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In studying the dramatic recent global decline of frog and salamander populations, researchers increasingly cite emerging diseases as major causes. Among these, one particularly mysterious new pathogen, the chytrid fungus Batrachochytrium dendrobatidis appears to be especially lethal, having been implicated in massive declines and waves of extinction in Central America and Eastern Australia.
A report published today in the October 4 issue of The Public Library of Science, includes findings that may give important new clues concerning this pathogenic organism's behavior in the wild, and a step towards understanding how it spreads.
The paper, written by Arizona State University biologist Richard Retallick, Hamish Ian McCallum from the University of Queensland, and Richard Speare from James Cook University, finds evidence for the persistence of the fungus in surviving populations of Taudactylus eungellensis, a species that had suffered a massive decline.
The species largely disappeared from rainforest streams in the mid-1980's, but surviving remnant populations sampled in the mid-1990's show the continued presence of the fungus in 15% to 18% of the sampled frogs. Later investigation showed that infected frogs had similar survival to uninfected frogs.
"This shows that frog populations can persist with an endemic infection of the chytrid fungus," said Retallick. "The presumption until now has been that when a population is hit by this pathogen it is wiped out. It doesn't appear to be that simple.
"In Central America and Eastern Australia, the same pattern has occurred – the frog populations have been healthy and all of a sudden there is a crash and some species are wiped out entirely. In Eastern Australia several species crashed to extinction, among them the Northern Gastric Brooding Frog, which lived in the same environment as T. eungellensis." Retallick said.
T. eungellensis populations crashed but survived in remnant colonies totaling about five percent of the original population. The decline was not directly observed, but the remaining populations were studied in the mid-1990's and appeared to have stabilized. Frog toes taken to identify and study individual frogs were later found to show evidence of the fungus.
"We found chytrid on apparently healthy frogs, and some of these frogs were recaptured later," Retallick said. "Our records show that frogs with the fungus can persist for three years, but we don't know whether these frogs coexist with the fungus during that time, or clear it and then live as normal healthy frogs. Understanding that would give us some insight into how the fungus operates in the wild."
The researchers found a steady rate of infection from year to year, but a higher rate of infection in samples taken during the cooler seasons, leading the researchers to speculate that temperature may be a factor affecting the virulence of the fungus and/or the ability of the frogs to survive it.
While the Gastric Brooding Frog went extinct, and T. eungellensis suffered its severe decline, yet another species in the area, Litoria lesueuri, was apparently completely unaffected by the fungus, despite a high rate of infection in that species. This suggests that some species may tolerate the pathogen, which raises questions about how the disease persists and spreads from frog to frog and region to region.
"T. eungellensis seems to have been 'lucky' in terms of having at least survived the fungus," Retallick noted, "but another species living beside it -- the Gastric Brooding Frog -- wasn't. If you do have a situation where all the frogs get infected and die, does the fungus die out too, or does it persist elsewhere in the environment?
"We don't yet know how or whether the fungus gets around in the environment and one possibility is that resistant species of frogs carry it around," Retallick said. "Because Litoria lesueuri populations carry high levels of infection of this fungus and the frogs show no sign of distress, this species may very well be a reservoir for the fungus."
Journal
PLOS Biology