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During her daily walk to the scientific station, biologist Miriam San José stoops near a shallow water body surrounded by dense vegetation and retrieves a small plastic sound device.

She had placed there overnight to record the distinctive calls of the Fowler's snouted treefrog, known by Galápagos scientists as an invasive species with effects that experts are starting to understand.

Although teeming with unique animals – such as centuries-old giant tortoises, swimming lizards, and the famous birds that sparked Darwin's theory of evolution – the island chain off the coast of South America had long remained devoid of amphibians.

During the 1990s, this shifted. Several tiny amphibians traveled from continental the mainland to the archipelago, likely as stowaways on transport vessels.

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

The population is expanding so rapidly that researchers have been finding it difficult to keep track, estimating numbers in the hundreds of thousands on each island, across developed and agricultural areas, but also in the protected natural reserve.

When the biologist marked frogs and attempted to recapture them in the following week and a half, she could locate only a single marked frog from time to time, indicating their populations were enormous.

They estimated six thousand frogs in a solitary pond. "Our estimates are still very low," states San José. "I'm quite certain there are additional numbers."

Acoustic Chaos and Rising Worries

The frogs' abundance is evident from the sound chaos they cause. "The amount of frogs and the sound – it's really incredible," says San José.

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

But nearby farmers say the sounds are so loud they prevent sleep at night.

"During the wet season, I constantly hear their croaks and they're really loud," says Jadira Larrea Saltos from the island.

"Initially it was a surprise, seeing the first frogs in the area," says the farmer, who started observing their large numbers about several years ago when one jumped on her palm as she was stepping out of her house.

Ecological Impact Stays Unclear

The noise isn't the fundamental problem, though. While the species has been in the islands for nearly three decades, experts still know limited information about its impact on the islands' delicately balanced land and water ecosystems.

On archipelagos, it is very typical for non-native organisms to prosper, as they have few of their natural predators. The Galápagos has 1,645 introduced types, many of which are seriously affecting the survival of its native ones.

A 2020 study indicates the invasive amphibians are hungry bug consumers, and might be disproportionately consuming rare insects found exclusively on the islands, or depleting the nutrition of the islands' rare avian species, affecting the ecosystem balance.

Unusual Traits and Control Challenges

The Galápagos frogs have shown some unusual characteristics, including living in slightly salty water, which is uncommon for frogs.

Their development process is also highly variable, with some larvae becoming frogs very quickly and others taking a long time: San José witnessed one which stayed as a larva in her laboratory for six months.

"We really don't know this part," she says, worried the larvae could be affecting the region's freshwater, a very scarce commodity in the islands.

Techniques to control the amphibians in the beginning of the century were largely ineffective. Park rangers tried capturing large numbers by manual methods and gradually raising the salt content of ponds in vain.

Research indicates 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 species.

Without solutions to more of the basic questions about their biology and impact, culling the amphibians might not even be the correct way to proceed, says San José.

Financial Obstacles for Research

While she expects the growing use of eDNA methods and genetic analysis will help her group understand of the invader, financial support for the research has been hard to come by.

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