For nearly 50 years, scientists have tried to explain the disappearance of frogs from Brazil’s Atlantic Forest, one of the world’s richest amphibian habitats. New research using more than 370 museum specimens suggests that chytridiomycosis, a disease caused by the fungus Batrachochytrium dendrobatidis, spread through the Boraceia Biological Station at the same time that amphibian populations collapsed in the late 1970s and early 1980s.
The study, conducted by researchers at the Smithsonian’s National Zoo Conservation Biology Institute (NZCBI) and the Center for Conservation Genomics at Pennsylvania State University (PSU), provides compelling evidence for a fungal outbreak at the site. It also shows how decades-old museum collections can help scientists reconstruct disease events that occurred before the pathogens involved were known.
A decades-old mystery in Brazil’s Atlantic Forest
Researchers recorded a dramatic decline in amphibian populations at Boraceia between 1979 and 1982. Some scientists previously proposed that climate change or extreme weather events might have triggered the disappearances, but the cause remained uncertain.
Chytridiomycosis became a leading suspect. The disease is caused by the microscopic fungus B. dendrobatidis, which scientists believe originated in Asia and spread around the world during the 1970s and 1980s. However, the fungus was not formally identified and described until 1999, long after catastrophic amphibian declines had begun.
By then, hundreds of amphibian species worldwide had been driven to extinction, while others had already disappeared. Because many populations collapsed before scientists knew that the fungus existed, determining whether it was responsible proved extremely difficult.
“Nearly 50 years later, we are still uncertain as to what caused many amphibian extinctions throughout history, making these declines one of the most persistent cold cases in wildlife epidemiology,” said Jack Boyett, an NZCBI graduate fellow and PSU doctoral candidate who led the study.
The team investigated the Boraceia decline by examining preserved frogs and other amphibians collected during expeditions by William Ronald Heyer, a former curator of amphibians and reptiles. Heyer conducted five collecting expeditions to Boraceia: two in the 1970s, before the major population decline, and three in the 1980s, afterward.
These specimens gave researchers an opportunity to compare amphibians from before and after the disappearance. By testing samples collected at different times, they could investigate when chytrid arrived and how rapidly it spread.
Museum specimens reveal a fungal outbreak
The researchers analyzed samples from more than 370 preserved amphibians representing 17 species. They searched for evidence of chytrid infection and tracked how frequently the fungus appeared over time.
No specimens collected before 1979 showed evidence of chytrid infection. In contrast, more than half of the specimens collected during the 1980s tested positive.
The sharp increase indicates that the pathogen reached local amphibian communities and spread around the same time that frog populations were collapsing.
The results also revealed a pattern that initially appeared to challenge the conclusion. Species that suffered the most severe declines showed no detectable infection in the preserved specimens, while species whose populations remained relatively stable frequently carried the fungus.
Researchers believe this difference may reflect how individual species respond to infection. Some frogs may have had natural resistance, allowing them to survive while carrying the fungus. More vulnerable species, however, may have died soon after becoming infected, leaving few or no infected individuals available for collection.
“It’s important to keep in mind that not all species suffered catastrophic losses at Boraceia,” Boyett said. “The species that did not decline may have been naturally resistant to the fungus and were able to become infected without suffering mass mortality. In contrast, the species that declined most dramatically were so susceptible that the infected frogs may have died before Heyer collected them.”
Together, the findings suggest that chytrid can spread quickly among amphibians with little previous exposure, with potentially devastating consequences for species that cannot tolerate the infection.
How historical collections can solve wildlife mysteries
For NZCBI genomicist Michael Campana, who contributed to the research, the findings demonstrate the scientific value of natural history museums.
Specimens preserved decades ago can contain biological evidence that researchers had no way to recognize at the time. Modern testing methods now allow scientists to revisit those collections and investigate outbreaks that began long before the responsible pathogens were discovered.
“Museum collectors could never have predicted how important their collections would become to modern conservation challenges, but without them we would never have been able to detect pathogens that went unnoticed for decades or reconstruct disease outbreaks that remained mysterious,” Campana said.
Understanding past outbreaks may also help scientists anticipate future threats. One growing concern is Batrachochytrium salamandrivorans, also known as B. salamandrivorans or Bsal. The fungus threatens salamander populations in Europe and may pose a serious risk to salamanders in North America.
By reconstructing how fungal diseases emerged, spread and affected different species, researchers hope to improve the ability to detect and respond to similar threats before they cause widespread damage.
Amphibians show signs of recovery
The findings also provide evidence of recovery at Boraceia despite the severe losses recorded decades ago. Working with researchers from the University of São Paulo, the team visited the site six times between 2022 and 2024 to survey its current amphibian population.
Researchers found 12 of the 17 species represented in Heyer’s museum collection. The other five have not been found, and their current status remains uncertain. Some may have disappeared from the site altogether.
Even so, the surveys indicate that most amphibian species at Boraceia have recovered from the severe declines of the early 1980s.
The fungus has not disappeared. Chytrid infections are still present among local frogs, but the infection patterns suggest that species once heavily affected may have developed greater resistance over successive generations.
That increased resistance could help explain why many populations have survived and recovered despite continued exposure to the pathogen. The findings also highlight the difference between infection and death: a species can carry a dangerous pathogen without suffering fatal population losses if its members are able to survive the infection.
What the findings mean for amphibian conservation
The evidence strongly implicates chytrid in Boraceia’s historical decline, but the researchers caution that the results apply specifically to this location. They cannot automatically explain amphibian disappearances elsewhere in the Atlantic Forest.
Nevertheless, the study offers a persuasive explanation for a long-standing local mystery and demonstrates how historical specimens can be used to reconstruct poorly understood disease outbreaks.
The investigation is continuing. Researchers are examining DNA recovered from museum specimens to identify genetic mechanisms that influenced how amphibians responded to chytrid infections in the past. These analyses could help explain why some species disappeared while others survived.
A separate project is using ancient environmental DNA to fill gaps in the historical record. The method searches for genetic traces left by organisms in their surroundings, potentially revealing evidence of species and pathogens that were never directly collected.
Together, these efforts may provide a clearer account of how chytrid fungi emerged, spread and affected amphibian biodiversity over the past century. Identifying the biological changes that helped some frogs survive and recover could also provide insights into protecting vulnerable species from future fungal outbreaks.
Source: www.sciencedaily.com
