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Though Bd swept through Central America from the 1980s to the 2000s, the analysis that demonstrated its effect on human health could be accomplished only recently, says Michael Springborn, the paper’s lead author and a professor and environmental and resource economist at UC Davis. “The data existed, but it wasn’t easily obtainable,” he says. Over the years, though, county-level disease records were digitized at the ministries of health in Costa Rica and Panama, providing an opportunity to combine that epidemiology in a particular statistical model with satellite images and ecological surveys revealing land characteristics and precipitation, as well as with data on amphibian declines.
“We always thought if we could link [the die-off] to people, more people would care,” Lips says. “We were pretty sure we could quantify changes in bugs, or frogs, or the water quality, or fish or crabs or shrimp. But making that connection to people was so difficult, because the effect was so diffuse, and it happened across such a large area.”
But precisely because Bd swept through Central America in a specific pattern, from northwest to southeast—“a wave that hit county after county over time,” Springborn says—it created a natural experiment that allowed the researchers to look granularly at Costa Rica and Panama before and after the fungal wave arrived. In the health records, they could distinguish that malaria rates were flat in counties (called cantons or distritos) before the Bd fungus tore through, then began to rise afterward. At the peak of the disease surge, six years from the arrival of Bd in an area, malaria cases rose five-fold.
And then they began to fall off again, beginning about eight years after the lethal fungus arrived. Researchers aren’t sure why, because most amphibian populations haven’t bounced back from the fungal onslaught. Though some populations appear to be developing resistance, most have not recovered their density or diversity. Since the fungus lingers in the environment, they remain at risk.
There’s a missing piece in the researchers’ analysis, which is that there is no contemporaneous data to prove that mosquito populations surged in a way that promoted malaria. The surveys they needed—of mosquito density during and after Bd’s arrival, in the 81 counties in Costa Rica and 55 in Panama—simply don’t exist. That makes it difficult for them to determine why malaria fell off again, particularly since frog populations haven’t revived. Springborn theorizes it might be due to human intervention, like governments or organizations noticing the malaria spike and spraying insecticides or distributing bed nets. Or it might be that ecosystems recovered even though the frogs did not, with other predator species taking advantage of the emptied niche to keep mosquito counts down.
But the fact that malaria rates came back down again doesn’t invalidate the findings’ importance. “For the most part, Bd has been a story of the consequences for amphibians, basically: Isn’t it too bad to lose this charismatic group of organisms?” says James P. Collins, an evolutionary ecologist and professor at Arizona State University. (Collins has some connection to this research; he oversaw a grant that the National Science Foundation made to Lips in the 1990s.) “It’s been an embedded assumption that reducing the world’s biodiversity is bound to be harmful. Connecting the dots to real implications for humans is a nice piece of evidence for understanding the consequences.”