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During her daily walk to the scientific station, biologist Miriam San José crouches near a shallow water body surrounded by thick vegetation and retrieves a compact green audio device.

She had placed there through the night to record the characteristic croaks of the Scinax quinquefasciatus, known by Galápagos scientists as an invasive species with effects that experts are just beginning to comprehend.

Although teeming with unique wildlife – including centuries-old giant tortoises, marine iguanas, and the well-known birds that inspired Darwin's evolutionary theory – the island chain near the shoreline of Ecuador had historically been free of frogs and toads.

During the 1990s, this changed. Some small amphibians traveled from continental the mainland to the islands, probably as hitchhikers on transport vessels.

DNA research suggest that, through time, there have been multiple accidental arrivals to the islands, and the frogs now have a strong presence on several islands: Isabela and Santa Cruz.

The population is expanding so quickly that scientists have been finding it difficult to monitor, estimating numbers in the hundreds of thousands on every island, across developed and farming areas, but also in the conservation Galápagos national park.

When the biologist marked frogs and attempted to find them in the subsequent week and a half, she could locate just one marked frog occasionally, suggesting their populations were enormous.

They calculated six thousand frogs in a solitary pond. "Our estimates are still very conservative," states San José. "I'm pretty sure there are additional numbers."

Deafening Noise and Rising Worries

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

For the scientists, their nocturnal mating calls are helpful in estimating their presence in remote areas, using recorders like the one near San José's office.

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

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

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

Environmental Consequences Stays Unclear

The noise 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 effect on the archipelago's delicately balanced land and water ecosystems.

On archipelagos, it is very common for invasive species to prosper, as they have none of their enemies. The islands counts over sixteen hundred invasive types, many of which are seriously affecting the safety of its endemic ones.

A 2020 research indicates the non-native frogs are hungry insect consumers, and might be unevenly consuming rare insects found exclusively on the archipelago, or reducing the food sources of the region's rare avian species, affecting the ecosystem balance.

Unique Characteristics and Control Challenges

The island frogs have shown some atypical characteristics, including living in slightly salty water, which is uncommon for amphibians.

Their metamorphosis stage is also highly inconsistent, with some larvae becoming frogs very rapidly and others taking a extended period: the researcher witnessed one which stayed as a larva in her lab for six months.

"We truly don't know this part," she says, worried the larvae could be impacting the islands' freshwater, a very scarce commodity in Galápagos.

Techniques to curb the frogs in the beginning of the century were largely unsuccessful. Conservation officers tried capturing significant quantities by hand and slowly increasing the salt content of lagoons in vain.

Research suggests spraying coffee – which is extremely toxic to amphibians – or using electrocution could assist, but these methods aren't always safe for other rare Galápagos organisms.

Without solutions to more of the fundamental questions about their biology and impact, removing the frogs might not even be the right way to proceed, says San José.

Funding Challenges for Study

While she hopes the growing use of eDNA methods and DNA examination will help her group make sense of the invasive species, financial support for the research has been difficult to obtain.

"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."