The Galápagos Islands Had No Native Amphibians. Then Hundreds of Thousands of Frogs Made Their Home

On her daily walk to the research facility, scientist Miriam San José stoops near a small pond covered by thick plants and collects a small plastic sound device.

The device was left there overnight to record the characteristic calls of the Fowler's snouted treefrog, known by Galápagos researchers as an invasive species with effects that experts are just beginning to comprehend.

Despite teeming with unique animals – such as centuries-old large turtles, swimming iguanas, and the famous birds that sparked Darwin's theory of evolution – the island chain near the coast of South America had historically been free of amphibians.

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

DNA studies suggest that, through time, there have been multiple accidental arrivals to the archipelago, and the amphibians now have a firm foothold on several locations: multiple locations.

The population is expanding so rapidly that researchers have been struggling to monitor, estimating numbers in the hundreds of thousands on every island, across developed and agricultural areas, but also in the conservation Galápagos national park.

When San José marked amphibians and attempted to recapture them in the following week and a half, she could locate only a single marked frog occasionally, suggesting their numbers were enormous.

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

Deafening Noise and Rising Worries

The frogs' abundance is evident from the acoustic chaos they create. "The number of frogs and the sound – it's truly incredible," says the scientist.

For the researchers, their nocturnal mating calls are useful in determining their existence in far-flung areas, using audio devices like the one outside the workplace.

But nearby farmers say the calls are so raucous they prevent sleep at night.

"In the rainy period, I constantly hear their calls and they're extremely loud," says Jadira Larrea Saltos from Santa Cruz.

"Initially it was a surprise, seeing the initial frogs in the area," says Larrea Saltos, who started observing their abundance about several years ago when one leaped on her palm as she was walking out of her house.

Ecological Impact Remains Unknown

The noise isn't the primary problem, however. While the species has been in the Galápagos for nearly 30 years, scientists still know limited information about its impact on the islands' precariously balanced terrestrial and aquatic environments.

On archipelagos, it is very common for invasive species to prosper, as they have none of their enemies. The islands has 1,645 introduced types, many of which are seriously disrupting the survival of its native ones.

A 2020 study indicates the non-native amphibians are hungry insect eaters, and might be disproportionately consuming uncommon bugs found only on the islands, or reducing the food sources of the region's uncommon avian species, affecting the ecosystem balance.

Unique Characteristics and Management Difficulties

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

Their metamorphosis process is also highly inconsistent, with some larvae becoming frogs very quickly and others taking a extended period: the researcher witnessed one which remained as a tadpole in her lab for half a year.

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

Methods to control the frogs in the early 2000s were mostly ineffective. Park rangers tried collecting significant quantities by manual methods and gradually raising the salt content of ponds in vain.

Studies suggests applying coffee – which is highly poisonous to frogs – or using electrocution could help, but these approaches aren't always secure for other rare island organisms.

Lacking answers to more of the fundamental questions about their lifestyle and impact, culling the amphibians might not even be the correct way to advance, says San José.

Financial Obstacles for Research

While she expects the increasing use of eDNA methods and genetic examination will assist her group make sense of the invader, financial support for the research has been difficult to obtain.

"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 manage."