High in the misty mountains of Honduras, the world looks primeval, with glistening pine trees draped in moss and leafy bromeliads. This is ideal habitat for tree frogs, and some of the species hiding in the foliage are found nowhere else in the world.
Unfortunately, a virulent fungus is also lurking in the region, and if it can't be stopped, these frogs may disappear forever.
The Cusuco spike-thumb frog (Plectrohyla dasypus) is one of several endangered species from the cloud forests of Honduras that scientists hope to save from extinction. [Photograph By Jonathan Kolby]
Over the past few decades, amphibians have been hammered by the invasive chytrid fungus. The pestilence seems to interfere with keratin in a frog's skin, making it hard for the animal to breathe and regulate electrolytes, often leading to a heart attack. Worldwide, hundreds of amphibian species have become endangered or gone extinct due to the fungus—a single forest in Panama lost 30 amphibian species in about a year.
American biologist Jonathan Kolby hopes to turn the tide in Honduras with an experimental rescue attempt. Using reclaimed shipping containers, he's currently setting up a center to research and treat frogs from the cloud forest of Cusuco National Park, which lies in the northwestern part of the country. This week the team kicked off an Indiegogo campaign to raise funds for the project.
When it opens, the Honduras Amphibian Rescue and Conservation Center will try "head starting" the frogs by catching individuals that have been infected with chytrid fungus and treating them using various methods. If they can get the frogs disease-free, they will release the animals back into the park, tagging them so they can monitor their progress.
It's an approach that has only been tried a few other times with frogs, and with mixed results. But the team hopes their efforts with three especially vulnerable species will bolster populations of the already endangered amphibians.
Scientists aren't sure how the frog-killing form of the chytrid fungus, Batrachochytrium dendrobatidis, swept across so much of the planet, although many suspect it may be traveling due to imports and exports of amphibians for food or the pet trade. It's also possible birds or even storms may move the fungus around.
Once the disease arrives in a region, tadpoles can pick up chytrid spores from the water where they live. But the disease becomes most aggressive as a tadpole starts becoming an adult through the process of metamorphosis, says Kolby. During that time, the frog's body suppresses its immune system to allow the change in forms, and that lets the fungus wreak havoc on its skin.
The exquisite spike-thumb frog (Plectrohyla exquisita) can grow to the size of a fist. [Photograph By Jonathan Kolby]
Chytrid tends to be most devastating in cool, moist environments, such as cloud forests, with infection rates particularly bad in Oceana and the Americas. The fungus has been running rampant in Honduras since at least the 1980s.
Kolby has counted 30 frog species in Cusuco National Park while surveying with the British conservation group Operation Wallacea. Of those, 14 are endangered. Kolby will focus on trying to rescue three, all of which are under heavy assault from chytrid.
Two of them are related species of spike-thumb frogs, which have a bone spur jutting off their hands that is thought to be used in territorial combat. The exquisite spike-thumb frog (Plectrohyla exquisite) can grow to the size of a fist, while the Cusuco spike-thumb frog (Plectrohyla dasypus) gets to be about half that size.
The third species, the mossy red-eyed frog (Duellmanohyla soralia), resembles the classic red-eyed tree frog that sometimes appears as a symbol of Central America—except the mountain frogs are a little smaller and “way cooler,” says Kolby. The tadpoles of this species are covered in metallic green spots and they swim upside down, perhaps so they can feed on tiny food particles on the water's surface.
One additional species, the miles' robber frog (Craugastor milesi), was thought to have already gone extinct on the mountain, likely because of the fungus, although Kolby reignited some hope when he found one individual in 2008 and another in 2013.
"If we find any more of those frogs, we'll probably bring them in and see if we can captive breed them, but it might already be too late and we may never see them again," says Kolby.
The decline of all these species may already be having a ripple effect in the ecosystem. Scientists suspect that the endangered palm viper (Bothriechis marchi), which primarily eats tree frogs, has been declining too.
The mossy red-eyed frog (Duellmanohyla soralia) has very unusual tadpoles that swim upside down. [Photograph By Jonathan Kolby]
To combat the fungus, Kolby's team will collect young animals from the forest and then treat them at their new biosecure facility, located at the Lancetilla Botanical Garden in Tela. Healthy frogs will be released back into the wild, with tiny tags on their legs so they can be reevaluated later. Full-grown adults of these species are less susceptible to chytrid, so it’s less likely they will get re-infected.
The scientists won't be the first to try to treat frogs for chytrid infection, and other projects have had varying degrees of success, says Louise Rollins-Smith, a biologist and professor at Vanderbilt University in Tennessee who has studied chytrid but is not affiliated with this work. The most popular approach is to use a strong anti-fungal drug, but that can be hard on the sensitive animals. Heating frogs past the tolerance point of the fungus is cheaper, if the frog can survive the higher temperatures.
No one has yet tried heating up frogs from Honduras, so Kolby and his colleagues are going to collect some and give it a go. The fact that he has sometimes seen the frogs soaking up direct sunlight suggests they can take the heat, he notes.
"But ultimately, we'll use whatever is least invasive and most financially sustainable," says Kolby.
In the best-case scenario, frogs that survive the fungus may pass down their resistance, or at least produce stronger offspring, helping the animals adapt to the threat faster than evolution alone. Even if that doesn’t happen, Kolby says the frogs will be more likely to survive if there are 10,000 adults out there producing tadpoles instead of just 1,000.
Tricky Track Record
The plan "is a good approach and is definitely worth trying," says Rollins-Smith. But Kolby's team has their work cut out for them. While such head-starting conservation programs have been used effectively for decades with birds, turtles, mammals, and other creatures, the technique has only rarely been tried with amphibians struggling with chytrid, and those projects have been hit or miss.
In Australia, the tiny black-and-yellow Southern corroboree frog was nearly pushed to extinction by the fungus. Scientists from the Taronga Zoo captured a few individuals before they disappeared, treated them, and released them. As a result, the species seems to be making a slow recovery.
The palm viper (Bothriechis marchi) is declining in response to a decrease in its base of prey: tree frogs. [Photograph By Jonathan Kolby]
A similar effort with Wyoming toads hasn’t gone as well, though, with many released individuals getting sick a second time. Still, adults of that species seem to be more susceptible to chytrid infection than adults of the species in Honduras, says Kolby.
"Ultimately," says Rollins-Smith, "for many frogs to survive, they are going to have to get stronger defenses, or the chytrid fungus is going to have to get less virulent."
In the meantime, Kolby is hoping he can lend a helping hand to at least a few endangered frogs.