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On her regular walk to the research facility, biologist Miriam San José crouches near a shallow water body surrounded by dense plants and retrieves a compact plastic audio device.

The device was left there through the night to capture the distinctive calls of the Scinax quinquefasciatus, recognized by Galápagos researchers as an invasive species with effects that scientists are just beginning to understand.

Although abounding with unique wildlife – including centuries-old giant tortoises, marine iguanas, and the famous finches that inspired Darwin's theory of evolution – the Galápagos archipelago near the coast of Ecuador had long remained free of amphibians.

In the late 1990s, this shifted. Several tiny amphibians traveled from mainland Ecuador to the archipelago, probably as hitchhikers on cargo ships.

Genetic studies suggest that, over the years, there have been multiple unintentional introductions to the archipelago, and the amphibians now have a firm presence on several locations: Isabela and Santa Cruz.

The numbers is expanding so rapidly that scientists have been finding it difficult to monitor, estimating numbers in the millions on every island, across developed and farming areas, but also in the conservation natural reserve.

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

They estimated six thousand frogs in a single pond. "Our estimates are still very low," says San José. "I am quite certain there are additional numbers."

Acoustic Chaos and Growing Concerns

The amphibians' proliferation is clear from the acoustic chaos they create. "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 far-flung areas, using audio devices like the one near San José's workplace.

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

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

"Initially it was a shock, seeing the initial frogs in the area," says the farmer, who started noticing their large numbers about three years ago when one jumped on her palm as she was walking out of her house.

Ecological Impact Remains Unclear

The noise isn't the fundamental problem, though. While the species has been in the Galápagos for almost three decades, experts still know limited information about its effect on the islands' delicately balanced terrestrial and aquatic ecosystems.

On islands, it is very common for invasive organisms to thrive, as they have few of their enemies. The Galápagos counts 1,645 introduced types, many of which are seriously affecting the survival of its endemic ones.

A 2020 research indicates the non-native amphibians are hungry bug consumers, and might be unevenly consuming uncommon insects found exclusively on the islands, or depleting the nutrition of the islands' rare birds, disrupting the ecosystem balance.

Unique Characteristics and Control Challenges

The Galápagos amphibians have exhibited some atypical characteristics, including surviving in brackish water, which is uncommon for amphibians.

Their metamorphosis stage is also highly inconsistent, with some larvae becoming frogs very quickly and others taking a extended period: San José observed one which stayed as a larva in her lab for half a year.

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

Techniques to control the frogs in the early 2000s were largely ineffective. Park rangers tried collecting significant quantities by hand and gradually raising the salt content of ponds in vain.

Research suggests spraying caffeine – which is highly toxic to amphibians – or using electrocution could help, but these approaches aren't always safe for other uncommon island species.

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

Financial Obstacles for Research

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

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