Although Bd was widespread in Central America from the 1980s to the 2000s, analysis demonstrating its effect on human health was only recently achieved, says Michael Springborn, the paper’s lead author and a professor and environmental and environmental economist. resources at UC Davis. “The data existed, but it wasn’t easy to come by,” he says. However, over the years, county-level disease registries were digitized in the Costa Rican and Panamanian ministries of health, providing an opportunity to combine that epidemiology in a particular statistical model with satellite images and studies. ecological data revealing land characteristics and precipitation, as well as data on amphibian declines.
“We always thought that if we could link [the die-off] people, more people would care,” says Lips. “We were pretty sure we could quantify changes in insects, frogs, water quality, fish, crabs, or shrimp. But making that connection with people was very difficult, because the effect was so diffuse and it happened over such a large area.”
But precisely because Bd spread across Central America in a specific pattern, from northwest to southeast, “a wave that hit county after county over time,” says Springborn, it created a natural experiment that allowed researchers to take a close look at Costa Rica. and Panama. before and after the fungus wave arrived. In the health records, they were able to make out that malaria rates were flat in the counties (called cantons or districts) before the Bd fungus spread and then started to rise. At the peak of the disease surge, six years after Bd arrived in an area, malaria cases increased fivefold.
And then they started falling off again, roughly eight years after the lethal fungus arrived. The researchers aren’t sure why, because most amphibian populations haven’t recovered from the fungal attack. Although some populations appear to be developing resistance, most have not regained their density or diversity. Since the fungus remains in the environment, they remain at risk.
There is a missing piece in the researchers’ analysis, and that is that there are no contemporary data to show that mosquito populations increased in a way that promoted malaria. The surveys they needed on the density of mosquitoes during and after the arrival of Bd, in the 81 counties of Costa Rica and 55 in Panama, simply do not exist. That makes it difficult for them to determine why malaria has dropped again, particularly because frog populations have not revived. Springborn theorizes that it could be due to human intervention, such as governments or organizations noticing the rise in malaria and spraying insecticides or distributing mosquito nets. Or it could be that ecosystems recovered even though the frogs did not, with other predator species taking advantage of the empty niche to keep mosquito counts down.
But the fact that malaria rates dropped again does not invalidate the importance of the findings. “For the most part, Bd has been a fallout story for amphibians, basically: Isn’t it so 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 National Science Foundation grant to Lips in the 1990s.) “It has been an entrenched assumption that reducing the world’s biodiversity will surely be harmful. Connecting the dots with real implications for humans is good evidence for understanding the consequences.”