For more than 10 years, the San Diego Zoo Wildlife Alliance has led an effort to bolster the population of a threatened species, the mountain yellow-legged frog.
But as the zoo’s facilities have reached capacity, local partners have ed in.
One of the most recent is Birch Aquarium at UC San Diego’s Scripps Institution of Oceanography in La Jolla, which welcomed its first batch of tadpoles late last summer.
Weeks later, the nearly 250 tadpoles were successfully reared into frogs. Their release into the wild from a simulated environment, or “captive space,” is likely not far away.
Mountain yellow-legged frogs are considered endangered because of threats from habitat destruction, drought and diseases. Less than 200 of them remain in the wild, according to the San Diego Zoo.
“I didn’t realize how dire the situation was until I started attending U.S. Fish and Wildlife [Service] annual meetings,” said Sean Bruce, assistant curator of fishes and invertebrates at Birch Aquarium. “And we have 250 in captivity just at Birch. So you’re looking at your frogs in your tank, going ‘This is as many as there are in the wild.’”
Mountain yellow-legged frogs, measuring about 3 inches long, are native to Southern California mountains. The batch of tadpoles Birch Aquarium received from the zoo came from San Bernardino.
Up-to-date population estimates are difficult to determine because of environmental restraints preventing access to sites where the frogs are thought to live.
Bruce said the aquarium hopes to release its frogs in June or July. When and where they are delivered depends on weather conditions such as rainfall and where they can safely return in the wake of fires earlier this year.
In the meantime, Birch is doing something eight of the zoo’s nine partner organizations have not done — housing the frogs in an outdoor environment.
Their aquatic habitat was designed to replicate conditions they will encounter in the wild. Currently, the frogs are in a “false winter scenario” called brumation — the amphibian and reptilian equivalent of hibernation.
By turning the water temperature to freezing, the process simulates when amphibians and reptiles conserve energy in the winter, when food is scarce and temperatures are low. During the winter, lab temperatures are reduced to 30. In the summer, the number is raised to about 60.
Birch Aquarium’s efforts will serve as a case study on whether this better equips frogs for survival in their natural habitats. It could prove especially important as their population recuperates.
“We’re releasing these frogs, but they’re going from this very sterile lab environment … to just right out into the wild,” Bruce said. “We’re hoping maybe raising the frogs in a more outdoor setting will make them heartier and make survival rates go up in the wild.”
The zoo and its partners ultimately hope to increase breeding and have the frogs delisted as an endangered species.
Throughout the process, the institutions have shared data and checked in on each group’s frog population growth. Each month, they are required to submit reports to the zoo and subsequently the state. The updates cover disease issues, mortality rates and more.
The life span of mountain yellow-legged frogs is unknown, but they typically reach reproductive age at 3 or 4 years. ♦
