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Who Cooks for You?

Emerging research shows how biodiversity is good for our health.

By Aaron Ferster

Photos courtesy of the U.S. Environmental Protection Agency

We began to hear the calls when the evenings were first warm enough to leave the windows open during dinner. It was that delightful time of year in the Washington area when the cold of winter is just about forgotten, but before the heat and humidity arrive. The last hues of sunlight linger just a bit longer each evening before surrendering to darkness.

A random lull in the family conversation was broken by an unmistakable sound coming from behind the house: hoo hoo to hoo, hoo hoo to hoo aw. The final verse trailed off with a noticeable warble.

The girls looked up, wide-eyed and open-mouthed. We all shared one thought: “OWL!”

We rushed to the window to scan the woods beyond the back fence. A few moments passed. Then we heard the call again. There! On the upper branches of a tall, thick-trunked pin oak not more than a few meters beyond the boundary between our suburban yard and the forest behind our house was the unmistakable silhouette of an owl.

The bird gazed in our direction for a beat, then looked away. We were able to steal a few more glances before it leaned forward off its perch, made a sharp, arching turn, and glided into the woods. Two silent wing beats later, our dinner guest was gone, seemingly evaporating before our eyes as it passed from open sunlight into the dappled shadow beyond. Like a ghost.

Owl Encounters

That was not our only encounter. Far from it. The owl interrupted dinner again the next night, and the night after that. In fact, by the time we surrendered to the summer heat and sealed the windows in favor of air conditioning, we just about expected owl calls to be part of the evening’s banter. A nesting pair had taken up residence somewhere just beyond our little cul-de-sac.

That was not our only encounter. Far from it. The owl interrupted dinner again the next night, and the night after that. In fact, by the time we surrendered to the summer heat and sealed the windows in favor of air conditioning, we just about expected owl calls to be part of the evening’s banter. A nesting pair had taken up residence somewhere just beyond our little cul-de-sac.

My favorite passage about barred owls in our field guide describes their distinctive “handle,” or call. The book compares it to a person’s insistently asking “Who cooks for you? Who cooks for you all?”

The girls found dozens of owl pellets beyond the fence. By picking them apart, we could decipher what the owls had been eating for dinner. It was mostly rodent bits; small jawbones with tiny, perfect teeth were common. Sometimes we found red scales and what looked like miniature lobster claws, a clue that our owls were plucking crayfish out of the creek below the nearby playground. It was fun to imagine the owls perched on the top of the swing set, watching for unsuspecting crayfish in the moonlight, and then swooping in for a meal.

We learned a lot from our new neighbors. And not long after their first appearance, I began to learn about a research program at my new job at the U.S. Environmental Protection Agency (EPA). The research explores how important such neighbors could be. As it turns out, the diversity of wild species and natural communities—biodiversity—that surrounds us could very well have an impact on public health. This vital relationship is the subject of a new exhibit in the Zoo’s Amazonia Science Gallery (see page 21).

A Link to Lyme Disease

“An emerging body of scientific evidence shows connections between the loss of biodiversity and human health, specifically the emergence of infectious diseases,” explains Montira Pongsiri, a scientist and colleague of mine at EPA. She helps lead the agency’s biodiversity and human health research. This collaborative effort (the Smithsonian is one partner) aims to better understand how human-caused changes to biodiversity are linked to public health risks, such as the transmission of disease-causing agents from animal hosts and vectors to people.

Much of the work is conducted by interdisciplinary research teams, supported in part by EPA research grants, bringing together a variety of experts—ecologists, biologists, public health officials, social scientists, and others—to integrate data on ecosystems, biodiversity, public health, and human-caused stressors to the environment.

One such expert is Richard S. Ostfeld, a self-described “disease ecologist” and a senior scientist at the Cary Institute of Ecosystem Studies in the Hudson River Valley in New York. Ostfeld’s research team is exploring how the risk of contracting Lyme disease seems to change with shifts in the diversity of species found in the forested landscapes of the northeastern United States.

Ostfeld’s field studies and computer models have revealed a predictable pattern linking the community of animals in local forested areas with the abundance of blacklegged ticks infected with the bacterium that causes Lyme disease. This relationship, referred to as “the dilution effect,” is an active area of research.

The effect depends largely on the relationship between black-legged ticks and white-footed mice. Hungry ticks are equal-opportunity suckers. They latch on to whatever source of blood meal happens to wander by: white-footed mouse, deer, barred owl, skunk, possum, or human.

The source of blood may not matter to the tick, but it turns out to be a major risk factor for people. To study this, Ostfeld and his research partners send field teams into the forest to find out what animals are living there. The teams set out baited live-traps and painstakingly inspect each animal caught for ticks. Any ticks they find are carefully removed and sent to a laboratory to determine if they harbor Borrelia burgdorferi, the bacterium responsible for Lyme disease.

