Deep in Costa Rica’s mist-shrouded cloud forest, hundreds of bright golden toads would appear suddenly each April to mate. It was a spectacular sight for those who witnessed it: the dazzling, mostly subterranean amphibians gathered en masse around pools of rainwater and fought aggressively for the right to copulate with the females before heading back underground.
“It was one of the truly great wildlife spectacles of the American tropics,” says ecologist Alan Pounds, resident scientist at the Tropical Science Center’s Monteverde Cloud Forest Biological Preserve, standing at the centre of the toads’ former habitat. “It somehow looked unreal.”
About 1,500 golden toads were observed in 1987 in the area of the highland forest where the entire species resided – the Children’s Eternal Rainforest. But by 1989, only a single male was left after the pools in which the toads congregated dried up. He is presumed to have died not long after. The species was certified as extinct in 2004 and is believed to be one of the earliest terrestrial extinctions linked to the climate crisis.
Pounds was among the first to report on the impacts of the climate crisis on natural populations in his investigation into the declines of amphibians at Monteverde, including that of the golden toad. He and colleagues published a report which featured on the front page of Nature in 1999 and argued that the declines were part of a constellation of human-driven biological changes also affecting birds and reptiles.
Earlier this year, the UN’s Intergovernmental Panel on Climate Change (IPCC) confirmed that the golden toad (Incilius periglenes) was one of two species (along with an Oceanian rodent) for which the climate crisis was “implicated as a key driver” in its extinction.
The report cited “successive extreme droughts” as the cause. Increasingly wet spells became interspersed with ever drier periods, leaving behind mossy enclaves, which are now largely devoid of noticeable frog and toad life.
“Before, we would see litters of frogs hopping across the trails,” Pounds says. But on a four-hour hike in late August, not one amphibian showed itself – though on one occasion a ribbit was heard.
Climate heating and deforestation continue to push clouds in the area higher upland – creating the conditions in which amphibians are more vulnerable to a potentially lethal exotic chytrid fungus that has wiped out dozens of species globally.
“The extreme conditions that result from climate change and their interaction with other forces can load the dice for outbreaks of certain diseases,” Pounds says. “Rarely is it the case in ecology that a single factor explains changes: everything is interconnected, and the empirical data show climate change is playing a key role.”
In the cloud forest, he says, weather monitoring has observed a steady 1C rise in temperature in the past 40 years. This has led to greater, but much more variable overall rainfall. In the 1970s, there were about 25 dry days a year but over the past decade there have been more than 100 annually.
“We searched many sites: the gaps between rocks, rivers, permanent and temporary pools, and the hiking paths,” says Gabriel Barboza, an amateur naturalist who took part in the forest inventory of species in the 1990s that confirmed the disappearance of the golden toad.
“Until around 1990, there was always mist here in the town of Santa Elena [several miles from Monteverde]. Sometimes, you could raise your arm and not see your hand, but that never happens any more. The clouds have moved higher up,” he says.
The fog and mist – neblina and llovizna, as they are called locally – remain, but have reduced to such a spectacular degree that tourists sometimes ask how to get to the cloud forest.
Visitors still have good reasons to come. The rainforest is home to about 3,000 plant species, some of which are unique to the area, including the most varieties of orchid found anywhere, and more than 750 types of trees. The ecosystem is one of the first in which plants were found to be absorbing water through their leaves in a process called foliar uptake.
The biodiversity hotspot is also home to more than 400 bird species, 100 species of mammal, including monkeys, jaguars and porcupines, as well as dozens of snakes and lizard species, and 60 surviving species of amphibian.
But there has been a wider erosion of amphibian diversity and abundance. Along with the golden toad, a breed of harlequin frog endemic to Monteverde remains missing after also disappearing in the late 1980s. Dozens of other amphibian species have vanished, in step with a wider decline in such populations across Central America.
Numerous searches for the unmistakable 5cm brightly coloured golden toad have been unsuccessful. As recently as last year, after rumours of sightings, the Monteverde Conservation League and the conservation group Re:wild led an expedition with local rangers, biologists and residents.
“The golden toad is the ghost that haunts Monteverde,” says documentary film-maker Trevor Ritland, who joined the searches and co-produced a short film on the subject. “Its demise is a warning to humankind: its killers may come for us and we can either learn or follow the golden toad into extinction.”
Although the searches for the iconic toad were unsuccessful, Ritland reports that they found several frog species that disappeared from the area around the same time. “The fact that those species are coming back suggests that Monteverde’s conservation efforts are paying off,” he says.
Since the golden toad lived underground and vanished suddenly without warning, gathering appropriate specimens for tests to determine the details of the collapse has been near-impossible. Accordingly, not everyone is convinced that the climate crisis alone was the cause of the toad’s demise.
Some academics have singled out the impact of the chytrid fungus, which was first detected on the Korean peninsula, and which can produce a fatal skin disease.
However, a 2019 paper combining experiments, field data and historical climate records reported that “widespread species declines, including possible extinctions, have been driven by an interaction between increasing temperatures and infectious disease”.
Pounds is convinced that while the fungus, periodic warming pattern El Niño and possibly the loss of forest well beyond the preserve’s boundary all played a part in the golden toad’s demise, global heating dealt the crucial blow.
“The disease is the bullet that kills, but climate change pulls the trigger,” he says.