The Galápagos Islands Lacked Any Indigenous Amphibians. Then Countless Numbers of Frogs Made Their Home

On her regular commute to the scientific station, scientist Miriam San José crouches near a shallow pond surrounded by dense plants and collects a small plastic audio device.

The device was left there overnight to capture the characteristic calls of the Fowler's snouted treefrog, recognized by Galápagos researchers as an invasive threat with consequences that scientists are starting to comprehend.

Although abounding with remarkable wildlife – including ancient giant tortoises, swimming lizards, and the well-known birds that sparked Charles Darwin's theory of evolution – the Galápagos archipelago near the shoreline of South America had long remained free of amphibians.

During the 1990s, this changed. Some tiny amphibians traveled from mainland Ecuador to the islands, likely as stowaways on transport vessels.

Genetic research indicate that, through time, there have been multiple accidental arrivals to the archipelago, and the frogs now have a strong foothold on several islands: multiple locations.

The population is expanding so rapidly that scientists have been struggling to keep track, estimating numbers in the millions on each island, across urban and agricultural areas, but also in the protected natural reserve.

When the biologist tagged frogs and attempted to recapture them in the subsequent week and a half, she could find only a single marked frog from time to time, suggesting their populations were massive.

They calculated 6,000 frogs in a single pond. "Our estimates are still very low," states the researcher. "I'm pretty sure there are even more."

Acoustic Chaos and Rising Worries

The frogs' abundance is evident from the sound disruption they cause. "The number of frogs and the sound – it's truly insane," comments San José.

For the scientists, their nocturnal mating calls are helpful in estimating their presence in far-flung areas, using recorders like the one outside San José's office.

But local agricultural workers say the calls are so loud they prevent sleep at night.

"In the rainy period, I regularly hear their calls and they're extremely loud," says a local coffee farmer from Santa Cruz.

"At first it was a surprise, seeing the initial frogs in the region," says Larrea Saltos, who started observing their abundance about three years ago when one leaped on her palm as she was walking out of her front door.

Ecological Impact Stays Unknown

The noise isn't the primary problem, however. While the species has been in the islands for almost three decades, scientists still know very little about its effect on the islands' precariously balanced terrestrial and aquatic environments.

On archipelagos, it is very common for invasive organisms to prosper, as they have none of their natural predators. The Galápagos has 1,645 introduced types, many of which are significantly affecting the safety of its native ones.

A 2020 research suggests the non-native frogs are voracious insect consumers, and might be disproportionately eating rare bugs found exclusively on the archipelago, or depleting the nutrition of the islands' uncommon avian species, disrupting the ecosystem balance.

Unique Characteristics and Control Challenges

The Galápagos frogs have shown some unusual traits, including surviving in slightly salty water, which is uncommon for amphibians.

Their metamorphosis process is also extremely variable, with some larvae becoming frogs very quickly and others taking a long time: the researcher witnessed one which remained as a larva in her laboratory for six months.

"We truly don't know this aspect," she says, concerned the larvae could be impacting the islands' freshwater, a very scarce resource in Galápagos.

Techniques to curb the frogs in the early 2000s were mostly unsuccessful. Conservation officers tried collecting large numbers by manual methods and gradually increasing the salt content of lagoons in vain.

Research suggests spraying caffeine – which is extremely toxic to amphibians – or using electrocution could assist, but these approaches aren't necessarily secure for other rare island species.

Lacking answers to more of the fundamental questions about their biology and effect, culling the amphibians might not even be the right way to advance, says San José.

Financial Obstacles for Research

While she expects the increasing use of eDNA methods and genetic examination will assist her team make sense of the invader, financial support for the research has been hard to obtain.

"Everybody wants to give support for preserving frogs," says San José. "But it's more difficult to find funding for an introduced frog that you might want to control."