California red-legged frogs
key terms
Natural selection: The evolutionary process by which successful genes are passed on to the next generation
Chytrid fungus: Fungus that grows on amphibians’ skin which is fatal and highly contagious
Native species: Plants and animals that naturally live in an area (without human interference)
Invasive species: Plants or animals brought in by humans that are harming the native ecosystem
Immune: Plant or animal is not harmed by illness or threat
Headstart program: Individuals from species are raised in captivity and released into the wild
Threats: Human actions that lead to a decline in plants/animals
save the frogs!
The northern CA red-legged frog (Rana draytonii) is an endangered species native to California, however they disappeared from Yosemite National Park for 50 years! The main threat that most likely led to their disappearance was the presence of an invasive species, American bullfrogs, and waste water pollution in Yosemite’s waters. Bullfrogs are especially threatening to this species, as they compete for food (sometimes preying on them as well) and are notoriously a carrier of Chytrid fungus, often spreading it to more vulnerable populations.
Thanks to a huge network of people who care, conservation efforts have significantly reduced ongoing threats to this species: there are no longer bullfrogs in Yosemite, and waste water pollution has been stopped! A headstart program at the San Francisco Zoo has helped their numbers recover, and there is now a healthy breeding population in Yosemite! We have even seen in them in our very own SF Botanical Gardens!!
Pictures taken by our Fall 2025 Animal Care Intern, Margay Mina Olson (pictured), who was able to help with a recent release of these endangered frogs with the SF Zoo!
adaptive powers of survival
Scientists have found that California Red-legged Frogs developed an immunity to Chytrid Fungus on their own!
Most likely, Chytrid Fungus decreased their populations in the wild until frogs developed traits which allowed them to be unaffected by the fungus. Natural selection took care of the rest!
Sources:
Us, P. 209/372-0200 C. (n.d.). California Red-Legged Frog - Yosemite National Park (U.S. National Park Service). Www.nps.gov. https://www.nps.gov/yose/learn/nature/caredfrog.htm
n.d.). Local Conservation Efforts. Retrieved November 17, 2025, from https://www.sfzoo.org/local-conservation-efforts-2/