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During her regular walk to the scientific station, biologist Miriam San José stoops near a small water body surrounded by dense plants and collects a small green sound device.

She had placed there overnight to record the distinctive calls of the Fowler's snouted treefrog, recognized by Galápagos scientists as an invasive species with effects that experts are just beginning to comprehend.

Despite teeming with unique wildlife – including centuries-old giant tortoises, marine lizards, and the well-known birds that inspired Darwin's evolutionary theory – the Galápagos archipelago off the shoreline of South America had long remained free of amphibians.

During the 1990s, this shifted. Several small amphibians traveled from mainland Ecuador to the archipelago, probably as stowaways on transport vessels.

Genetic research suggest that, through time, there have been multiple unintentional arrivals to the islands, and the frogs now have a strong presence on two islands: Isabela and Santa Cruz.

The population is growing so rapidly that scientists have been struggling to keep track, estimating numbers in the millions on each island, across developed and farming areas, but also in the protected natural reserve.

When San José tagged frogs and attempted to recapture them in the subsequent week and a half, she could locate just one tagged frog occasionally, suggesting their populations were massive.

They estimated 6,000 frogs in a solitary pond. "The calculations are still very low," states the researcher. "I'm pretty sure there are even more."

Deafening Noise and Growing Concerns

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

For the researchers, their nocturnal mating calls are useful in determining their presence in remote areas, using audio devices like the one near San José's office.

But nearby farmers say the sounds are so raucous they keep them up at night.

"In the wet season, I regularly hear their calls and they're extremely loud," says Jadira Larrea Saltos from Santa Cruz.

"At first it was a shock, observing the first frogs in the region," says Larrea Saltos, who started noticing their abundance about three years ago when one jumped on her palm as she was walking out of her house.

Ecological Impact Stays Unclear

The noise isn't the primary problem, however. 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 terrestrial and aquatic environments.

On islands, it is very typical for invasive organisms to prosper, as they have few of their enemies. The islands counts 1,645 introduced types, many of which are seriously disrupting the survival of its native ones.

A 2020 study suggests the non-native amphibians are hungry insect consumers, and might be unevenly eating uncommon bugs found only on the islands, or depleting the nutrition of the islands' rare birds, affecting the food chain.

Unique Characteristics and Management Difficulties

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

Their development stage is also highly variable, with some larvae becoming frogs very quickly and others taking a extended period: San José witnessed one which remained as a tadpole in her laboratory for six months.

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

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

Studies indicates applying caffeine – which is extremely toxic to amphibians – or using electrocution could assist, but these approaches aren't always secure for other uncommon Galápagos species.

Without answers to more of the fundamental issues about their biology and effect, culling the frogs might not even be the right way to proceed, says the biologist.

Financial Obstacles for Research

While she expects the growing use of environmental DNA techniques and DNA analysis will assist her team make sense of the invasive species, funding for the project has been hard to obtain.

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