Letting Biodiversity Get Under Our Skin
Some aspects of dirty living can be healthy. A new study posits that the decline of plant and animal diversity in cities may be linked to the recent surge of allergies and other chronic inflammatory diseases.
By Rob Dunn
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.
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 ancestors were to visit our cities and suburbs, they would wonder where the plants and animals have gone. What have we done with all the birds?
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, 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 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.
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 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.
The possible link between biodiversity and human health has been around for a while. Half a dozen theories—biophilia, nature deficit disorder, the deficiency theory of disease, the dilution effect, and more—describe the ways in which the loss of a connection to biological richness might cause us to ail. Elements of these theories are at the core of modern ecology. Less biodiverse systems—be they grasslands, forests, or the biomes of tiny life on our skin and in our guts—are less resilient and at greater risk of invasion (whether by pathogens or weeds) than more diverse systems.
Allergies were not part of the story until the early 1980s, and even then, they were considered separately, as though part of another tale with a different beginning and different ends. Epidemiologists began to notice differences between the immune systems of city kids and farm kids. Farm kids were less likely to have allergies.
Many explanations were suggested. But David Strachan, an epidemiologist at St. George’s University of London, had a curious idea, which he called the hygiene hypothesis. The key was bacteria; the lock was our immune system.
Perhaps urban kids were too distant from microbial nature for their immune systems to develop properly. Farm kids work in the dirt. They touch farm animals. They are exposed to more life and the microbes that animals harbor. It was a wild, speculative idea. It also increasingly appears to have been right.
In laboratories, mice without skin bacteria failed to develop normal immune systems. Add skin bacteria back, and their defenses were restored. Bacteria seem to be part of the useful “dirtiness”.
What can be said with certainty is that, as we have become more urban and as we have transformed the world, we have also become experts at replacing habitats filled with many species with habitats populated by just a few. We plant inert cement where forests once grew. We clean and scrub our houses with antibiotic wipes. We overuse antibiotics to clean out pathogens in our bodies. We overuse antimicrobials to clean everything else.
The primary role of the immune system is to distinguish deadly species from good species . . . and in this way, the immune system is our sixth sense—our inner taxonomist. And this inner taxonomist needs to see a lot of species to learn to distinguish good from bad from innocuous.
The word “clean” seems wholesome, but what it usually means is kill. We kill some species and favor others. We are reducing diversity in our daily lives, even on our bodies, in exactly the same way that we are reducing it in the world. We manage our own flesh as we manage the earth.
This parallel caught Hanski’s attention, and he wondered whether he could take the hygiene hypothesis a step further. Could the loss of biodiversity—the number of kinds of species, not the presence of some particular form—lead our immune systems to break in such a way that they can no longer distinguish wholesome friends from ancient enemies?
As we wait for more understanding, we continue to simplify the world. We will become more urban and thus more likely to suffer from allergies and autoimmune diseases, at least if Hanski is right. And if he is right, there may also be a way forward, a way out of our sick and simple morass. Could we rewild the places around us, plant a richness of species in our backyards and so raise healthier children covered in more kinds of bacteria? As a country boy who is living now in the city, raising two children, I hope so. Whatever we do, we will be measured by our immune systems and our microbes, which in their function or dysfunction seem to record the richness of our lives.
Rob Dunn is a science writer and biologist in the Department of Biology at North Carolina State University.
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