Galápagos Had No Native Amphibians. Then Hundreds of Thousands of Amphibians Arrived

During her daily walk to the scientific station, scientist Miriam San José stoops near a shallow pond surrounded by dense plants and collects a small plastic audio recorder.

She had placed there overnight to record the distinctive croaks of the Scinax quinquefasciatus, recognized by Galápagos researchers as an non-native species with effects that scientists are just beginning to understand.

Despite teeming with unique animals – including ancient large turtles, marine lizards, and the well-known birds that sparked Darwin's theory of evolution – the Galápagos archipelago off the coast of South America had long remained free of frogs and toads.

In the late 1990s, this shifted. Several small tree frogs traveled from continental the mainland to the islands, probably as stowaways on transport vessels.

DNA studies indicate that, through time, there have been multiple accidental arrivals to the islands, and the frogs now have a strong foothold on several islands: multiple locations.

The numbers is expanding so quickly that scientists have been finding it difficult to monitor, estimating populations in the hundreds of thousands on each island, across developed and agricultural areas, but also in the protected Galápagos national park.

When the biologist marked amphibians and attempted to find them in the following week and a half, she could locate just one marked frog from time to time, suggesting their numbers were enormous.

They calculated 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 Growing Concerns

The frogs' abundance is clear from the sound chaos they create. "The number of frogs and the noise – it's truly insane," comments San José.

For the researchers, their nightly vocalizations are useful in determining their existence in far-flung areas, using recorders like the one outside the workplace.

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

"In the wet season, I constantly hear their calls and they're extremely loud," says Jadira Larrea Saltos from the island.

"At first it was a shock, seeing the first frogs in the region," says Larrea Saltos, who started noticing their abundance about three years ago when one jumped on her palm as she was stepping out of her house.

Ecological Impact Remains Unknown

The noise isn't the primary problem, however. While the species has been in the Galápagos for almost three decades, scientists still know limited information about its effect on the islands' delicately balanced land and water ecosystems.

On archipelagos, it is very common for invasive organisms to prosper, as they have few of their natural predators. The Galápagos has over sixteen hundred introduced species, many of which are seriously disrupting the survival of its native ones.

A 2020 study suggests the invasive frogs are hungry bug eaters, and might be disproportionately consuming uncommon bugs found exclusively on the archipelago, or reducing the nutrition of the region's uncommon birds, affecting the food chain.

Unique Characteristics and Control Challenges

The island frogs have shown some atypical characteristics, including living in slightly salty water, which is uncommon for frogs.

Their metamorphosis stage is also highly variable, with some larvae becoming frogs very quickly and others taking a long time: the researcher observed one which stayed as a larva in her lab for six months.

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

Methods to control the frogs in the early 2000s were mostly unsuccessful. Conservation officers tried collecting large numbers by hand and slowly raising the salt content of ponds in vain.

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

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

Financial Obstacles for Study

While she expects the increasing use of environmental DNA methods and genetic examination will help her group understand of the invader, financial support for the project has been difficult to obtain.

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