The Galápagos Islands Lacked Any Indigenous Amphibians. Then Countless Numbers of Frogs Arrived

On her daily commute to the research facility, scientist Miriam San José crouches near a small pond covered by dense vegetation and retrieves a compact green audio device.

The device was left there through the night to capture the characteristic croaks of the Fowler's snouted treefrog, known by local scientists as an non-native species with effects that experts are starting to comprehend.

Despite teeming with remarkable wildlife – including centuries-old large turtles, marine lizards, and the well-known finches that inspired Charles Darwin's theory of evolution – the island chain near the shoreline of South America had long remained free of frogs and toads.

In the late 1990s, this shifted. Several tiny tree frogs made their way from continental the mainland to the islands, probably as hitchhikers on cargo ships.

DNA studies suggest that, over the years, there have been repeated unintentional introductions to the islands, and the amphibians now have a firm foothold on several islands: Isabela and Santa Cruz.

The population is expanding so quickly that researchers have been struggling to monitor, estimating populations in the hundreds of thousands on each island, across developed and farming areas, but also in the protected Galápagos national park.

When the biologist marked frogs and attempted to recapture them in the following 10 days, she could locate only a single tagged frog from time to time, suggesting their numbers were enormous.

They calculated six thousand frogs in a single pond. "Our estimates are still very low," states San José. "I'm pretty sure there are additional numbers."

Deafening Noise and Rising Worries

The frogs' abundance is clear from the acoustic chaos they cause. "The number of frogs and the sound – it's really insane," says the scientist.

For the scientists, their nightly mating calls are useful in determining their existence in far-flung areas, using recorders like the one outside San José's office.

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

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

"At first it was a shock, observing the initial frogs in the region," says Larrea Saltos, who started noticing their large numbers about three years ago when one jumped on her hand as she was stepping out of her house.

Ecological Impact Stays Unknown

The noise isn't the primary problem, however. While the amphibians has been in the Galápagos for almost three decades, scientists still know very little about its impact on the archipelago's delicately balanced terrestrial and aquatic environments.

On islands, it is very typical for non-native organisms to thrive, as they have few of their natural predators. The Galápagos counts 1,645 introduced types, many of which are seriously disrupting the survival of its endemic ones.

A recent research indicates the invasive frogs are hungry insect consumers, and might be unevenly consuming rare bugs found only on the archipelago, or reducing the nutrition of the islands' rare birds, affecting the food chain.

Unusual Traits and Management Difficulties

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

Their metamorphosis process is also extremely variable, with some tadpoles becoming frogs very quickly and others taking a extended period: San José witnessed one which remained as a larva in her lab for half a year.

"We truly don't know this part," she says, worried the tadpoles could be impacting the region's clean water, a very limited resource in the islands.

Methods to control the amphibians in the beginning of the century were largely ineffective. Conservation officers tried collecting significant quantities by hand and gradually increasing the salt content of lagoons in vain.

Research indicates applying caffeine – which is extremely toxic to frogs – or using electrocution could assist, but these approaches aren't necessarily secure for other rare island organisms.

Without solutions to more of the fundamental questions about their biology and impact, removing the frogs might not even be the correct way to proceed, says San José.

Financial Obstacles for Study

While she expects the increasing use of environmental DNA methods and DNA analysis will help her group understand of the invader, financial support for the project has been hard to come by.

"Everyone wants to give funding for protecting frogs," says the researcher. "But it's harder to find financial backing for an introduced frog that you might want to manage."