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During her daily walk to the research facility, biologist Miriam San José crouches near a shallow water body surrounded by thick plants and retrieves a small green audio recorder.

She had placed there overnight to record the characteristic croaks of the Fowler's snouted treefrog, recognized by Galápagos researchers as an invasive threat with consequences that scientists are starting to comprehend.

Although abounding with remarkable wildlife – including centuries-old large turtles, marine iguanas, and the well-known finches that inspired Charles Darwin's evolutionary theory – the island chain off the shoreline of Ecuador had long remained free of amphibians.

In the late 1990s, this changed. Some tiny amphibians traveled from mainland Ecuador to the archipelago, probably as stowaways on transport vessels.

DNA research suggest that, over the years, there have been multiple accidental introductions to the islands, and the frogs now have a firm foothold on two islands: Isabela and Santa Cruz.

The population is expanding so rapidly that scientists have been finding it difficult to monitor, calculating populations in the hundreds of thousands on every island, across urban and farming 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 tagged frog from time to time, suggesting their numbers were enormous.

They calculated 6,000 frogs in a solitary pond. "The calculations are still very conservative," states San José. "I'm quite certain there are additional numbers."

Acoustic Chaos and Rising Worries

The frogs' proliferation is clear from the sound disruption they cause. "The amount of frogs and the noise – it's truly incredible," says San José.

For the researchers, their nightly vocalizations are helpful in estimating their existence in remote areas, using recorders like the one outside San José's office.

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

"In the wet season, I constantly hear their croaks and they're really loud," says a local coffee farmer from Santa Cruz.

"At first it was a shock, seeing the first frogs in the area," says the farmer, who started observing their large numbers about three years ago when one jumped on her palm as she was walking out of her front door.

Ecological Impact Remains Unknown

The noise isn't the fundamental problem, though. While the species has been in the Galápagos for almost 30 years, scientists still know limited information about its effect on the archipelago's precariously balanced land and water ecosystems.

On islands, it is very common for non-native organisms to prosper, as they have few of their enemies. The Galápagos counts over sixteen hundred invasive species, many of which are significantly affecting the survival of its native ones.

A 2020 study indicates the invasive amphibians are hungry bug consumers, and might be unevenly consuming rare insects found exclusively on the archipelago, or depleting the food sources of the region's uncommon birds, affecting the food chain.

Unique Characteristics and Management Difficulties

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

Their metamorphosis stage is also highly variable, with some tadpoles turning into frogs very rapidly and others taking a long time: the researcher observed one which stayed as a tadpole in her laboratory for half a year.

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

Techniques to control the amphibians in the early 2000s were mostly ineffective. Park rangers tried collecting significant quantities by hand and slowly raising the salt content of ponds in without success.

Studies suggests applying coffee – which is extremely poisonous to frogs – or using electrocution could assist, but these methods aren't necessarily secure for other rare island species.

Without answers to more of the fundamental questions about their biology and effect, removing the amphibians might not even be the correct way to proceed, says the biologist.

Financial Obstacles for Study

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

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