Frogs, salamanders and other amphibians have been battling a plague for decades. A global fungal pandemic affecting these creatures has driven roughly 90 amphibian species to extinction and killed significant numbers of 400 other species, according to Wired’s Maryn McKenna.
Now, researchers have linked this amphibian die-off to a spike in malaria cases in Costa Rica and Panama during the 1990s and 2000s, respectively. The decline in amphibians likely led to a surge in malaria-spreading mosquitoes, since amphibian tadpoles eat mosquito larvae, according to New Scientist’s Clare Wilson.
The findings, published last week in the journal Environmental Research Letters, are the first to show that the amphibian deaths have impacted humans, per Wired.
“This paper is a wake-up call,” John Vandermeer, an ecologist at the University of Michigan who was not involved in the study, tells Wired. “It makes the point that the problem is not just that we’re losing biodiversity, and biodiversity is wonderful and pretty and beautiful. It’s that the loss of biodiversity does have secondary consequences on human welfare—in this particular case, human health.”
The fungus, known as chytrid or Bd (for Batrachochytrium dendrobatidis), appeared in northwest Costa Rica in the early 1980s, per New Scientist. From there, it moved south and east, spreading through Panama in the 2000s.
It devastated amphibian numbers, which in turn caused declines in the animals’ predators, such as snakes, and surges in insects that the disappearing amphibians would typically have eaten, per Wired.
To measure the impact on humans, the researchers used data from individual counties, called cantons or distritos, to track malaria’s spread across Costa Rica and Panama, writes New Scientist. While the spread occurred decades ago, the data only became obtainable recently, as county-level disease records were digitized, Michael Springborn, lead author of the paper and an environmental and resource economist at the University of California, Davis, tells Wired.
The researchers combined this malaria data with statistics on the amphibian decline, satellite images and ecological surveys to tease out the impact of dying frogs and salamanders on human health, per Wired.
One year after amphibians began to decline in an area, malaria cases linked to the Bd fungus started showing up, according to the paper. Initially, the number of cases rose for three years before plateauing for several more. The authors write that cases peaked about six years after amphibians started dying, and in Panama, malaria cases increased fivefold, per New Scientist. About eight years after Bd arrived, its effect on malaria cases started to decline and soon became statistically insignificant, per the paper.
During the surge, amphibian deaths accounted for an estimated one-half to two-thirds of all malaria cases, per New Scientist.
Though the researchers think that mosquito populations affected malaria cases, they couldn’t prove that relationship because the necessary data on mosquito density doesn’t exist, according to Wired. As a result, the researchers are unsure why malaria cases began to drop after eight years, even though frog populations hadn’t recovered. Springborn tells Wired it could be because humans took more steps to stop the spread of malaria, or because other predators may have stepped in to eat the mosquitoes.
The study was motivated by the threat of a similar fungal pathogen, which could spread through the international wildlife trade, according to a press release. Springborn says in the release that updating trade regulations on species that host these diseases could help prevent the illnesses from spreading among wildlife—and might even have benefits for human health.
“The costs of putting those protective measures in place are immediate and evident,” Springborn says in the release. “But the long-term benefits of avoiding ecosystem disruptions like this one are harder to assess but potentially massive, as this paper shows.”