(originally commissioned for the Living Alongside Wildlife blog)
More than an experiment
In the 1990s, researchers embarked on an ambitious plan to restore Mountain Yellow-legged Frogs to California’s highest lakes.
The Mountain Yellow-legged Frog (now split into two species) was once the most numerous vertebrate in the Sierra Nevada Mountains. The steep streams that pour off the Sierra Nevada’s slopes are a barrier to fish movement, so frogs thrived in the empty niche. Early surveyors said the lakeshores were so thick with frogs that it was impossible to avoid stepping on them as you walked.
But sometime around the 1960s their populations began to crash. Basins which historically had been full of the frogs now held few, often none at all. Soon they were gone from 95% of their former range.
Theories proliferated as to what was behind the decline. A few researchers suggested that increased UV radiation from the weakened ozone layer could be the issue, while others found a correlation between struggling frog populations and vehicular pollution. Some hypothesized that a novel disease had hit the frogs. Pesticide exposure may have been a factor, as the worst-hit populations were downwind of the intensively farmed Central Valley.
By the nineties the bulk of researchers had coalesced around one primary theory: introduced trout were eating the frogs to extinction.
Surveying a deserted landscape
Flash forward to May 2018. I had spent ten days surveying the deserts of eastern California and was ready to move on to the forests that form the backbone of the state. With my father I traveled through the San Bernardino Mountains, the San Gabriels, the Tehachapis, Breckenridge, and up into the southern Sierras.
Frogs were in short supply. On most hikes I was lucky to encounter a single chorus frog or toad. Only once did I find a Southern Mountain Yellow-legged Frog. As I described in my first article, it was tucked away in a remote high-elevation stream in the San Gabriel Mountains, part of a population down to its last dozen or so frogs. Nowhere else did I even cross the path of a ranid (true frog).
Before the trip I had known this would be a likely result. Southern Mountain Yellow-legged Frogs are basically gone from these elevations. Despite low expectations, I still went through something of a mourning process as we hiked pristine streams and gorgeous meadows yet found nary a frog jumping into the water. My father and I had to content ourselves with several lizard species, salamanders in the wettest spots, and the occasional Mountain Garter Snake.
When White fishermen first arrived in California, they met an entirely different disappointment – the fishless lakes and streams of the Sierra Nevada. Their response was to introduce trout to the waters. These rainbow and brook trout survived by competing with frogs for food and even eating the tadpoles themselves. At first fish were only introduced to the most accessible lakes, but in the 1950s the Department of Fish and Wildlife began employing helicopters to plant trout in remote waterways. As sport fish thrived, frogs disappeared. By the end of the twentieth century, 90% of Mountain Yellow-legged frog habitat was inundated with predatory trout.
One step forward, two steps back
Dr. Vance Vredenburg was one of many biologists who saw the dots connect between trout and the frog declines. As the habitat was still intact, he suggested that removing trout could allow frogs to recover. In 1996 he chose the 10,000-foot-high Sixty Lakes Basin as a place to test the theory. Though Southern Mountain Yellow-legged Frogs had been extirpated from many trout-filled lakes in the basin, populations remained in the ponds where fish were absent.
Dr. Vredenburg’s research team used gill nets to extract fish from five more of the basin’s lakes. Once the fish were gone, the frogs returned. Even lakes that still contained trout began to harbor frogs so long as they were adjacent to fishless lakes. Similar studies in other basins confirmed the theory. Conservationists excited by the results began to remove fish from high-elevation lakes across the Sierras.
Sixty Lakes Basin became one of the most vibrant Mountain Yellow-legged Frog metapopulations in the Sierras.
