California Red-legged Frogs Reintroduced to Historic Range in Southern California

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The California red-legged frog (Rana draytonii) has been absent from Southern California for over 20 years. California red-legged frogs were in decline statewide since the 1970s and ultimately disappeared from a 250-mile stretch between southern California and northern Baja California largely due to habitat loss, fungal disease, and predation by non-native species. The species was listed under the Endangered Species Act in 1996. 

Earlier this year, the scientists collected eggs from a genetically similar population of red-legged frogs in the Sierra San Pedro Mártir mountain range in Mexico and transported them to Riverside and San Diego counties, California. The eggs and newly hatched tadpoles were placed into ponds in protective cages that will allow them to grow and develop into frogs safely before they are fully released into the wild. To ensure a successful reintroduction, invasive species were removed from the release sites and locations will continue to be closely monitored to protect from any new invaders or other threats. 

The effort to reintroduce the frog to Southern California has been two decades in the making and is the result of a collaboration by a binational group of scientists representing agencies and organizations from the United States and Mexico, including the Conservación de Fauna del Noroeste (FAUNO), San Diego Natural History Museum, The Nature Conservancy, U.S. Fish and Wildlife Service, and the USGS.  

“This reintroduction has been something we have all been working towards for many years,” said Liz Gallegos, a WERC biologist involved in the effort. “Finally seeing red-legged frog eggs and tadpoles in ponds at these two sites in southern California is incredibly exciting and a testament to the hard work of all partners involved.” 

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The USGS has been part of the effort since 1999, when the agency began extensive surveys for the frog throughout southern California to document and monitor populations. The USGS is a pioneer in the development of novel techniques to recover this species and the agency has led translocation of egg masses elsewhere in the state, including in the Santa Monica Mountains National Recreation Area. In 2014, USGS research on the frog’s genetics concluded that extirpated California red-legged frog populations south of Los Angeles County had been most closely related to populations in Baja California. These results and other genomic data prompted the idea to reintroduce genetically similar individuals from the Mexico populations back to the United States. 

More recently, the USGS provided expertise in preparing the receiver sites in Southern California, including removing non-native invasive species. The USGS will continue to monitor the introduced eggs and tadpoles in the receiver cages at both sites. 

The California red-legged frog is just one of many amphibians that USGS science has helped conserve. The USGS team involved in the red-legged frog reintroduction worked with the Los Angeles Zoo and other federal and state agencies to release captive-bred southern mountain yellow-legged frog tadpoles, another federally endangered species, to southern California streams in 2018 and 2019.  Both efforts are part the Amphibian Research and Monitoring Initiative (ARMI), a USGS program founded in 2000 to an integrated assessment of amphibian status and trends and the factors that may be causing declines.  

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Read the press release from USGS partners here.

Read an article about the reintroduction in the LA Times here.

Learn more about USGS research on amphibians in Southern CA here.

Learn about the Amphibian Research and Monitoring Initiative (ARMI) website.