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On her daily commute to the scientific station, scientist Miriam San José stoops near a small water body surrounded by thick plants and retrieves a small plastic audio device.

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

Although abounding with remarkable wildlife – such as centuries-old giant tortoises, swimming iguanas, and the famous birds that sparked Darwin's theory of evolution – the Galápagos archipelago off the shoreline of South America had long remained devoid of frogs and toads.

In the late 1990s, this changed. Some small amphibians traveled from mainland the mainland to the islands, likely as stowaways on cargo ships.

DNA research suggest that, over the years, there have been repeated unintentional introductions to the islands, and the amphibians now have a firm foothold on two islands: multiple locations.

The population is growing so quickly that researchers have been struggling to monitor, calculating numbers in the millions on every island, across urban and agricultural areas, but also in the protected Galápagos national park.

When San José marked frogs and attempted to recapture them in the subsequent 10 days, she could find just one marked frog from time to time, suggesting their numbers were enormous.

They estimated 6,000 frogs in a single pond. "The calculations are still very conservative," states the researcher. "I'm pretty sure there are additional numbers."

Deafening Noise and Rising Worries

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

For the scientists, their nocturnal mating calls are useful in determining their presence in remote areas, using recorders like the one outside the workplace.

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

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

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

Ecological Impact Stays Unknown

The sound isn't the fundamental problem, though. While the amphibians has been in the Galápagos for nearly 30 years, scientists still know very little about its effect on the archipelago's precariously balanced terrestrial and aquatic environments.

On archipelagos, it is very typical for invasive species to prosper, as they have few of their enemies. The islands has 1,645 invasive species, many of which are seriously affecting the survival of its native ones.

A 2020 study indicates the invasive frogs are hungry insect consumers, and might be disproportionately eating uncommon bugs found only on the archipelago, or reducing the food sources of the region's rare avian species, affecting the food chain.

Unique Characteristics and Control Challenges

The island amphibians have exhibited some unusual characteristics, including surviving in slightly salty water, which is rare for amphibians.

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

"We really don't know this aspect," she says, worried the larvae could be affecting the islands' clean water, a very scarce commodity in Galápagos.

Techniques to control the amphibians in the beginning of the century were largely ineffective. Park rangers tried capturing large numbers by manual methods and slowly raising the salt content of ponds in without success.

Research suggests applying caffeine – which is extremely toxic to frogs – or using electrocution could assist, but these methods aren't necessarily secure for other uncommon Galápagos organisms.

Lacking answers to more of the basic issues about their biology and effect, removing the frogs might not even be the correct way to proceed, says the biologist.

Funding Challenges for Research

While she hopes the increasing use of eDNA methods and DNA examination will assist her group make sense of the invasive species, funding for the research has been difficult to come by.

"Everybody wants to give funding for preserving frogs," says the researcher. "But it's harder to find funding for an invasive frog that you might want to control."