Written By
Recently, 115 Foothill yellow-legged frogs, hailing from the Oakland Zoo, called the Plumas National Forest their new home. Little did these frogs know they were the part of a historical conservation moment – the first ever population of captive-reared Foothill yellow-legged frogs released into the wild.
This colossal effort was the outcome of a collaborative partnership involving the zoo, U.S. Forest Service, Garcia and Associates, U.S. Fish and Wildlife Service, Pacific Gas and Electric, California Department of Fish and Wildlife and a research biologist.
“The lessons these tiny frogs have taught us about captive rearing will set the stage for this species’ conservation,” said Service biologist Kat Powelson. “They’re part of an experiment showing us how to successfully captive rear, translocate and release their more imperiled brothers and sisters, especially along the Southern California coast.”
In hopes of improving the health of Foothill yellow-legged frogs across their range, the U.S. Fish and Wildlife Service’s At-Risk Species Team has recently begun working with a well-developed, voluntary partnership that has been working tirelessly on its conservation since 2002. The strategies and successes developed by these experts can be used to inform conservation of the species across its range.
Historically ranging throughout California and part of Southern Oregon, Foothill yellow-legged frogs were listed as endangered along the South Coast, most of the Sierra Nevada foothills and threatened in the Feather River drainage under the California Endangered Species Act earlier this year.
The Cresta reach of the North Fork Feather River population has dwindled over recent decades. Close monitoring efforts by PG&E and Garcia began in 2002 and documented the decline. This was a concern, as not only do the frogs serve as key indicators for ecosystem health, adults eat insects like mosquitos and yellow-jackets populations.
Kevin Wiseman, a Garcia herpetologist who has been surveying these frogs for almost 20 years explained, “We’re still trying to understand threats to this population; however, we know that changes in water flow during their breeding season and invasive predators like signal crayfish are some of the challenges facing this species.”
In 2016, surveys conducted by Garcia and the U.S. Forest Service concluded that the population was on the brink of local extinction, instigating the start of an active management project involving instream rearing efforts.
Colin Dillingham, district wildlife biologist for the U.S. Forest Service stated, “We knew we had at least two egg masses which means it could’ve been as low as four animals. We went from nine breeding sites on the river down to just one. I’ve had two populations of frogs under my management go extinct – and I couldn’t let this happen to this population, as well.”
To help with conservation goals, instream rearing projects then took place on the river from 2017-2019 with support from Sarah Kupferberg, a University of California, Berkeley affiliated researcher who began studying the species 30 years ago as part of her PhD.
“I studied their ecology and tinkered a lot with how to raise them both in the field and in the lab,” said Kupferberg. “Eventually, I came up with many techniques that were used for the instream and captive-rearing projects.”
The combined efforts proved to be highly successful, boosting the population to at least 12 additional female frogs whose egg masses hatched and surviving offspring returned to the breeding site over the years. While the ultimate conservation goal for the population is a reproductive output of 40 egg masses a year, this is still a step in the right direction, noted Dillingham.
“The success that the partnership in the Feather River has had is exciting,” said Powelson. “That’s why we were thrilled to collaborate with them to learn how and if these frogs could be raised in captivity, which would add another strategy to our management ‘tool kit’ for this frog.”
To further conservation efforts in the Feather River, 327 tadpoles were collected from 16 different egg masses to maximize genetic diversity and sent to the Oakland Zoo.
Margaret Rousser, Oakland Zoo’s conservation manager explained, “This group of frogs arrived at the zoo as tadpoles last summer. The 40 mm-long, young adults being released this year were PIT (Passive integrated transponder) tagged for tracking purposes and skin swabbed to ensure no presence of Chytrid, a deadly amphibian disease. The remaining frogs, who are still too small, will wait until next summer.”
While the zoo raised mountain and Sierra Nevada yellow-legged frogs, both of which are closely related to the Foothill yellow-legged frogs, in the past, the Foothill frogs proved to be a little more particular when it came to their housing conditions and their disease susceptibility.
“It was challenging to have a brand new species that’s never been reared in captivity before – there’s no template or standard operating procedure,” said Rousser. “Thankfully, we had several people on speed dial like Sarah Kupferberg we could consult with.”
Aside from their preferences in captivity, this species historically occupied a diversity of habitats. “They occurred across much of California, including ephemeral streams in Southern California, all way up the coast into Oregon and across the Sierra Foothills,” said Powelson. “It’s amazing how they are able to persist in different landscapes.”
“I couldn’t imagine it working out if we didn’t have partnerships,” said Wiseman. “The success from our efforts depended on individuals providing different expertise. Everyone has their strengths to add to the pot and each play critical roles with recovering this population.”
The zoo also plans to produce a husbandry manual so that the captive rearing can be used as a conservation tool for populations of Foothill yellow-legged frog that are even more imperiled, noted Rousser.
“I think there’s something a little magical about this animal; I have yet to meet someone who isn’t incredibly nice that works with this frog,” said Kupferberg. “The frogs themselves aren’t big nor flashy, yet there is just something about them that attracts people who harmonize with each other. It’s encouraging when you have an efficient and collaborative team of people all working towards the same goal and in return, seeing positive outcomes.”
