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Saving Wildlife is Good for Your Health (But It’s Complicated)

Posted by Richard Conniff on June 19, 2015

(Photo: Wolfgang Kaehler/LightRocket via Getty Images)

(Photo: Wolfgang Kaehler/LightRocket via Getty Images)

I live in one of the towns that gave Lyme disease its name, and yet I also love wildlife. So I rejoiced a few years ago when a study argued that maintaining healthy natural habitats with a rich diversity of wildlife can help keep people healthy, too, by protecting us from infectious diseases. Now two new studies out this week support this theory—though skeptics say they still have their doubts.

The basic idea, first proposed by ecologists Richard Ostfeld and Felicia Keesing, is called the “dilution effect,” and it works like this: for a given disease affecting multiple species—like Lyme disease or malaria—some host animals readily transmit the disease organism to the tick or mosquito that will carry it to the next victim. But other species are dead ends.

In the case of Lyme disease in the western United States, for example, western gray squirrels readily contract the bacteria and pass it on to ticks. But when those same ticks feed on western fence lizards, it kills the Lyme-causing bacteria in the ticks’ blood. That makes the lizards a bad host for Lyme, and good for us. In theory, the greater the biodiversity in any habitat, the more chances there are that a “bad” host will get the bacterium or virus and therefore fail to pass it on, diluting the impact of the disease.

The first of the new studies, published this week in the journal Proceedings of the National Academy of Sciences (PNAS), reviewed dozens of past studies to see whether there was evidence for the dilution effect in 47 disease organisms that exclusively infect wildlife hosts and another 14 that also sicken humans. The result was “broad evidence that biodiversity inhibits the abundance of both types of natural enemies.” The researchers found evidence even for the idea that a diversity of wildlife reduces agricultural pests.

Support for “this idea that biodiversity can buffer against disease outbreaks,” was so strong, said lead author David Civitello, a disease ecologist at the University of South Florida, that it highlights the need to “move beyond debates over the generality of the dilution effect” and get down to understanding the specific mechanisms that drive the effect.

The second study, also published in PNAS this week, looked at the health of people living near strictly protected areas in the Brazilian Amazon and found good news: They had significantly lower rates of malaria, acute respiratory infections, and diarrhea than their counterparts living in more developed circumstances. This was true even after weeding out the effect of factors like rainfall, income, and health services, said Subhrendu Pattanayak, a co-author of the paper and a professor of public policy and environmental economics at Duke University.

The dilution effect might be one possible mechanism making protected areas good for the neighborhood. But Pattanayak also suggested that simpler factors may be at work. “If you have standing forests, you might have fewer breeding sites for mosquitoes,” he said. Roads in particular mean more malaria, because the edge habitat they create makes for better mosquito breeding sites. Because protected areas don’t allow forests to be cleared and burned, there’s less smoke in the air, Pattanayak said, so “you might not see those respiratory infections.” But he added that the root causes of diseases are difficult to pin down: It’s not just a case of “do one thing and get the other thing. It’s a lot more complex.”

At least one prominent critics of the dilution effect idea says that’s the sticking point. “I’m just not sure that you can generalize to the extent that you have one overwhelming message that biodiversity reduces disease,” said Daniel Salkeld, a disease ecologist at Colorado State University. He was the lead author of a 2013 paper that found “very weak support at best” for the dilution effect. Instead, that paper argued, disease risk depends on the specific local host species, vector species (that is, the ticks, mosquitoes, and other organisms that transmit a disease from host to host), and the nuances of how they live and interact.

At least from a public health perspective, Salkeld said, it makes more sense to evaluate diseases and their causes on a case-by-case basis. “There’s going to be exceptions, and if you’re doing a public health initiative, you want to know whether your case is an exception or not,” he said.

Maintaining biodiversity may well reduce the overall risk that people will contract diseases, and that’s both a good thing and another reason to protect wildlife habitat. “You can manage habitats so that you reduce disease,” Salkeld said. But for a specific, targeted public health project, it may be more effective to “just do larvicide treatment for mosquitoes.” In certain cases, biodiversity could actually increase the infectious disease risk to humans, as a 2014 paper suggested.

So what’s the bottom line? The dilution effect clearly holds promise. But it’s too soon to be trying to sell habitat protection as a broad public health initiative. Even Civitello and his co-authors acknowledge that “biodiversity does not inhibit every enemy in every system.”

What’s needed now is research on mechanisms, so conservationists will have the specific details to say that if you log here, or build a road here, you are liable to make people over here very sick. And that sustainable use project you have in mind? It might not be so sustainable if you factor in the effects on human health. With that kind of know-how, protecting habitat and biodiversity could yet become a specific tool for society to apply with confidence.

Geoffrey Giller contributed reporting for this story.

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