Galápagos Lacked Any Native Amphibians. Then Countless Numbers of Frogs Invaded

During her daily commute to the research facility, scientist the researcher crouches near a shallow pond covered by dense vegetation and retrieves a small plastic audio device.

The device was left there overnight to record the characteristic calls of the Scinax quinquefasciatus, recognized by Galápagos researchers as an non-native species with effects that scientists are starting to understand.

Although abounding with unique wildlife – including centuries-old large turtles, swimming iguanas, and the famous finches that inspired Darwin's theory of evolution – the island chain near the shoreline of Ecuador had historically been free of amphibians.

In the late 1990s, this shifted. Several small tree frogs traveled from mainland Ecuador to the islands, 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 presence on several locations: Isabela and Santa Cruz.

The numbers is growing so quickly that researchers have been finding it difficult to monitor, calculating populations in the millions on each island, across developed and agricultural areas, but also in the conservation Galápagos national park.

When the biologist tagged amphibians and attempted to recapture them in the following 10 days, she could find only a single tagged frog from time to time, suggesting their populations were massive.

They calculated six thousand frogs in a solitary pond. "The calculations are still very low," says the researcher. "I am pretty sure there are even more."

Acoustic Chaos and Growing Concerns

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

For the scientists, their nocturnal vocalizations are useful in estimating their existence in remote areas, using audio devices like the one near San José's office.

But local agricultural workers say the calls are so raucous they prevent sleep at night.

"During the wet season, I constantly hear their croaks and they're extremely loud," says Jadira Larrea Saltos from Santa Cruz.

"At first it was a shock, seeing the first frogs in the region," says the farmer, who started noticing their abundance about several years ago when one jumped on her hand as she was stepping out of her front door.

Ecological Impact Stays Unknown

The noise isn't the fundamental problem, though. While the species has been in the Galápagos for almost three decades, scientists still know limited information about its effect on the archipelago's delicately balanced terrestrial and aquatic environments.

On islands, it is very typical for invasive species to thrive, as they have few of their enemies. The islands counts over sixteen hundred introduced types, many of which are significantly affecting the survival of its native ones.

A 2020 study suggests the invasive frogs are hungry insect eaters, and might be unevenly consuming rare bugs found exclusively on the islands, or reducing the food sources of the islands' uncommon birds, affecting the food chain.

Unique Characteristics and Control Challenges

The Galápagos amphibians have exhibited some atypical traits, including surviving in slightly salty water, which is rare for amphibians.

Their development stage is also highly variable, with some larvae becoming frogs very rapidly and others taking a long time: the researcher witnessed one which remained as a larva in her laboratory for six months.

"We truly don't know this part," she says, concerned the tadpoles could be impacting the region's freshwater, a very scarce commodity in Galápagos.

Techniques to curb the frogs in the early 2000s were mostly ineffective. Park rangers tried collecting large numbers by hand and slowly increasing the salt content of lagoons in vain.

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

Lacking solutions to more of the basic questions about their biology and impact, culling the frogs might not even be the right way to advance, says San José.

Funding Challenges for Study

While she expects the increasing use of environmental DNA techniques and DNA examination will help her team make sense of the invasive species, funding for the project has been hard to obtain.

"Everybody wants to give support for preserving frogs," says San José. "But it's harder to find financial backing for an invasive frog that you might want to control."