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During her regular walk to the research facility, scientist Miriam San José stoops near a small water body covered by thick plants and retrieves a compact plastic sound recorder.

She had placed there overnight to record the distinctive croaks of the Scinax quinquefasciatus, known by local scientists as an invasive threat with consequences that experts are just beginning to understand.

Although abounding with unique animals – including ancient giant tortoises, swimming lizards, and the famous birds that sparked Charles Darwin's theory of evolution – the island chain off the shoreline of Ecuador had historically been free of frogs and toads.

During the 1990s, this changed. Several tiny tree frogs made their way from continental Ecuador to the archipelago, probably as hitchhikers on cargo ships.

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

The population is growing so rapidly that researchers have been struggling 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 San José marked frogs and attempted to find them in the subsequent week and a half, she could find only a single marked frog occasionally, suggesting their numbers were enormous.

They calculated six thousand frogs in a single pond. "The calculations are still very conservative," says the researcher. "I am pretty sure there are additional numbers."

Acoustic Chaos and Growing Concerns

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

For the researchers, their nightly vocalizations are useful in estimating their presence in far-flung areas, using recorders like the one near San José's workplace.

But local agricultural workers say the calls are so loud they prevent sleep at night.

"During the rainy period, I constantly hear their calls and they're really loud," says Jadira Larrea Saltos from Santa Cruz.

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

Ecological Impact Stays Unclear

The sound isn't the fundamental problem, however. While the species has been in the islands for almost 30 years, scientists still know very little about its effect on the archipelago's delicately balanced terrestrial and aquatic ecosystems.

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

A 2020 research indicates the non-native amphibians are hungry insect eaters, and might be unevenly eating rare bugs found exclusively on the archipelago, or depleting the nutrition of the islands' rare birds, disrupting the food chain.

Unusual Traits and Control Challenges

The island frogs have exhibited some unusual characteristics, including living in slightly salty water, which is uncommon for amphibians.

Their metamorphosis stage is also extremely inconsistent, with some tadpoles becoming frogs very quickly and others taking a long time: San José witnessed one which stayed as a tadpole in her laboratory for half a year.

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

Methods to control the amphibians in the beginning of the century were mostly unsuccessful. Park rangers tried collecting significant quantities by manual methods and gradually raising the salinity of lagoons in without success.

Studies suggests applying caffeine – which is extremely poisonous to amphibians – or using electrical methods could help, but these methods aren't necessarily secure for other rare Galápagos organisms.

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

Funding Challenges for Research

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

"Everybody wants to give funding for protecting frogs," says the researcher. "But it's more difficult to find funding for an invasive frog that you might want to control."