Galápagos Had No Native Amphibians. Until Hundreds of Thousands of Amphibians Invaded

On her daily commute to the scientific station, biologist Miriam San José crouches near a shallow pond covered by dense plants and collects a compact green sound recorder.

The device was left there through the night to record the distinctive calls of the Scinax quinquefasciatus, known by local scientists as an invasive threat with consequences that experts are just beginning to understand.

Despite teeming with remarkable wildlife – such as centuries-old giant tortoises, swimming iguanas, and the famous finches that sparked Charles Darwin's theory of evolution – the Galápagos archipelago near the coast of South America had historically been devoid of frogs and toads.

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

DNA studies suggest that, over the years, there have been multiple accidental arrivals to the islands, and the amphibians now have a strong foothold on several locations: Isabela and Santa Cruz.

The numbers is growing so rapidly that scientists have been struggling to keep track, calculating numbers in the millions on every island, across developed and farming areas, but also in the protected Galápagos national park.

When the biologist tagged amphibians and attempted to recapture them in the subsequent week and a half, she could locate only a single tagged frog from time to time, indicating their numbers were massive.

They estimated six thousand frogs in a single pond. "Our estimates are still very low," says the researcher. "I am pretty sure there are even more."

Deafening Noise and Growing Concerns

The frogs' abundance is clear from the sound disruption they cause. "The number of frogs and the sound – it's truly incredible," comments San José.

For the scientists, their nightly mating calls are helpful in estimating their presence in remote areas, using audio devices like the one outside San José's office.

But local farmers say the calls are so loud they keep them up at night.

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

"Initially it was a surprise, observing the first frogs in the region," says the farmer, who started observing their abundance 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 primary problem, though. While the species has been in the islands for almost three decades, scientists still know very little about its impact on the islands' precariously balanced land and water ecosystems.

On archipelagos, it is very common for invasive organisms to prosper, as they have none of their natural predators. The islands has over sixteen hundred invasive types, many of which are significantly disrupting the survival of its native ones.

A recent study indicates the non-native frogs are voracious insect consumers, and might be disproportionately eating rare bugs found exclusively on the archipelago, or reducing the nutrition of the islands' uncommon avian species, disrupting the food chain.

Unique Characteristics and Management Difficulties

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

Their development process is also extremely variable, with some tadpoles turning into frogs very rapidly and others taking a long time: San José witnessed one which remained as a tadpole in her lab for six months.

"We really don't know this part," she says, worried the tadpoles could be affecting the region's freshwater, a very limited commodity in Galápagos.

Techniques to control the amphibians in the beginning of the century were mostly unsuccessful. Conservation officers tried collecting large numbers by manual methods and gradually increasing the salinity of lagoons in without success.

Research indicates spraying caffeine – which is highly poisonous to amphibians – or using electrocution could help, but these methods aren't always safe for other rare Galápagos species.

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

Funding Challenges for Study

While she hopes the increasing use of environmental DNA techniques and DNA examination will assist her group make sense of the invader, financial support for the project has been difficult to come by.

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