The Dilution Effect

By Jill Hahn / Special To The Tab

 

Here’s a story about a blacklegged tick, a white-footed mouse, an unpleasant

bacterium, and how reducing biodiversity in our own backyards can literally make us

sick.

Lyme disease, which now accounts for about 90

percent of the vector-borne disease (spread by an

animal carrier) in the U.S., is caused by the

bacterium Borrelia burgdorferi, transmitted by the

bite of an infected blacklegged tick, Ixodes

scapularis. A new-born tick does not carry the

bacterium. In order to acquire the bacterium, the

larval tick must take its blood meal from an infected

animal. And here’s where the I. scapularislarvae are

the kind of eaters you wish your children were: they

are not picky. They will feed on a wide variety of

mammalian, bird, or even reptile hosts. The larva

takes one blood meal from the host it happens to

encounter, and then molts into its next stage, called

a nymph.

Most people contract Lyme disease from the nymphal stage of the blacklegged tick,

partly because the nymph is small and hard to spot, partly because it is active in June

and July, when we’re likely to be out enjoying the woods.

What determines whether the nymphal tick that just bit you is likely to give you Lyme

disease? Dr. Richard Ostfeld, senior scientist at the Institute of Ecosystem Studies

(Millbrook, NY), conducted a series of elegant, if messy, experiments to find out. Since

a nymph can only acquire B. burgdorferi during its larval meal, Dr. Ostfeld’s first task

was to determine whether feeding on different animals resulted in differing proportions

of infected nymphs. To do this, he and his colleagues trapped individuals from every

potentially important bird and mammal species in his study site in Duchess County,

NY. This list included deer, robins and other songbirds, white-footed mice, chipmunks,

raccoons, possums, skunks, shrews, and squirrels. The animals (deer excepted) were

caged for 72 hours. Any tick larvae attached to them fell off into pans of water under

the cages. They were collected (a dirty job, because more than just ticks dropped into

those pans during the 72 hours) and tested for the presence of the Lyme disease

bacteria.

Dr. Ostfeld discovered that over 90 percent of the ticks that fed on white-footed mice

tested positive for B. burgdorferi. 40-55 percent of the ticks from shrews or chipmunks

tested positive, and the proportion of positive ticks collected from the other species

ranged from around 15 percent to less than 2 percent. So the host species a larval tick

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Photo courtesy of Frontiers in Ecology

and the Enivronment

White-footed mouse female

and pups

fed on dramatically affected whether the resulting nymph would be able to transmit

Lyme disease.

Since different species of animal have differing abilities to pass the Lyme disease

bacterium to the tick, this suggests that increased host biodiversity might lower the

prevalence of infected ticks. Dr. Ostfeld dubbed this hypothesis the Dilution Effect.

Ecologists know that, as you fragment forest into smaller and smaller pieces, the

number of animal species found in those forested "islands" declines. If the Dilution

Effect holds true, then the proportion of infected nymphal ticks should increase as

forest area goes down and biodiversity decreases. Dr. Ostfeld and his colleagues set

out to test that prediction.

They measured the density of nymphal ticks in forest fragments of different sizes

(ranging from less than two acres to almost 19 acres) by dragging drop cloths through

the forest and counting the number of nymphs collected. When Dr. Ostfeld tested the

ticks, he discovered that, as the Dilution Effect predicted, the proportion of ticks

infected with B. bergdorferi increased as the forested area decreased.

Why would this be? In the smaller forest fragments,

many potential host species disappeared. One

species, however, whose numbers conspicuously

explode as forest area decreases is the white-footed

mouse. White-footed mice, as Dr. Ostfeld had

already shown, are incredibly efficient at infecting

ticks with Lyme disease.

What does this mean for human health? Simply put,

biodiversity protects us from Lyme disease. If you go

hiking, say, in the White Mountains of New

Hampshire, and you get bitten by a blacklegged tick

nymph, you know that tick had a wide variety of

species from which it could have taken its larval

meal, and most of those wouldn’t be likely to infect it

with the Lyme disease bacterium. On the other

hand, if you’re out on a small plot of forested land in

your suburban hometown - especially if it’s smaller than about five acres - that nymph

that bit you most likely got its last meal from a white-footed mouse, and most likely did

contract the bacterium during that meal. So your chances of contracting Lyme disease

from a tick bite are much higher in the forest fragment near your house than in the

National Forest.

If those cute little white-footed mice are the problem, why not simply get rid of them?

Attempts to eradicate rodents to a level at which they can no longer transmit disease

are notoriously unsuccessful. Attempting to rid the woods of ticks is a similarly

Sisyphean task. There is another solution: let the Dilution Effect work for us by

changing the way we manage our landscape.

Dr. Ostfeld’s work has shown that the loss of biodiversity through the fragmentation of

our native forests has real health consequences. The Dilution Effect holds true for

Lyme disease and may play a role in other vector-borne diseases as well. It’s time to

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stop thinking of biodiversity as something that would be nice to preserve, but of no

practical value. The next time a local interest in your community wants to break up an

existing parcel of forest into smaller pieces for the sake of development, think about

the health consequences, and think twice.

Jill Hahn, a Newton Highlands resident, is a biologist, a writer, and a mom. All three

roles contribute to her passion about environmental issues. She can be reached at

jkkhahn@comcast.net.

This article is archived at www.greendecade.org/tabarchive.asp.