The Human Ecosystem
Conditions that vary
Abiotic (or non-living) factors vary among your body's ecosystems—often by a lot. Below are some examples. Can you think of more?
- Temperature—skin is cooler, especially on extremities like hands and feet
- pH—skin, upper GI tract, and vagina are acidic
- Water—skin has dry, exposed areas and moist creases; areas inside the body are much more moist than skin
- Oxygen—pockets around teeth have little oxygen, skin has the most
- Available nutrients—oils on the skin, food in the gut, dead cells, secretions
- UV light—certain areas of the skin have more exposure than others
- Substrate—hard (teeth) vs. soft (cheeks, tongue) areas of the mouth, for example
Learning from Earth's ecology
Ecosystems are constantly changing. Forests are burned by fires. Species introduced from other other ecosystems compete for resources. Ecologists study these phenomena, and they come up with strategies for understanding and managing them. It turns out that some of these strategies apply to the body's ecosystems as well.
In the mid-1800s, Australia was overrun with non-native rabbits, which wreaked havoc on the native vegetation. Similarly, harmful bacteria invade the sinuses, outcompeting the resident species for resources.
After a forest fire, species gradually return and repopulate the area. After antibiotic treatment, the few microbes that remain must re-build the ecosystem of the colon.
Lots of microbes can gain access to an ecosystem. But only those with the proper adaptations—that is, those who can use the available resources and withstand environmental challenges—will survive and reproduce. Just like a fern from a wet forest will die if it's moved to a desert, microbes from your forearm will die in your stomach. Those that are best-equipped for the environment will take hold. In other words, the environment imposes selection.
Likewise, changing an ecosystem can affect who lives there. In the gut, for example, one important abiotic factor is food. Changing our diet influences the balance of microbes living in our guts.
Costello, E.K., Stagaman, K., Dethlefsen, L., Bohannan, B.J.M. & Relman, D.A. (2012). The application of ecological theory toward an understanding of the human microbiome. Science, 336(6086), 1255-1262. doi: 10.1126/science.1224203
Faust, K., Sathirapongsasuti, J.F., Izard, J., Segata, N., Gevers, D., Raes, J. & Huttenhower, C. (2012). Microbial co-occurrence relationships in the human microbiome. PLoS Computational Biology, 8(7), e1002606. doi: 10.1371/journal.pcbi.1002606
Grice, E.A. & Segre, J.A. (2011). The skin microbiome. Nature Reviews Microbiology, 9, 244-253. doi: 10.1038/nrmicro2537