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On her daily commute to the scientific station, biologist Miriam San José crouches near a shallow pond covered by dense plants and collects a small plastic audio device.

She had placed there overnight to capture the distinctive calls of the Fowler's snouted treefrog, recognized by local scientists as an non-native threat with consequences that scientists are starting to understand.

Although teeming with unique animals – such as centuries-old large turtles, swimming lizards, and the famous birds that inspired Charles Darwin's theory of evolution – the Galápagos archipelago near the coast of South America had long remained devoid of frogs and toads.

In the late 1990s, this shifted. Some small tree frogs made their way from continental Ecuador to the archipelago, likely as stowaways on cargo ships.

DNA studies indicate that, through time, there have been repeated unintentional arrivals to the islands, and the amphibians now have a strong presence on several islands: Isabela and Santa Cruz.

The population is expanding so rapidly that scientists have been struggling to monitor, estimating populations in the hundreds of thousands on each island, across urban and farming areas, but also in the protected natural reserve.

When the biologist marked frogs and attempted to recapture them in the following week and a half, she could locate only a single marked frog from time to time, indicating their populations were enormous.

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

Deafening Noise and Rising Worries

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

For the scientists, their nocturnal vocalizations are useful in estimating their presence in remote areas, using audio devices like the one outside the office.

But local farmers say the calls are so raucous they keep them up at night.

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

"Initially it was a surprise, observing the initial frogs in the region," says Larrea Saltos, who started noticing their large numbers about several years ago when one jumped on her hand as she was stepping out of her front door.

Ecological Impact Stays Unclear

The noise isn't the fundamental problem, however. While the amphibians has been in the Galápagos for nearly three decades, experts still know limited information about its impact on the islands' precariously balanced terrestrial and aquatic ecosystems.

On islands, it is very common for non-native species to prosper, as they have few of their natural predators. The Galápagos counts 1,645 invasive species, many of which are significantly affecting the safety of its endemic ones.

A recent research suggests the non-native amphibians are hungry bug eaters, and might be disproportionately consuming uncommon insects found only on the islands, or reducing the food sources of the region's uncommon avian species, disrupting the ecosystem balance.

Unusual Traits and Control Challenges

The island frogs have shown some unusual traits, including surviving in brackish water, which is rare for frogs.

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

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

Techniques to control the frogs in the beginning of the century were largely unsuccessful. Conservation officers tried collecting large numbers by manual methods and slowly raising the salt content of ponds in vain.

Studies indicates spraying caffeine – which is extremely toxic to amphibians – or using electrical methods could help, but these methods aren't always secure for other rare Galápagos species.

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

Funding Challenges for Study

While she hopes the growing use of environmental DNA methods and genetic examination will assist her group make sense of the invader, financial support for the project has been hard to come by.

"Everybody wants to give funding for preserving frogs," says San José. "But it's harder to find funding for an introduced frog that you might want to manage."