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During her regular commute to the scientific station, biologist Miriam San José crouches near a shallow pond surrounded by dense vegetation and collects a compact green audio recorder.

The device was left there overnight to capture the distinctive croaks of the Fowler's snouted treefrog, known by Galápagos researchers as an invasive species with consequences that experts are just beginning to understand.

Although teeming with unique animals – such as centuries-old giant tortoises, marine lizards, and the famous birds that inspired Charles Darwin's evolutionary theory – the Galápagos archipelago near the coast of Ecuador had historically been free of frogs and toads.

In the late 1990s, this shifted. Some tiny tree frogs made their way from mainland Ecuador to the islands, likely as hitchhikers on transport vessels.

Genetic research indicate that, over the years, there have been multiple unintentional arrivals to the archipelago, and the frogs now have a strong foothold on two locations: multiple locations.

The population is expanding so rapidly that scientists have been struggling to keep track, calculating populations in the millions on each island, across urban and farming areas, but also in the protected Galápagos national park.

When the biologist marked amphibians and attempted to find them in the subsequent 10 days, she could find just one tagged frog from time to time, suggesting their numbers were massive.

They calculated six thousand frogs in a single pond. "Our estimates are still very low," says the researcher. "I am quite certain there are additional numbers."

Acoustic Chaos and Growing Concerns

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

For the researchers, their nocturnal mating calls are helpful in estimating their presence in far-flung areas, using audio devices like the one outside San José's office.

But local agricultural workers say the calls are so loud they prevent sleep at night.

"During the wet season, I regularly hear their calls and they're extremely loud," says Jadira Larrea Saltos from the island.

"Initially it was a surprise, seeing the initial frogs in the region," says Larrea Saltos, who started observing their abundance about three years ago when one jumped on her hand as she was stepping out of her house.

Ecological Impact Remains Unknown

The sound isn't the primary problem, however. While the species has been in the Galápagos for nearly three decades, scientists still know very little about its impact on the archipelago's precariously balanced terrestrial and aquatic environments.

On archipelagos, it is very common for non-native species to thrive, as they have few of their natural predators. The Galápagos has over sixteen hundred introduced species, many of which are significantly affecting the safety of its native ones.

A 2020 research indicates the non-native frogs are hungry insect consumers, and might be unevenly consuming 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 island amphibians have shown some unusual characteristics, including surviving in brackish water, which is rare for frogs.

Their metamorphosis stage is also highly variable, with some larvae becoming frogs very rapidly and others taking a long time: San José witnessed one which stayed 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' freshwater, a very scarce resource in Galápagos.

Methods to curb the amphibians in the early 2000s were largely ineffective. Park rangers tried capturing large numbers by hand and gradually raising the salinity of lagoons in vain.

Research suggests spraying coffee – which is highly toxic to frogs – or using electrical methods could help, but these approaches aren't always safe for other uncommon island species.

Lacking answers to more of the fundamental questions about their lifestyle and impact, removing the amphibians might not even be the right way to advance, says San José.

Funding Challenges for Study

While she expects the increasing use of eDNA techniques and genetic analysis will help her team understand of the invader, funding for the research has been difficult to obtain.

"Everyone wants to give funding for preserving frogs," says the researcher. "But it's harder to find funding for an introduced frog that you might want to manage."