After more than seven decades, an iconic amphibian from the San Bernardino mountains returns to its natural habitat. In August 2025, over 350 mountain yellow-legged frogs were released at Bluff Lake, an event considered the largest reintroduction effort for this endangered species. The initiative marked a milestone for the Birch Aquarium at Scripps Institution of Oceanography at UC San Diego and for conservation in the United States.
Out of the released specimens, 220 were bred at the facilities of Birch Aquarium, while the rest came from the San Diego Zoo Wildlife Alliance and the University of California, Los Angeles (UCLA). This joint effort reflects the importance of interinstitutional cooperation to save species at critical risk.
The reintroduction operation was carried out under strict measures to ensure the adaptation of the amphibians. The teams transported the frogs in controlled conditions and distributed them at different points in Bluff Lake, using both direct releases into the water and a gradual process in temporary habitats. The goal was to maximize survival and evaluate which strategy is most effective in the long term.
Additionally, each specimen received a radiofrequency identification microchip, allowing detailed monitoring. This tracking technology provides key data to adjust protocols and improve the success odds in future recovery programs.
Release of the mountain yellow-legged frog in the United States. Photo: Birch Aquarium.
A new achievement for conservation
The release at Bluff Lake was not an isolated effort. In 2023, 70 frogs were reintroduced, constituting the program’s first trial. With the recent event in 2025, the project’s magnitude multiplies, reinforcing the viability of a self-sustaining population in the area.
The collaborative work of institutions like Birch Aquarium, San Diego Zoo Wildlife Alliance, UCLA, Big Bear Alpine Zoo, and The Wildlands Conservancy, alongside state and federal agencies, demonstrates the relevance of scientific cooperation in addressing the biodiversity crisis.
Looking ahead, the teams plan to progressively increase the number of released specimens and diversify the species’ genetic lines. Continuous monitoring will be crucial to measure the success of the different applied strategies and to ensure that the frogs establish stable and resilient populations in their original habitat.
The recovery of the mountain yellow-legged frog is not just a local triumph but a global reminder of the need to restore ecosystems damaged by human activity. Projects like this demonstrate that, with science, cooperation, and commitment, it is possible to reverse decades of biodiversity loss and offer a second chance to species that seemed doomed to oblivion.
Release of the mountain yellow-legged frog in the United States. Photo: Birch Aquarium.
What is the ecological role of this species?
The mountain yellow-legged frog (Rana muscosa) is an endemic amphibian from southern California and a crucial piece in the region’s ecological balance. Its presence favors the natural control of insects and serves as an indicator of the health of mountain aquatic ecosystems. However, its population drastically declined during the 20th century due to habitat loss, water body pollution, and the introduction of predatory species.
Since 2002, the mountain yellow-legged frog has been listed as an endangered species under the U.S. Endangered Species Act. Its disappearance from Bluff Lake, recorded since 1951, symbolized the fragility of local ecosystems. The reintroduction of over 350 individuals represents, therefore, an advance in the ecological restoration of the San Bernardino mountains.
The return of this frog also contributes to bolstering regional biodiversity. Amphibians play a fundamental role in the food chain, being prey for birds, reptiles, and mammals, and at the same time, predators of invertebrates. Their recovery not only rescues a species but also favors the resilience of the entire ecosystem.
