The mountain yellow-legged frog once thrived in the Sierra Nevada, but today there is less than 100.

The yellow-legged frog is a popular topic with researchers and was once an abundant amphibian species but has declined 95% since 1995.

The yellow-legged frog is a keystone species meaning that they are both predator and prey. They eat insects, eggs, tadpoles and even other frogs. In return, they are consumed by native garter snakes.

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The frogs also play an important role in the ecological balance of an ecosystem including controlling insect populations.

However, the frogs have several obstacles they are facing.

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In the late 1800s, non-native trout were stocked in naturally fishless high alpine lakes. These trout quickly became competitors for food and the trout also preyed on the yellow-legged frog eggs, tadpoles and young adults.

In the 1950s, aerial stocking gained momentum which allowed the California Department of Fish and Wildlife to stock remote high alpine lakes by plane with an intent to create more recreational fisheries.

Researchers saw a dramatic decline in the frog population after the fish were introduced according to the Tahoe Institute of Natural Science.

“In most high elevation lakes, the fish are not native,” said Sarah Hockensmith, Outreach Manager of TINS.

Hockensmith explained that another contributing factor to frog’s decline is pesticides. Frogs breathe through their skin so the introduction of pesticides are extremely harmful to these sensitive amphibians and causes a chemical imbalance. Along with the other factors, the frog’s habitats are threatened by grazing livestock, which has been an ongoing debate in the Sierra Nevada.

Researchers found dead frogs not being consumed which led to the discovery of amphibian chytrid fungus. Amphibian chytrid fungus attaches to the keratin of frogs and tadpoles restricting their ability to absorb water and breathe.

This fungus has spread across the Sierra for the last 40-50 years and has wiped out tons of amphibians.

Ample funding, time and scientific attention is required to be listed as an endangered species.

The frog was listed in 2014 as endangered federally and through the state.

According to the Center for Biological Diversity Scientists, the yellow-legged frog could be extinct within decades.

Research biologist at the University of California Sierra Nevada Aquatic Laboratory, Roland Knapp, has dedicated over 20 years researching the frogs and believes population recovery is still possible.

Knapp explained that a fish-less habitat along with increased resistance to chytrid fungus can allow populations to rebound and increase.

Knapp’s research findings have shown the frogs being able to adapt to the disease over time. The frogs had reduced susceptibility to chytrid fungus.

“The last 10-15 years have been truly remarkable with frog populations,” Knapp said. “I have a lot of hope. I wouldn’t have said that 10 years ago.”

Knapp has said he had been watching the decline since the mid 1990s when the frog population was critically low. He explained that these frogs have evolved millions of years as the top predator and taking out the top predator can have devastating effects on an ecosystem similar to the wolves’ impact in Yellowstone National Park.

Recognizing a changing ecosystem is difficult when the ecosystem is not terrestrial. Knapp explained that if you were to stick your head underwater, you can see directly how different the ecosystem is between lakes with and without fish.

He said a balance needs to be found to make sure these frogs are protected by halting the stocking of fish or removing fish from selective lakes that are critical for their habitat. He says a change needs to be made in the way lakes are managed by keeping some fish-less.

“Every lake in the Sierra is someone’s favorite fishing lake,” he said.

He hopes to collectively agree to set a few lakes aside as habitat for native species. The goal is for the frogs to recover while having minimal effect on recreation.

Knapp says progress is being made, “They are moving slowly but surely away from extinction.”

Actively adding the frogs back into fishless lakes is another important component.

“We can find that balance as a society.”

He explained that to protect natural character, top predators need to be protected that have outsized effects on an ecosystem.

The U.S. Forest Service Lake Tahoe Basin Management Unit has also spent time and effort surveying the fish and frogs in efforts to balance the ecosystem.

Fish were even removed from some of the lakes by the CDFW, especially those that provide minimal recreation value.

“If people don’t understand what’s going on, there will be no change,” Hockensmith said. “These frogs are an important keystone species that help control an environment.”

The disappearance of the frogs could have unforeseen consequences.

“There could be a chain reaction that we might not quite fully understand,” Hockensmith said. “There will be an impact whether people completely understand or not.”

Other local frogs in the Tahoe Basin include Sierran tree frogs and bullfrogs. Sierran tree frogs are mostly seen in the meadows after spring snow melts.

They hibernate over winter and can look similar to the yellow-legged frogs.

Bullfrogs are also popular and are not native to the Sierra.