Wild populations of frogs appear to have evolved resistance to a lethal skin-eating fungal pathogen. Now, researchers report that moving some of these resistant frogs to habitats where the fungus has killed all the others is an effective way to restore their populations.
The fungus Batrachochytrium dendrobatidis (Bd) 鈥� a chytrid fungus 鈥� is the pathogen responsible for one of the most deadly wildlife diseases, having decimated the populations of hundreds of amphibian species, and contributed to the extinction of many others. The fungus infects the skin and ultimately causes heart failure.
Researchers have tried a to protect vulnerable amphibians, including vaccinating them against the fungus and altering their microbiomes, all to mixed effect. at the University of California, Santa Barbara, and his colleagues took a different approach, focusing on the potential for the two species of mountain yellow-legged frog (Rana muscosa and Rana sierrae) in California to evolve resistance to the fungus through natural selection.
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Though Bd has spread in other parts of the world since the early 20th century, the mountain yellow-legged frogs that were once abundant across the mountain lakes and ponds of California鈥檚 Sierra Nevada were spared from the fungus until the middle of the century. Instead, most of their now critically endangered populations were killed by trout introduced in the late 19th century.
Knapp and his colleagues were working on getting rid of the trout to restore the frogs in Yosemite National Park when the fungus began tearing through frog populations, driving them down to less than 10 per cent of their historical range. 鈥淚t was a horrible rollercoaster,鈥� he says. The researchers were forced to rethink their restoration efforts: if frogs bred in captivity were simply introduced to areas cleared of trout they would be killed by the fungus.
The researchers began moving frogs from wild populations that appeared to be resistant to the fungus to areas where all the frogs had been lost. Doing so meant the researchers were assisting a natural process known as that can see populations recover through the spread of beneficial genes. Over the next 15 years, starting in 2006, they moved hundreds of frogs 鈥� sometimes by helicopter, sometimes in snow-filled backpacks 鈥� between remote mountain lakes.
The researchers also compared the genomes of frogs from resistant populations with those of frogs that had not been exposed to Bd. They found the resistant frogs differed in several genes associated with the immune system, suggesting the source of their resistance to the fungus was evolved, rather than acquired immunity or some feature of their particular habitats, says Knapp. It also suggested that resistance could be inherited by subsequent generations.
The effort appears to have been effective. The researchers now report that nine of the 12 sites where frogs were reintroduced have reproducing populations. Their ecological modelling also suggests that in half of the sites those populations are robust enough to have a greater-than-not chance of survival for at least the next 50 years. The researchers are now starting similar efforts in Sequoia and Kings Canyon national parks in California.
鈥淔or so long it looked just like gloom and doom and just losing one population after another,鈥� says at the University of Pittsburgh in Pennsylvania. 鈥淭hey have a really convincing line of evidence that that wasn鈥檛 the end of the story.鈥�
Other species have started showing signs of resisting the pathogen, and this approach might work for them, says Richards-Zawacki.
bioRxiv
