The Galápagos Islands Had No Indigenous Amphibians. Then Hundreds of Thousands of Frogs Arrived

During her daily walk to the scientific station, scientist Miriam San José stoops near a small water body covered by dense plants and collects a compact green sound recorder.

She had placed there overnight to capture the characteristic croaks of the Scinax quinquefasciatus, recognized by Galápagos researchers as an invasive threat with effects that experts are starting to understand.

Although abounding with remarkable wildlife – including centuries-old giant tortoises, swimming lizards, and the well-known birds that inspired Darwin's theory of evolution – the Galápagos archipelago off the shoreline of South America had historically been free of frogs and toads.

During the 1990s, this changed. Several tiny amphibians made their way from continental the mainland to the islands, likely as stowaways on transport vessels.

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

The numbers is growing so quickly that scientists have been struggling to monitor, estimating populations in the millions on each island, across developed and agricultural areas, but also in the conservation Galápagos national park.

When San José tagged frogs and attempted to recapture them in the subsequent week and a half, she could locate just one tagged frog occasionally, indicating their numbers were enormous.

They calculated 6,000 frogs in a solitary pond. "Our estimates are still very low," says the researcher. "I'm quite certain there are even more."

Acoustic Chaos and Growing Concerns

The amphibians' proliferation is clear from the acoustic chaos they cause. "The number of frogs and the sound – it's really insane," comments San José.

For the scientists, their nightly vocalizations are helpful in determining their presence in far-flung areas, using recorders like the one near the office.

But nearby farmers say the sounds are so loud they keep them up at night.

"During the rainy period, I regularly hear their croaks and they're really loud," says a local coffee farmer from the island.

"Initially it was a shock, seeing the initial frogs in the region," says the farmer, who started observing their abundance about several years ago when one jumped on her palm as she was walking out of her house.

Environmental Consequences Stays Unknown

The noise isn't the fundamental problem, though. While the amphibians has been in the islands for almost 30 years, experts still know very little about its effect on the islands' delicately balanced land and water ecosystems.

On archipelagos, it is very typical for invasive organisms to prosper, as they have none of their natural predators. The Galápagos counts 1,645 invasive types, many of which are significantly disrupting the survival of its endemic ones.

A recent study suggests the invasive frogs are voracious bug eaters, and might be disproportionately consuming uncommon bugs found only on the archipelago, or reducing the nutrition of the islands' rare avian species, affecting the ecosystem balance.

Unique Characteristics and Control Challenges

The Galápagos amphibians have shown some atypical characteristics, including living in brackish water, which is rare for amphibians.

Their metamorphosis process is also extremely inconsistent, with some larvae becoming frogs very rapidly and others taking a extended period: the researcher observed one which remained as a larva in her laboratory for half a year.

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

Methods to control the amphibians in the early 2000s were mostly unsuccessful. Conservation officers tried collecting large numbers by hand and gradually increasing the salt content of lagoons in vain.

Research indicates applying coffee – which is extremely toxic to frogs – or using electrocution could assist, but these methods aren't always safe for other rare island organisms.

Without solutions to more of the fundamental issues about their biology and effect, culling the amphibians might not even be the correct way to advance, says the biologist.

Funding Challenges for Study

While she hopes the increasing use of eDNA methods and genetic examination will help her team make sense of the invasive species, financial support for the research has been difficult to come by.

"Everyone wants to give funding for preserving frogs," says the researcher. "But it's harder to find funding for an introduced frog that you might want to manage."