The researchers were not dissuaded. They tried again in 2012, but instead of just removing and treating the tadpoles, they also applied low concentrations of a common commercial disinfectant to some of the ponds and the rocky crannies around them.

The following year, tadpoles and frogs in the three ponds that were disinfected and whose tadpole residents were treated did not have any signs of B.d. Three years on, those three ponds are still fungus free, and the researchers have gone on to apply the treatment to the remaining two ponds as well.

“They set out to eliminate a major threat to the survival of a very special frog, and they were successful,” said David Wake, an amphibian specialist and evolutionary biologist at the University of California, Berkeley, who was not involved in the research. “It took years, but it worked — so far.”

The hope now is that this success will raise interest in mitigation efforts beyond Majorca. “The method is really cheap and easy to use, so why not try it in other places?” said Jaime Bosch, a senior research scientist at the National Museum of Natural Sciences in Madrid and one of the paper’s authors.

“We can’t just stand still and do nothing, watching amphibian after amphibian go extinct.”

Conditions on Majorca, however, are particularly well suited to such interventions, Dr. Wake said. Not only is the island isolated, but the frogs breed in satellite sites largely cut off from one another, and without other amphibian species hopping about that could reintroduce the pathogen or complicate tadpole treatment.

“Doing such a study almost anywhere else might be dauntingly difficult,” Dr. Wake said.

The special circumstances on the island, and the threat B.d. posed to the protected Majorcan midwife toads, justified the use of fungicides in the environment, said Deanna Olson, a research ecologist at the United States Forest Service’s Pacific Northwest Research Station, who was not involved in the work. But it’s a strategy that some other scientists see as extreme.

“That would be considered very controversial in a wider application,” Dr. Olson said. “Eradication of B.d. in the wild has been discussed, but implementation has been stalled due to the widespread effects of antifungals on an extremely important component of ecosystems: fungi.”