southern California.
The southern California populations have been put on the endangered species list, and it’s likely their northern relatives will be added soon; but the protections that come with such listings probably won’t be able to help the mountain
yellow-legged frog.
To explain the deaths of so many frogs, scientists originally pointed to the trout that people have been introducing to the Sierras since the 1850s. When the first European fishermen arrived in the mountains we now call home, they found the pickings slim; 99 percent of the lakes and ponds above 7,000 feet were fishless. The unfulfilled anglers and federal and state governments soon set about remedying the situation, and for the last century and a half the introduced trout have been devouring the frogs. Mountain yellow-legged frogs are particularly vulnerable because they spend an exceptional amount of their lives in the water and, like the introduced fish, can only live in basins that don’t completely freeze in the winter.
However, the trout aren’t the only culprit behind the decline of the frogs, they are just the most conspicuous. In 1997 a fungus called Batrachochytrium dendrobatidis – or ‘chytrid fungus’ for short – was found in a population of mountain yellow-legged frogs just outside of Sequoia National Park. For the past decade, scientists have watched the fungus steadily spread in an easterly direction through the amphibians of the Sierra Nevada. Yosemite toads, Western toads, and Pacific tree frogs all become ill from it, but chytrid is an aquatic fungus, and no sierra frogs spend more of their life in the water than the yellow-legged species. For them, so much exposure to the fungus is proving lethal.
There are few populations of mountain yellow-legged frogs in the Sierra Nevada that have not yet contracted the chytrid fungus.
That’s grim news; when chytrid does enter a population, the entire group generally dies off within three years. Before dying, the individual frogs often become lethargic and don’t move away when people walk up to them. They hold their legs in an odd, distended position and the underside of their thighs blush bright red. They become so weak that they can’t right themselves when they’re turned on their backs. Scientists aren’t exactly sure how the fungus eventually kills frogs, but they have noticed that chytrid lives in the skin of the amphibians who contract it. Since frogs breathe through their skin, researchers have surmised that they may be dying from suffocation.
Scientists don’t know where chytrid came from and haven’t figured out how to stop it, but they have found it on every continent other than Antarctica, and in each place the symptoms are the same. According to Roland Knapp, biologist at the Sierra Nevada Aquatic Research Laboratory, chytrid may be one of the sources of the widespread amphibian extinctions taking place all over the world. Currently, one third of the 6,000 known amphibian species have been described as threatened, endangered, or critically endangered.
As Knapp put it, ‘We’re in the middle of a gigantic extinction event the likes of which we’ve never seen.’ The mountain yellow-legged frogs of the Sierra Nevada are a thread in a much larger pattern.
Many thanks to Deborah Urich, east zone aquatist on the Tahoe National Forest; Gary Fellers, research biologist with the USGS; Vance Vredenburg, assistant professor of biology at UC Berkeley; and Roland Knapp for their help with this article.
Looking through the clear, new ice of autumn in search of mountain yellow-legged frogs? If you’re lucky you’ll see them slumbering ‘til the spring with bioboy@moonshineink.com.
