Posted on behalf of Emma Marris.
If you were consoled by the news last year that the tropical rainforests are growing back, and that these new forests are “almost as biodiverse as untouched forest”, prepare to be unconsoled.
Even as those secondary forests are growing back, forests both old and new continue to be cut down. And this deforestation is not only driving species extinct and emitting lots of climate-changing carbon dioxide, it is also increasing malaria rates in deforested areas.
Researchers looked at satellite imagery of forest cover in Acre, Brazil, along with reports from health workers there. They conclude in the US Centers for Disease Control and Prevention journal Emerging Infectious Diseases that deforestation bumps up malaria rates in the area by almost 50%.
The Anopheles mosquitoes that carry malaria, in this case A. darlingi, just love the sunlit pools and puddles left in the wake of a good clear cut. Their non-malaria-transmitting brethren prefer the undisturbed forest pools in the shade.
The University of Wisconsin press release quotes co-author Sara Olson as saying “It appears that deforestation is one of the initial ecological factors that can trigger a malaria epidemic.”
This isn’t an entirely new idea. Back in 2008, similar results were found in Kenya, even though the malarial mosquito there is a different species.
The tree-planting nonprofit, Trees for the Future has been using the idea that forests combat malaria to encourage donations. They claim that trees suck up standing water in which mosquitoes breed, provide habitat for the birds and spiders that eat mosquitoes and can even drive them away all by themselves, as in the case of the Indian neem tree (Azadirachta indica), which has seeds that kill mosquito larvae.
So preserving rainforest could prevent malaria, as well as “slow forest loss, conserve biodiversity and preserve local cultures,” turning a win-win-win into a win-win-win-win. And planting trees, especially neem trees, could beat back malaria in places where it is rampant.
Except of course it is never quite that simple. According to the authors of a 2008 review of studies on the topic in Africa, “The relationship between malaria transmission, forest cover and deforestation is complex.” They report on a “generally accepted (though largely qualitative) opinion that deforestation increases the risk of malaria transmission in Africa and tropical America but decreases it in Asia.” Different malaria-carrying mosquitoes are adapted to different microhabitats; deforestation is often followed by agriculture, which, depending on the crop, can be good or bad for these species (rice paddies are some larvae’s dream home, for example).
If ecology and epidemiology were simple, we wouldn’t need ecologists and epidemiologists to unravel such tangles. But for the people in the studied districts in Brazil, deforestation was sickening.
Image: Barry Carlsen, University of Wisconsin, Madison