The Galápagos Islands Had No Indigenous Amphibians. Then Countless Numbers of Frogs Made Their Home
During her daily commute to the scientific station, scientist the researcher stoops near a shallow pond surrounded by dense vegetation and collects a compact green audio device.
She had placed there overnight to record the characteristic croaks of the Fowler's snouted treefrog, known by Galápagos researchers as an non-native species with effects that experts are starting to comprehend.
Despite teeming with remarkable animals – including ancient giant tortoises, marine lizards, and the well-known birds that sparked Darwin's theory of evolution – the Galápagos archipelago near the coast of Ecuador had long remained devoid of amphibians.
During the 1990s, this shifted. Some tiny amphibians made their way from continental Ecuador to the islands, probably as stowaways on cargo ships.
DNA studies suggest that, through time, there have been multiple accidental arrivals to the islands, and the amphibians now have a firm foothold on several islands: multiple locations.
The population is expanding so quickly that scientists have been struggling to monitor, estimating numbers in the millions on every island, across urban and agricultural areas, but also in the protected Galápagos national park.
When the biologist marked amphibians and attempted to recapture them in the following 10 days, she could find only a single tagged frog occasionally, indicating their numbers were enormous.
They estimated 6,000 frogs in a solitary pond. "Our estimates are still very low," says the researcher. "I am pretty sure there are even more."
Deafening Noise and Growing Concerns
The amphibians' abundance is evident from the acoustic chaos they cause. "The amount of frogs and the noise – it's really incredible," comments the scientist.
For the researchers, their nocturnal vocalizations are useful in estimating their presence in far-flung areas, using audio devices like the one outside San José's workplace.
But nearby farmers say the sounds are so raucous they prevent sleep at night.
"In the rainy period, I constantly hear their calls and they're extremely loud," says Jadira Larrea Saltos from Santa Cruz.
"Initially it was a surprise, seeing the first frogs in the region," says the farmer, who started noticing their large numbers about three years ago when one jumped on her hand as she was stepping out of her front door.
Ecological Impact Stays Unknown
The sound isn't the fundamental problem, however. While the species has been in the islands for nearly 30 years, experts still know limited information about its effect on the islands' precariously balanced terrestrial and aquatic ecosystems.
On islands, it is very common for invasive species to prosper, as they have few of their enemies. The islands counts 1,645 invasive species, many of which are seriously affecting the safety of its endemic ones.
A 2020 study indicates the non-native amphibians are hungry insect eaters, and might be unevenly consuming uncommon bugs found exclusively on the islands, or depleting the nutrition of the region's uncommon avian species, affecting the food chain.
Unique Characteristics and Control Challenges
The island frogs have shown some unusual traits, including living in brackish water, which is rare for amphibians.
Their metamorphosis stage is also extremely inconsistent, with some larvae becoming frogs very rapidly and others taking a long time: San José observed one which stayed as a larva in her lab for half a year.
"We truly don't know this aspect," she says, worried the tadpoles could be impacting the islands' clean water, a very scarce commodity in Galápagos.
Methods to control the amphibians in the beginning of the century were largely unsuccessful. Park rangers tried capturing significant quantities by hand and gradually raising the salt content of lagoons in vain.
Studies indicates spraying coffee – which is extremely toxic to amphibians – or using electrical methods could assist, but these methods aren't always secure for other rare Galápagos species.
Without answers to more of the fundamental issues about their biology and effect, culling the amphibians might not even be the right way to proceed, says the biologist.
Funding Challenges for Research
While she hopes the increasing use of environmental DNA methods and genetic analysis will assist her group make sense of the invader, financial support for the project has been hard to come by.
"Everyone wants to give funding for protecting frogs," says the researcher. "But it's more difficult to find financial backing for an invasive frog that you might want to control."
