ABOUTbiologist on the way to his office at the Charles Darwin Research Station Miriam San Jose crouches near a shallow pond shrouded in vegetation and reaches deep into the foliage, pulling out a small green plastic voice recorder.
She left it there overnight to capture the infamous croaking of the Fowler's tree frog (Cinnax pentagonal)known to Galapagos scientists as an invasive threat, the consequences of which researchers are just beginning to understand.
Despite the abundance of wildlife – centuries-old giant tortoisesswimming iguanas and finches that gave rise to Darwin's theory of evolution – The Galapagos archipelago off the coast of Ecuador has long been free of amphibians. Until recently, frogs, toads, newts or salamanders did not waddle or jump on volcanic islands.
At the end of the 1990s the situation changed. A few small tree frogs have made their way from the mainland Ecuador to the islands, probably as stowaways on cargo ships.
Genetic studies show that there have been repeated accidental introductions into the archipelago over the years, and the frogs are now firmly established on two islands: Isabela and Santa Cruz. The population is growing so fast that scientists are struggling to track it, estimating it in the hundreds of thousands on each island, in urban and agricultural areas, and in the protected Galapagos National Park.
When San Jose tagged the frogs and tried to catch them over the next 10 days (a method commonly used to estimate animal populations), she was only able to find one tagged frog at times, suggesting their populations were huge. They estimate that there were 6,000 frogs in one pond. “Our estimates are still very conservative,” San Jose says. “I’m almost sure there are even more.”
The number of frogs is evidenced by the acoustic chaos they cause. “The amount of frogs and the noise is really crazy,” San Jose says.
For scientists, their nocturnal mating calls help assess their presence in remote areas using recording devices like the one outside the San Jose office. But local farmers say the calls are so raucous they are keeping them awake at night.
“During the rainy season, I constantly hear their calls, and they are very loud,” says Jadira Larrea Saltos, a coffee farmer at Finca La Envidia in Santa Cruz.
“At first it was a surprise to see the first frogs in the area,” said Larrea Saltos, who began noticing their abundance about three years ago when one jumped on her arm as she walked out the front door. There are now more poultry on her property, which she believes has helped keep the population at bay.
However, noise is not the main problem. Although this species has been present in the Galapagos Islands for almost 30 years, scientists still know very little about its impact on the archipelago's delicately balanced terrestrial and aquatic ecosystems.
Invasive species often thrive on islands because they have no natural predators. Galapagos Islands has 1,645 invasive speciesmany of which seriously compromise the safety of endemic species. avian Vampire flies bleed, rare bird chicks dry upand the hardy blackberry plant is invading the forests of Scalesia.
A A 2020 study found that invasive frogs are voracious insectivores.and may disproportionately consume rare insects found only in the archipelago or deplete the food sources of the islands' rare birds, disrupting the food chain. According to the ecologist, since the frogs' diet is dominated by moths, they may even influence pollination on the islands. Maria del Mar Moretta-Urdialeswho took part in the study.
Galapagos frogs have some unusual traits, including living in brackish water, which is unusual for amphibians. Their metamorphosis process is also extremely variable, with some tadpoles turning into frogs very quickly, while others take a long time: San Jose observed one tadpole that remained in her laboratory as a tadpole for six months.
“We don't really know that part,” she says, concerned that the tadpoles could affect the islands' fresh water, a very scarce resource in the Galapagos Islands. And although research shows that the Galapagos swimming beetle is now consuming large numbers of tadpoles in its diet, this does not appear to have any effect on their population.
Methods to control the frogs in the early 2000s were largely unsuccessful. Park rangers tried in vain to capture large numbers of individuals by hand and gradually increase the salinity of the lagoons. Research offers to spray coffee – which is highly toxic to frogs – or using electric current may help, but these methods are not necessarily safe for other rare Galapagos species.
Without answers to basic questions about their biology and impact, San Jose said, culling the frogs might not even be the right decision.
While she hopes the growing use of environmental DNA and genetic analysis will help her team understand the invader, funding for the project has been difficult to come by. “Everyone wants to fund frog conservation,” San Jose says. “But it's harder to find funding for an introduced frog that you might want to control.”
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