If the amphibians die, this could have a direct impact on human health: A study shows connections between massive declines in amphibian populations in Central America and a sharp increase in malaria cases in the region. According to the researchers, the reason for this is that frogs, salamanders and the like normally keep the mosquito populations that transmit malaria in check. Due to the mass death of these amphibians, the mosquitoes were apparently able to spread more strongly again.
From the early 1980s to the mid-1990s, the chytrid fungus (Batrachochytrium dendrobatidis, Bd) spread in Costa Rica, infesting frogs, salamanders, and other amphibians, killing many of them. In the 2000s, the fungus made its way to Panama and caused the collapse of entire populations there as well. Worldwide, this pathogen has already led to the extinction of at least 90 amphibian species and the decline of at least 500 other species. Although the chytrid fungus caused the largest loss of biodiversity ever recorded for any pathogen, this gradual mass extinction of amphibians received little public attention.
Amphibian extinction affects human health
However, a new study now shows that amphibian extinctions in Central America may have had direct impacts on human health: "We combined ecological surveys, satellite data and public health records and used this data to suggest a link between the Bd-induced collapse of amphibian populations and an increase in human malaria cases,” reports a research team led by Michael Springborn from the University of California at Davis.
As the fungus gradually spread to Costa Rica and Panama, the researchers were able to determine when amphibian populations collapsed in each district. "For each district, we could see that shortly after the Bd-induced amphibian decline, malaria cases were significantly elevated for several years," the researchers said. According to their analysis, at the peak of the increase, up to one additional person per 1,000 inhabitants contracted malaria each year, which would not have been the case if the amphibian population had been intact. "This is a significant increase considering that the number of cases per 1,000 people nationwide during the period of our study was about 1.5 for Costa Rica and 1.1 for Panama," the researchers add.
Fewer frogs, more mosquitoes
According to the researchers, the reason for the increase is that the decline in amphibians has led to an increased number of mosquitoes that transmit malaria. Although data on the actual number of mosquitoes in corresponding years is lacking, it is known that many frogs, salamanders and other amphibians eat hundreds of mosquito eggs every day. It is therefore plausible that a massive decline in natural predators has led to larger mosquito populations and thus to increased transmission of malaria.
As a control, the researchers also tried to establish connections to other factors such as temperature, precipitation and tree density. However, none of these factors showed such strong temporal and spatial connections to the malaria outbreaks as the amphibian die-off. Although Springborn and his colleagues cannot rule out the possibility that other, as yet unknown, factors may have contributed to the outbreaks, they believe that amphibian declines are the most likely explanation.
Threat also from animal trade
In view of their results, the researchers call for greater attention being paid to the protection of wild animal populations in the future. "Stable ecosystems are important for all sorts of aspects of human well-being, including the regulation of processes important to disease prevention and health," Springborn said. "If we allow for massive ecosystem disruptions, it can have a significant impact on human health in ways that are difficult to predict in advance and difficult to control once they occur."
An important protective measure, for example, are new trade regulations that tighten control of international wildlife trade and thus limit the spread of diseases. Another fungal pathogen called Batrachochytrieum salamandrivorans, or Bsal for short, is currently threatening to penetrate ecosystems via the global trading markets. It is the closest relative of the chytrid fungus. "The costs of implementing such safeguards are immediate and obvious, while the long-term benefits are more difficult to assess," says Springborn. "However, as our study shows, the benefits can be enormous when it comes to avoiding ecosystem disruption."
Source: Michael Springborn (University of California, Davis, USA) et al., Environmental Research Letters, doi: 10.1088/1748-9326/ac8e1d