Galápagos Lacked Any Indigenous Amphibians. Until Hundreds of Thousands of Amphibians Made Their Home

On her regular commute to the scientific station, biologist the researcher stoops near a shallow pond surrounded by thick plants and collects a compact plastic sound device.

She had placed there through the night to record the distinctive calls of the Fowler's snouted treefrog, known by Galápagos scientists as an non-native species with consequences that experts are just beginning to understand.

Although teeming with unique wildlife – including centuries-old giant tortoises, swimming lizards, and the famous birds that inspired Charles Darwin's theory of evolution – the island chain near the shoreline of South America had historically been devoid of frogs and toads.

In the late 1990s, this changed. Several small amphibians made their way from mainland the mainland to the islands, probably as hitchhikers on transport vessels.

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

The numbers is expanding so quickly that researchers have been finding it difficult to monitor, estimating populations in the hundreds of thousands on every island, across developed and agricultural areas, but also in the protected Galápagos national park.

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

They estimated 6,000 frogs in a solitary pond. "Our estimates are still very low," states the researcher. "I am pretty sure there are even more."

Acoustic Chaos and Growing Concerns

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

For the researchers, their nocturnal vocalizations are helpful in determining their existence in far-flung areas, using recorders like the one near the workplace.

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

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

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

Environmental Consequences Remains Unclear

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

On islands, it is very common for non-native organisms to thrive, as they have few of their natural predators. The islands has over sixteen hundred introduced species, many of which are seriously affecting the survival of its endemic ones.

A 2020 research indicates the non-native amphibians are hungry bug eaters, and might be disproportionately consuming uncommon bugs found exclusively on the islands, or depleting the nutrition of the islands' rare birds, disrupting the ecosystem balance.

Unusual Traits and Control Challenges

The island frogs have exhibited some atypical traits, including surviving in brackish water, which is rare for frogs.

Their metamorphosis stage is also highly inconsistent, with some larvae becoming frogs very rapidly and others taking a extended period: the researcher observed one which remained as a tadpole in her lab for six months.

"We truly don't know this part," she says, worried the larvae could be affecting the islands' clean water, a very scarce resource in the islands.

Methods to control the frogs in the beginning of the century were mostly unsuccessful. Conservation officers tried collecting large numbers by hand and slowly raising the salt content of ponds in without success.

Studies suggests spraying caffeine – which is highly poisonous to frogs – or using electrical methods could help, but these methods aren't always secure for other uncommon Galápagos species.

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

Funding Challenges for Research

While she hopes the growing use of eDNA techniques and genetic examination will assist her group make sense of the invader, funding for the project has been difficult to obtain.

"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 control."