Galápagos Lacked Any Native Amphibians. Until Hundreds of Thousands of Frogs Arrived

On her regular walk to the scientific station, scientist Miriam San José stoops near a small pond surrounded by dense vegetation and collects a compact plastic sound device.

She had placed there overnight to capture the characteristic croaks of the Scinax quinquefasciatus, recognized by local researchers as an invasive threat with effects that scientists are just beginning to understand.

Although teeming with unique wildlife – including centuries-old large turtles, marine lizards, and the famous finches that inspired Charles Darwin's theory of evolution – the Galápagos archipelago off the shoreline of Ecuador had long remained free of frogs and toads.

In the late 1990s, this changed. Some small tree frogs made their way from continental Ecuador to the islands, probably as stowaways on cargo ships.

DNA studies indicate that, over the years, there have been repeated accidental arrivals to the islands, and the frogs now have a strong presence on several locations: Isabela and Santa Cruz.

The population is expanding so rapidly that scientists have been finding it difficult to keep track, estimating numbers in the millions on every island, across developed and agricultural areas, but also in the protected natural reserve.

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

They estimated six thousand frogs in a solitary pond. "The calculations are still very low," states San José. "I am quite certain there are additional numbers."

Acoustic Chaos and Growing Concerns

The frogs' abundance is evident from the acoustic disruption they cause. "The amount of frogs and the noise – it's truly insane," comments the scientist.

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

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

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

"At first it was a surprise, seeing the initial frogs in the region," says the farmer, who started noticing their large numbers about three years ago when one leaped on her palm as she was stepping out of her front door.

Ecological Impact Remains Unclear

The noise isn't the fundamental problem, however. While the species has been in the islands for almost three decades, experts still know very little about its effect on the islands' delicately balanced terrestrial and aquatic environments.

On islands, it is very typical for invasive species to prosper, as they have none of their natural predators. The islands counts 1,645 introduced species, many of which are seriously disrupting the safety of its endemic ones.

A recent study suggests the non-native amphibians are voracious bug consumers, and might be disproportionately eating rare bugs found exclusively on the archipelago, or reducing the food sources of the region's rare avian species, disrupting the food chain.

Unusual Traits and Management Difficulties

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

Their metamorphosis process is also extremely inconsistent, with some larvae turning into frogs very quickly and others taking a extended period: the researcher observed one which stayed as a tadpole in her laboratory for half a year.

"We really don't know this part," she says, concerned the larvae could be affecting the islands' freshwater, a very limited commodity in the islands.

Methods to curb the frogs in the beginning of the century were largely ineffective. Conservation officers tried collecting significant quantities by hand and gradually increasing the salt content of lagoons in vain.

Studies suggests spraying caffeine – which is extremely toxic to frogs – or using electrocution could assist, but these approaches aren't necessarily safe for other uncommon island organisms.

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

Funding Challenges for Study

While she hopes the growing use of eDNA methods and genetic analysis will help her group understand of the invasive species, funding for the research has been hard to come by.

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