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During her regular commute to the scientific station, scientist Miriam San José crouches near a shallow water body surrounded by thick plants and collects a compact green sound device.

She had placed there through the night to record the distinctive croaks of the Scinax quinquefasciatus, known by Galápagos scientists as an non-native threat with consequences that scientists are just beginning to comprehend.

Despite teeming with unique wildlife – including centuries-old large turtles, swimming lizards, and the well-known finches that sparked Charles Darwin's evolutionary theory – the Galápagos archipelago off the coast of South America had long remained free of amphibians.

During the 1990s, this shifted. Some small tree frogs traveled from continental Ecuador to the archipelago, probably as hitchhikers on cargo ships.

DNA studies indicate that, through time, there have been repeated accidental arrivals to the archipelago, and the amphibians now have a strong foothold on several locations: multiple locations.

The population is growing so rapidly that researchers have been struggling to monitor, estimating numbers in the millions on each island, across urban and agricultural areas, but also in the conservation Galápagos national park.

When San José marked frogs and attempted to find them in the subsequent 10 days, she could locate only a single tagged frog occasionally, indicating their numbers were enormous.

They calculated 6,000 frogs in a single pond. "The calculations are still very conservative," says the researcher. "I'm quite certain there are even more."

Deafening Noise and Rising Worries

The amphibians' proliferation is evident from the acoustic chaos they cause. "The amount of frogs and the noise – it's really incredible," says San José.

For the scientists, their nocturnal mating calls are helpful in determining their existence in far-flung areas, using audio devices like the one near San José's workplace.

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

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

"Initially it was a surprise, seeing 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 walking out of her house.

Environmental Consequences Stays Unclear

The noise isn't the fundamental problem, though. While the species has been in the islands for almost 30 years, experts still know very little about its impact on the archipelago's precariously balanced land and water ecosystems.

On islands, it is very common for invasive species to thrive, as they have few of their enemies. The Galápagos counts over sixteen hundred introduced types, many of which are significantly affecting the survival of its endemic ones.

A 2020 research suggests the non-native frogs are hungry bug eaters, and might be unevenly eating rare bugs found only on the islands, or depleting the nutrition of the region's uncommon avian species, disrupting the food chain.

Unusual Traits and Control Challenges

The Galápagos frogs have shown some unusual traits, including surviving in brackish water, which is uncommon for amphibians.

Their metamorphosis stage is also extremely variable, with some larvae turning into frogs very quickly and others taking a long time: San José observed one which remained as a tadpole in her lab for six months.

"We really don't know this aspect," she says, concerned the tadpoles could be impacting the islands' clean water, a very limited resource in the islands.

Techniques to curb the amphibians in the early 2000s were largely unsuccessful. Conservation officers tried collecting large numbers by hand and gradually raising the salinity of lagoons in vain.

Research indicates applying coffee – which is highly poisonous to frogs – or using electrical methods could help, but these approaches aren't necessarily safe for other uncommon Galápagos 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 advance, says San José.

Funding Challenges for Study

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

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