Environment

Is Dilution the Solution?

Scientists study how biodiversity affects the spread of animal-borne disease.

June 4, 2009
American robins are one of many wildlife species that can thrive in developed areas near humans, and spread diseases to them. [Credit: Colin Purrington, flickr.com] Below left, Researchers survey scat to estimate the local population of deer. [Credit: Lynne Peeples]
American robins are one of many wildlife species that can thrive in developed areas near humans, and spread diseases to them. [Credit: Colin Purrington, flickr.com] Below left, Researchers survey scat to estimate the local population of deer. [Credit: Lynne Peeples]

Several studies published over the last few months have bolstered the case for the dilution effect in both West Nile and Lyme diseases. Washington University’s Allan looked at birds and mosquitoes along a gradient of rural-to-urban sites in the St. Louis area. As he moved from habitats with few humans to those with higher densities he found a simultaneous drop in biodiversity and rise in West Nile prevalence. “As diversity goes up, disease goes down,” says Allan, whose study was published in January 2009 in the journal Oecologica.

His findings align with work by John Swaddle, a biologist at the College of William and Mary in Virginia, published just a few months earlier in the journal PLoS ONE. Rather than counting species over tracts of land, Swaddle used public bird counts and health department reports to compare bird diversity between pairs of adjacent counties with and without human cases of West Nile. This way, he was able to factor out other potential explanations, such as differences in weather. Again, less biodiversity meant more disease.

Not all West Nile research supports such a direct relationship, however. A recent study in the Chicago area found no association between the number of species and cases of West Nile. Published in Oecologica in March 2009, it addressed the dilution effect at a finer scale than previous state or countywide analyses.

The lack of perfect consistency among the studies hints at some unanswered questions. “We have good patterns,” says A. Marm Kilpatrick, an infectious disease ecologist at the University of California, Santa Cruz. “But we don’t fully understand the mechanisms for those patterns.”

Many ecologists believe the dilution effect is more complicated than simply saying greater diversity leads to less disease. Growing evidence suggests that individual species do play a significant role. American robins, for example, appear to be one of the key hosts for West Nile. “Even if they only make up 1 to 10 percent of the birds in a site, they’re often 40 to 60 percent of the blood meals,” says Kilpatrick.

In the Lyme disease model, Ostfeld of the Cary Institute recently found that the total number of species was less predictive of disease than the actual identities and abundance of those species. The presence of white-footed mice was most vital to transmission, according to a study he co-authored in Ecology in October 2008.

This ambiguity inspired Ostfeld to continue digging deeper into disease dynamics, which is why he and his team are working in Dutchess County this year. Until the researchers are ready to begin trapping and transporting forest animals this summer, they remain busy counting creatures — and droppings.

Manipulating species, and counting feces

On this warm spring afternoon, Ostfeld’s colleague Shannon Duerr has found an unusually large number of droppings on Tract Number 36, including one circle boasting seven piles. This likely means “lots of deer, lots of mice, lots of ticks,” she says. The tally also means she can put her name on the Deer Poop Count Leader Board back at the Institute — a humorous “competition” that helps keep this messy and monotonous field work bearable.

The team’s latest research suggests that in addition to some species being unusually good hosts, others are really bad ones. Originally, a species was thought to “dilute” simply by providing a meal for ticks, but not infecting them. Now, certain animals have been found to act as especially powerful diluters because they are inhospitable, or even poisonous, hosts for ticks. “It looks like some species actually go out there and kill lots of ticks,” says Ostfeld, highlighting the opossum as one of these deadly hosts. At the other extreme, he says, white-footed mice are great for both boosting and infecting tick populations.

Just as Santa Cruz’s Kilpatrick doesn’t recommend hunting American robins — his research suggests that when robins disperse, mosquitoes’ next choice is people — Ostfeld’s work doesn’t mean that ecologists will be introducing furry “ecological traps” into urban parks anytime soon. “Imagine air-dropping opossums,” says Ostfeld. “I don’t think that would fly.”

On an experimental scale, however, Ostfeld is doing his own species manipulation in Dutchess County. His team hopes that a better understanding of the connections between habitat size, biodiversity and the spread of pathogens will eventually lead to actions that could dilute not only West Nile and Lyme disease, but also new diseases before they arise.

