Although Bd swept through Central America from the 1980s through the 2000s, it was only recently that the analysis showing its impact on human health could be conducted, says Michael Springborn, the paper’s lead author and a professor and environmental and resource economist at the U.C. Davis. “The data existed, but it wasn’t easy to get,” he says. Over the years, county-level disease records have been digitized in the Costa Rican and Panamanian health ministries, providing an opportunity to combine this epidemiology in a specific statistical model with satellite imagery and ecological surveys that also revealed land characteristics and rainfall, as with data on amphibian declines.
“We always thought about linking up [the die-off] for the people, more people would be interested in it,” says Lips. “We were pretty confident that we could quantify changes in bugs or frogs or water quality or fish or crabs or shrimp. But making that connection with the people was so difficult because the effect was so diffuse and spread over such a large area.”
But precisely because Bd swept through Central America from northwest to southeast in a specific pattern — “a wave that hit district after district over time,” Springborn says — a natural experiment emerged that allowed researchers to identify Costa Rica and Panama to consider in detail before and after the arrival of the mushroom wave. From the medical records, they could see that malaria rates in the districts (called cantons or distritos) were flat before the Bd fungus broke through and started to rise after that. At the peak of the outbreak, six years after Bd’s arrival in an area, malaria cases rose five-fold.
And then they started falling off again, about eight years after the deadly fungus appeared. Researchers aren’t sure why, as most amphibian populations have not recovered from the fungal attack. Although some populations appear to be developing resistance, most have not regained density or diversity. Because the fungus lingers in the environment, they remain at risk.
One part missing from the researchers’ analysis is that there are no concurrent data proving that mosquito populations were increasing in a way that promoted malaria. The surveys they needed – on mosquito densities during and after Bd’s arrival in the 81 counties in Costa Rica and 55 in Panama – simply don’t exist. This makes it difficult for them to determine why malaria has returned, especially since frog populations have not recovered. Springborn suspects this could be due to human intervention, such as governments or organizations noticing the malaria outbreak and spraying insecticides or distributing bed nets. Or it could be that ecosystems were recovering even though frogs weren’t, and other predatory species took advantage of the emptied niche to keep mosquito numbers down.
But the fact that malaria rates have fallen again doesn’t negate the importance of the results. “Bd was mostly a story about the consequences for amphibians, basically: Isn’t it a shame to lose this charismatic group of organisms?” says James P. Collins, evolutionary ecologist and professor at Arizona State University. (Collins has a connection to this research; he oversaw a grant the National Science Foundation awarded Lips in the 1990s.) “There was an ingrained assumption that reducing the world’s biodiversity was bound to be harmful. Connecting the dots with real-world human impact is nice proof of understanding the consequences.”