Biodiversity and human health: Why more species mean fewer cases of Lyme disease

The United Nations has proclaimed 2010 to be the International Year of Biodiversity. "Biodiversity" is shorthand for the variety of life on Earth, from genes to ecosystems, and is often quantified by the number of species in a specified area. If biodiversity seems a bit arcane to you, you're not alone.

Of the many environmental crises going on today — global warming, air and water pollution, invasive species, and so on — the biodiversity crisis might have the lowest profile. But it might well be the most critical to human well-being. It is certainly a factor in our increasing risk of contracting Lyme disease here in the Hudson Valley.

Right now we are losing species at rates at least 100 times faster than the fastest extinction rates documented in the fossil record — and this includes the extinction pulse that wiped out the dinosaurs 65 million years ago.

The main culprit is the destruction of natural habitats on which species rely. Clearing a swath of tropical forest to graze cattle or grow soybeans, bulldozing a mountaintop to mine coal, damming a river to irrigate cornfields, can cause dozens or even thousands of species to disappear for good. Biodiversity loss is often seen as unfortunate but necessary collateral damage from economic development.

Many who are concerned about species loss have argued it is our ethical and moral responsibility to protect our fellow inhabitants of Earth from extinction. Such a stance, however, has rarely slowed down development projects likely to drive species extinct, because protestors are easily cast as a fringe element — tree-huggers who care more about plants and animals than about people.

Now we know better.

That's because ecologists have discovered that biodiversity provides humanity with many "ecosystem services" that can't otherwise be engineered. Natural ecosystems clean the air, filter water, suck greenhouse gases from the atmosphere, provide genetic stock for agriculture, contribute chemicals that cure cancer, and so on. We might pay to filter water, for example, but ecosystems do it free of charge. And more diverse ecosystems are better at providing these services than less diverse ones.

More recently we have discovered that biodiversity protects us against exposure to Lyme disease, West Nile virus, and other diseases. Of these illnesses, Lyme disease has been best characterized, so I'll focus on it. We get exposed to this potentially debilitating disease when a blacklegged tick infected with the bacterium Borrelia burgdorferi bites us. The more infected ticks roaming around in places we hike, garden, watch birds or walk the dog, the greater our risk of exposure.

My laboratory has spent more than 10 years learning what regulates the abundance of infected ticks in Dutchess County habitats. Ticks have four life stages — egg, larva, nymph and adult — and nymphs are responsible for the vast majority of Lyme disease cases. And we have found the number of infected nymphs is determined by which mammal and bird hosts are available for larval ticks to feed on.

Larval blacklegged ticks are not picky eaters and will feed on pretty much anything with fur or feathers, from mice to raccoons to robins to deer. But tick larvae feeding on white-footed mice are at least twice as likely to survive and become infected as are those feeding on any other hosts. Some hosts — such as opossums and gray squirrels — groom off and kill ticks by the hundreds and are unlikely to infect them with the Lyme disease bacterium.

Mice thrive in disturbed, degraded, fragmented forests where their predators are scarce or absent. Forest fragmentation also tends to eliminate the species such as opossums and squirrels that protect us by killing ticks. But the same fragmentation boosts mice populations, which are excellent at feeding and infecting ticks. If we can avoid land-use practices that fragment our forests, we will have a more diverse community of mammals and birds and fewer infected ticks crawling around in the woods.

In this International Year of Biodiversity, it's important to realize the disappearance of species is not limited to far-away tropical places with limited direct bearing on our daily lives. Biodiversity loss is also a concern for us in the Hudson Valley, and managing our landscapes to support many species can help protect our health.

Cary Institute of Ecosystem Studies | Millbrook, New York 12545 | Tel (845) 677-5343

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