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On her daily commute to the scientific station, scientist Miriam San José stoops near a shallow pond covered by dense plants and retrieves a compact plastic audio device.

The device was left there through the night to capture the characteristic calls of the Scinax quinquefasciatus, recognized by Galápagos scientists as an invasive threat with effects that experts are starting to comprehend.

Despite abounding with unique wildlife – such as ancient large turtles, swimming lizards, and the well-known birds that sparked Darwin's theory of evolution – the island chain near the coast of South America had long remained free of amphibians.

During the 1990s, this shifted. Several small tree frogs made their way from continental the mainland to the islands, probably as stowaways on cargo ships.

DNA research suggest that, over the years, there have been multiple unintentional introductions to the archipelago, and the amphibians now have a strong presence on several islands: Isabela and Santa Cruz.

The numbers is growing so rapidly that researchers have been struggling to monitor, estimating populations in the hundreds of thousands on every island, across developed and farming areas, but also in the conservation Galápagos national park.

When San José marked frogs and attempted to recapture them in the following week and a half, she could find just one tagged frog from time to time, indicating their populations were massive.

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

Deafening Noise and Rising Worries

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

For the scientists, their nocturnal mating calls are helpful in determining their existence in far-flung areas, using recorders like the one outside the workplace.

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

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

"Initially it was a shock, observing the first frogs in the region," says the farmer, who started observing their abundance about three years ago when one leaped on her palm as she was walking out of her house.

Environmental Consequences Stays Unclear

The sound isn't the fundamental problem, however. While the species has been in the islands for almost three decades, scientists still know very little about its impact on the archipelago's delicately balanced land and water ecosystems.

On archipelagos, it is very typical for non-native organisms to thrive, as they have none of their natural predators. The Galápagos has over sixteen hundred introduced species, many of which are significantly affecting the safety of its endemic ones.

A 2020 study suggests the non-native frogs are hungry bug eaters, and might be disproportionately consuming uncommon bugs found exclusively on the islands, or reducing the nutrition of the islands' uncommon birds, affecting the food chain.

Unusual Traits and Control Challenges

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

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

"We really don't know this part," she says, concerned the tadpoles could be affecting the islands' clean water, a very scarce resource in the islands.

Methods to control the amphibians in the beginning of the century were mostly ineffective. Park rangers tried collecting large numbers by hand and slowly raising the salinity of lagoons in without success.

Studies suggests spraying coffee – which is highly poisonous to frogs – or using electrical methods could assist, but these methods aren't necessarily safe for other uncommon island organisms.

Without answers to more of the basic issues about their lifestyle and impact, removing the amphibians might not even be the right way to advance, says the biologist.

Financial Obstacles for Study

While she expects the increasing use of eDNA techniques and genetic analysis will assist her team understand of the invasive species, financial support for the project 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 introduced frog that you might want to control."