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During her daily walk to the research facility, scientist the researcher crouches near a small water body surrounded by thick plants and retrieves a compact green sound recorder.

The device was left there overnight to capture the characteristic calls of the Fowler's snouted treefrog, recognized by local scientists as an non-native threat with consequences that experts are just beginning to comprehend.

Although teeming with remarkable animals – such as centuries-old large turtles, swimming lizards, and the well-known finches that inspired Darwin's evolutionary theory – the Galápagos archipelago off the shoreline of South America had long remained devoid of amphibians.

In the late 1990s, this shifted. Some tiny amphibians traveled from continental Ecuador to the archipelago, probably as hitchhikers on transport vessels.

Genetic research suggest that, through time, there have been repeated accidental arrivals to the archipelago, and the amphibians now have a strong foothold on two islands: multiple locations.

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

When San José tagged amphibians and attempted to recapture them in the subsequent 10 days, she could locate only a single tagged frog occasionally, suggesting their numbers were enormous.

They estimated six thousand frogs in a solitary pond. "Our estimates are still very low," says San José. "I'm pretty sure there are even more."

Acoustic Chaos and Growing Concerns

The frogs' abundance is evident from the sound chaos they create. "The amount of frogs and the sound – it's really incredible," says San José.

For the researchers, their nocturnal mating calls are useful in determining their presence in far-flung areas, using recorders like the one outside the office.

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

"During the rainy period, I constantly hear their croaks and they're really loud," says a local coffee farmer from Santa Cruz.

"At first it was a surprise, seeing the initial frogs in the area," says the farmer, who started noticing their large numbers about three years ago when one leaped on her hand as she was stepping out of her front door.

Ecological Impact Remains Unknown

The noise isn't the primary problem, however. While the species has been in the islands for nearly 30 years, experts still know limited information about its effect on the islands' delicately balanced land and water ecosystems.

On islands, it is very common for invasive species to thrive, as they have none of their enemies. The Galápagos has 1,645 invasive species, many of which are seriously affecting the survival of its native ones.

A recent research suggests the non-native amphibians are voracious insect eaters, and might be disproportionately consuming rare insects found only on the islands, or depleting the nutrition of the islands' uncommon birds, disrupting the food chain.

Unusual Traits and Management Difficulties

The island frogs have shown some atypical traits, including living in slightly salty water, which is rare for frogs.

Their metamorphosis process is also highly variable, with some larvae turning into frogs very rapidly and others taking a long time: the researcher observed one which remained as a larva in her lab for six months.

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

Techniques to control the amphibians in the early 2000s were largely ineffective. Park rangers tried capturing large numbers by manual methods and slowly raising the salt content of ponds in vain.

Studies indicates spraying coffee – which is highly toxic to frogs – or using electrical methods could help, but these approaches aren't always secure for other rare island organisms.

Lacking answers to more of the fundamental issues about their biology and impact, culling the frogs might not even be the right way to proceed, says San José.

Financial Obstacles for Research

While she expects 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 obtain.

"Everyone wants to give support for protecting frogs," says San José. "But it's more difficult to find funding for an introduced frog that you might want to control."