New research helped solve the cold case of why frogs at the Boracéia Biological Station in Sao Paulo, Brazil, were dying off in the late 1970s using museum samples at the Smithsonian’s National Museum of Natural History. This clay robber frog was seen near the site in 2023.
Photo Credit: Jack Boyette / Pennsylvania State University
(CC BY-NC-ND 4.0)
Scientific Frontline: Extended "At a Glance" Summary: Decades-Old Amphibian Decline Linked to Chytrid Fungus
The Core Concept: A retrospective epidemiological study utilizing museum specimens confirmed that the emergence of the chytrid fungus (Batrachochytrium dendrobatidis) coincided with severe amphibian die-offs at Brazil’s Boracéia Biological Station in the late 1970s and 1980s.
Key Distinction/Mechanism: Unlike contemporary field studies, this research analyzed DNA from preserved museum specimens (collected by William Ronald Heyer) to retroactively trace the arrival and infection prevalence of the pathogen across a timeline before and after the sudden population declines.
Origin/History: Severe amphibian population declines occurred in Brazil's Atlantic Forest between 1979 and 1982. The chytrid fungus, believed to have originated in Asia and swept the globe during this era, was not formally described until 1999, leaving historical die-offs unexplained until now.
Major Frameworks/Components:
- Pathogen Invasion: Specimens collected prior to 1979 showed no evidence of chytrid, whereas over half of the 1980s specimens tested positive, indicating a rapid transmission through naive host populations.
- Disease Tolerance Patterns: Species that suffered catastrophic declines exhibited no detectable infections in the museum samples (hypothesized to have died prior to collection due to high susceptibility), while species that did not decline showed high infection rates, suggesting natural tolerance.
- Pathogen Resilience and Evolution: Modern sampling (2022–2024) indicates a recovery among persisting species; infection patterns suggest the evolution of widespread pathogen tolerance.
- Genomics and Retrospective Epidemiology: The application of genomic sequencing techniques on archival biological samples to solve historical wildlife mortality events.
Branch of Science: Wildlife Epidemiology, Conservation Genomics, Herpetology, Evolutionary Biology.
Future Application: The methodological blueprint—combining museum collections with genomic tools and ancient environmental DNA—can be used to reconstruct historical disease dynamics, aiding in the anticipation and management of emerging threats like Batrachochytrium salamandrivorans in Europe and North America.
Why It Matters: Establishing a definitive link between the chytrid fungus and historical mass mortalities resolves a nearly 50-year-old ecological cold case in a critical biodiversity hotspot, while demonstrating how historical data informs modern epidemiological forecasting and wildlife conservation strategies.
In the 1970s, researchers began to observe drastic amphibian declines in Brazil’s Atlantic Forest, a global hotspot of frog biodiversity. For decades, these disappearances remained a mystery. Now, new research from scientists at Penn State and the Smithsonian's National Zoo and Conservation Biology Institute’s (NZCBI) Center for Conservation Genomics, published this week in the journal Molecular Ecology, points to amphibian chytridiomycosis, a devastating infectious disease, as the likely culprit.
Chytridiomycosis, commonly known as chytrid, is caused by the microscopic fungal pathogen Batrachochytrium dendrobatidis. Believed to have originated in Asia, the chytrid fungus likely swept the globe in the 1970s and 1980s, but scientists did not formally describe the pathogen until 1999. At that point, amphibian populations around the world had suffered catastrophic declines, with hundreds of species pushed to the brink of extinction or beyond. Because many of these declines occurred before the pathogen’s discovery, however, establishing whether the chytrid fungus was responsible has remained a challenge.
"After almost 50 years, we still aren’t confident what caused many historical amphibian die-offs, making these declines some of the most enduring cold cases in wildlife epidemiology,” said Jack Boyette, an NZCBI graduate fellow and doctoral candidate in biology in the Penn State Eberly College of Science, who led the study. “In Brazil’s Atlantic Forest, amphibian die-offs at the Boracéia Biological Station between 1979 and 1982 have been attributed by some scientists to climate change or extreme weather events. While environmental factors could have played a role, our findings most directly implicate the chytrid fungus, revealing that regional invasion of the pathogen coincided with these unexplained amphibian disappearances.”
To crack the case, Boyette and his colleagues turned to the Smithsonian’s National Museum of Natural History, which holds the collections of William Ronald Heyer, a former curator of amphibians and reptiles. Heyer collected specimens at Boracéia across five field campaigns: two in the 1970s, prior to the sudden amphibian declines, and three in the 1980s, during the aftermath of the die-offs. These collections offered a rare opportunity to trace the pathogen’s arrival across a critical period of amphibian declines. NZCBI researchers collected samples from more than 370 museum specimens representing 17 species to determine when the chytrid fungus first appeared at Boracéia and how infection prevalence varied through time.
