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On her regular walk to the research facility, biologist the researcher stoops near a small pond covered by dense plants and collects a compact plastic sound device.

The device was left there overnight to record the characteristic croaks of the Scinax quinquefasciatus, recognized by Galápagos researchers as an invasive species with effects that experts are starting to understand.

Although teeming with remarkable animals – including centuries-old large turtles, marine lizards, and the well-known birds that inspired Charles Darwin's theory of evolution – the Galápagos archipelago off the shoreline of South America had historically been free of amphibians.

During the 1990s, this shifted. Some tiny tree frogs traveled from continental Ecuador to the archipelago, probably as hitchhikers on transport vessels.

Genetic studies indicate that, through time, there have been repeated unintentional introductions to the islands, and the frogs now have a strong presence on several islands: multiple locations.

The numbers is expanding so rapidly that scientists have been finding it difficult to monitor, estimating 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 recapture them in the subsequent week and a half, she could locate just one marked frog from time to time, indicating their numbers were massive.

They calculated 6,000 frogs in a single pond. "Our estimates are still very conservative," states San José. "I am pretty sure there are additional numbers."

Acoustic Chaos and Rising Worries

The amphibians' abundance is clear from the sound chaos they create. "The number of frogs and the noise – it's really incredible," says the scientist.

For the researchers, their nocturnal vocalizations are helpful in estimating their existence in remote areas, using audio devices like the one near San José's office.

But nearby farmers say the calls are so raucous they prevent sleep at night.

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

"Initially it was a surprise, observing the initial frogs in the area," says the farmer, who started noticing 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 primary problem, though. While the species has been in the islands for almost three decades, scientists still know limited information about its impact on the archipelago's delicately balanced terrestrial and aquatic ecosystems.

On archipelagos, it is very typical for invasive organisms to thrive, as they have none of their enemies. The islands counts 1,645 invasive types, many of which are significantly affecting the safety of its native ones.

A 2020 research indicates the non-native amphibians are voracious insect eaters, and might be disproportionately eating rare insects found exclusively on the archipelago, or reducing the food sources of the region's rare birds, disrupting the ecosystem balance.

Unique Characteristics and Control Challenges

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

Their metamorphosis stage is also extremely inconsistent, with some larvae becoming frogs very quickly and others taking a long time: the researcher witnessed one which remained as a tadpole in her lab for six months.

"We truly don't know this aspect," she says, worried the larvae could be affecting the islands' freshwater, a very limited resource in Galápagos.

Techniques to control the amphibians in the early 2000s were mostly unsuccessful. Conservation officers tried collecting large numbers by manual methods and slowly increasing the salt content of lagoons in vain.

Research indicates applying coffee – which is highly poisonous to frogs – or using electrocution could help, but these approaches aren't necessarily safe for other rare island species.

Without solutions to more of the basic questions about their biology and effect, removing the amphibians might not even be the right way to advance, says the biologist.

Financial Obstacles for Research

While she expects the increasing use of eDNA methods and DNA examination will help her team understand of the invasive species, funding for the project has been hard to come by.

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