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On her regular walk to the research facility, scientist the researcher stoops near a small water body surrounded by dense vegetation and retrieves a small green sound recorder.

She had placed there overnight to capture the distinctive croaks of the Scinax quinquefasciatus, recognized by Galápagos researchers as an non-native species with effects that experts are just beginning to understand.

Despite abounding with unique wildlife – such as centuries-old large turtles, swimming lizards, and the well-known birds that inspired Darwin's evolutionary theory – the Galápagos archipelago off the shoreline of South America had historically been devoid of amphibians.

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

DNA studies suggest that, through time, there have been repeated unintentional arrivals to the archipelago, and the frogs now have a strong presence on two islands: Isabela and Santa Cruz.

The population is growing so quickly that researchers have been finding it difficult to monitor, calculating populations in the hundreds of thousands on each island, across developed and agricultural areas, but also in the conservation Galápagos national park.

When San José marked frogs and attempted to recapture them in the subsequent week and a half, she could find only a single marked frog occasionally, suggesting their numbers were massive.

They calculated six thousand frogs in a single pond. "The calculations are still very low," states San José. "I am quite certain there are even more."

Acoustic Chaos and Rising Worries

The frogs' abundance is clear from the acoustic disruption they cause. "The number of frogs and the sound – it's really incredible," comments San José.

For the researchers, their nocturnal vocalizations are helpful in determining their presence in far-flung areas, using recorders like the one near San José's office.

But local farmers say the calls are so raucous they keep them up at night.

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

"Initially it was a shock, seeing the first frogs in the area," says Larrea Saltos, who started noticing their large numbers about three years ago when one jumped on her palm as she was walking out of her house.

Environmental Consequences Remains Unclear

The noise isn't the fundamental problem, however. While the species has been in the Galápagos for nearly three decades, experts still know limited information about its impact on the islands' precariously balanced terrestrial and aquatic ecosystems.

On islands, it is very common for invasive organisms to prosper, as they have none of their enemies. The Galápagos counts 1,645 introduced species, many of which are seriously affecting the safety of its native ones.

A 2020 research suggests the non-native frogs are voracious bug consumers, and might be disproportionately eating rare insects found exclusively on the islands, or reducing the nutrition of the region's uncommon avian species, disrupting the ecosystem balance.

Unique Characteristics and Control Challenges

The island amphibians have exhibited some unusual traits, including living in brackish water, which is uncommon for frogs.

Their metamorphosis stage is also extremely inconsistent, with some tadpoles turning into frogs very quickly and others taking a long time: San José observed one which remained as a larva in her lab for six months.

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

Methods to curb the amphibians in the early 2000s were largely ineffective. Park rangers tried collecting large numbers by manual methods and gradually raising the salt content of ponds in vain.

Studies suggests applying caffeine – which is extremely poisonous to frogs – or using electrical methods could assist, but these approaches aren't always safe for other uncommon island organisms.

Without solutions to more of the fundamental questions about their lifestyle and effect, removing the frogs might not even be the correct way to advance, says the biologist.

Financial Obstacles for Study

While she hopes the increasing use of eDNA techniques and DNA analysis will assist her group make sense of the invasive species, funding for the research has been hard to come by.

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