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On her daily commute to the scientific station, scientist Miriam San José stoops near a small pond surrounded by dense vegetation and collects a compact green audio device.

She had placed there overnight to capture the characteristic calls of the Fowler's snouted treefrog, known by Galápagos scientists as an invasive threat with consequences that experts are just beginning to understand.

Although teeming with unique wildlife – including ancient large turtles, marine iguanas, and the famous birds that sparked Charles Darwin's evolutionary theory – the island chain off the shoreline of Ecuador had historically been free of frogs and toads.

During the 1990s, this shifted. Some small amphibians made their way from continental the mainland to the archipelago, probably as hitchhikers on transport vessels.

DNA studies indicate that, through time, there have been multiple accidental arrivals to the archipelago, and the amphibians now have a strong foothold on two locations: Isabela and Santa Cruz.

The numbers is expanding so rapidly that researchers have been finding it difficult to keep track, calculating populations in the hundreds of thousands on every island, across urban and agricultural areas, but also in the protected Galápagos national park.

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

They estimated 6,000 frogs in a single pond. "The calculations are still very conservative," states San José. "I'm quite certain there are even more."

Deafening Noise and Growing Concerns

The amphibians' abundance is clear from the acoustic disruption they create. "The number of frogs and the noise – it's really insane," comments San José.

For the researchers, their nocturnal vocalizations are helpful in determining their presence in far-flung areas, using recorders like the one outside the workplace.

But local agricultural workers say the sounds are so raucous they keep them up at night.

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

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

Environmental Consequences Stays Unclear

The sound isn't the fundamental problem, however. While the species has been in the islands for almost 30 years, scientists still know very little about its impact on the islands' delicately balanced terrestrial and aquatic ecosystems.

On archipelagos, it is very common for non-native organisms to thrive, as they have none of their enemies. The Galápagos counts 1,645 invasive types, many of which are significantly affecting the safety of its native ones.

A 2020 research indicates the non-native amphibians are voracious insect eaters, and might be disproportionately eating rare insects found exclusively on the islands, or depleting the nutrition of the region's rare birds, affecting the ecosystem balance.

Unusual Traits and Management Difficulties

The Galápagos frogs have exhibited some atypical characteristics, including living in slightly salty water, which is rare for amphibians.

Their development stage is also extremely variable, with some larvae becoming frogs very rapidly and others taking a long time: the researcher observed one which remained as a tadpole in her laboratory for six months.

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

Techniques to curb the amphibians in the beginning of the century were largely unsuccessful. Conservation officers tried collecting significant quantities by manual methods and gradually increasing the salinity of ponds in without success.

Research indicates applying caffeine – which is highly toxic to amphibians – or using electrical methods could help, but these approaches aren't always safe for other rare Galápagos species.

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

Funding Challenges for Study

While she hopes the increasing use of eDNA methods and genetic examination will help her group make sense of the invasive species, funding for the research has been hard to obtain.

"Everybody wants to give support for protecting frogs," says San José. "But it's harder to find financial backing for an introduced frog that you might want to control."