The Galápagos Islands Lacked Any Native Amphibians. Then Countless Numbers of Amphibians Invaded

On her daily commute to the scientific station, scientist the researcher crouches near a small pond covered by thick vegetation and collects a compact green audio recorder.

The device was left there overnight to record the characteristic calls of the Scinax quinquefasciatus, recognized by local researchers as an invasive species with effects that scientists are starting to understand.

Although teeming with unique animals – such as ancient large turtles, swimming lizards, and the well-known finches that inspired Darwin's evolutionary theory – the Galápagos archipelago off the coast of Ecuador had long remained free of amphibians.

During the 1990s, this shifted. Some small tree frogs made their way from continental Ecuador to the archipelago, likely as hitchhikers on cargo ships.

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

The numbers is expanding so rapidly that scientists have been struggling to keep track, estimating numbers in the millions on every island, across urban and agricultural areas, but also in the protected Galápagos national park.

When the biologist tagged amphibians and attempted to recapture them in the subsequent 10 days, she could find only a single tagged frog from time to time, indicating their numbers were enormous.

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

Acoustic Chaos and Rising Worries

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

For the scientists, their nightly mating calls are helpful in estimating their presence in remote areas, using recorders like the one near the office.

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

"In the rainy period, I regularly hear their calls and they're extremely loud," says a local coffee farmer from Santa Cruz.

"Initially it was a shock, seeing the initial frogs in the region," says the farmer, who started noticing their large numbers about several years ago when one leaped on her hand as she was walking out of her front door.

Ecological Impact Stays Unclear

The sound isn't the fundamental problem, however. While the amphibians has been in the islands for nearly 30 years, scientists still know limited information about its effect on the islands' delicately balanced terrestrial and aquatic environments.

On archipelagos, it is very common for non-native species to thrive, as they have none of their natural predators. The islands counts over sixteen hundred introduced types, many of which are seriously affecting the survival of its endemic ones.

A recent study suggests the invasive amphibians are hungry insect consumers, and might be unevenly consuming uncommon insects found only on the islands, or depleting the food sources of the islands' rare birds, affecting the ecosystem balance.

Unique Characteristics and Management Difficulties

The island amphibians have exhibited some unusual traits, including living in slightly salty water, which is uncommon for frogs.

Their metamorphosis stage is also highly variable, with some tadpoles becoming frogs very quickly and others taking a extended period: San José observed one which stayed as a tadpole in her laboratory for six months.

"We really don't know this aspect," she says, worried the tadpoles could be affecting the region's clean water, a very scarce resource in the islands.

Methods to control the frogs in the early 2000s were largely ineffective. Park rangers tried capturing significant quantities by hand and gradually increasing the salinity of ponds in without success.

Studies suggests spraying coffee – which is highly toxic to amphibians – or using electrical methods could help, but these methods aren't necessarily secure for other rare island species.

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

Funding Challenges for Study

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

"Everybody wants to give support for protecting frogs," says San José. "But it's harder to find financial backing for an introduced frog that you might want to manage."