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During her daily walk to the scientific station, biologist Miriam San José crouches near a small pond surrounded by dense plants and collects a compact green sound recorder.

She had placed there overnight to capture the characteristic calls of the Scinax quinquefasciatus, known by Galápagos scientists as an non-native species with consequences that scientists are starting to understand.

Although abounding with unique animals – including centuries-old giant tortoises, swimming lizards, and the well-known birds that inspired Charles Darwin's evolutionary theory – the Galápagos archipelago off the shoreline of South America had long remained free of amphibians.

In the late 1990s, this shifted. Some small tree frogs traveled from mainland the mainland to the archipelago, probably as stowaways on cargo ships.

Genetic research suggest that, through time, there have been repeated accidental introductions to the islands, and the frogs now have a strong foothold on several locations: Isabela and Santa Cruz.

The population is growing so quickly that researchers have been struggling to keep track, estimating numbers in the hundreds of thousands on every island, across developed and agricultural areas, but also in the protected Galápagos national park.

When the biologist marked 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 solitary pond. "The calculations are still very conservative," says the researcher. "I'm quite certain there are even more."

Acoustic Chaos and Growing Concerns

The frogs' abundance is evident from the sound disruption they create. "The number of frogs and the noise – it's truly incredible," says San José.

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

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

"In the wet season, I regularly hear their croaks and they're really loud," says Jadira Larrea Saltos from the island.

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

Environmental Consequences Stays Unclear

The sound isn't the fundamental problem, though. While the species has been in the Galápagos for nearly 30 years, experts still know limited information about its impact on the archipelago's precariously balanced terrestrial and aquatic environments.

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

A 2020 study suggests the non-native frogs are hungry insect consumers, and might be unevenly eating uncommon bugs found exclusively on the islands, or reducing the nutrition of the islands' uncommon avian species, disrupting the ecosystem balance.

Unusual Traits and Management Difficulties

The Galápagos frogs have exhibited some atypical characteristics, including surviving in brackish water, which is rare for amphibians.

Their metamorphosis stage is also highly inconsistent, with some larvae turning into frogs very quickly and others taking a long time: the researcher witnessed one which stayed as a larva in her lab for half a year.

"We really don't know this part," she says, worried the tadpoles could be impacting the islands' freshwater, a very scarce commodity in Galápagos.

Methods to control the amphibians in the beginning of the century were mostly ineffective. Park rangers tried capturing significant quantities by manual methods and slowly raising the salt content of ponds in vain.

Research indicates spraying caffeine – which is extremely poisonous to amphibians – or using electrocution could assist, but these methods aren't necessarily secure for other uncommon Galápagos species.

Lacking solutions to more of the fundamental issues about their lifestyle and impact, removing the frogs might not even be the correct way to advance, says San José.

Financial Obstacles for Research

While she hopes the increasing use of eDNA techniques and DNA examination will help her group understand of the invader, funding for the research has been difficult to come by.

"Everyone 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."