The results were stark—specimens collected prior to 1979 showed no evidence of chytrid infection, while more than half of the 1980s specimens tested positive. Surprisingly, the species that suffered the most dramatic declines had no detectable infections, while the species that never declined were highly infected. Although these infection patterns may seem counterintuitive, the researchers hypothesize that they reflect natural patterns of disease tolerance among the amphibian community.
“It’s important to keep in mind that not all species suffered catastrophic losses at Boracéia,” Boyette said. “The species that didn’t decline may have naturally tolerated the fungus, allowing them to become infected without suffering mass mortality. By contrast, the species that declined most dramatically may have been so susceptible that infected frogs died before Heyer could collect them.”
Together, the researchers said, these findings suggest the chytrid fungus spread rapidly through the amphibian community at Boracéia in the 1980s, shedding light on how quickly this emerging disease can sweep through a community of naive hosts. The use of museum collections is a game changer for researchers seeking to reconstruct historical disease impacts, according to Michael Campana, a genomicist at NZCBI and co-author of the study.
“Museum collectors could not have anticipated how important their collections would be to our modern conservation challenges, but without them, we would never be able to detect pathogens that went unnoticed for decades and reconstruct disease outbreaks that would otherwise remain a mystery,” Campana said.
Reconstructing historical disease dynamics may also help researchers anticipate future disease outcomes. The team said that this is especially relevant as the emergence of another fungal pathogen, Batrachochytrium salamandrivorans, threatens salamanders in Europe and could pose a serious risk to salamander populations in North America.
At Boracéia, however, the story today is one of resilience. The team returned to Boracéia between 2022 and 2024, alongside researchers from the University of São Paulo in Brazil, to sample modern amphibian populations. Of the 17 species represented in Heyer’s collection, the researchers could only find 12 at the site. The fate of the other five species remains uncertain, although their continued absence raises concerns that they may have been lost locally, the research team said. Reassuringly, however, most amphibian species appear to have recovered since the early 1980s. While the chytrid fungus still infects frogs at Boracéia, infection patterns suggest the evolution of widespread pathogen tolerance among species that historically declined, likely promoting their persistence and recovery in the decades since the declines.
“Brazil is one of the world’s great biodiversity hotspots, home to thousands of animals and plants found nowhere else on Earth, many of which are now facing serious declines,” said Gui Becker, associate professor of biology at Penn State and senior author of the paper. “Amphibians are the most threatened group of vertebrates, yet the history of amphibian declines in Brazil has remained remarkably nebulous. For decades, studies produced mixed evidence about whether the chytrid fungus was actually involved in these declines. Our study provides the clearest evidence yet that chytrid was tightly linked to amphibian losses in the region where those declines were most severe.”
While the researchers acknowledge that the site-specificity of the study means the results cannot be extrapolated to the broader Atlantic Forest, they do solve the mystery of Boracéia’s disappearing frogs and provide a blueprint for other researchers to conduct similar disease retrospectives in the future.
The work is far from over, the team said. Researchers are now using DNA collected from museum samples to investigate the genomic mechanisms that shaped historical amphibian responses to chytrid, while other ongoing projects aim to fill gaps in historical specimen records using ancient environmental DNA. Together, these efforts could reveal a more detailed picture of chytrid emergence, spread, and impacts over the past century, deepening understanding of its historical toll on biodiversity and helping to anticipate future impacts in a world where fungal pathogens remain a major threat.
Funding: This research was funded by the U.S. National Science Foundation under award numbers DEB-2227340, DEB-2413542, BII-2120084, IOS-2131060, and DGE-1255832, and the Smithsonian Institution Fellowship Program.
Disclaimer: This content is solely the responsibility of the authors and does not necessarily represent the views of the funders.
Published in journal: Molecular Ecology
Authors: Jack L. Boyette, Tamilie Carvalho, David Rodriguez, Wesley J. Neely, Daniel Medina, Renato A. Martins, Robert J. Ossiboff, Zaira Gomez, William C. Vuyk, Shannon M. Buttimer, Karen A. Paniagua Torres, Ananda Brito de Assis, Carlos A. Navas, Carly R. Muletz-Wolz, Michael G. Campana, Corinne L. Richards-Zawacki, C. and Guilherme Becker
Source/Credit: Pennsylvania State University
Edited by: Scientific Frontline
Reference Number: epi102126_01
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