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During her regular commute to the scientific station, biologist the researcher stoops near a small water body covered by thick plants and collects a compact green sound recorder.

She had placed there overnight to capture the distinctive calls of the Fowler's snouted treefrog, known by local scientists as an invasive threat with effects that scientists are starting to understand.

Despite teeming with unique animals – including ancient giant tortoises, marine iguanas, and the famous birds that inspired Charles Darwin's evolutionary theory – the Galápagos archipelago off the coast of South America had long remained free of frogs and toads.

During the 1990s, this changed. Several tiny tree frogs traveled from mainland Ecuador to the islands, likely as stowaways on cargo ships.

DNA studies suggest that, through time, there have been multiple unintentional arrivals to the archipelago, and the frogs now have a firm presence on several islands: Isabela and Santa Cruz.

The population is expanding so rapidly that researchers have been finding it difficult to keep track, estimating numbers in the hundreds of thousands on each island, across urban and agricultural 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 find just one marked frog occasionally, indicating their numbers were massive.

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

Deafening Noise and Growing Concerns

The amphibians' abundance is clear from the acoustic chaos they create. "The number of frogs and the noise – it's truly incredible," says San José.

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

But nearby agricultural workers 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 the island.

"Initially it was a surprise, seeing the first frogs in the area," says Larrea Saltos, who started noticing their abundance about several years ago when one leaped on her palm as she was stepping out of her house.

Environmental Consequences Stays Unclear

The noise isn't the primary problem, though. While the amphibians has been in the Galápagos for nearly 30 years, scientists still know limited information about its effect on the islands' precariously balanced land and water ecosystems.

On archipelagos, it is very typical for non-native organisms to prosper, as they have few of their natural predators. The Galápagos has 1,645 introduced species, many of which are significantly affecting the safety of its endemic ones.

A 2020 study suggests the invasive amphibians are hungry insect consumers, and might be unevenly consuming rare insects found only on the islands, or reducing the food sources of the islands' uncommon birds, disrupting the ecosystem balance.

Unique Characteristics and Control Challenges

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

Their metamorphosis process is also highly variable, with some tadpoles turning into frogs very quickly and others taking a extended period: the researcher witnessed 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 impacting the region's freshwater, a very scarce commodity in the islands.

Methods to curb the amphibians in the beginning of the century were largely ineffective. Park rangers tried capturing significant quantities by manual methods and slowly increasing the salinity of ponds in vain.

Research suggests applying coffee – which is extremely toxic to frogs – or using electrocution could help, but these approaches aren't always secure for other rare Galápagos organisms.

Without answers to more of the basic questions about their biology and effect, culling the amphibians might not even be the right way to proceed, says the biologist.

Funding Challenges for Research

While she hopes the increasing use of eDNA techniques and DNA analysis will help her team understand of the invasive species, financial support for the research has been difficult to obtain.

"Everyone 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."