Habitat loss is messing with these frogs’ poison supply - Vox

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This story is part of Down to Earth, a Vox reporting initiative on the science, politics, and economics of the biodiversity crisis.

For many creatures, from stick bugs to leaf-litter toads, the best way to stay alive is to blend in. Then there are poison frogs, which use precisely the opposite strategy. Painted in vivid reds, golds, and blues, these cracker-sized amphibians pop against the dull forest floor.

Their strategy works because — as you might guess — poison frogs are poisonous. If a snake, bird, or other predator swallows one of these frogs, they’ll spit it out and, if all goes to plan, never try to eat one again. The frogs’ bright color is a warning sign that says: Remember: You’ve tried this before, and it didn’t go so well.

But as researchers are learning, there may be a problem with that approach.

Amphibians are among the most vulnerable animals, in part because they face a large number of threats, from the deadly chytrid fungus to poaching. But none is more potent than habitat loss. Deforestation is devastating the tropics — surging 12 percent in 2020 compared to 2019 — shrinking the places where poison frogs live. And now, some researchers suspect it could actually be affecting, and perhaps even dimming, the poison that keeps these amphibians alive.

It all has to do with the remarkable way that these frogs become poisonous in the first place.

If you were to buy a pet poison frog (which, for the record, I am not recommending), chances are it wouldn’t actually be poisonous. That’s because, unlike venomous snakes and spiders, these amphibians — found in the forests of Central and South America, and in Madagascar — don’t manufacture toxins themselves. Instead, researchers believe, they get them from their natural diet of ants and mites.

Certain ants and mites contain organic compounds called alkaloids, some of which are toxic. When the frogs eat them, they’re able to absorb the toxic alkaloids and concentrate them in glands under their skin. “They rely on a really healthy habitat full of ants and mites to be able to acquire their toxins,” said Lauren O’Connell, an assistant professor at Stanford University whose lab is currently studying the amphibians. (It’s not clear where the alkaloids originate.)

A golden poison frog

Peter Gercke/picture alliance via Getty Images

Some of the poison frog toxins, including epibatidine and batrachotoxin, are lethal — a fact not lost on some Indigenous groups of South America. The Emberá people of western Colombia have tipped blowgun darts with the batrachotoxin found in at least three species of poison frog, including the golden poison frog — perhaps the most poisonous animal alive. (Poison dart frogs are a subset of poison frogs.)

Other toxins aren’t deadly but taste foul to predators. When a snake or bird tosses back a frog, the predator will likely vomit it up, said Juan Santos, a herpetologist and assistant professor at St. Johns University. Even if the frog dies in the process, he said, the incident can benefit other members of its species, because the predator will learn to avoid similar-looking animals when hunting for its next meal.

But these chemical defenses depend on a frog’s local menu — and poison frogs may lose their poison if they can’t get their fill of alkaloid-wielding ants and mites.

While ants appear to be everywhere and bothered by nothing, they’re highly sensitive to changes in their environment. So much so that, like the proverbial canary in the coal mine, they often help scientists measure the quality of habitats. Changes to land tend to change the makeup of ant populations, and deforestation can reduce the total number of ant species in a given area.

There’s recent evidence that deforestation — which is common in tropical countries where these amphibians reside — could be altering the ant populations that poison frogs rely on, and thus the amounts and types of toxins they carry.

In a study published last April, researchers analyzed Diablito poison frogs — “little devils” that are patterned like lava — in a forest and a nearby deforested cattle pasture in northwestern Ecuador. They found that the two habitats harbored distinct communities of ants. And overall, frogs in the forest ate more ants.

A Diablito poison frog

Eva Fischer

Those differences appeared to affect the quantity of toxins found in the frogs: Poison frogs in the pasture had a significantly lower toxin load, the analysis revealed.

According to Nora Moskowitz, a doctoral student at Stanford and the study’s lead author, that’s likely because the pasture had fewer toxin-containing ants that frogs rely on to accrue their poison. (Moskowitz described using plastic cups to catch the frogs, which brings its own challenges. “I’ve cried over missing a frog before,” she said.)

Frogs in the pasture also spent less time foraging, likely because the habitat is hotter and drier, meaning that they ate less overall, she said. “Forest frogs ate a much higher percentage of ants, and as far as we know, that’s where the majority of their alkaloids are coming from,” she said.

So does that mean deforestation is making poison frogs less poisonous? Not necessarily.

The study showed that frogs in the deforested land had a smaller toxin load, but that doesn’t mean they were less poisonous. The researchers didn’t measure how potent the toxins were to predators, and researchers still don’t understand the impact of all the different kinds of alkaloids, said Rebecca Tarvin, an assistant professor of biology at the University of California, Berkeley who was not involved in the study. “It’s clear that land use changes the chemical profiles,” Tarvin said. “We just don’t quite understand what that means.”

The Diablito poison frogs were collected in a forest and cattle pasture in Santo Domingo de los Tsáchilas, a province in northwestern Ecuador.

Eva Fischer

Santos also pointed out that the research captures just one point in time and only assesses a single species. That makes it hard to draw any firm conclusions about the impact of deforestation on poison frogs.

But if poison frogs are becoming less toxic, that would be a serious problem. These frogs evolved bright colors specifically so that predators can recognize them. “They’ve bought into their coloration genetically,” O’Connell said. So if they do lose their potency, she added, “there’s going to be some point at which the predators can see that they’re not as toxic and won’t avoid them anymore.”

While poison frogs look delicate and are highly visible, they may actually be somewhat more resilient than other types of frogs. In general, they don’t seem to be all that susceptible to chytrid fungus, which has devastated frog populations over the last three decades (though researchers still know little about how the fungus is affecting the frogs).

Poaching of poison frogs is still a problem, but some organizations are now breeding them in captivity to create legal markets. WIKIRI, a company based in Ecuador, sells amphibians such as the Diablito poison frog as pets, partly in an effort to undermine illicit trade, and puts some of its profits toward habitat restoration.

The company WIKIRI in Ecuador raises poison frogs and other amphibians in captivity to sell legally into the pet trade.

Wikiri

But the future of these frogs remains highly uncertain. Deforestation in the tropics has been especially severe in several regions home to poison frogs, including Brazil, Bolivia, and Peru. And there’s the other big threat that’s only getting worse: climate change. “We are really worried about the impacts of climate on these amphibians,” Santos said.

Scientists fear that unchecked deforestation and global warming could dry out large swaths of the Amazon rainforest over the next several decades, transforming them into savannas, Santos said. That kind of sweeping change could impact entire ecosystems and accelerate the pace of extinctions.

“My concern is that 50 years from now these [frogs] will just be a footnote or a page in a book about things that used to be around us,” he said. “That is always in the back of my mind. The race to extinction started a long time ago. We are now seeing extinctions in real-time.”

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