Our skin is encased in a snug microbial suit, from our scalps to the tips of our toes. Bacteria begin to colonize our skin from the moment we are born, and they continue to coat us throughout life. They do us many favors. They moisturize our skin to keep it supple; they unleash anti-microbial toxins to ward off pathogens that might make us ill. Scientists know that our skin is home to many species, but they can't yet say exactly how many--or why some species are found more often on the elbow than on the chin. Two years ago at a conference in North Carolina, I ran into Rob Dunn, a biologist who was conducting a survey of this menagerie. He was interested in the life found in one particular spot on the human body: the belly button. At the conference, he was handing out Q-tips people could use to swab their navels, which he and his colleagues could then study to tally up the species dwelling there. Five months later, Dunn sent me a preliminary report: "You, my friend, are a wonderland." I was the proud host of 53 different types of bacteria, including some decidedly weird creatures, such as a microbe only known from the ocean, and another from the soils of Japan. I was only one of many human hosts to offer up our navel's residents to Dunn's scrutiny. Today, Dunn and his colleagues published a scientific paper on the biological diversity found in 60 bellybuttons in the journal PLOS One. They show that the diversity of my navel was not freakish. Even in a tiny divot of human flesh, dozens or even hundreds of species of bacteria can coexist. All told, Dunn and his colleagues identified 2368 different species living in our 60 belly buttons. The average person had 67 species, with the number ranging from a low of 29 species to a swarming high of 107. Out of those 2368 species, the majority--1458--are new to science. A few of them are very common, while most are exquisitely rare. Dunn and his colleagues found that eight types of bacteria made up nearly half the microbes the scientists detected. Each of them was present on over seventy percent of us. But the vast majority of the species--2188 all told--lived on six or fewer people. Most were found only on a single individual. It's possible that the rare microbes are only visitors, dropping by for a short stay in our navels before dying out or traveling on. The most common species the scientists found may have long-term leases, having evolved adaptation that help them thrive in the bellybutton's distinctive habitat. Dunn and his colleagues found that these abundant species were also closely related to each other compared to the rarer ones. It's a pattern similar to the one found in rain forests, were only a few lineages of trees dominate, with many species only contributing a few trees. Your belly button, in other words, really is a jungle.
For more information, read Dunn's account of the study.
P.S. I refer to these bacteria as belonging to "species." It's a convenient term but, when it comes to bacteria, not a precise one. Feel free to mentally substitute "operational taxonomic unit" or "phylotype."