In the late 1970s, frogs in Brazil’s Atlantic Forest, one of the most biodiverse places on Earth, began disappearing at a alarming pace. For years, researchers offered competing explanations, from unusually severe cold snaps to acid rain. A new study built on preserved museum specimens now points to a fungus that was not even named until decades later. The findings are promising, but the authors say they answer the question for one site, not the entire forest. ([1])
A gap in the collection record
The research centers on Boracéia Biological Station in São Paulo state. William Ronald Heyer, a former Smithsonian curator, gathered and preserved 370 frogs from 17 species there during the 1970s and 1980s. The collection has a conspicuous hole: no animals were gathered between 1979 and 1982, the precise window when numbers fell sharply.
Jack Boyette, the study’s first author and a Smithsonian graduate fellow, described the timing as a stroke of luck for science. The collection, he said, offers a rare before-and-after snapshot, even with the missing years in the middle. Researchers in the study include scientists from the United States and Brazil.
What the preserved skins revealed
When the team examined the specimens for the fungus, the pattern was clear. Frogs collected before 1979 showed no sign of the pathogen, while more than half of those gathered in the 1980s tested positive. ([2])The pathogen, Batrachochytrium dendrobatidis, causes chytridiomycosis, a skin disease that can be fatal to amphibians. Its rapid appearance in the records suggests it moved through the local frog community quickly once it arrived.
Boyette framed the result as a matter of timing rather than proof of every mechanism. “While environmental factors could have played a role, our findings most directly implicate the chytrid fungus,” he said. ([2])Scientists consider the fungus to have originated in Asia and likely spread around the world during the 1970s and 1980s, yet it was not formally described until 1999. ([3])
Caveats and a surprising recovery
The researchers have been careful about how far their conclusions reach. Their evidence comes from one research station, and they warn that it may not explain declines elsewhere in the Atlantic Forest. ([1])([4])
Outside experts have also urged caution. Lucas Ferrante, a researcher at the University of São Paulo who was not part of the study, pointed out that finding the pathogen after a population crash does not by itself show it caused the deaths. ([5])
The story does not end in loss. Modern sampling indicates that most amphibian species at the site have recovered since the early 1980s. The chytrid fungus remains present, yet infection patterns hint that frogs which once declined may have developed a degree of tolerance to it. ([2])The team returned to Boracéia between 2022 and 2024 to sample living populations, and those visits form the modern half of the comparison. ([3])
What remains unsettled
- Whether chytrid explains die-offs at other Atlantic Forest sites, which the study does not test directly.
- How much climate and extreme weather contributed, since they were long proposed as explanations for losses at Boracéia. ([6])
- Why some species recovered while others never returned to the area.
The “cold case” is closer to a solution than it was a decade ago, but it is not closed. The Boracéia specimens offer a powerful example of how museum collections can answer questions that field teams could not reach in real time. For conservationists, the same lesson carries a warning: a pathogen that spreads quietly through naive populations can change an ecosystem before anyone knows what to look for.
