Museum frogs point to chytrid fungus in a decades-old Atlantic Forest die-off

By the source's description, Heyer's two 1970s campaigns at Boracéia came before the declines and his three 1980s campaigns came in their aftermath ^[3]. The die-offs ran from 1979 to 1982 ^[2], and specimens from before 1979 showed no sign of infection ^[7]. That puts the fungus's arrival between the last 1970s campaign and the first 1980s one, if the earlier samples were large enough to catch it.

The team sampled more than 370 specimens from 17 species ^[4], roughly 22 or more per species on average ^[16]. The article does not report counts by species, by campaign or by infection status. How firmly a clean 1970s result excludes an earlier, low-level presence depends on how many frogs were in it.

Across species, the results are odd. The species that declined most had no detectable infections, and the species that never declined were highly infected ^[13]. The authors hypothesize that this reflects natural disease tolerance in the community ^[14]. Jack Boyette, who led the study published in Molecular Ecology ^[10], extended the idea to the hardest-hit species. "By contrast, the species that declined most dramatically may have been so susceptible that infected frogs died before Heyer could collect them," he said ^[15].

Boyette's explanation is a hypothesis, and his wording is "may have". It explains why the hardest-hit species look clean. It also means those species supply no direct evidence of infection, so the case for chytrid at Boracéia rests on timing across the community, which the authors read as rapid spread through naive hosts ^[9].

Some scientists had attributed the Boracéia die-offs to climate change or extreme weather ^[2]. Boyette said: "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" ^[8]. Coincidence in time is the evidence offered. In our view "likely culprit" is the strength the data earn: the fungus was absent before and common after, and the step from presence to cause is inferred.

The pathogen was formally described in 1999 ^[1], about 20 years after the die-offs began ^[12]. For declines that predate its discovery, the source says, establishing the fungus's role has remained a challenge ^[11]. Six visits between 2022 and 2024 found 12 of the 17 species ^[5]. The fate of the other five is uncertain, though their continued absence raises concerns that they may have been lost locally ^[6].