A red-legged froglet peeking out in a restoration pond that is part of a cross-border effort to bring back the species in both Southern California in the United States and Baja California in Mexico. — AP
BRAD Hollingsworth crouches at the edge of a pond in Southern California, rubber boots sunk into the mud, but he doesn’t step into the water.
Instead, the herpetologist – an expert in reptiles and amphibians – retrieves a small recording device, the size of a deck of cards, and removes a tiny memory card holding 18 hours of sound.
Back at the San Diego Natural History Museum, Hollingsworth feeds the data into artificial intelligence software.
Within minutes, the AI identifies the animals visiting the pond: owl hoots, woodpecker pecks, coyote howls and tree frog ribbits.
But there’s no croak from the invasive bullfrog, which has decimated the native red-legged frog over the past century.
It was another good day for the team’s mission to restore the red-legged frog and the broader ecosystem spanning the US-Mexico border.
At 5-13cm, red-legged frogs are the largest native frogs in the West.
Once abundant along the California coast and into Baja California, they are widely believed to be the frog in Mark Twain’s The Celebrated Jumping Frog of Calaveras County.
Their crimson hind legs were eaten during the Gold Rush. But bullfrogs, introduced for food in the late 19th and early 20th centuries, preyed on the red-legged frogs and brought disease.
Habitat loss to drought, homes and dams further pushed them to the brink.
Herpetologist Bennet Hardy (left) holding a red-legged froglet as biologist Jorge Valdez holds a device to read a microchip in the froglet on a ranch outside of El Coyote, Mexico. — AP
Hollingsworth estimates red-legged frogs have disappeared from 95% of their historical range in Southern California.
Robert Fisher of the US Geological Survey once searched 400km from Los Angeles to the border, finding just one frog in 2001 and none after that.
In 2006, Hollingsworth and Fisher traced a small population to Baja California, where Anny Peralta, a student at the time, joined the team.
Inspired, Peralta co-founded Fauna del Noroeste, a non-profit that builds ponds in Mexico to boost the frog population for eventual reintroduction in the United States.
Transporting frog eggs northward proved challenging. During the Covid-19 pandemic, Peralta and US scientists secured permits for pilot transport of coolers containing eggs, which then continued by road after border inspection.
Over the past five years, Hollingsworth has monitored the ponds using AI.
On Jan 30, the software picked up the red-legged frog’s breeding call – a quiet grunting reminiscent of rubbing a thumb on a balloon.
“It felt like a big burden off my shoulder because we were thinking the project might be failing,” Hollingsworth said.
Over the following nights, more calls were heard, and by March, the first egg mass was found beneath one microphone.
These frogs had not only hatched from the Mexican eggs but were now reproducing in the United States.
Conservationists increasingly rely on AI to track species, monitor breeding and assess environmental changes. Tools initially developed for birdsong analysis are now applied to amphibians, allowing scientists to build audio landscapes and study behaviour and reproduction.
The team hopes to expand technology further, using satellite-linked audio to alert them to predators – particularly bullfrogs – and monitor frog movements, which are otherwise difficult to detect.
Cold-blooded creatures evade thermal cameras, but AI audio provides a reliable window into their activity.
The software also saves time: previously, hours of recordings had to be painstakingly listened to over the cacophony of other wildlife.
“There’s tree frogs calling, cows mooing, motorcycles zooming by,” Hollingsworth said. “Owls, ducks, all this noise – and you have to pick out the red-legged frog.”
The red-legged frog’s recovery is part of broader cross-border conservation efforts.
Mexican grey wolves have returned to parts of the southwestern United States and Mexico, and California condors now soar from Baja to Northern California.
Currently, scientists estimate more than 100 adult red-legged frogs inhabit Southern California ponds and tadpoles are appearing at new sites.
Egg masses continue to be transported from Baja, where the population has grown from 20 to as many as 400 adults, with the goal of establishing thriving populations on both sides of the border.
The restored ponds are already having ecological benefits, including fewer disease-carrying mosquitoes.
In Baja, ponds built by Peralta’s organisation teem with froglets, their tiny eyes bobbing on the fern-covered surface. These frogs may one day supply eggs for relocation to the United States.
“They don’t know about borders or visas or passports,” Peralta said. “This is just their habitat and these populations need to reconnect. I think this shows that we can restore this ecosystem.” — AP
