The Benefits of Biodiversity in Our Homes

Contrary to conventional wisdom, an overemphasis on cleanliness may be hindering the benefits of biodiversity in our homes.


| March/April 2013



A buck, trees, and plants emerge from cupped hands.

If the germ theory is the idea that the presence of bad species can make you sick, the growing sense seems to be that the opposite can also be true.

Illustration By Yuta Onoda

We live at the crossroads of three global megatrends, three barreling and intertwined juggernauts of modernity. The first is the massive migration of humanity to the world’s cities. By 2050, two-thirds of all humans on Earth will live in urban areas.

The second is the loss of biodiversity. Species are disappearing, both from the places where we live and from the earth as a whole. If our hairy ancestors were to visit our cities and suburbs, they would wonder how the escalators work, but they would also question where the plants and animals have gone. What have we done with all the birds? Some, like the Carolina parakeet, are just gone. Others live on, but at a distance—geographically removed from our daily lives, far away from the majority of people.

And then there’s the third trend—the one that, at first glance, seems not to belong with the others. The prevalence of allergies and chronic inflammatory diseases among urban populations in developed countries has skyrocketed in recent years. Incidences of asthma, Crohn’s disease, multiple sclerosis, and even depression (which can have an immune component) are on the rise.

The parallels in geography and timing between urbanization, the loss of biodiversity, and the rise in immune-system problems raise an intriguing—and troubling—question. Could our distance from nature and our chronic immunological discontent be related? Some now say … yes.

In May 2012, a team of Finnish ecologists, allergy specialists, molecular biologists, and immunologists led by Ilkka Hanski at the University of Helsinki announced the results of a study comparing the allergies of adolescents living in houses surrounded by biodiversity to those of adolescents surrounded by simplicity—the modern landscape of cement and grass. They found that those individuals who lived in houses surrounded by a greater diversity of life were themselves covered with different kinds of microbes. They were also less likely to show the telltale immunological signs of allergies.

In the years to come, we may regard these results as a new threshold to our understanding biodiversity. What Hanski and others have posited—that the loss of contact with a diversity of other species is making us sick—is almost unprecedented in the long history of our medical understanding of the body. It is the opposite of the germ theory of disease. If the germ theory is the idea that the presence of bad species can make you sick, the growing sense seems to be that the opposite can also be true. We can get sick because of the absence of good species—or even just the absence of the diversity of species.