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During her regular walk to the scientific station, biologist Miriam San José crouches near a small water body surrounded by dense plants and retrieves a compact green sound recorder.

The device was left there overnight to record the characteristic calls of the Fowler's snouted treefrog, known by Galápagos researchers as an non-native threat with effects that scientists are just beginning to comprehend.

Despite abounding with remarkable wildlife – such as centuries-old large turtles, swimming iguanas, and the famous birds that inspired Darwin's evolutionary theory – the island chain off the coast of South America had historically been devoid of amphibians.

During the 1990s, this changed. Several tiny tree frogs traveled from continental Ecuador to the islands, likely as hitchhikers on cargo ships.

DNA studies suggest that, through time, there have been repeated accidental introductions to the archipelago, and the amphibians now have a firm foothold on two locations: Isabela and Santa Cruz.

The population is expanding so rapidly that researchers have been finding it difficult to keep track, estimating populations in the hundreds of thousands on every island, across urban and farming areas, but also in the conservation Galápagos national park.

When San José marked frogs and attempted to recapture them in the subsequent 10 days, she could locate just one tagged frog from time to time, indicating their populations were enormous.

They calculated six thousand frogs in a solitary pond. "Our estimates are still very conservative," states San José. "I am quite certain there are even more."

Acoustic Chaos and Rising Worries

The amphibians' proliferation is evident from the sound chaos they cause. "The number of frogs and the sound – it's really incredible," comments San José.

For the scientists, their nightly vocalizations are useful in determining their presence in remote areas, using recorders like the one outside San José's workplace.

But local farmers say the calls are so raucous they prevent sleep at night.

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

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

Environmental Consequences Stays Unclear

The noise isn't the primary problem, though. While the amphibians has been in the islands for almost 30 years, scientists still know very little about its impact on the archipelago's precariously balanced terrestrial and aquatic environments.

On islands, it is very common for invasive organisms to prosper, as they have none of their natural predators. The islands counts 1,645 introduced types, many of which are seriously disrupting the survival of its endemic ones.

A 2020 research suggests the non-native frogs are voracious bug consumers, and might be unevenly eating uncommon insects found only on the islands, or reducing the food sources of the islands' uncommon avian species, disrupting the food chain.

Unusual Traits and Control Challenges

The Galápagos amphibians have shown some atypical traits, including surviving in slightly salty water, which is uncommon for amphibians.

Their development stage is also highly inconsistent, with some larvae turning into frogs very rapidly and others taking a long time: the researcher observed one which stayed as a tadpole in her laboratory for half a year.

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

Methods to control the amphibians in the beginning of the century were mostly ineffective. Park rangers tried collecting large numbers by manual methods and gradually increasing the salinity of ponds in vain.

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

Without answers to more of the fundamental issues about their biology and effect, culling the frogs might not even be the correct way to proceed, says San José.

Funding Challenges for Research

While she hopes the growing use of environmental DNA methods and DNA analysis will assist her team make sense of the invasive species, funding for the research has been difficult to come by.

"Everybody wants to give funding for protecting frogs," says the researcher. "But it's more difficult to find financial backing for an invasive frog that you might want to manage."