Where butterflies decline, chronic inflammatory diseases increase. Where native plants flourish, kids have fewer allergies. A new book explains why.

Eastern comma

As unseasonably warm temperatures descended on us this week, I threw open the windows and let the microbes in. I walked among the meadow plants to recruit protective bacteria onto my skin. And when the fleeting shadow of a winter-emerging Eastern comma butterfly fluttered into view, I pondered the established links between wildlife habitat and reduced risk of disease.

That was my reaction to finishing the book Never Home Alone, a sweeping look at the little noticed and often underappreciated life in our manmade nests. Though a New York Times headline said it “will make you terrified of your own house,” ecologist Rob Dunn’s latest work sparked only wonder and appreciation as I read the scientific case for something many of us already intuit: A sterile backyard can be hazardous to your health.

Among the studies demonstrating the need to rewild our outdoor spaces, one found a significant correlation between reduced risk for allergies and the prevalence of native plants in backyards. Others revealed similar results for children whose houses contained a diversity of environmental microbes. At the broader landscape level, a researcher examined statistical trends and linked the vanishing of butterflies with a rise in chronic inflammatory diseases among people.

Those are just a few of the revelations Dunn explores in his plea for more ecological balance in the places where we eat and sleep. Calling for an end to our all-out war on the tiniest organisms living among us, he inspires affection for everyone from the wasps who parasitize cockroaches to the bacterial detritivores chowing down on the millions of skin flakes we shed daily.

But to me the connection between native plants and human health is the most critical lesson of the book, in part because it’s something we can address almost immediately. Almost two decades ago, I reported on research showing that farm children were less likely to develop allergies than urban kids—an interesting finding but one that seemed difficult to incorporate into most lifestyles. We can’t all move to the country, after all, and most of us aren’t going to follow the cheeky advice of one medical journal editor who told readers at the time to “eat more dirt.” What we can do is plant native plants anywhere and everywhere we live, from the backyards of suburbia to the balconies and rooftops of downtowns. Taken cumulatively, the reasons for doing so—more food and shelter for wildlife, more filtration of stormwater runoff and environmental pollutants, and now, more natural protections for our own immune systems—have never been so compelling.

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Common checkered skipper

Other books have made persuasive cases for basking in Big Nature or nurturing organisms too small to see. The effects of walking among trees on mental health are beautifully narrated in Florence Williams’s The Nature Fix. The importance of soil and gut microbiomes to our physical health come to life in David Montgomery and Anne Biklé’s expansive The Hidden Half of Nature. In Never Home Alone, Dunn brings these two concepts together, detailing the role that microbes in biodiverse backyards and homes can play in preventing disease.

In a study linking the presence of uncommon native plants with lower allergy rates among teenagers, researchers in Finland also examined the children’s skin. They found a greater diversity of bacteria, among them a kind known to help modulate immune responses. Different species of plants associate with different microbial partners, and the protective bacteria could have landed on the children as they played in backyards. It also could have floated through windows as they slept. Regardless of how these helpful organisms had become a part of human microbial communities, the study established “a direct relationship between exposure to native plant diversity,” writes Dunn, “and the effect of native plant diversity on Gammaproteobacteria on the skin … which in turn triggers the peacekeeping pathway of the immune system and keeps inflammation in check.”

That’s a significant finding in a world where the incidence of chronic inflammatory diseases has doubled every two decades since 1950, especially in wealthier countries. In the U.S., allergies have increased by 50 percent and asthma by one-third over the past 20 years, Dunn notes. “The status quo is that we are exposed to far different species than we used to be, far fewer because we have diminished the biological diversity of the world around us and because we spend nearly all of our time indoors, a realm we appear to be making ever less diverse,” he writes. “As a result, Crohn’s disease, asthma, allergies, multiple sclerosis, and their kin have become far more common.”

Throughout the book, Dunn contends that our skewed view of the smallest creatures as pathogens and pests is contributing to our downfall. Out of the billions of microbial species, less than a hundred cause most of the world’s infectious diseases. The vast majority of the others are likely harmless or even helpful, but those in our homes stand little chance of survival in the face of all the toxins we throw at them. This, in turn, threatens to encourage stronger and ever more resistant versions of the few microorganisms that can harm us. In other words, by sealing up and scrubbing and spraying our homes to prevent intruders, we lock out our longtime allies as well. Describing the need to cultivate and nurture beneficial microorganisms while gently weeding out the ones that can hurt us, Dunn writes that “our perspective on risk has changed in ways that emphasize the dangers of gardening and largely ignore the dangers of war.”

