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During her regular commute to the scientific station, scientist Miriam San José stoops near a small water body surrounded by dense plants and collects a small plastic audio device.

The device was left there overnight to capture the characteristic calls of the Scinax quinquefasciatus, known by local scientists as an invasive species with consequences that experts are just beginning to comprehend.

Although teeming with remarkable wildlife – including centuries-old giant tortoises, marine lizards, and the famous finches that sparked Darwin's evolutionary theory – the Galápagos archipelago near the shoreline of South America had historically been devoid of frogs and toads.

In the late 1990s, this changed. Some small tree frogs made their way from continental Ecuador to the archipelago, probably as hitchhikers on cargo ships.

Genetic studies suggest that, over the years, there have been multiple accidental arrivals to the islands, and the frogs now have a strong foothold on two islands: Isabela and Santa Cruz.

The numbers is expanding so quickly that researchers have been struggling to keep track, calculating numbers in the hundreds of thousands on each island, across urban and farming areas, but also in the protected natural reserve.

When San José marked frogs and attempted to find them in the subsequent 10 days, she could find only a single marked frog from time to time, indicating their populations were enormous.

They calculated six thousand frogs in a single pond. "Our estimates are still very conservative," states the researcher. "I'm pretty sure there are additional numbers."

Deafening Noise and Rising Worries

The amphibians' abundance is clear from the acoustic chaos they create. "The amount of frogs and the noise – it's really incredible," says San José.

For the scientists, their nightly mating calls are helpful in estimating their presence in remote areas, using recorders like the one outside San José's workplace.

But local farmers say the sounds are so loud they prevent sleep at night.

"In the rainy period, I regularly hear their croaks and they're extremely loud," says Jadira Larrea Saltos from the island.

"At first it was a shock, observing the first frogs in the region," says Larrea Saltos, who started noticing their large numbers about three years ago when one leaped on her palm as she was stepping out of her front door.

Ecological Impact Remains Unclear

The sound isn't the fundamental problem, though. While the amphibians has been in the Galápagos for nearly three decades, scientists still know limited information about its effect on the islands' delicately balanced terrestrial and aquatic ecosystems.

On archipelagos, it is very common for invasive organisms to thrive, as they have few of their enemies. The islands has over sixteen hundred introduced species, many of which are significantly affecting the safety of its endemic ones.

A 2020 research suggests the invasive amphibians are voracious bug eaters, and might be disproportionately eating uncommon insects found exclusively on the islands, or reducing the food sources of the region's rare birds, disrupting the food chain.

Unusual Traits and Control Challenges

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

Their development stage is also highly inconsistent, with some larvae turning into frogs very rapidly and others taking a long time: San José observed one which remained as a tadpole in her lab for six months.

"We really don't know this aspect," she says, concerned the larvae could be impacting the region's clean water, a very scarce commodity in the islands.

Techniques to curb the amphibians in the beginning of the century were mostly unsuccessful. Conservation officers tried capturing large numbers by hand and gradually increasing the salt content of ponds in without success.

Research suggests spraying coffee – which is highly toxic to amphibians – or using electrocution could assist, but these methods aren't always safe for other rare Galápagos organisms.

Without answers to more of the basic issues about their biology and effect, removing the amphibians might not even be the correct way to proceed, says San José.

Financial Obstacles for Study

While she expects the increasing use of eDNA techniques and genetic examination will assist her team make sense of the invasive species, financial support for the project has been hard to come by.

"Everybody wants to give support for protecting frogs," says the researcher. "But it's harder to find funding for an introduced frog that you might want to manage."