Lessons From The Field: The Frog Blog

Mountain yellow-legged (Rana Muscosa) and Sierra Nevada yellow-legged frogs (Rana Sierrae) are genetically separated into two different species, though the frogs face similar threats and play similar roles in their ecosystems.

The Mountain and Sierra Nevada yellow-legged frogs used to be one of the most abundant vertebrate in the Sierra Nevada. There are historical reports of hikers seeing thousands of frogs basking in the sun on the edges of lakes, and marshy shallows filled with tadpoles. After glaciers carved steep hanging valleys with waterfalls pouring down into the major river canyons, fish were blocked from upstream travel. Without any fish around, these frogs evolved as a top predator in the alpine aquatic world. 

Although they are at the top of the aquatic ecosystem, they are in the middle of the larger alpine food chain. Snakes and birds consider the frogs to be very tasty treats and the frogs serve an important role as transfers of nutrients from the aquatic to the terrestrial.

The Mountain and Sierra Nevada yellow-legged frogs were officially listed as federally endangered in 2014, the first year I began working on the National Park Service frog restoration project. They are extirpated from a little over 90% of their historic range. 

The story of their decline however, begins more than 100 years earlier. Starting in the 1890's people began stocking trout in historically fishless Sierra alpine lakes and streams. This practice continued until the 1990's when research showed how adding these highly efficient new predators disrupted the balance of alpine watersheds, impacting not only the frogs, but also the entire aquatic food chain from macroinvertebrates to zooplankton and phytoplankton. While fish stocking stopped in national parks, fish populations persisted in basins with available spawning habitat. Physical fish removal began in Sequoia Kings in 2001 and was initially a great success, with frog populations rapidly recovering. 

Then a new threat arrived: Batrachochytrium dendrobatidis—a fungal infection more commonly known as chytrid or Bd. This fungus is causing global amphibian declines and extinctions. Frog populations infected with chytrid experience rapid and often complete die-offs. It is currently unknown exactly how chytrid arrived in the Sierra or how the infection is still spreading in the mountains. There are populations in Yosemite that recovered after experiencing chytrid die-offs, and the frogs that survived are resistant to the fungus! These frogs are being used to help repopulate basins where the frogs have been extirpated. Other efforts to help these frogs fight chytrid include captive rearing and release of wild-born frogs, as well as ongoing research on treating frogs in situ to help them survive chytrid die-offs. 
 

Winters are long and summers are short up at 10,000 and 11,000 feet. The slow-growing season means these frogs spend three to four years as tadpoles before metamorphosing into adult frogs. During the winters months the frogs and tadpoles survive by burrowing into the muck of lakes that are so deep they do not completely freeze through. The adult frogs can live for around 20 or so years!

The project I was working on was almost entirely wilderness field based. After a few weeks of training and net mending, the crew spent over three months working and living in off-trail basins removing fish. My field partners and I would work 10 days on, four days off in sites that were 16-21 miles from a trailhead. We used gill nets and electrofishers to remove the introduced trout, returning the fish to the water to decompose in order to keep the nutrients in the system. 

In my fourth year on the project I served as the assistant crew lead. In addition to helping train and support the field crews I also went on survey trips and swabbed frogs to test for the spread of chytrid in new basins and frog populations. 
 

During the four years I spent on the restoration project in Sequoia and Kings National Parks, I was able to participate in some of the capture and release for captive rearing. Because metamorphosing into adults takes so much energy for a frog, they are very vulnerable to the chytrid fungus at this time and almost all infected frogs die shortly after. To help with this, the park service is partnering with zoos to experiment with captive rearing. In 2015 I helped catch newly metamorphosed young frogs and third- and fourth-year tadpoles. We put them on a helicopter to start their journey to the San Francisco and Oakland Zoos. In the zoos, they were safely raised, exposed to chytrid and then cleared of chytrid with medication. Then, in 2017 I helped release the adult frogs back into the same lakes I had caught them two years prior. It was amazing to see these frogs hop back into the water and explore their true home. 

Just recently, a similar restoration project helped reintroduce the threatened California red-legged frog back to its home of Yosemite National Park. The California red-legged frogs and the Mountain/Sierra Nevada yellow-legged frogs are similar in that their decline was linked to introduced species (such as bullfrogs) and human activity in the mountains. 

They are both part of the Rana genus, more commonly known as pond frogs or brown frogs, and are both susceptible to the Chytrid fungus. Red-legged frogs do not seem to be as affected though, which is a great sign for their recovery. A main difference between the frogs is that the red-legged frogs are found all throughout California, while the yellow-legged frogs are only found in the high mountains. This makes the red-legged frog habitat much more accessible than the yellow-legged because of the elevation difference.

Additionally, the red-legged frogs were totally extirpated from the park, and the zoo-reared frogs were taken from private property. Luckily, we still have wild yellow-legged frogs in both parks.

I want my students at NatureBridge to know that an ecosystem’s balance depends on all creatures, including frogs! I also want them to think about how easy it is for us to disrupt the balance of ecosystems, even if we mean no harm.

The people stocking fish in the lakes thought they were making the mountains a better place, and thought people would care more about their preservation if they could enjoy fishing. It takes a lot more work to clean up our mistakes than it does to make them.