The Galápagos Islands Had No Native Amphibians. Until Countless Numbers of Frogs Made Their Home
On her daily walk to the scientific station, scientist Miriam San José crouches near a small pond covered by thick vegetation and retrieves a small green audio recorder.
The device was left there through the night to capture the distinctive calls of the Scinax quinquefasciatus, known by local scientists as an invasive species with consequences that experts are starting to comprehend.
Despite teeming with unique animals – including centuries-old giant tortoises, marine lizards, and the famous finches that sparked Darwin's theory of evolution – the Galápagos archipelago off the shoreline of South America had long remained free of amphibians.
During the 1990s, this shifted. Some small tree frogs traveled from continental the mainland to the archipelago, likely as stowaways on transport vessels.
DNA studies indicate that, over the years, there have been repeated accidental arrivals to the archipelago, and the frogs now have a firm foothold on several locations: multiple locations.
The population is expanding so quickly that scientists have been struggling to monitor, calculating numbers in the hundreds of thousands on every island, across developed and agricultural areas, but also in the protected natural reserve.
When the biologist marked amphibians and attempted to recapture them in the following 10 days, she could locate only a single marked frog from time to time, suggesting their populations were massive.
They estimated six thousand frogs in a single pond. "Our estimates are still very conservative," says the researcher. "I'm quite certain there are even more."
Deafening Noise and Rising Worries
The frogs' proliferation is clear from the sound chaos they create. "The amount of frogs and the sound – it's truly incredible," says San José.
For the researchers, their nightly mating calls are helpful in estimating their presence in remote areas, using audio devices like the one near San José's office.
But local farmers say the sounds are so loud they prevent sleep at night.
"In the rainy period, I regularly hear their calls and they're extremely loud," says a local coffee farmer from Santa Cruz.
"At first it was a surprise, observing the initial frogs in the area," 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.
Ecological Impact Stays Unclear
The noise isn't the primary problem, though. While the species has been in the Galápagos for nearly 30 years, scientists still know very little about its effect on the archipelago's precariously balanced terrestrial and aquatic environments.
On archipelagos, it is very common for invasive species to prosper, as they have none of their enemies. The Galápagos has 1,645 invasive species, many of which are significantly disrupting the survival of its native ones.
A recent study suggests the non-native frogs are voracious insect eaters, and might be disproportionately eating uncommon insects found exclusively on the archipelago, or depleting the food sources of the islands' uncommon birds, affecting the ecosystem balance.
Unique Characteristics and Management Difficulties
The Galápagos frogs have exhibited some atypical characteristics, including living in brackish water, which is rare for amphibians.
Their development process is also extremely inconsistent, with some tadpoles becoming frogs very rapidly and others taking a extended period: the researcher observed one which stayed as a tadpole in her laboratory for half a year.
"We really don't know this part," she says, worried the larvae could be affecting the region's freshwater, a very limited commodity in the islands.
Techniques to curb the amphibians in the beginning of the century were mostly unsuccessful. Park rangers tried collecting large numbers by hand and gradually increasing the salinity of ponds in without success.
Research suggests spraying caffeine – which is extremely poisonous to amphibians – or using electrocution could help, but these methods aren't necessarily secure for other uncommon Galápagos organisms.
Without answers to more of the basic issues about their biology and impact, culling the frogs might not even be the right way to proceed, says the biologist.
Funding Challenges for Research
While she expects the increasing use of eDNA methods and DNA analysis will help her group understand of the invasive species, funding for the research has been hard to obtain.
"Everybody wants to give funding for protecting frogs," says the researcher. "But it's more difficult to find funding for an introduced frog that you might want to manage."
