In the 1990s and 2000s, the raucous hum of many tropical forests across Central America went quiet.
An amphibian-killing fungus — Batrachochytrium dendrobatidis (Bd) — swept across the region, and eventually the globe, snuffing out the ribbits and croaks of millions and millions of frogs, toads and salamanders in what may be the greatest recorded loss of biodiversity from a single disease. This fungal pandemic harmed over 500 amphibian species, driving dozens to extinction.
That colossal loss reverberated beyond amphibians to impact human health in surprising ways. In Panama and Costa Rica, malaria cases increased five-fold as Bd wiped out mosquito-eating frogs, toads and salamanders, according to research published this week in Environmental Research Letters. “The study found about one-half to two-thirds of that increase in malaria cases were tied to amphibian decline — drawing one of the clearest lines yet between biodiversity and public health.”
Grid spoke with Karen Lips, a tropical ecologist at the University of Maryland at College Park and co-author of the new study about its findings and why we should view the extinction and biodiversity crises as a public health crisis.
This interview has been edited for length and clarity.
Grid: I imagine that many people wouldn’t necessarily think that biodiversity would relate to public health. Why are they wrong, and what caused you to look for a link between amphibian loss and malaria?
Karen Lips: I think this is the essential problem with the biodiversity crisis. We’ve kind of made it an issue of saving panda bears and other beautiful, charismatic things. But the reality is that biodiversity is the foundation of life on the planet, including us.
When we were studying the amphibian extinction crisis, we were really interested to see if there was any way we could link that to a direct impact on humans. The loss seemed like such a huge thing. We thought there must be a connection, but we weren’t sure what that might be at first. We eventually found this correlation between changes in frogs and changes in malaria.
G: The chytrid fungus caused this massive decline in amphibians. Can you tell me a bit about what it is and why it’s had such an impact?
KL: Chytrid fungi are small, microscopic, aquatic organisms that probably came from Asia, but we have introduced them around the world, probably through trade in pets. And where they were introduced, they’re exotic species, and the native species don’t really have any defenses against them.
Chytrid fungi can infect pretty much every species of frog on the planet, more than 1,000 species. It can kill them basically by giving them a really bad skin infection that prevents the frog or the salamander from getting enough oxygen and from getting enough water, which they do through their skin. And so they die.
The impact has been huge. It’s been on every continent in the world [except Antarctica], and has caused maybe 70 to 80 percent declines in the numbers of frogs and salamanders. So you go from thousands and thousands of frogs in a pond to, like, two, and we’re seeing extinction of species all around the world. it all happened quite quickly.
G: Why did you think that this amphibian loss might have an impact on human health?
KL: I had been studying the frog extinction in Panama and Costa Rica for many years. And we’ve shown that when the frogs disappeared, lots of things changed: snake diversity went down, there were effects on steam ecology, lots of things. We thought it might have also affected people, and we started looking for preliminary information but there weren’t any.
We started thinking about possible connections. We know that frogs eat insects of all sorts. And it may be that because there was such a huge loss so quickly, that would result in an increase in insects. If there’s no frogs, there’s more bugs, and that could increase diseases that are carried by insects such as malaria. We looked at a really fine scale at when chytrid came through an area and the number of malaria cases. And it turned out that there was a pretty significant increase in malaria cases for about 6 or 8 years after chytrid was introduced. Compared to what normal [malaria] levels are, it’s an obvious big shift. It’s pretty crazy to think about.
G: Why do you think that happened?
KL: This is the essential question and more research will have to be done. What we think is that the chytrid fungus arrived, it was new and wiped out 80 to 90 percent of frog biodiversity, and the natural predators of mosquitoes were gone, allowing mosquitoes to increase. Because mosquitoes are the main vector for malaria, there were more malaria cases.
The key thing is that in this analysis we tried to control for all the known things normally associated with malaria, like deforestation or climate. We controlled for that and there were no major changes in any of that, the one thing that changed in that period was the change in the frog. We can’t say 100 percent that that was the causal link, but it suggests this might be happening.
G: Might there be other public health impacts of amphibian decline?
KL: Certainly, there are lots of other insects out there that carry diseases. Leishmaniasis is carried by these little flies; other mosquito species carry other diseases. We were limited by what data were available, but there definitely could be broader consequences.
G: What do you think this study says about the broader connection between biodiversity and public health?
KL: I always use the phrase, “we are all connected.” People cannot survive without a healthy environment. This concept of One Health, where people’s health is directly connected to the health of the environment and to the health of the animals and plants that live in that environment. And whether you’re talking about food or infectious diseases, or you know, building materials on the environment, we rely on a healthy environment to provide these goods and services. And I think more and more this is becoming better explained in the media, but it’s still something that doesn’t get as much attention as it should.
And it just means that conserving, protecting and managing healthy environments is really important for all of us everywhere on the planet.
Thanks to Alicia Benjamin for copy editing this article.