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

During her regular commute to the research facility, scientist Miriam San José stoops near a shallow water body covered by thick plants and retrieves a small green sound recorder.

The device was left there through the night to capture the distinctive croaks of the Scinax quinquefasciatus, recognized by Galápagos researchers as an invasive species with consequences that scientists are starting to understand.

Although teeming with unique wildlife – such as ancient large turtles, marine lizards, and the famous finches that inspired Darwin's evolutionary theory – the island chain near the coast of South America had long remained devoid of amphibians.

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

Genetic research indicate that, over the years, there have been repeated unintentional introductions to the archipelago, and the amphibians now have a firm foothold on several islands: Isabela and Santa Cruz.

The numbers is growing so quickly that researchers have been struggling to monitor, calculating numbers 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 following 10 days, she could locate just one marked frog from time to time, indicating their populations were enormous.

They calculated six thousand frogs in a single pond. "The calculations are still very conservative," states San José. "I'm pretty sure there are even more."

Acoustic Chaos and Growing Concerns

The frogs' abundance is evident from the acoustic chaos they create. "The number of frogs and the noise – it's really incredible," says San José.

For the researchers, their nightly vocalizations are helpful in determining their existence in remote areas, using audio devices like the one near the workplace.

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

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

"At first it was a surprise, observing the initial frogs in the region," says Larrea Saltos, who started observing their abundance about three years ago when one jumped on her palm as she was walking out of her front door.

Ecological Impact Stays Unclear

The sound isn't the primary problem, however. While the amphibians has been in the Galápagos for almost three decades, experts still know very little about its effect on the islands' delicately balanced land and water environments.

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

A 2020 study suggests the invasive frogs are hungry insect consumers, and might be disproportionately eating uncommon insects found exclusively on the archipelago, or reducing the nutrition of the region's rare avian species, disrupting the food chain.

Unique Characteristics and Management Difficulties

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

Their metamorphosis process is also extremely variable, with some tadpoles becoming frogs very rapidly and others taking a extended period: the researcher witnessed one which stayed as a tadpole in her laboratory for six months.

"We really don't know this aspect," she says, concerned the larvae could be affecting the region's clean water, a very limited resource in Galápagos.

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

Research indicates applying caffeine – which is highly poisonous to frogs – or using electrocution could assist, but these methods aren't always secure for other rare island species.

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

Funding Challenges for Study

While she expects the increasing use of eDNA techniques and DNA examination will help her team make sense of the invasive species, funding for the research has been hard to come by.

"Everybody wants to give funding for preserving frogs," says San José. "But it's harder to find financial backing for an invasive frog that you might want to control."