That is, until August 2004, when a frog-killing fungal disease called chytridiomycosis (chytrid) entered the basin’s waters and spread from one lake to another. Dr. Vredenburg watched disaster unfold in front of him. “When I witnessed the die-off of mountain yellow-legged frogs from this chytrid fungus, I saw literally tens of thousands of dead frogs littering the shorelines of these beautiful pristine lakes.” As published in “Dynamics of an emerging disease drive large-scale amphibian population extinctions”, the fungus eliminated 98% of the yellow-legged frogs in Sixty Lakes Basin. Twenty-seven of its 33 populations were wiped out in the first five years alone. Sister projects in Milestone Basin and Barrett Lakes Basin suffered the same fate.
This story repeated itself across the Sierras, and testing suggests that chytrid had been responsible for previous Mountain Yellow-legged Frog declines. In fact, the disease has become an existential threat to frogs across the globe, famously causing the extinction of Costa Rica’s Golden Toad and dozens of other species.
Scientists believe that chytrid originated with a non-lethal strain of the fungus native to the Korean peninsula. That strain can exist in local frogs without killing them, but around 100 years ago it mutated into deadly forms that were then spread globally by international trade and the Korean War. Infected amphibians from pet markets now drive its global spread, while wading birds, fishermen, hikers, and even researchers may accidentally move the disease locally to one new waterway after another.
I began with a narrative of trout, and then flipped to fungus. Which one drove the Mountain Yellow-legged Frogs to near-extinction?
It is likely that both are responsible. Trout decimate frog populations, and so does chytrid. The combination is nearly insurmountable. Air pollution, pesticide exposure, and climate change may also weaken the frogs’ resilience. And when development and high-impact recreation have eliminated many connecting populations, the isolated survivors in-between are that much more susceptible to extinction events.
The two Mountain Yellow-legged Frog species, Sierra Nevada Yellow-legged Frog (Rana sierrae) and Southern Mountain Yellow-legged Frog (Rana muscosa), are not the only Sierran frogs to suffer. The Yosemite Toad (Anaxyrus canorus) has also been listed as an endangered species. Strangely, it is not as susceptible to trout predation nor chytrid infection, so scientists continue to investigate the reasons for its decline. The Foothill Yellow-legged Frog (Rana boylii) has nearly disappeared from the Sierras, only hanging on in a few places in the lower elevations. It was brought down by a mix of dams, habitat fragmentation, and introduced predators. And the California Red-legged Frog (Rana draytonii) has been reduced to a dozen tiny populations spread out here and there in the northern foothills.
Ways forward in a beautiful land
In the final week we made it up to the northern Sierras. The streams, lakes, waterfalls, meadows, trees, cliffs, everything were gorgeous beyond compare. Yet just as in the southern Sierras, the frogs were gone. Even when I hiked through perfect-looking habitat, such as one beautiful flooded meadow bisected by a stream, I was distressed to see huge trout dashing through the waters. Sierra Nevada Yellow-legged Frogs and Yosemite Toads still have viable populations at 9,000 feet and above, but those lakes were snowed over. Across three days in the northern Sierras, the Sierran Chorus Frog was the only frog species I saw.
My poor luck does not tell the whole story. Scientists have noted that some frogs are developing a degree of resistance to the chytrid fungus. And highly complex mid-elevation meadow systems, those with a combination of streams, ponds, flooded meadow and forest habitat, may result in frog populations more resilient to infection than the isolated high-elevation lakes.
Unfortunately, recreational impact and trout introductions have left healthy mid-elevation meadow systems in short supply. But a combination of meadow restoration, fish removal, and the development of fungal resistance may one day allow prime habitat to become viable across the Sierra Nevada. In Yosemite Park there has already been a documented increase in mountain yellow-legged frog populations, leading Roland Knapp of UCSB to affirm that, “given sufficient time and the availability of intact habitat, the frogs can recover despite the human-caused challenges they face.”
Given the availability of intact habitat.
It is striking that wildlife suffer from human impact even in one of California’s most pristine ecosystems. But while remoteness does not protect the frogs completely, it does allow a pathway to hope. Through all they’ve endured, the habitat is still there, and as long as the habitat is still there the frogs can come back. Reintroductions are in process with several species and have already shown some success. These waterways may one day be filled with frogs again.
