Galápagos Had No Native Amphibians. Until Countless Numbers of Frogs Arrived

During her daily walk to the scientific station, biologist the researcher stoops near a shallow pond surrounded by dense plants and collects a compact green sound recorder.

She had placed there through the night to capture the distinctive calls of the Scinax quinquefasciatus, known by Galápagos researchers as an invasive threat with consequences that scientists are just beginning to comprehend.

Although abounding with remarkable animals – including ancient large turtles, marine iguanas, and the well-known finches that sparked Charles Darwin's theory of evolution – the island chain near the shoreline of South America had historically been free of amphibians.

In the late 1990s, this changed. Several small tree frogs made their way from continental the mainland to the archipelago, likely as hitchhikers on transport vessels.

Genetic research suggest that, over the years, there have been multiple unintentional introductions to the islands, and the frogs now have a firm presence on several islands: Isabela and Santa Cruz.

The population is expanding so rapidly that researchers have been finding it difficult to keep track, calculating populations in the hundreds of thousands on each island, across urban and farming areas, but also in the protected natural reserve.

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

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

Acoustic Chaos and Growing Concerns

The frogs' proliferation is evident from the acoustic chaos they cause. "The number of frogs and the noise – it's really incredible," comments San José.

For the researchers, their nightly vocalizations are useful in estimating their presence in remote areas, using recorders like the one outside San José's workplace.

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

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

"Initially it was a shock, seeing the initial frogs in the region," says the farmer, who started observing their large numbers about several years ago when one jumped on her palm as she was walking out of her house.

Ecological Impact Remains Unclear

The sound isn't the fundamental problem, however. While the species has been in the Galápagos for nearly three decades, scientists still know limited information about its impact on the islands' precariously balanced land and water environments.

On archipelagos, it is very common for non-native organisms to prosper, as they have few of their enemies. The Galápagos counts over sixteen hundred invasive types, many of which are seriously disrupting the safety of its endemic ones.

A 2020 study indicates the invasive amphibians are voracious bug consumers, and might be unevenly eating uncommon insects found only on the islands, or reducing the nutrition of the islands' rare birds, disrupting the food chain.

Unusual Traits and Control Challenges

The island amphibians have exhibited some atypical traits, including living in brackish water, which is uncommon for amphibians.

Their development stage is also extremely variable, with some tadpoles turning into frogs very quickly and others taking a extended period: San José witnessed one which remained as a tadpole in her laboratory for half a year.

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

Techniques to curb the frogs in the beginning of the century were largely ineffective. Conservation officers tried collecting significant quantities by hand and gradually raising the salt content of lagoons in without success.

Studies suggests spraying coffee – which is highly toxic to frogs – or using electrocution could help, but these methods aren't necessarily secure for other rare Galápagos species.

Without answers to more of the basic questions about their biology and effect, removing the amphibians might not even be the correct way to advance, says San José.

Funding Challenges for Research

While she expects the increasing use of environmental DNA methods and genetic examination will help her team make sense of the invader, funding for the research has been difficult to come by.

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