The Frogs That Vanished from Boracéia
AFor more than forty years, a puzzle has hung over a patch of forest in south-eastern Brazil. Boracéia Biological Station, in the state of São Paulo, lies within the Atlantic Forest, one of the richest regions for amphibians anywhere on the planet. Between 1979 and 1982, many of the frogs living there declined sharply, and some species seemed to disappear altogether. At the time, nobody could say with any confidence what had caused the losses. Scientists had collected frogs at the station on several expeditions during the 1970s and 1980s, so there was a record of what had lived there before and after the losses.
BOver the years, several explanations were put forward. Some researchers argued that a changing climate, or a run of unusually extreme weather, might have pushed the populations into decline. Disease was much harder to investigate, because the organism now regarded as the most likely culprit had not yet been identified by science. The fungus Batrachochytrium dendrobatidis, which causes an amphibian disease known as chytridiomycosis, was not formally described until 1999. It attacks the skin, which frogs depend on to take in water and essential salts, and severe infections can kill them. By the time the fungus had been named, many dramatic frog declines around the world had already taken place, and the evidence that might have explained them appeared to be lost.
CA new study, led by Jack Boyette and published in the journal Molecular Ecology, has now looked back in time to solve the mystery. Instead of relying only on modern fieldwork, the team turned to frogs that had been collected at Boracéia decades earlier and preserved in a museum. Many of these specimens had been gathered by the herpetologist W. Ronald Heyer and are kept at the Smithsonian's National Museum of Natural History in Washington. A preserved frog can still carry traces of the fungus on its skin, so it is possible to check whether an animal was infected when it was caught. Altogether, the researchers tested more than 370 preserved frogs belonging to 17 species for signs of chytrid infection.
DThe results revealed a striking pattern. None of the frogs collected before 1979 showed any trace of the fungus. Among those collected in the 1980s, however, more than half tested positive. This timing matches the period of the declines closely, suggesting that the arrival of the fungus coincided with the beginning of the crisis. In Boyette's words, the findings "most directly implicate the chytrid fungus," although he acknowledged that environmental factors may also have played a part. Because the specimens came from the same place over a long period, they offered a rare chance to compare conditions before and after the declines.
EOne result, however, seemed puzzling at first. The species that suffered the heaviest declines showed no infections at all in the museum collection. The researchers believe this is probably because these frogs died very quickly once they were infected, so that few sick individuals were ever caught and preserved. By contrast, species whose numbers remained relatively stable often carried the fungus. This suggests that these frogs were able to tolerate the infection and live with it, rather than being killed by it. In other words, the absence of the fungus from a species' preserved specimens does not prove that the disease left that species untouched.
FTo find out what has happened since the crisis, the team made six visits to Boracéia, working together with the University of São Paulo. Their surveys, carried out between 2022 and 2024, found 12 of the 17 species that had been recorded there in the past. The other five have not been seen, and their fate remains unknown. Their status is uncertain, and some of them may have disappeared from the site entirely. Most of the surviving species have recovered, even though the fungus is still present in the forest. According to the researchers, this suggests that the frogs have developed greater tolerance of the disease over several generations.
GThe study also highlights the value of museum collections, which are sometimes seen as little more than records of the past. Michael Campana, one of the scientists involved, noted that such collections can preserve evidence of diseases that went unnoticed for decades, allowing researchers to reconstruct outbreaks long after they occurred. Specimens gathered for one purpose may therefore answer questions that their collectors could never have imagined. Nevertheless, the authors are careful not to overstate their conclusions. Their findings apply to Boracéia, and they do not automatically explain the losses that have been reported in other parts of the Atlantic Forest.