Amphibian biologists watched in horror in 2004 as the frogs of El Copé, Panama, began dying by the thousands. The culprit: Batrachochytrium dendrobatidis, a deadly fungus more commonly known as chytrid. Within months, roughly half of the frog species native to the area went locally extinct.
A new study suggests that, within a decade, the species remaining in El Copé developed the ability to coexist with chytrid fungus. In a later field study, the researchers found that frogs infected with the fungus survived at a nearly identical rate compared with uninfected frogs.
The results, published in the current issue of Ecological Applications, suggest that the frogs underwent ecological and/or evolutionary changes that enabled the community as a whole to persist, despite severe species losses. According to the researchers, the results could mean good news for other hotspots of amphibian biodiversity hit hard by the chytrid fungus, such as South America and Australia.
“Our results are really promising because they lead us to conclude that the El Copé frog community is stabilizing and not drifting to extinction,” said Graziella DiRenzo, the lead author and a postdoctoral researcher at Michigan State University (formerly at University of Maryland). “That’s a big concern with chytrid worldwide. Before this study, we didn’t know a lot about the communities that remain after an outbreak.”
DiRenzo and her colleagues returned to the same small, 2-square-kilometer field site in El Copé every year from 2010 to 2014. They broke the field site down into smaller, 20-meter subsites, repeatedly sampling the subsites several days in a row within a season. Each time, the researchers tested individual frogs for the presence of the fungus while assessing the severity of any disease symptoms.
The researchers then developed a novel model to assess disease dynamics in communities beset by an outbreak. The frequent, repeated sampling of frogs in the field allowed the team to minimize biases in the model and enabled the researchers to conclude that infected frogs were surviving at the same rate as uninfected frogs. This surprising result strongly suggested that the frog species remaining in El Copé developed the ability to tolerate the fungus and survive its deadly effects.
“Our statistical model allowed us to estimate amphibian survival and disease dynamics in a case where the small size of the remaining amphibian community prohibits the use of more traditional analysis methods,” said Elise Zipkin, assistant professor at MSU and co-author on the study. “This new modeling framework offers unprecedented opportunities to examine the factors impacting small and declining populations decimated by disease.”
The researchers suggest that the El Copé frog community stabilized through an effect known as “eco-evolutionary rescue.” In this scenario, some species may have evolved tolerance to the fungus while other highly infectious species died off and stopped contributing to the spread of the pathogen. The fungus itself may have also become less virulent and the frog community as a whole may have undergone other types of restructuring.
The researchers note that, because the frog community in El Copé had been well-studied for years before the 2004 outbreak, the research site provides a rare window to assess changes to a frog community as a result of widespread chytrid infection. If the community has stabilized here, the researchers say, it is likely that other hard-hit frog communities elsewhere in the world may have undergone similar adaptations – even where disease has reduced the overall number of species and/or individuals.
“The frogs of El Copé are not doing great, but they’re hanging on. The fact that some species survived is the most important thing,” said Karen Lips, University of Maryland biology professor and co-author. “If enough frog species in a given place can survive and persist, then hopefully someday a vibrant new frog community will replace what was lost.”