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On her regular walk to the scientific station, biologist Miriam San José crouches near a shallow water body surrounded by dense vegetation and retrieves a small green audio recorder.

She had placed there overnight to capture the distinctive croaks of the Fowler's snouted treefrog, known by local researchers as an non-native species with effects that experts are starting to comprehend.

Although teeming with unique animals – such as centuries-old large turtles, swimming lizards, and the well-known birds that inspired Darwin's evolutionary theory – the Galápagos archipelago off the shoreline of Ecuador had historically been devoid of frogs and toads.

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

DNA studies suggest that, over the years, there have been repeated unintentional arrivals to the islands, and the frogs now have a strong foothold on two islands: multiple locations.

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

When the biologist tagged amphibians and attempted to find them in the subsequent week and a half, she could find only a single marked frog occasionally, indicating their populations were enormous.

They calculated six thousand frogs in a solitary pond. "Our estimates are still very low," says the researcher. "I'm pretty sure there are additional numbers."

Deafening Noise and Growing Concerns

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

For the scientists, their nightly mating calls are helpful in estimating their existence in far-flung areas, using recorders like the one outside the office.

But local agricultural workers say the sounds are so raucous they prevent sleep at night.

"In the wet season, I constantly hear their croaks and they're really loud," says a local coffee farmer from the island.

"At first it was a shock, observing the initial frogs in the area," says the farmer, who started noticing their abundance about several years ago when one leaped on her palm as she was stepping out of her house.

Environmental Consequences Stays Unclear

The sound isn't the primary 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 common for non-native organisms to thrive, as they have none of their natural predators. The Galápagos has over sixteen hundred invasive species, many of which are seriously affecting the safety of its endemic ones.

A 2020 study indicates the non-native frogs are hungry insect consumers, and might be unevenly consuming rare bugs found only on the archipelago, or depleting the food sources of the islands' rare avian species, disrupting the food chain.

Unique Characteristics and Management Difficulties

The island frogs have exhibited some atypical characteristics, including surviving in brackish water, which is uncommon for amphibians.

Their metamorphosis stage is also extremely inconsistent, with some larvae becoming frogs very rapidly and others taking a long time: the researcher witnessed one which remained as a larva in her lab for half a year.

"We truly don't know this aspect," she says, concerned the tadpoles could be affecting the region's freshwater, a very scarce commodity in Galápagos.

Techniques to curb the amphibians in the beginning of the century were mostly unsuccessful. Conservation officers tried capturing large numbers by hand and gradually increasing the salt content of ponds in vain.

Studies suggests spraying caffeine – which is highly toxic to amphibians – or using electrical methods could help, but these approaches aren't necessarily safe for other uncommon island organisms.

Without answers to more of the fundamental questions about their biology and impact, removing the amphibians might not even be the correct way to advance, says San José.

Financial Obstacles for Study

While she hopes the increasing use of environmental DNA methods and DNA analysis will assist her team make sense 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 invasive frog that you might want to manage."