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During her daily walk to the research facility, scientist Miriam San José stoops near a small water body covered by dense plants and retrieves a small plastic audio device.

The device was left there through the night to capture the characteristic calls of the Fowler's snouted treefrog, recognized by local researchers as an non-native threat with effects that scientists are starting to comprehend.

Despite teeming with unique wildlife – including centuries-old large turtles, swimming iguanas, and the well-known finches that inspired Charles Darwin's theory of evolution – the Galápagos archipelago near the coast of Ecuador had long remained devoid of amphibians.

In the late 1990s, this shifted. Some tiny tree frogs traveled from mainland the mainland to the archipelago, probably as stowaways 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 several islands: multiple locations.

The population is growing so quickly that scientists have been struggling to keep track, estimating numbers in the millions on every island, across developed and farming areas, but also in the protected natural reserve.

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

They estimated six thousand frogs in a single pond. "Our estimates are still very conservative," says the researcher. "I am pretty sure there are additional numbers."

Acoustic Chaos and Rising Worries

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

For the scientists, their nocturnal vocalizations are helpful in estimating their existence in remote areas, using recorders like the one near the workplace.

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

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

"Initially it was a surprise, seeing the first frogs in the area," says the farmer, who started noticing their abundance about several years ago when one jumped on her palm as she was stepping out of her house.

Ecological Impact Remains Unknown

The sound isn't the fundamental problem, however. While the species has been in the islands for almost three decades, scientists still know very little about its impact on the archipelago's precariously balanced land and water environments.

On archipelagos, it is very common for non-native species to thrive, as they have none of their enemies. The Galápagos has 1,645 introduced species, many of which are seriously disrupting the safety of its endemic ones.

A recent research suggests the non-native frogs are hungry insect eaters, and might be disproportionately eating uncommon insects found exclusively on the islands, or depleting the food sources of the region's rare birds, affecting the ecosystem balance.

Unique Characteristics and Control Challenges

The Galápagos amphibians have shown some atypical characteristics, including surviving in slightly salty water, which is rare for amphibians.

Their metamorphosis process is also extremely variable, with some tadpoles turning into frogs very quickly and others taking a long time: San José witnessed one which remained as a tadpole in her lab for half a year.

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

Methods to curb the amphibians in the early 2000s were mostly unsuccessful. Park rangers tried capturing significant quantities by manual methods and slowly raising the salinity of ponds in without success.

Research suggests applying caffeine – which is extremely poisonous to amphibians – or using electrical methods could assist, but these methods aren't always safe for other uncommon island species.

Lacking solutions to more of the fundamental issues about their biology and effect, removing the amphibians might not even be the correct way to proceed, says the biologist.

Financial Obstacles for Research

While she hopes the growing use of environmental DNA methods and genetic examination will assist her team make sense of the invader, funding for the project has been difficult to obtain.

"Everybody wants to give funding for protecting frogs," says the researcher. "But it's harder to find financial backing for an invasive frog that you might want to control."