Frog diversity at the northern borders
Our time in the Sierras was disappointing, but luck shifted as we made our way north to Shasta. Shasta Lake is a manmade reservoir created in 1945 by the construction of Shasta Dam, which flooded nearly 30,000 acres of Winnemem Wintu homeland and prime wildlife habitat. Thankfully, the unique rock systems of the area support so many rivers and creeks in close proximity that even with the dam, amphibian diversity remains robust.
Arriving at the meandering roads that track the lake’s arms, my father and I soon encountered a Boreal Toad (Bufo boreas boreas) under a rock. Just a couple feet away we lifted another rock to find a Striped Racer (Masticophis lateralis). Nice start!
We stopped the car at the first likely-looking stream and started hiking. A splash around the bend alerted us to a frog’s presence. Ears pricked, I followed the sound to a secluded pool where a Foothill Yellow-legged Frog floated in the waters. After nearly a week without seeing a ranid, the sighting brought significant joy.
As we made our way from stream to stream, additional searches revealed Shasta Black Salamander (Aneides iecanus), Oregon Ensatina (Ensatina eschscholtzii oregonensis) and Coastal Giant Salamander (Dicamptodon tenebrosus) in good numbers in the sheltered canyons. Forest Alligator Lizard (Elgaria multicarinata multicarinata) and Northwestern Fence Lizard (Sceloporus occidentalis occidentalis) roamed the sunny spots on the side of the road.
Shasta Lake’s limestone outcrops are also home to three species of Shasta Salamander (Hydromantes sp.), ancient cave-lovers found nowhere else on Earth. Shasta Salamanders are at the center of an effort to prevent a planned $1.4 billion expansion of the Shasta Dam, a move that would destroy 3,000 more acres of their habitat and flood much of the Winnemem Wintu’s remaining historic lands.
At night I hiked out of the campground and found Sierran Chorus Frogs calling in numbers from the lake shore. Millipedes meandered across the forest floor and toads hunted on the roads. The quiet darkness rewarded me with a Ringtail sniffing around an abandoned campsite, the first I’d ever encountered in the wild and my most exciting mammal sighting of the trip. Later I spotted it again, this time making its way through the forest understory. The disappointments of the Sierras had disappeared.
Which is not to say that the Shasta system does not have its own concerns. While Foothill Yellow-legged Frogs are more common in Shasta County, the dams have still cut into their numbers. And California’s worst-ever fire season burned hundreds of thousands of acres just months after I visited. The Carr Fire devastated Whiskeytown Lake, where I had previously found Foothill Yellow-legged Frog and 14 other herp species. Reptiles and amphibians can adapt to normal burns, but the size and intensity of modern fires eradicates entire populations. And because roads, dams, and development leave areas isolated, sometimes there is nothing left to recolonize the spots that were lost.
One particularly sensitive local resident is the Coast Tailed Frog (Ascaphus truei). Uniquely adapted to clear, cold, fast-flowing waters in old-growth forest, it can’t tolerate logging, road construction, or fires, all of which fill its habitat with sediment. Canopy loss is also a problem for the frogs when the reduction in tree cover leads to an increase in water temperature.
Some distance north of Shasta my father and I embarked on a five-mile hike with the requisite elements – old unlogged forest and fast streams. Underneath a waterfall at the turnaround point I uncovered two Coast Tailed Frogs side-by-side in a rocky pool.
California’s northernmost counties are also home to the Cascades Frog, which fills the same montane niche that the Sierra Nevada Yellow-legged Frog holds a bit to the south. A unique population of Cascades Frog is going extinct in the Lassen region of northeastern California, and no one is certain why. Drought, the introduction of trout, pesticide exposure, and disease all may contribute to the problem. The other California population, residing in remote regions of the Trinity Alps and Siskiyou Mountains, is more robust but also in trouble. Cascades Frogs are doing better to the north in Oregon and Washington’s Cascade Mountains, though declines have been noted there as well.
