YOSEMITE NATIONAL PARK — Giddy scientists have spotted scores of tiny tadpoles and egg sacs in ponds, creek beds and marshy meadows in Yosemite Valley this spring, the first evidence in a half-century that the rare, sensitive California red-legged frog is breeding after being driven to extinction in the lush meadows of this spectacular park.

The discovery of the newborn amphibians coincided with the release Friday of 142 adult frogs raised at the San Francisco Zoo, an event Yosemite officials called a major turning point in the push by park biologists to bring back native wildlife and restore historic habitat for a host of threatened and endangered species.

The reddish puddle hoppers were let loose in four wetland ponds under a brilliant blue sky within sight of roaring Yosemite Falls and towering El Capitan some 50 years after the local population was killed off by invasive American bullfrogs, ravenous raccoons and a deadly introduced fungus, all the result of human stupidity or neglect.

“Putting red-legged frogs back into Yosemite is a pretty big deal, because now that bullfrogs have been eradicated, we sort of had this empty niche,” said Rob Grasso, an aquatic biologist for the park, who is in charge of the release program.

“One of the early milestones to know that it was successful is if you see evidence of breeding, and that’s what we did. ... The significance is if we can get the frogs established in Yosemite Valley, hopefully we can use this as a pinch point to start releasing in other known historic localities.”

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Grasso said 20 egg batches have been spotted since March, including eight masses in Cook’s Meadow underneath Yosemite Falls, which are clearly the offspring of the 125 frogs released a year ago. That would translate to about 50,000 tadpoles, but he expects there are probably three or four times that many in the park now. That may sound like a lot, he said, but only 1% to 3% of the frogs that are born survive.

Still, the appearance of a naturally born generation of polliwogs marks the first time breeding has been documented in Yosemite since biologists began releasing adults in 2017.

The reintroduction program is the result of a unique collaboration between the National Park Service, Yosemite Conservancy, San Francisco Zoo, the U.S. Fish and Wildlife Service, California Department of Fish and Wildlife, and NatureBridge. The group established a frog-rearing facility at San Francisco Zoo in 2016 and began raising the species in a former bald-eagle exhibit that was reconfigured for frogs. So far, 4,000 red-legged frog eggs and tadpoles and 500 adult frogs have been released. All of the reintroduced frogs have microchips inserted under their skin, so they can be identified if they die or are captured.

As many as 400 more zoo frogs will be released in Yosemite in June, some of them fitted with electronic transmitter bracelets, officials said. The zoo has also been raising pond turtles for release into Yosemite lakes, rivers and meadows.

The California red-legged frog, known scientifically as Rana draytonii, is the largest native frog in the Western United States, ranging in size from 2 to 5 inches long. The amphibian, which has distinctive reddish skin with iridescent green patches, was the type of frog Mark Twain was talking about when he wrote the “Celebrated Jumping Frog of Calaveras County.” Rather than a “ribbit” sound, the red-legged frog uses a series of short soft underwater grunts, which Grasso said sound like someone rubbing a finger on a balloon.

The species, which is listed as threatened under the U.S. Endangered Species Act, has undergone a major decline across California, disappearing from 70 percent of its habitat, which once covered the Sierra Nevada, coast range and foothills from north to south. They were harvested for food in the late 1800s and early 1900s, a major reason for their decline.

The death toll in Yosemite was particularly precipitous and, experts say, totally avoidable. Once common in the creeks and meadows, the reddish animals first began to disappear in the 1950s when bullfrogs were introduced into a reflection pond at the the Ahwahnee Hotel. The survivors were then attacked by an uncommonly large number of raccoons drawn to Yosemite in the 1970s by open refuse sites, which also attracted crowds of pesky bears.

Meanwhile, the frogs were being infected by a worldwide epidemic of a fungal disease, which was also killing local newts, the Yosemite toad and, most of all, their cousins, the endangered yellow-legged frog. Park biologists said lakes and waterways went from thousands of frogs to none in just a few years.

The demise of the two frog species was part of what wildlife biologists say is a worldwide collapse of amphibians, which many fear are going the way of the dinosaur.

The terrible trend, though, has been reversed, at least in Yosemite. Park officials have since eradicated the bullfrogs, closed the refuse sites, and restored the shrubby river and stream bank habitat frogs like. Both frog species have been making a comeback — the yellow-legged population has increased sevenfold over the past two decades, according to biologists.

The seeds for the comeback came in the form of frog eggs found in a small lake on a farm in El Dorado County. The park has also begun reintroduction programs for the other missing frog and toad species.

The comeback, Grasso said, is important, because red-legged frogs “are a sentinel of land and water quality.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite