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During her regular walk to the research facility, biologist Miriam San José stoops near a small pond covered by dense vegetation and retrieves a compact plastic audio device.

The device was left there overnight to capture the characteristic calls of the Scinax quinquefasciatus, known by Galápagos scientists as an invasive threat with effects that experts are just beginning to understand.

Although teeming with unique animals – such as centuries-old large turtles, marine lizards, and the well-known birds that inspired Darwin's theory of evolution – the island chain off the shoreline of South America had long remained free of frogs and toads.

During the 1990s, this shifted. Some tiny amphibians made their way from mainland the mainland to the archipelago, likely as hitchhikers on transport vessels.

DNA research suggest that, through time, there have been repeated unintentional introductions to the archipelago, and the frogs now have a strong foothold on several islands: multiple locations.

The population is growing so rapidly that scientists have been finding it difficult to monitor, calculating numbers in the millions on each island, across developed and agricultural areas, but also in the conservation Galápagos national park.

When the biologist marked amphibians and attempted to find them in the following 10 days, she could find only a single marked frog from time to time, indicating their populations were enormous.

They calculated 6,000 frogs in a solitary pond. "The calculations are still very low," states San José. "I'm quite certain there are additional numbers."

Acoustic Chaos and Growing Concerns

The amphibians' proliferation is evident from the acoustic disruption they cause. "The amount of frogs and the sound – it's really insane," comments San José.

For the researchers, their nocturnal mating calls are useful in determining their presence in remote areas, using audio devices like the one near the workplace.

But nearby farmers say the sounds are so loud they keep them up at night.

"In the rainy period, I regularly hear their croaks and they're really loud," says a local coffee farmer from the island.

"Initially it was a surprise, observing the first frogs in the region," says the farmer, who started observing their large numbers about several years ago when one leaped on her hand as she was walking out of her house.

Environmental Consequences Remains Unclear

The noise isn't the primary problem, though. While the species has been in the Galápagos for almost 30 years, scientists still know very little about its effect on the archipelago's delicately balanced land and water ecosystems.

On archipelagos, it is very typical for invasive species to thrive, as they have few of their enemies. The islands has 1,645 introduced species, many of which are seriously affecting the survival of its native ones.

A 2020 study suggests the invasive amphibians are hungry insect eaters, and might be disproportionately eating rare insects found only on the islands, or reducing the nutrition of the region's uncommon avian species, disrupting the ecosystem balance.

Unusual Traits and Management Difficulties

The island amphibians have exhibited some unusual characteristics, including surviving in slightly salty water, which is rare for frogs.

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

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

Techniques to curb the amphibians in the beginning of the century were largely ineffective. Park rangers tried capturing significant quantities by manual methods and gradually raising the salt content of lagoons in without success.

Research suggests applying coffee – which is extremely poisonous to amphibians – or using electrocution could assist, but these methods aren't necessarily secure for other rare Galápagos species.

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

Funding Challenges for Research

While she hopes the increasing use of eDNA methods and genetic analysis will help her group understand of the invader, financial support for the project has been difficult to come by.

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