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

On her daily commute to the scientific station, biologist the researcher stoops near a shallow pond surrounded by dense vegetation and retrieves a compact plastic sound device.

The device was left there through the night to record the characteristic croaks of the Scinax quinquefasciatus, known by Galápagos researchers as an non-native species with consequences that experts are starting to understand.

Despite teeming with unique wildlife – such as ancient large turtles, swimming lizards, and the well-known finches that inspired Charles Darwin's theory of evolution – the island chain off the shoreline of Ecuador had long remained devoid of frogs and toads.

In the late 1990s, this changed. Several small amphibians traveled from continental the mainland to the archipelago, probably as stowaways on transport vessels.

DNA studies suggest that, through time, there have been repeated accidental introductions to the archipelago, and the frogs now have a firm foothold on two locations: multiple locations.

The population is growing so rapidly that scientists have been struggling to keep track, estimating numbers in the hundreds of thousands on every island, across urban and farming areas, but also in the protected Galápagos national park.

When San José tagged frogs and attempted to recapture them in the following week and a half, she could locate only a single tagged frog from time to time, suggesting their populations were massive.

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

Acoustic Chaos and Rising Worries

The amphibians' abundance is clear from the acoustic disruption they cause. "The amount of frogs and the sound – it's truly incredible," says the scientist.

For the researchers, their nocturnal vocalizations are useful in estimating their presence in far-flung areas, using audio devices like the one near San José's workplace.

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

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

"At first it was a shock, observing the initial frogs in the area," says the farmer, who started observing their abundance about several years ago when one jumped on her palm as she was walking out of her front door.

Environmental Consequences Remains Unclear

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

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

A recent research suggests the invasive frogs are hungry insect eaters, and might be disproportionately eating uncommon bugs found exclusively on the islands, or depleting the food sources of the islands' rare birds, affecting the food chain.

Unique Characteristics and Control Challenges

The Galápagos frogs have exhibited some unusual traits, including surviving in brackish water, which is rare for amphibians.

Their metamorphosis stage is also extremely inconsistent, with some tadpoles becoming frogs very quickly and others taking a extended period: San José observed one which remained as a larva in her lab for six months.

"We truly don't know this aspect," she says, worried the larvae could be impacting the region's clean water, a very limited commodity in Galápagos.

Methods to control the amphibians in the beginning of the century were mostly unsuccessful. Park rangers tried capturing significant quantities by hand and slowly raising the salinity of lagoons in vain.

Research suggests spraying coffee – which is extremely poisonous to frogs – or using electrocution could help, but these approaches aren't always secure for other rare Galápagos organisms.

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

Financial Obstacles for Research

While she hopes the growing use of eDNA techniques and DNA examination will assist her group make sense of the invasive species, financial support for the project has been hard to obtain.

"Everyone 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."