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Red admiral

That attitude has also meant that the diversity of life right under our noses (and sometimes in them) goes unstudied. As Dunn writes, “we have blinders on blocking our view of the wildlife that is innocuous and close at hand.” He and his collaborators have set out to change that, exploring the microscopic life in showerheads and the insects in every nook and cranny of our indoor habitats. They’ve demonstrated that the microbial life on the hands and in the kitchens of bakers around the world affects the taste of their bread. They’ve discovered bacteria in the bellies of camel crickets that can break down the toxic byproduct of paper mills. And they’ve identified a new yeast in wasps’ nests that is now used in commercially produced beer.

Though he’s explored places where few other scientists have dared—or cared—to go, Dunn knows he hasn’t even scratched the surface of the microbiota lining every inch of our lives. Our homes are filled with extreme habitats, where hot water heaters, freezers and ovens invite species “once thought to live only in the deep sea, on glaciers, or in remote salt deserts,” he writes. At the same time, the places where we spend the most time, especially some urban apartments, are increasingly devoid of the microorganisms we coevolved with in the world outside our doors.

What can we do to change that? Dunn is careful to note that he’s not advocating for an end to antibiotics and life-saving vaccines that “tame the most dangerous beasts.” “But … we also need to find ways to allow the rest of biodiversity to flourish around us,” he writes, invoking the sentiments of the world’s first microbiologist. “We need, like Antony van Leeuwenhoek, to find joy and wonder in the bacteria, fungi, and insects in our daily lives.”

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Red-banded hairstreak

When my husband and I began exploring the possibility of installing solar panels on our south-facing roof, an architect friend recommended tighter insulation instead to save energy and cut costs. The idea gave me flashbacks to the illnesses I contracted while working long days in a cubicled fortress with no access to fresh air, and I had to wonder: Wouldn’t a sealed bubble of a house trap harmful indoor air pollutants as well?  Now that I’ve read Never Home Alone, I’m just as worried about the flip side—about the microbes that would be barred from entry, and about the loss of all that lovely stuff in the biodiverse backyards of those healthy Finnish kids.

I thought of this again as I closed the windows in preparation for the return of colder temperatures, and I hoped our microbial allies had found their way in during the brief warm spell. I also began dreaming of spring and the growing list of native plants we’ll nurture for our wild neighbors who need them as much as we do. When we first moved to our home, we were delighted to see one kind of butterfly enjoying our newly planted flowers, a phenomenon I’d never experienced while growing up. Little did I know how simple it would be to provide sanctuary for many more. Since we began nurturing native plants, the number of butterfly species finding sustenance on our two acres has grown to nearly 30.

Recently I was dismayed to learn that that one-third of those species are now uncommon sights in our county. Such statistics should concern us all in light of the observations of Tari Haahtela, an epidemiologist with a soft spot for butterflies. A lead author of the Finnish studies of allergies in children, he sounded a little-heard alarm in 2009, writing a paper that connected loss of butterfly habitat with degradation of human health.

To reverse the trend, Haahtela proposed a simple but profound solution: “Let us take care of the butterflies!” he wrote in his letter to the journal Allergy. “Their disappearance indicates loss of life. When the last tiger is killed in India, it may hit the headlines, however, shrinkage of biodiversity also takes place in the micro-world close to us, but without notice. Preserving biodiverse life might have a preventive effect on allergy and other diseases of modern civilization.”

Inviting the mini-wilderness back into our lives doesn’t have to be hard. Dunn proposes a few simple steps: Leave the windows open. Get a dog. Wash dishes by hand and avoid turning on the air conditioning as long as possible. Buy fresh food, bake sourdough bread, and most of all, head outdoors. “Plant a greater diversity of plants outside of your home and interact with those plants,” Dunn advises. “Tend them. Watch them. Take a nap on them.”

In other words, don’t just take care of your garden. Open yourself up to nature, and let the plants—and the butterflies and all the life you cannot see—take care of you, too.

(Photos: Nancy Lawson. Of the five butterflies shown, only one is a common sight across the county where I live.)