Variable harlequin frogs (Atelopus various) and many other amphibians in the lush forests of Panama have been slammed by a globe-creeping fungus. The murderous pathogen attacks frogs, toads, salamanders and wormlike creatures called caecilians, and when it reached Panama early in this century the frogs vanished along streams where they had once been abundant. That's why the scientific community took notice this year when researchers announced that some of the vanished frogs were popping up again.
There's a simple explanation, potentially, for the apparent comeback: evolution. This might be natural selection working at lightning speed, an example of survival of the fittest frogs.
That would have implications beyond Panama, beyond frogs, beyond amphibians. The planet's biodiversity is threatened by habitat destruction, climate change, hunting and fishing, the spread of invasive species and all the other ways that seven billion human beings alter and degrade the environment.
Experts warn that we're witnessing a mass extinction event, the first such die-off since an object from space 66 million years ago hit the Earth and ended the long reign of dinosaurs.
No one expects natural selection to solve a problem as massive as the loss of biodiversity, but it might save some animals and plants here and there.
The rebound of the frogs was reported in March in the journal Science by two scientists, Jamie Voyles of the University of Nevada at Reno and Corinne Richards-Zawacki of the University of Pittsburgh, who had had worked in Panama in the early 2000s amid abundant frogs species, only to see the fungus arrive and essentially destroy the objects of their research. Research published last week concluded that the fungus originated in East Asia in the 1900s, and pointed to expanding global trade and the market in exotic pets as likely factors in the pandemic.
Voyles and Richards-Zawacki went back to Panama in 2012, trekking through the forest on a search for frogs.
“We were really in the mud, long treks, pretty discouraged. Then we came across a brilliant yellow Adelopus sitting on a mossy boulder looking healthy and happy,” said Voyles. They found more frogs. Nine species across three separate sites showed signs of a recovery from the die-off.
The researchers' initial assumption was that the fungus had grown less virulent. That's another feature of natural selection and disease
: The most vicious pathogens in nature will often evolve into milder forms. That's a good strategy , because pathogens that don't exterminate their host species are able to live longer and spread more widely.
The researchers, however, ruled out that hypothesis. They took uninfected frogs raised in captivity and exposed them to the Batrachochytrium dendrobatidis (Bd) chytrid fungus obtained and frozen early in the century
. They exposed other frogs to the more recently isolated fungus. Each batch of pathogens killed infected frogs at the same rate.
That put the focus on the frogs themselves. The frogs have a community of bacteria and fungi that live on their skin, and perhaps these microbes had found new ways to kill or compete with the deadly fungus. The frogs also secrete infection-fighting proteins, and that's where classic natural selection could be showing its hand: The frogs with the best immune systems might have survived while less-resistant frogs were culled from the population. Laboratory tests showed that frogs captured recently were more resistant to the fungus than frogs bred from populations captured before the fungus arrived in Panama.
When Voyles, Richards-Zawacki and their collaborators published their findings, they wrote that the “evolution in host defenses” might explain the apparent recovery. They did not specify which defenses, exactly. They framed their conclusion cautiously. They noted that they couldn't rule out that some frogs had migrated up the slope from lower elevations, making it look as though some of the original population survived, though they called that unlikely. The title of the paper did not assert causation, but rather focused on one slice of their findings: “Shifts in disease dynamics in a tropical amphibian assemblage are not due to pathogen attenuation.”
Karen Lips, a University of Maryland biologist who has studied amphibians at the same sites in Panama, said she is skeptical that evolution could have come to the rescue for so many frogs.
“They’re claiming that multiple species have simultaneously evolved some sort of resistance to the pathogen — which is even more unlikely,” she said.
This is science in action: Bold claims get close scrutiny. And nature isn't static, predictable, unambiguous. It's worth noting that the subjects of the research are mobile. Lots of things are jumping around here.
Voyles and Richards-Zawacki are the first to admit that they can't say how, exactly, resistance to the Bd fungus might be playing out at the level of the individual frog. And Voyles points out that the study is limited geographically and cannot be used to extrapolate a broad truth about a global problem.
“This is only a very small subset of all of the species in the area. We are not seeing this wholesale rebound of all amphibian species. We are seeing a few, a small number, of species are showing this promising sign,” Voyles said.
Meanwhile there are other battle zones in the fight against pathogenic fungi. A chytrid fungus similar to Bd that kills salamanders has already swept through Europe and threatens to arrive in the United States, home to a huge salamander population. The U.S. Fish & Wildlife Service in 2016 banned the import or interstate trade of 201 species of salamanders, but experts have called for further restrictions on the global trade in exotic amphibians.
Efforts to fight the Bd fungus in the wild have had limited success. One of the few triumphs happened on Spain's island of Majorca, but it was hardly an elegant process. Scientists retrieved tadpoles from ponds, drained the ponds, and treated the tadpoles with antifungals in the laboratory. When that wasn't sufficient to rid the island of Bd, they hit the ponds with powerful disinfectants.
That brute-force, chemically intensive effort can't work at the level of a forest. There doesn't seem to be a high-tech fix so far for the Bd pandemic, said Brian Gratwicke, a biologist at the Smithsonian Conservation Biology Institute. He said researchers have found ways to kill the fungus in test tubes using antifungal bacteria, but efforts to transfer such bacteria to the skin of frogs haven't been successful in the lab. He said that manipulating what lives on amphibians “is going to take a lot more understanding of skin microbial ecology.”
In any case, scientists are not about to stand back and hope that natural selection saves the day.
“If you go extinct before there's been a chance for natural selection to occur, it's too late,” Gratwicke said. “Extinction’s forever.”