While people know more about animal-borne diseases now than they did several decades ago, Ostfeld sees a continuing need to alert the public to the important role biodiversity plays in keeping disease in check. “People care about their health a lot,” he says. “The notion that biodiversity might influence their probability of getting sick is a strong motivator.”

Public awareness could not only help prevent disease, Ostfeld says, it could also promote conservation — especially because the species involved are charismatic animals that are attractive to humans. “We’re talking about furry, feathered, colorful, warm fuzzy creatures,” he says. “People care about those as well.”

Related on Scienceline:

Preserving amphibian biodiversity.

What species give you Lyme disease?

Why are mosquitoes selective in who they bite?

About the Author

Lynne Peeples

Lynne Peeples is a freelance journalist focusing on health and the environment. She graduated from NYU’s Science, Health and Environmental Reporting Program, where she was the editor-in-chief of Scienceline. She has also written for Scientific American online, Audubon Magazine, The Harvard Gazette and Amstat News. Before NYU, Lynne worked at Harvard University crunching numbers for HIV clinical trials and environmental health studies, while teaching an introductory biostatistics course. She also holds an M.S. in Biostatistics from Harvard and a B.A. in Mathematics from St. Olaf College. Her resume and clips can be found at: http://www.lynnepeeples.com

Discussion

7 Comments

Awesome article. I actually think the dilution effect would work. This makes sense. I was bitten in my backyard and have been sick with Lyme for 13 years now. The problem with healthcare is an entirely different story but this problem with Lyme effects everyone, even athletes like this runner http://www.beatlymediseae.com who apparently went hiking and was bitten and remained sick for 4 years until she finally recovered from it. Once people are bitten it’s so hard to get over these nasty pathogens. Thanks so much for the article.

Jacey Barrows says:

Ops, I was writing too fast and didn’t put the her website right. it’s http://www.beatlymedisease.com

also there are good wikipedia information online about what to do about Lyme Disease.
Again, thank you for the article and the research!

mary says:

A good web page that has a interactive map that tracks Tick- Borne vectors is…. http://WWW.DOGSANDTICKS.COM. It tracks by State and County.Our Four leg family members have ticks that often love to bite the owner.One bite could change both your pets life and yours.I look forward to the results of this research.

Socrates says:

Ostfeld has a long history of “science” that keeps trying to get the focus off the deer as carriers of the ticks. For people who don’t want the deer seen as the problem, Ostfeld is the answer.

Deer are the primary reason why there are so many black-legged ticks (a/k/a deer ticks), but deer don’t carry the disease itself. Therefore, Ostfeld “studies” the role of the white-footed mouse and other small creatures, since they do in fact serve as Lyme reservoirs. He ignores plenty of evidence that if you reduce the deer population to the appropriate threshold (10-15 per square mile), you get the tick population down dramatically and the Lyme disease drops with it.

No other forest animal is present in sufficient numbers and densities to serve the role of the deer in transporting huge numbers of ticks, giving the adult female her last blood meal before she drops off and lays 2,000 to 3,000 tick eggs and starts the tick life cycle all over again.

Secret hidden in plain sight: We would still have a huge deer overpopulation problem even if they were not implicated in Lyme disease. Most of our worst Lyme areas have deer densities of 60 deer per square mile or higher, four to six times what it should be if you want to reduce the disease, not to mention the forest destruction caused by the deer. (The deer destroy the habitat of other small forest creatures, songbirds, native wildflowers, etc., and leave invasive plant species as monocultures, the only things that will survive the deer overbrowsing.)

I’ve been teaching my students and colleagues for years about the importance of biodiversity in mitigating the evolution and spread of highly virulent forms of microbes. Indeed as species diversity declines, the populations of those that remain increase exponentially to take advantage of the resources and space left unfilled… if microbes are competing for resources (hosts) then natural selection should favor more virulent strains as host availability increases (e.g., burn your bridges behind you so your competition can’t follow). To make matters worse, those species that can coexist with us, are immunologically similar enough that viruses can “lock-in” on us and spread interspecifically. Perhaps biodiversity is driven by this natural checks and balance system as imposed by a rapidly changing and adaptive microbial community.

Think it such as a journal or a magazine from general interest proper to browse. Funny, witty, controversial, entertaining, useful, acerbic, thoughtful, serious, curious, unexpected comebacks are usually welcome.

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