The results have implications for conservation: If more amphibians in the wild have higher-than-expected levels of the deadly fungus, then they could be more vulnerable to collapse due to additional environmental stressors, such as climate change.
"We found that wild frogs that appeared healthy actually carried fairly high levels of infection, and frogs that appeared sick had extremely high infection levels," said study lead author and Penn State University disease ecologist Jeffrey Voyles. "This suggests that the fungus may cause infections more frequently or at a larger scale than we currently understand, which has implications for the ongoing extinction crisis in amphibians."
For the study, the team collected blood samples from 126 individual amphibians, spanning 21 species and found at six locations across California, from the Sierra Nevada Mountains down to Big Sur.
Blood samples were analyzed to create the most comprehensive picture yet of infection prevalence and load across wild amphibian populations in California. The team's findings revealed more cases of infection than expected and an overall increasing trend of Batrachochytrium dendrobatidis (Bd), the microscopic fungal pathogen that causes the disease chytridiomycosis, which is linked to amphibian population declines worldwide.
"The number of infected individuals and overall infection load are increasing — that is, the amount of fungal DNA in infected individuals continues to go up," said Voyles. "What we're seeing with this fungus is similar to what we saw with white-nose syndrome in bats: We saw increasing incidences of the pathogen in bats, and then a few years later we started seeing massive die-offs. This is a slow-moving train that could lead to significant population declines in multiple species."
The researchers note that several species with high infection loads appear to be doing just fine. For example, in the high mountain lakes within the Sierra Nevada Mountains, some populations of Pacific chorus frogs (Pseudacris regilla) show extremely high infection levels but do not seem to be declining. In contrast, California red-legged frogs (Rana draytonii) were consistently infected but had low infection loads, and they are considered "near threatened" by the International Union for Conservation of Nature.
"These results point toward the possibility that amphibian species may differ in their susceptibility to Bd," said Voyles. "We suspect that environmental factors, such as cold temperatures and low UV radiation may provide refugia that enable certain species to coexist with the fungus indefinitely. But, warmer temperatures due to climate change could wipe out these protective factors, enabling Bd to kill large numbers of amphibians throughout the state."
The researchers say their findings, published in the journal PLOS ONE, contribute important information that will aid future research and conservation efforts aimed at protecting California amphibians from Bd.
"We hope that ongoing monitoring of infection prevalence and load can contribute to the early detection of large-scale die-offs," said Voyles. "If populations of vulnerable species start showing rapid increases in infection load, that could potentially signal a catastrophic event that's about to happen."
