September 18, 2012 by Louis Sahagu,
Physorg
To reach one of the last wild populations
of the mountain yellow-legged frog on Earth, Adam Backlin and Elizabeth
Gallegos tramped down a no-nonsense trail, scaled cliffs and barged through
nettles along a vein of water in a scowling canyon deep in the San Gabriel
Mountains.
Finally, the U.S. Geological Survey field
biologists reached the headwaters of the Mojave River, about 15 miles west of
Wrightwood. They forded pools and crawled through underbrush to net as many of
the endangered frogs as possible and methodically record their vital
statistics.
Recent efforts to save the frogs have
included restricting public access and ridding the water of predatory trout.
Now, the fist-sized amphibians are breeding in numbers not seen in decades.
When Backlin and Gallegos visited recently, several hundred adult frogs shared
the knee-deep pools with hundreds of wiggling tadpoles and dime-sized
babies.
"Whoa!" Backlin shouted, diving
head first into a tangle of branches shading a small patch of water boiling
with frogs. With one sweep of the net, he pulled out half a dozen. In less than
three hours, the biologists captured 71 adults of the species scientists know
as Rana muscosa.
Fifty-two had been tagged during previous
forays into the canyon. But 19 were new frogs. Two years ago, this 1.5-mile
stretch of spring water and ice melt was thought to hold about five. With skin
as permeable as a mop, the species is susceptible to a skin fungus linked to
amphibians vanishing around the world. And the fungus and its waterborne
zoospores have been detected in mountain yellow-legged frogs.
Batrachochytrium
dendrobatidis
is a chytrid fungus first identified in 1998. It causes a thickening of the
skin, which impairs gas exchange and the animal's ability to absorb water,
triggering rapid, mass die-offs.
"Exactly how they these frogs are
surviving the fungus is a mystery," Backlin said. "One theory is that
after the fungus first swept through the San Gabriels in the 1960s - wiping out
up to 90 percent of the entire frog population - survivors somehow developed an
immunity." Biologists will study possible defense mechanisms as they
search for a cure to the fungus.
The life and times of mountain
yellow-legged frogs embody the challenges facing species - and wildlife
biologists - in Southern California. For thousands of years, the frogs thrived
in almost all of the creeks cascading down the San Bernardino, San Gabriel and
San Jacinto mountains. Since the 1960s, however, the species has been decimated
by fires, mudslides, pesticides, fungal infections and loss of habitat, as well
as the appetites of garter snakes, raccoons and nonnative fish, bullfrogs and
crayfish.
Recent efforts by zoos in Los Angeles,
San Diego and Fresno to reintroduce captive-bred frogs into their ancestral
haunts have had limited success. For the time being, the creation of trout-free
zones in hard-to-reach streams is one of the most effective survival strategies.
For example, in a remote corner of the San Jacinto Mountains, the frogs are
starting to recolonize sections of Fuller Mill Creek where fish were removed by
the California Department of Fish and Game and the U.S. Forest Service. Usually
found on sunny banks and rocks near riffles, the 3-inch-long frogs are named
for the bright yellow extending from the undersides of the hind legs onto the
lower abdomen.
"Amphibians in general, and frogs in
particular, are resilient - if you give them a chance, they rebound," said
Sam Sweet, professor of ecology and evolution at UC Santa Barbara. "One
reason they produce huge numbers of eggs is that life is so uncertain for an
egg or tadpole. All those eggs compensate for years when creeks dry up, or
predators and disease move in and wipe them all out."
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