Globally, amphibians are going extinct at alarming rates.
New research indicates amphibians that use poisons or other chemical defenses may have higher historic extinction rates than those without such defenses.
Human-caused factors may now be overpowering historic extinction rates.
Amphibians occupy almost every ecological niche, from the highest tropical trees to the most fetid pools of desert water. Brightly colored and cryptically camouflaged, they have evolved an astounding array of defenses – about half of all amphibians are poisonous. But despite their adaptability, these animals are in serious trouble, all over the world.
And now, it seems, their best defense may be their biggest weakness. According to a recent study published in Proceedings of the National Academy of Sciences, poisonous amphibians may be more likely to go extinct than their benign counterparts.
These results surprised study authors Kevin Arbuckle and Michael Speed of the University of Liverpool in the United Kingdom. They designed their study to test a hypothesis in evolution, called escape-and-radiate. Scientists first used this to describe competition between poisonous plants and the caterpillars that eat them. It holds that natural selection favors adaptations that help prey escape predators, because those traits will be passed down to future generations. At the same time, any traits that help predators catch their prey will be passed down, too.
“It’s been described as an evolutionary arms race, and rightly so,” Arbuckle told Mongabay.
According to Arbuckle, few studies had actually tested this hypothesis in animals. Amphibians, with their diverse defenses against predators and their well-studied fossil record, seemed to be the perfect natural laboratory. “Plus,” Arbuckle added, “they’re exceptionally cool.”
Arbuckle and Speed examined historical data from nearly 3,000 amphibian species. Of those, 857 species had information about whether they used chemical defenses.
Evolution is driven by two factors, Arbuckle explained: diversification, (how often new traits arise in a species) and extinction. The escape-and-radiate hypothesis predicts that amphibians armed with defenses like poisonous skin will diversify faster than those without – and in this study, they did. However, the poisonous amphibians also went extinct more quickly.
“We didn’t expect to find higher extinction rates,” said Arbuckle. “The theoretical framework doesn’t predict this.”
Malcolm McCallum, a researcher at Texas A&M University who studies amphibian extinction, told Mongabay he thinks Arbuckle and Speed’s conclusions are plausible. “Making poisons takes a lot of energy – they’re very costly. It could make the animals more vulnerable.”
The researchers are concerned about what their findings might mean for fragile amphibian populations around the world today.
“Amphibians have been described as the ‘canary in a coal mine,’” McCallum said. He noted amphibians are often the first species affected by changes to their environment.
The study by Arbuckle and Speed examined historical data, so they intend to do another study looking at present-day extinction rates. “A lot of the problems amphibians face now are related, in one way or another, to humans,” said Arbuckle. It’s possible what made amphibians go extinct in the past might not be driving their numbers down today. A new study might reveal those distinctions.
Human influences impact amphibians in a number of ways, including habitat destruction. “Most amphibians live a dual lifestyle in water and land environments, so they’re exposed to human threats in multiple areas,” said McCallum.
Some adaptations that help amphibians live in water and on land also make them more vulnerable to pollution. Human skin works like a barrier, keeping our insides in and chemicals out. Amphibians, however, have semi-permeable skin. This helps them absorb oxygen – effectively breathing through their skin. Unfortunately, that permeability also means amphibians can easily pick up chemicals in their environment. It also makes them susceptible to chytrid, a group of fungi spread by humans. When chytrid grows on amphibian skin, it blocks their ability to breathe.
It’s a perfect storm that’s causing amphibian extinctions worldwide.
“As a group, amphibians are probably the most in danger of going extinct, but really, everything is interconnected. The bottom line,” said McCallum, “is we’re screwing up the environment badly.”
Arbuckle, K., Speed, M. P. (2015). Predator defenses predict diversification rates. The Proceedings of the National Academy of Sciences, 191, 12597-13602 dx.doi.org/10.6084/m9.figshare.1248938
Erin E.A. Ross is a graduate student in the Science Communication Program at the University of California, Santa Cruz. Other Mongabay stories produced by UCSC students can be found here