On Science: Your own personal ecosystem
This article first appeared in the St. Louis Beacon, Dec. 17, 2008 - Scientists who study ecology view the world as a patchwork quilt of different environments, all bordering on and interacting with one another.
Consider for a moment a patch of Missouri forest, the sort of place a deer or turkey might live. Ecologists call the collection of creatures that live in a particular place a community — all the animals, plants, fungi and microorganisms that live together in a Missouri forest, for example, are the forest community. Ecologists call the place where a community lives its habitat — the soil, and the water flowing through it, are key components of the forest habitat. The sum of these two, community and habitat, is an ecological system, or ecosystem.
An ecosystem is a largely self-sustaining collection of organisms and their physical environment. An ecosystem can be as large as a Missouri forest or as small as a Missouri cave.
You may not realize it, but you yourself are a walking ecosystem. Microscopic bacteria live on you and inside you. Insects and spiders lurk about unseen. Most of the time, you share your body harmoniously with these other inhabitants, a balanced ecological community. A good understanding of how you as a resident of Missouri should treat our state’s other nonhuman inhabitants can be gained by looking more closely at the neighbors with whom you share your body.
To start with, there are about 3.3 pounds of bacteria living in your gut, 100 trillion tiny individuals (that’s 10 times as many bacterial cells as human cells!) going about their microbial affairs without so much as a by-your-leave from you. There are at least 500 different species, although most are from one of two phyla, the Firmicutes and the Bacteroides.
From a bacterium’s point of view, your body is a vast framework within which its community thrives, much as you might view the state of Missouri. Every time you eat, you provide your bacterial community with food, like rain and sunshine falling on a Missouri farm. But like sharecroppers, your bacteria pay their rent: individual bacterial cells break down carbohydrates for you that you are unable to metabolize, so that you can share in the feast. Other bacterial cells make essential vitamins like K and B12 that you cannot make for yourself. At least as important, all these bacteria occupy a lot of space, crowding out any harmful bacteria that attempt to take up residence.
Your skin supports about 1 trillion more bacteria (even more if you don’t wash frequently). An amalgamation of more than 180 different species (the most common include Staphylococcus, Streptococcus and Corynebacterium), these bacteria make their living by metabolizing your sweat. They gain food energy by doing this, and in the process produce body odor for you. Of more practical use to you, this community of skin bacteria keeps your body safe by competing with dangerous pathogens for nutrients.
If you are a woman, your gut and skin are not the only parts of your body where bacteria reside. Beneficial bacteria – mostly members of the Lactobacillus family -- also inhabit your vagina. There they secrete lactic acid, which fends off hostile invaders like pathogenic Candida yeast.
Still other bacteria live on your tongue and teeth, and it is particularly difficult to show these unwanted guests the door. Even if you brush your teeth diligently, these Streptococcus bacteria will still refuse to leave. They arrive on your teeth soon after they break the surface, and stay as long as the teeth do. Unfortunately, they are not well-behaved guests. If you don’t keep them firmly in line by brushing your teeth regularly, a biofilm of Streptococcus bacteria 500 cells thick soon forms on the unbrushed surfaces. This is NOT good for your body. Biofilm bacteria ferment sugars that they snare from candy and sweets on their way to your stomach, pumping out lactic acid in the process of fermentation that decays your tooth enamel – tooth decay is how they repay you for your body’s hospitality!
Nor are all the denizens of your body tiny microbes. You are home to several animals as well. One of them lives in human hair, especially the unwashed variety -- a flat, wingless insect called a head louse. Less than a tenth of an inch long, head lice suck on your blood, cementing their eggs (what we Virginia kids used to call nits) to your hair. Head lice have been companions of humans for a long time – lice eggs have been found attached to a strand of human hair 10,000 years old. A second blood-sucking human companion is the body louse; you are usually not aware of its presence unless its biting becomes over-enthusiastic, causing skin rash.
Not every animal inhabiting your body is a louse. Tiny, eight-legged mites of the genus Demodex (not an insect at all but an arachnid – a spider!), less than half a millimeter long, nestle head-down inside the follicles of your eye lashes, hair follicles, and by the oil glands in your skin pores, feasting unnoticed in these hideaways on skin cells. Nearly everyone has them, and lots of them. Each follicle may house up to 25 of these tiny animals.
So tomorrow morning when you look in the mirror, say hello to your many friends. They inhabit your body with you, and like you share its fate. When you thrive they do too, and when you die they die. You are their world.
George B. Johnson's "On Science" column looks at scientific issues and explains them in an accessible manner. There is no dumbing down in Johnson's writing; rather he uses analogy and precise terms to open the world of science to others.
Johnson, Ph.D., professor emeritus of Biology at Washington University, has taught biology and genetics to undergraduates for more than 30 years. Also professor of genetics at Washington University’s School of Medicine, Johnson is a student of population genetics and evolution, renowned for his pioneering studies of genetic variability.
He has authored more than 50 scientific publications and seven texts, including "BIOLOGY" (with botanist Peter Raven), "THE LIVING WORLD" and a widely used high school biology textbook, "HOLT BIOLOGY."
As the founding director of The Living World, the education center at the St Louis Zoo, from 1987 to 1990, he was responsible for developing innovative high-tech exhibits and new educational programs.