Zoo-Raised Threatened Species Released In Yosemite National Park

SAN FRANCISCO — On June 27, the San Francisco Zoo's Conservation team completed its first successful reintroduction of an endangered toad species back to Yosemite National Park.

For over a decade, the Yosemite toad, native to the Sierra Nevada, has been recognized as a federally threatened species.

Found in primarily high-elevation areas, these amphibians are known for their bumpy skin and their yellow to darker green color. Yosemite toads also tend to walk, rather than hop.

After experiencing a 50% population decline due to the Rim Fire of 2013 that engulfed a large portion of land near Yosemite, the amphibian species became a vulnerable population within the park, which makes up a significant portion of their habitat range.

"Biologists stated that they were not likely to recolonize in the meadow where they were last seen without active reintroductions," said Rob Grasso, a park aquatic ecologist from Yosemite, adding that it makes this current effort an important indicator of the program's success.

Since 2022, the San Francisco Zoo's conservation team has reared a large number of the amphibians to re-establish a self-sustaining population in the national park.

These toads were fed a diet of crickets and vitamin supplements for over two years. After individual examinations to ensure that each toad was ready for wildlife release, they were inserted with a microchip that contained a unique 15-digit code for identification. The toads were then transported via helicopter and released at a site determined to be suitable for their habitat.

Rochelle Stiles, a field conservation manager with SF Zoo, said that no other accredited zoos or aquariums had attempted to bolster these threatened toads.

"We learned the essential components of their development through working with them every day for two years," said Stiles in a statement.

Yosemite toads play a crucial role in the Sierra Nevada's ecosystem and its complex food web.

Their reintroduction to the environment could restore the population balance of invertebrates and small vertebrates that the toads consume, as well as continuing to serve as prey for snakes, birds and other larger predators in the area.

Their reintroduction will be closely monitored through their aforementioned microchips, as well as through radio transmissions. Transmitters were attached to 30 of the toads through tubing and surgical suture threads around their hips. This allows their movements to be tracked using a radio receiver and antenna. This radio telemetry project is being conducted by Tiffany May, a Yosemite National Park biologist and a San Francisco State University Master's student, alongside the zoo's conservation team.

However, there are concerns about the toads' survivability, given that their ideal breeding conditions require pools of snowpack runoff and spring recharge, which are growing increasingly rare due to climate change. In a statement released by the San Francisco Zoo, researchers are worried that meadows will dry up before tadpoles are given the chance to metamorphose.

Whether or not the breeding conditions are ideal for the Yosemite toad will be revealed through the team's long-term monitoring of the species from the National Park Service. This reintroduction project will take place over the next five years and future toad releases will also be raised at the zoo's wellness and conservation center.

The zoo's project is a partnership with the National Park Service, Yosemite Conservancy, U.S. Fish & Wildlife Service, California Department of Fish & Wildlife and the U.S. Geological Survey.

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