Repeated results show that white-footed mice and eastern chipmunks are very efficient, as compared with gray squirrels and opossums, at transmitting B. burgdorferi to black-legged ticks. It follows then that the risk of someone’s being exposed to Lyme disease from a tick bite would be higher if that same tick had previously fed on a white-footed mouse or a chipmunk, instead of some other kind of animal. Without a full complement, or diversity, of critters, there are fewer tick hosts available to “dilute” the transmission of disease from tick to human.

From a public health perspective, the dilution effect boils down to this: The more white-footed mice there are (and the less diversity there is in the animal community), the higher the risk of people’s getting Lyme disease. And a relative abundance of white-footed mice is exactly what happens in the typical scenario of human-caused landscape changes in the northeastern United States. Forests are cleared and fragmented in favor of development. Some animals, particularly predators that rely on large tracts of intact habitat and abundant resources, have a hard time of it, perhaps disappearing. Other species, such as white-footed mice, thrive.

As biodiversity declines, what’s left is an animal community dominated by creatures that just happen to be the among the most efficient at transmitting the bacterium responsible for causing Lyme disease, currently the most frequently reported animal-borne disease in the United States.

Scientific Sleuths

Anyone living in the eastern third or so of the United States is well aware of the apparent increase in Lyme disease incidence, but it is not the only one that appears to be strongly influenced by changes in biodiversity. In an analysis of both epidemiological and ecological studies conducted over the past several years, Pongsiri and research partner Joe Roman, a biologist at the University of Vermont’s Gund Institute for Ecological Economics, identified malaria, West Nile virus, schistosomiasis, hantavirus pulmonary syndrome, and Lyme disease as all having links to biodiversity change, decline, and extinction. Three of these diseases—Lyme disease, malaria, and West Nile virus—are being studied as part of the biodiversity and human health research effort by EPA and its partners.

To investigate the dynamics of mosquito- borne diseases such as malaria, researchers at EPA and the Smithsonian Tropical Research Institute in Panama are monitoring mosquito species diversity at both intact and disturbed forest plots. The scientists hope to discern how landscape changes relate to the pathogens responsible for mosquito-borne diseases.

To start, the researchers are establishing a baseline survey to compare mosquito diversity (composition, abundance, and distribution) across habitats with different levels of forest disturbance.

What researchers learn will be added to the Smithsonian’s planned VectorMap initiative to provide of a picture mosquito-species richness and diversity across the globe. This research, coupled with an understanding of the relationship between diversity and land-use change, has the potential to provide critical information for developing land-use and integrated pest management strategies aimed at reducing the risk of mosquito-borne disease.

Closer to home, research teams are exploring how the diversity of wild bird species may be linked to the spread and transmission of West Nile virus. Its emergence in the United States was only confirmed in 1999 when a Bronx Zoo pathologist, concerned about a rash of crow deaths too close to the zoo for her comfort, convinced public health officials to reinvestigate a handful of human cases originally misdiagnosed as St. Louis encephalitis.

“The finding of West Nile virus in New York stunned experts, who have no theory so far on how the strain found its way here,” reported Jennifer Steinhauer in the New York Times.

Tools for Human Health

While the emergence of the West Nile virus is still a mystery, its transmission is linked to changes in biodiversity. Researchers from the University of California at Los Angeles are combining satellite-based remote sensing data with feather analysis to document the role that migratory birds play in disease transmission. Another team, from Rutgers University in New Jersey, is studying how the prevalence of West Nile virus is related to plant, mosquito, and avian diversity.

“The work that we and our research partners do can have an important impact on how people view and understand biodiversity,” says Pongsiri. “The overall goal is to provide information that can be applied to develop tools and models of disease risk that individuals and decision-makers can use to reduce and maybe even prevent disease.”

One such tool was recently released by a team at the Yale School of Public Health. It’s a smartphone app that provides a map of infected-tick density. Hikers who find a high infected-tick density overlapping their favorite trail can access a list of recommended precautions, such as applying insect repellent, wearing long pants and sleeves, and carefully inspecting themselves for ticks after they get out of the woods. The program also provides tick images so that people can identify different species.

The next step is providing tools to help entire communities make decisions about land use and environmental protection that incorporate a full understanding of the link between biodiversity and public health. “One of the societal benefits that biodiversity could provide,” says Pongsiri, “is the regulation of disease. If we can develop the scientific foundation to support this, we want to quantify the benefit of biodiversity in public health terms. This can help inform and improve decision- and policy-making for the environment and for human health.”

That environment includes the woods beyond my house. Could the owls that return each spring to ask “Who cooks for you?” and hunt mice be boosting the health of my family by protecting us from disease carriers? Perhaps. It’s a powerful, personal argument for advancing what we know about the importance of biological diversity.

-- AARON FERSTER served as the Zoo's lead exhibit writer for ten years before becoming a science writer at EPA.