Galápagos Had No Native Amphibians. Until Countless Numbers of Frogs Made Their Home

On her daily commute to the research facility, scientist Miriam San José crouches near a shallow water body covered by dense plants and collects a small plastic audio device.

The device was left there through the night to capture the distinctive calls of the Scinax quinquefasciatus, known by Galápagos scientists as an non-native species with consequences that experts are starting to comprehend.

Although teeming with remarkable wildlife – such as centuries-old giant tortoises, marine iguanas, and the well-known finches that inspired Charles Darwin's evolutionary theory – the island chain off the shoreline of Ecuador had long remained devoid of amphibians.

In the late 1990s, this changed. Some small amphibians made their way from continental Ecuador to the archipelago, probably as hitchhikers on cargo ships.

Genetic research suggest that, over the years, there have been repeated accidental introductions to the archipelago, and the amphibians now have a strong presence on several locations: Isabela and Santa Cruz.

The population is growing so rapidly that researchers have been struggling to monitor, estimating numbers in the millions on every island, across urban and agricultural areas, but also in the conservation Galápagos national park.

When the biologist tagged frogs and attempted to find them in the subsequent week and a half, she could locate only a single tagged frog from time to time, indicating their numbers were enormous.

They estimated 6,000 frogs in a single pond. "Our estimates are still very low," says the researcher. "I am quite certain there are additional numbers."

Acoustic Chaos and Rising Worries

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

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

But nearby agricultural workers say the calls are so raucous they prevent sleep at night.

"During the rainy period, I regularly hear their calls and they're extremely loud," says a local coffee farmer from Santa Cruz.

"At first it was a surprise, seeing the initial frogs in the area," says the farmer, who started observing their large numbers about several years ago when one leaped on her hand as she was walking out of her house.

Environmental Consequences Remains Unclear

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

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

A 2020 study indicates the non-native amphibians are voracious insect consumers, and might be unevenly eating rare bugs found exclusively on the archipelago, or depleting the nutrition of the islands' uncommon avian species, affecting the food chain.

Unique Characteristics and Management Difficulties

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

Their development stage is also extremely variable, with some larvae turning into frogs very quickly and others taking a long time: the researcher observed one which stayed as a larva in her laboratory for half a year.

"We really don't know this part," she says, worried the tadpoles could be affecting the islands' freshwater, a very limited resource in the islands.

Techniques to control the frogs in the early 2000s were mostly unsuccessful. Conservation officers tried collecting significant quantities by hand and slowly raising the salt content of lagoons in vain.

Research indicates spraying coffee – which is highly toxic to frogs – or using electrical methods could assist, but these approaches aren't always safe for other uncommon island species.

Lacking solutions to more of the basic issues about their lifestyle and impact, removing the amphibians might not even be the right way to advance, says San José.

Funding Challenges for Study

While she hopes the increasing use of eDNA techniques and genetic examination will assist her team understand 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 harder to find funding for an introduced frog that you might want to manage."