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On her daily commute to the scientific station, biologist Miriam San José stoops near a small water body covered by dense vegetation and retrieves a compact green audio recorder.

The device was left there overnight to record the distinctive croaks 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 ancient giant tortoises, marine lizards, and the famous birds that sparked Darwin's evolutionary theory – the island chain off the coast of Ecuador had historically been free of amphibians.

In the late 1990s, this changed. Several tiny amphibians made their way from mainland the mainland to the archipelago, probably as hitchhikers on transport vessels.

Genetic studies suggest that, through time, there have been multiple accidental arrivals to the archipelago, and the amphibians now have a strong presence on several locations: multiple locations.

The numbers is growing so quickly that scientists have been finding it difficult to keep track, calculating populations in the hundreds of thousands on each island, across urban and agricultural areas, but also in the conservation natural reserve.

When the biologist marked amphibians and attempted to find them in the subsequent 10 days, she could locate just one tagged frog occasionally, indicating their populations were enormous.

They estimated 6,000 frogs in a single pond. "Our estimates are still very low," states San José. "I am quite certain there are even more."

Acoustic Chaos and Growing Concerns

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

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

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

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

"Initially it was a shock, observing the initial frogs in the region," says the farmer, who started observing their large numbers about three years ago when one leaped on her palm as she was stepping out of her front door.

Ecological Impact Remains Unknown

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

On islands, it is very typical for non-native species to thrive, as they have few of their enemies. The Galápagos counts over sixteen hundred invasive species, many of which are seriously affecting the safety of its native ones.

A 2020 study indicates the non-native frogs are voracious insect consumers, and might be disproportionately consuming rare bugs found only on the islands, or reducing the food sources of the region's uncommon avian species, disrupting the food chain.

Unique Characteristics and Control Challenges

The island frogs have exhibited some atypical traits, including living in brackish water, which is rare for frogs.

Their metamorphosis stage is also highly variable, with some larvae turning into frogs very rapidly and others taking a extended period: San José witnessed one which stayed as a tadpole in her lab for half a year.

"We really don't know this aspect," she says, worried the larvae could be impacting the region's clean water, a very scarce resource in the islands.

Methods to control the frogs in the early 2000s were mostly ineffective. Park rangers tried collecting significant quantities by manual methods and slowly increasing the salinity of ponds in vain.

Studies suggests spraying caffeine – which is extremely toxic to frogs – or using electrical methods could help, but these approaches aren't always secure for other uncommon Galápagos species.

Without answers to more of the fundamental questions about their lifestyle and effect, removing the amphibians might not even be the right way to advance, says the biologist.

Financial Obstacles for Research

While she hopes the increasing use of eDNA techniques and genetic analysis will assist her group make sense of the invasive species, financial support for the research has been difficult to obtain.

"Everyone wants to give funding for preserving frogs," says San José. "But it's harder to find financial backing for an invasive frog that you might want to manage."