Only a few miles from the tailed frogs’ waterfall, we encountered a lone Cascades Frog at the edge of a woodland pond.
The last stragglers
The Northern Leopard Frog (Lithobates pipens) once lived along California’s eastern border, but is now extirpated from the state. The grassy meadows it frequented were prime targets for ranchers. Their livestock gnawed down the grassland, leading to a drying out of the soil that the leopard frogs couldn’t survive. Development, pesticides, pollution, and the introduction of bullfrogs also played a role.
At the beginning of our trip my friend Matt and I had crossed into western Nevada, where two populations of the species remain. We visited a site where leopard frogs eek out an existence on recovered ranchland. Unfortunately, even that population is inundated with predatory bullfrogs. A preserve manager described various control methods to us, but the bullfrogs appear resilient to extermination.
At least 20-30 frogs must have jumped from the banks as we walked. Only one or two of the jumpers were leopard frogs, the rest invasive bullfrogs that threaten their existence in Nevada just as they helped wipe them out in California. Naturalists associated with the preserve are attempting to relocate some frogs to new sites where bullfrogs are absent.
Conservationists press on. The Washington Department of Fish and Wildlife has undertaken a Northern Leopard Frog Reintroduction Recovery Project in Washington state. Through habitat enhancement, head-starting, and bullfrog reduction, they hope to bring a robustness to their leopard frog populations so that they avoid the fate of California and Nevada’s frogs.
Such efforts can only succeed where habitat survives. That is the ultimate issue. When habitat persists, frogs may one day recover. When habitat is bulldozed, planted over, dammed, drained, or built upon…what hope is left?
Thomson, Robert C.
California Amphibian and Reptile Species of Special Concern.
University of California Press. Kindle Edition.
Nafis, Gary. California Herps: A guide to the reptiles and amphibians of California. http://www.californiaherps.com/
“Pesticides and amphibian population declines in California, USA”, paper published in Environmental Toxicology and Chemistry by DW Sparling
“Effects of nonnative fish and habitat characteristics on lentic herpetofauna in Yosemite National Park, USA”, paper published in Biological Conservation by Roland Knapp
“Reversing introduced species effects: Experimental removal of introduced fish leads to rapid recovery of a declining frog”, paper published in Proceedings of the National Academy of the Sciences by Vance Vredenburg
“Removal of nonnative fish results in population expansion of a declining amphibian (mountain yellow-legged frog, Rana muscosa)”, paper published in Biological Conservation by Knapp, Boliano, and Vredenburg
“Dynamics of an emerging disease drive large-scale amphibian population extinctions”, paper published in Proceedings of the National Academy of the Sciences by Vredenburg, Knapp, Tunstall and Briggs.
“Recent Asian origin of chytrid fungi causing global amphibian declines”, paper published in Science by Simon O’Hanlon et al.
“Ground Zero of Amphibian ‘Apocalypse’ Finally Found”, article published in National Geographic by Michael Greshko
“Large-scale recovery of an endangered amphibian despite ongoing exposure to multiple stressors”, paper published in Proceedings of the National Academy of the Sciences by Knapp et al.
“Opportunities and Constraints of Sierra Nevada Meadow Restoration”, a University of San Franciso Master’s Thesis by Allie Sennett
“Preliminary Restoration of Mountain Yellow-legged Frogs Environmental Assessment”, report for the Department of the Interior
“Disease and climate effects on individuals drive post‐reintroduction population dynamics of an endangered amphibian”, paper published in Disease Ecology by Maxwell Joseph and Roland Knapp
“Tiny salamanders could stand in the way of massive dam raising project”, article published in the Redding Record Searchlight by Damon Arthur
I want to thank Vance Vredenburg for the use of a figure and photo from his research. Quotes from Vredenburg and Knapp were taken from “Scientists try to save this frog species from being wiped out by fungus”, an article by Public Radio International.
Much thanks again to Shanti Mathias for editing the piece and making suggestions.