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

On her daily commute to the scientific station, biologist Miriam San José stoops near a small pond surrounded by dense plants and retrieves a compact plastic audio device.

The device was left there overnight to capture the characteristic croaks of the Scinax quinquefasciatus, known by Galápagos scientists as an invasive threat with consequences that scientists are just beginning to understand.

Despite abounding with remarkable animals – including centuries-old large turtles, swimming lizards, and the well-known finches that sparked Darwin's evolutionary theory – the island chain off the coast of South America had long remained devoid of frogs and toads.

During the 1990s, this changed. Some tiny tree frogs made their way from continental the mainland to the islands, probably as hitchhikers on cargo ships.

Genetic studies indicate that, through time, there have been multiple accidental arrivals to the islands, and the amphibians now have a firm presence on two locations: Isabela and Santa Cruz.

The numbers is growing so quickly that researchers have been finding it difficult to monitor, calculating populations in the millions on every island, across urban and agricultural areas, but also in the conservation Galápagos national park.

When the biologist marked frogs and attempted to recapture them in the subsequent 10 days, she could locate just one tagged frog from time to time, indicating their populations were massive.

They calculated six thousand frogs in a solitary pond. "The calculations are still very conservative," states the researcher. "I'm quite certain there are additional numbers."

Deafening Noise and Growing Concerns

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

For the researchers, their nightly mating calls are useful in determining their existence in far-flung areas, using recorders like the one outside San José's office.

But local agricultural workers say the sounds are so loud they keep them up at night.

"During the wet season, I constantly hear their croaks and they're really loud," says a local coffee farmer from Santa Cruz.

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

Environmental Consequences Remains Unclear

The sound isn't the fundamental problem, however. While the species has been in the islands for nearly three decades, experts still know very little about its impact on the islands' precariously balanced land and water environments.

On archipelagos, it is very typical for non-native organisms to thrive, as they have few of their natural predators. The islands has 1,645 invasive types, many of which are significantly disrupting the survival of its native ones.

A 2020 research indicates the invasive frogs are hungry insect consumers, and might be unevenly consuming uncommon insects found exclusively on the archipelago, or reducing the food sources of the region's uncommon birds, disrupting the food chain.

Unique Characteristics and Control Challenges

The Galápagos frogs have shown some atypical traits, including living in slightly salty water, which is uncommon for amphibians.

Their development stage is also highly variable, with some tadpoles turning into frogs very rapidly and others taking a extended period: the researcher witnessed one which remained as a larva in her lab for half a year.

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

Techniques to curb the amphibians in the early 2000s were largely ineffective. Park rangers tried capturing significant quantities by hand and gradually increasing the salt content of lagoons in vain.

Research suggests spraying coffee – which is extremely toxic to frogs – or using electrocution could assist, but these approaches aren't always secure for other rare island species.

Without answers to more of the basic questions about their biology and impact, culling the frogs might not even be the right way to advance, says San José.

Financial Obstacles for Research

While she expects the increasing use of environmental DNA methods and DNA analysis will help her group understand of the invader, financial support for the research has been difficult to come by.

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