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On her regular commute to the scientific station, biologist Miriam San José stoops near a shallow water body surrounded by dense vegetation and collects a compact green sound recorder.

She had placed there through the night to record the distinctive croaks of the Fowler's snouted treefrog, known by local scientists as an non-native threat with effects that experts are starting to understand.

Although teeming with remarkable animals – including centuries-old large turtles, marine iguanas, and the well-known birds that inspired Darwin's theory of evolution – the island chain off the shoreline of South America had long remained devoid of frogs and toads.

During the 1990s, this shifted. Some tiny amphibians traveled from mainland the mainland to the archipelago, likely as hitchhikers on transport vessels.

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

The numbers is expanding so rapidly that scientists have been struggling to monitor, calculating numbers in the millions on each island, across developed and farming areas, but also in the protected natural reserve.

When the biologist tagged amphibians and attempted to recapture them in the following 10 days, she could locate just one tagged frog from time to time, indicating their populations were massive.

They estimated 6,000 frogs in a solitary pond. "Our estimates are still very conservative," says San José. "I'm pretty sure there are additional numbers."

Acoustic Chaos and Rising Worries

The frogs' abundance is clear from the sound disruption they create. "The number of frogs and the noise – it's really insane," says the scientist.

For the researchers, their nocturnal vocalizations are helpful in determining their existence in remote areas, using audio devices like the one near the workplace.

But local agricultural workers say the calls are so loud they keep them up at night.

"During the rainy period, I constantly hear their calls and they're extremely loud," says Jadira Larrea Saltos from Santa Cruz.

"At first it was a surprise, observing the first frogs in the region," says the farmer, who started observing 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 Unknown

The sound isn't the primary problem, however. While the species has been in the islands for nearly 30 years, experts still know limited information about its effect on the islands' delicately balanced land and water ecosystems.

On islands, it is very typical for non-native organisms to prosper, as they have none of their enemies. The islands has 1,645 introduced types, many of which are significantly disrupting the safety of its endemic ones.

A 2020 research suggests the invasive frogs are voracious insect consumers, and might be disproportionately consuming rare insects found exclusively on the archipelago, or depleting the food sources of the region's uncommon avian species, affecting the ecosystem balance.

Unusual Traits and Management Difficulties

The island frogs have shown some unusual traits, including living in slightly salty water, which is uncommon for frogs.

Their development process is also highly variable, with some tadpoles turning into frogs very rapidly and others taking a long time: the researcher witnessed one which remained as a tadpole in her lab for six months.

"We truly don't know this aspect," she says, concerned the tadpoles could be affecting the islands' clean water, a very limited resource in Galápagos.

Techniques to curb the amphibians in the beginning of the century were mostly ineffective. Park rangers tried capturing large numbers by manual methods and gradually raising the salinity of lagoons in without success.

Research indicates spraying coffee – which is highly toxic to amphibians – or using electrical methods could assist, but these methods aren't always safe for other uncommon Galápagos species.

Without solutions to more of the basic issues about their lifestyle and impact, culling the frogs might not even be the right way to advance, says San José.

Funding Challenges for Study

While she hopes the increasing use of eDNA techniques and genetic examination will assist her team make sense of the invader, funding for the research has been hard to obtain.

"Everyone wants to give funding for protecting frogs," says the researcher. "But it's more difficult to find financial backing for an introduced frog that you might want to manage."