Does Biodiversity Really Protect Us From Disease?

November 15, 2013

Engorged deer ticks. Image credit: Dann Thombs/Flickr through a Creative Commons license.

Safe bet: Conservation will never have its pulse checked, history taken, pain charted and soul plumbed any more painstakingly than reporter Paul Voosen did it this week for The Chronicle of Higher Education.

The piece — titled “Who is Conservation For?” — is an elegantly crafted and exhaustive narrative of how conservation biology veered away from its biodiversity-centric, spiritualist roots (personified in the piece by Society of Conservation Biology founder Michael Soulé) and came to embrace ecosystem services (represented here by the Stanford University and Natural Capital Project’s Gretchen Daily). Ecosystem services, that is, as both a dominant analytical paradigm and the best way for conservation to stay relevant to the rest of the world.

Yes, Voosen captures the sometimes bitter debate between the two camps over which path conservation should follow — a story and a dynamic now quite familiar to many insiders. (“We fight with each other because we can’t succeed in fighting the real enemies, because they’re much more powerful,” he quotes Soulé as saying.) More interestingly, he also goes out to Vancouver Island to see how some of the Natural Capital Project‘s ecosystem service models are feeding into decision-making — and comes away less than inspired:

So this is what conservation science looks like. The Natural Capital scientists are essentially consultants, advising the planners, who then cajole interests to follow their zoning guidelines through regulatory action or peer pressure. It all reminded me of a writing term: the ladder of abstraction. At the top sits the Blue Marble view of the world; at the bottom, one human’s concrete actions. We were stranded in the ladder’s middle, vague, questionably effective. Certainly the conversations have shifted; no one had talked about the importance of sea grass for stopping coastal erosion before Wood and company arrived. But would all these plans change the coast’s future?

Well, that’s the gamble, isn’t it?

For my money, though, “Who is Conservation For?” takes a back seat to another Voosen piece from this week, a blog post on whether the “dilution effect” — the hypothesis that a rich level of biodiversity serves as a buffer between people and disease transmission from the animal world — really holds water.

It’s the tale of how hungry conservationists are for a way to sell our work to wide audiences; and how science sometimes gets in the way of reaching such Holy Grails.

The dilution effect has been building in currency for about a decade, led by the disease ecologist Richard Ostfeld, who found that the lower a forest’s biodiversity levels, the greater the incidence of Lyme disease in deer ticks in that forest.

Scientists have applied the dilution effect to other ecosystems, seemingly with success, and the idea went mainstream in a New York Times Sunday Review piece last year. It seemed like a slam dunk argument for prioritizing biodiversity protection — nature prevents killer diseases!

But as Voosen reports, other scientists have recently conducted experiments whose results chip away at the generalizability of the dilution effect and even at its replicability for Lyme disease in deer ticks.

Some have been openly skeptical of the idea of the dilution effect, saying it makes no sense based on their experience in other ecosystems. Research continues, and Ostfeld’s claims over Lyme reduction may yet stand up. However, as Voosen writes, with a touch of (earned) ominousness:

It now seems unlikely that a conservation group would build its actions on disease prevention as an ecosystem service. And that’s probably a good thing. Conservation science will get a thorough examination this next decade, like climate science has before it. The United Nations is now putting together the Intergovernmental Platform on Biodiversity and Ecosystem Services, modeled explicitly on its influential climate panel. Its first meeting occurred this year.

The platform will present conservationists with an unparalleled chance to influence policy. But it also presents risks, as several scientists noted last year in an editorial in Conservation Biology.“We tell compelling stories,” they wrote, but care is needed in which stories the science chooses to tell. Some, they warned, “will not stand up to careful scientific scrutiny.”


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  1. The most comprehensive analysis of the dilution effect (reduced disease transmission with increasing diversity) to date by a team of prominent scientists showed that the dilution effect is statistically supported in 80% of 152 direct tests in the field and lab ( [See especially the Supplementary Online Material for details]). Since that review paper appeared in June 2012, at least a dozen other studies demonstrating dilution have appeared in leading scientific journals. The dilution effect applies to diseases of plants, animals, and humans, and to a wide variety of pathogen types, modes of transmission, and ecosystems. It does not apply to all disease systems all of the time, but available evidence suggests that, for any randomly chosen disease, we can be about 80% sure that biodiversity will reduce its transmission risk.

    In its attempt to be even-handed, the blog post by Voosen conforms to the journalistic tradition of “teaching the controversy” even when the evidence for each side is quite unequal. For instance, rather than describing the analysis of 152 statistical tests published in Nature, Voosen emphasized a single, unpublished study that appears to compare Lyme disease risk in one diverse plot to that in one less diverse plot.

    Several lab groups are rigorously pursuing the mechanisms underlying the dilution effect, the conditions under which it is and is not expected to occur, and how theory interacts with data. These efforts should carry more weight than the anecdotes pro or con or the impressions of individual investigators based on natural history. As our understanding of these issues grows, we might reach a point where conservation groups will advocate specific actions on disease prevention as an ecosystem service.

    1. Thanks for the clarification, Dr. Ostfeld. To be fair to Paul Voosen, he did not emphasize a “single, unpublished study”; rather, he cited the Salkeld et al (2013) metaanalysis published in Ecology Letters, a forthcoming modeling paper in Ecology, and several other studies and analyses.

  2. Increasing biodiversity while maintaining, strengthening, and expanding mutually symbiotic networks is the way. In design schools we are taught the “principles of design”. One principle is Unity with Diversity. I believe this is key to ecosystem health.

    Also, when studying ecosystems we must make sure that we are respectful to every living cell, otherwise we erode the mutually symbiotic network because the relationships within a mutually symbiotic network are based on trust. If we study an ecosystem without respecting every living being, we introduce a reason for mistrust and start a chain reaction of defensiveness/offensiveness throughout the system. The ONLY way to accurately study something is by loving.