TAMPA — For years, biologists at the University of South Florida have worked with their peers worldwide in trying to understand the global demise of amphibian populations and how to reverse it.
This month, there is a potential breakthrough.
Researchers affiliated with USF have found that frogs can acquire resistance to a deadly fungus that has had a catastrophic effect on amphibians. The findings of professors Jason Rohr of USF and Taegan McMahon, who recently earned her doctorate at USF and is now at the University of Tampa, were displayed on the cover of the July 10 issue of Nature, one of the world's most prestigious scientific journals.
“This is definitely a beginning step,” said McMahon. “It's a very exciting beginning step, but there's a lot more work that needs to be done to know if we can actually implement actual management plans using this.”
The spread of a fungus, Batrachochytrium dendrobatidis, has been identified as a major culprit in the decimation of amphibian populations around the world. How bad is it? Scientists say that more than one-third of the nearly 7,000 amphibian species are listed as threatened or are already extinct.
It's difficult to imagine during Florida's rainy season, with the sounds of frogs and toads piercing the night, but observers from Australia to Central America have described once-healthy amphibian habitats falling completely silent.
The implications are huge. Amphibians play a major role in world ecology. And the human connection is significant, said Rohr.
Important products have come from amphibian secretions, he said, from adhesives to medications treating high blood pressure, pain and cancer.
“Most humans don't like to be bitten by flies and insects, and amphibians are important predators,” Rohr said. “As a result, they can not only reduce those sorts of annoyances, but also the threat of disease.”
In the northeastern forests of the U.S., amphibians comprise the largest amount of biomass — greater than mammals, birds, and other vertebrates, Rohr said. And those amphibians are key indicators of environmental health — the proverbial “canary in a coal mine.”
❖ ❖ ❖
With that backdrop, the Tampa researchers hypothesized that frogs might be exposed to the deadly chytrid fungus in the lab, “cured” of ensuing disease, then build up immunity.
The chytrid fungus dies in high temperatures, so the researchers were able to infect frogs, raise the temperature of their enclosures and, in turn, the frogs' body temperatures. The fungus eventually disappeared.
“What we found was that given previous exposures, amphibians could mount an immune response, almost like a response that we would get if we were vaccinated against a pathogen,” McMahon said. “We found that we have the potential to vaccinate frogs in the lab to this chytric fungus, which may lead to some positive management plans ... and some hope for amphibians.”
Rohr and McMahon found that with each subsequent exposure to the fungus, frogs built up a stronger immunity.
They had further success. Immunized toads also developed a behavioral response: When placed in an enclosure with the fungus present on one side but not the other, 70 percent of the toads who had been exposed to the fungus avoided the contaminated side, while “na´ve” toads moved freely about the chamber, risking infection.
Finally, in follow-up experiments, the researchers learned the amphibians didn't have to be exposed to a live pathogen. Exposure to the dead fungus resulted in the same levels of acquired resistance.
That development “is really quite promising, because you never want to have to expose a population to a live pathogen, and you're certainly never going to get approvals to release a live pathogen into the environment,” Rohr said.
When biologists talk about “vaccinating” amphibians, they don't mean chasing them with a syringe. It might mean cultivating and killing large quantities of the chytrid fungus then spreading the dead fungus in ponds.
❖ ❖ ❖
In the wake of the publication of their findings, Rohr and McMahon have received some pushback from scientists who question whether such a management program would be practical or effective. No offense taken, they said.
“That's very healthy,” McMahan said. “In science, it's good to have people ask those questions, because it makes you really focus your next step and try to make it the most appropriate to answer the kinds of questions that are out there.”
Rohr said the two have a healthy amount of skepticism themselves.
“We'll be the first ones to say there are a lot of additional questions that need to be answered,” he said.
To that end, they will be applying for a National Science Foundation grant to expand on what they've learned. They'll attempt to determine how long the frogs' immunity to the chytrid fungus lasts, the optimal concentration of the fungus and how long exposure has to occur to produce immunity, and whether they can create immunity in the tadpole stage.
Jamie Barichivich, a wildlife biologist with the U.S. Geological Survey, was impressed with the USF research, and said he would like to see if they could achieve the same results with additional species of frogs and toads, particularly those in the worst states of decline.
The USGS has also been studying the affect of the chytrid fungus on amphibians.
“It's very nice to see the two different kinds of responses — the behavioral and the immunological,” Barichivich said. “That means there are some real possibilities.”
The two USF professors allowed themselves a bit of basking in the honor of publication in such a prestigious journal.
Their work has been covered by the New York Times and England's The Guardian.
The Nature article was five years in the making and involved 10 USF researchers and others from Allegheny College, Vanderbilt University and Oakland University.
McMahon acknowledged doing a “happy dance” when she heard the news it was accepted for publication. Rohr said appearing on the cover is “an additional feather in the cap. We were really proud.”
But the focus remains on reversing the frightening decline of the world's frogs, toads, lizards and other amphibians.
“We hope that amphibian conservation research in the future, hopefully in the next decade or so, is going to move toward this dire story and start to identify some management options that could improve the prospects for these amphibian populations,” said Rohr.
“Hopefully, the model will say, yes, this is doable.”