On a humid summer afternoon, Steven Whitfield, a wildlife researcher at Florida International University, leaves the extensive trail system at La Selva Biological Station in Costa Rica and wades deeper into the rainforest. "I'll watch out for snakes for you," he says. I stop in my tracks. "You should probably also look out for them yourself, though," he adds.
We soon meet a red-and-blue poison dart frog chirping loudly on a log. I lean in closer to get a photograph, and the chirping stops. "If you get too close, he'll run away," Whitfield tells me, "but if you stay still for a second, he'll start calling again." Sure enough, the chirping resumes, and another poison dart frog soon hops into view.
Venturing into a swamp within the jungle, we are greeted by a bullfrog poking its head through vegetation, as well as a pair of mating toads. The Costa Rican rainforest boasts of life, and Whitfield was impressed by how many frogs he saw when he first visited the site eight years ago.
Which is why he couldn't believe it when his colleague, Kristen Bell, said the La Selva frogs were declining. Bell had counted frogs at La Selva using the same methods as studies done in the 1970's and reported far fewer frogs. Whitfield was skeptical: "I told Kristen that she was crazy." The golden toad and the harlequin frog had gone extinct from the mountaintops of Monteverde, but no one had yet detected widespread declines in lowlands regions like La Selva, which is shielded from deforestation and other types of man-made destruction.
To settle their argument, Whitfield and Bell reconstructed a long-term dataset on amphibians at La Selva, spanning 35 years and 17 species, and showed that the numbers of frogs have gradually dropped by about 75% since the early 1970's (PNAS, 104:8352-6, 2007). Lizards and salamanders have declined as well.
"The argument I had with Kristen is one that I lost," Whitfield says. "We're not sure at all what caused these declines," he says. "We were caught off guard." La Selva spans only 1615 hectares, but its declines pose a puzzle: Amphibians have been declining worldwide, but usually rapidly - within half a year, typically. But frogs at La Selva mysteriously died over three decades.
Researchers have long blamed the invasive and lethal chytrid fungus as the primary culprit for killing amphibians worldwide, but little is known about the prevalence of chytrid at La Selva, or when it first arrived. Whitfield has been conducting monthly tests for chytrid for over a year, and he hopes to find out which species are infected, as well as which seasons allow chytrid to flourish. He is also attempting to manipulate the amount of leaf litter, on which some species of frog thrive, and reconstruct a long-term dataset on the depth of leaf litter, reasoning that less litter could limit frogs' survival. The culprit may also be pesticides from nearby banana plantations or global warming, which only makes matters worse for the cold-blooded animals, who are vulnerable to even minor climate changes.
La Selva is the only place in the tropics with data on amphibians going back to the 1970s, but now such data is starting to be collected elsewhere as well. The University of Costa Rica just began a long-term monitoring program at Peninsula de Osa, another lowland site. "We are collaborating with scientists from US and Europe," says Mahmood Sasa, a researcher at the Clodomiro Picado Institute in Costa Rica, "but we need more access to local funds to pursue our own scientific agendas."
Nightfall arrives, and the rainforest takes on a subdued, eerie sort of charm. It's harder to find frogs in the dark. "It's really depressing, all these species going extinct…" Whitfield's voice trails off, and he grows silent. We shine our flashlights on the forest floor and make our way out of the Costa Rican wilderness, sharing the trail with leafcutter ants and listening to the cries of howler monkeys in the canopy.