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During her regular walk to the scientific station, scientist Miriam San José stoops near a small pond covered by thick vegetation and retrieves a compact green audio device.

The device was left there through the night to record the characteristic croaks of the Fowler's snouted treefrog, known by Galápagos scientists as an invasive threat with effects that scientists are starting to comprehend.

Despite teeming with unique wildlife – including ancient large turtles, marine iguanas, and the well-known birds that sparked Darwin's evolutionary theory – the Galápagos archipelago off the shoreline of Ecuador had historically been devoid of frogs and toads.

During the 1990s, this changed. Several tiny tree frogs traveled from mainland Ecuador to the archipelago, likely as stowaways on cargo ships.

Genetic studies suggest that, through time, there have been repeated accidental introductions to the islands, and the amphibians now have a strong foothold on two locations: multiple locations.

The numbers is growing so rapidly that researchers have been finding it difficult to monitor, calculating populations in the hundreds of thousands on every island, across developed and farming areas, but also in the protected Galápagos national park.

When San José marked frogs and attempted to recapture them in the following week and a half, she could locate only a single tagged frog from time to time, indicating their numbers were massive.

They calculated 6,000 frogs in a solitary pond. "Our estimates are still very conservative," states the researcher. "I am pretty sure there are even more."

Deafening Noise and Rising Worries

The frogs' proliferation is clear from the sound disruption they cause. "The number of frogs and the sound – it's truly insane," says the scientist.

For the scientists, their nightly vocalizations are useful in estimating their presence in far-flung areas, using recorders like the one outside the workplace.

But nearby farmers say the sounds are so raucous they keep them up at night.

"During the wet season, I regularly hear their calls and they're really loud," says a local coffee farmer from Santa Cruz.

"Initially it was a surprise, observing the first 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.

Ecological Impact Remains Unknown

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

On islands, it is very typical for invasive organisms to prosper, as they have few of their enemies. The islands counts 1,645 invasive types, many of which are seriously disrupting the survival of its native ones.

A recent study suggests the invasive amphibians are voracious insect consumers, and might be disproportionately eating uncommon insects found only on the archipelago, or depleting the nutrition of the region's uncommon avian species, disrupting the ecosystem balance.

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 metamorphosis stage is also extremely variable, with some larvae turning into frogs very quickly and others taking a extended period: the researcher observed one which remained as a tadpole in her laboratory for half a year.

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

Methods to control the amphibians in the early 2000s were mostly ineffective. Conservation officers tried capturing significant quantities by hand and gradually raising the salinity of ponds in vain.

Studies suggests spraying coffee – which is extremely toxic to amphibians – or using electrical methods could assist, but these methods aren't necessarily secure for other rare Galápagos species.

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

Funding Challenges for Study

While she expects the growing use of eDNA techniques and DNA analysis will help her group understand of the invader, financial support for the research has been hard to come by.

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