The Galápagos Islands Had No Native Amphibians. Until Hundreds of Thousands of Frogs Invaded

On her regular walk to the research facility, scientist the researcher stoops near a shallow pond surrounded by dense vegetation and retrieves a compact green audio recorder.

She had placed there overnight to record the distinctive calls of the Scinax quinquefasciatus, known by local scientists as an non-native threat with effects that scientists are starting to comprehend.

Despite abounding with unique animals – such as centuries-old giant tortoises, swimming iguanas, and the well-known finches that sparked Darwin's evolutionary theory – the Galápagos archipelago near the shoreline of South America had historically been devoid of frogs and toads.

In the late 1990s, this shifted. Some small tree frogs made their way from continental the mainland to the archipelago, likely as stowaways on transport vessels.

Genetic research indicate that, over the years, there have been multiple accidental arrivals to the archipelago, and the frogs now have a strong presence on several islands: Isabela and Santa Cruz.

The population is growing so quickly that researchers have been struggling to keep track, calculating populations in the hundreds of thousands on each island, across urban and agricultural areas, but also in the conservation Galápagos national park.

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

They estimated 6,000 frogs in a single pond. "The calculations are still very conservative," states the researcher. "I'm pretty sure there are even more."

Acoustic Chaos and Growing Concerns

The frogs' proliferation is evident from the sound disruption they create. "The number of frogs and the sound – it's really insane," says San José.

For the scientists, their nocturnal mating calls are helpful in determining their existence in remote areas, using audio devices like the one near San José's office.

But local agricultural workers say the calls are so raucous they keep them up at night.

"In the wet season, I regularly hear their croaks and they're really loud," says Jadira Larrea Saltos from Santa Cruz.

"Initially it was a shock, observing the initial frogs in the area," says Larrea Saltos, who started observing their large numbers about several years ago when one jumped on her hand as she was stepping out of her front door.

Environmental Consequences Remains Unknown

The noise isn't the primary problem, though. While the species has been in the Galápagos for nearly three decades, scientists still know very little about its effect on the islands' precariously balanced terrestrial and aquatic ecosystems.

On islands, it is very typical for invasive species to thrive, as they have few of their enemies. The Galápagos has over sixteen hundred introduced species, many of which are seriously affecting the safety of its native ones.

A recent study suggests the invasive frogs are hungry bug eaters, and might be disproportionately consuming rare insects found only on the archipelago, or reducing the food sources of the region's rare birds, disrupting the food chain.

Unusual Traits and Management Difficulties

The Galápagos amphibians have shown some unusual characteristics, including living in slightly salty water, which is uncommon for amphibians.

Their development stage is also extremely variable, with some larvae becoming frogs very rapidly and others taking a long time: the researcher witnessed one which stayed as a tadpole in her lab for six months.

"We truly don't know this part," she says, concerned the tadpoles could be impacting the islands' freshwater, a very scarce commodity in the islands.

Techniques to curb the frogs in the early 2000s were largely ineffective. Park rangers tried capturing large numbers by manual methods and slowly increasing the salt content of ponds in without success.

Studies indicates applying caffeine – which is highly poisonous to frogs – or using electrical methods could assist, but these methods aren't necessarily safe for other rare island organisms.

Lacking solutions to more of the basic questions about their biology and effect, culling the frogs might not even be the right way to proceed, says San José.

Financial Obstacles for Research

While she hopes the increasing use of eDNA techniques and genetic analysis will assist her group understand of the invader, financial support for the research has been difficult to obtain.

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