“No organism is an island​—each one has a relationship to other organisms, directly or indirectly.”​—“Symbiosis—​An Introduction to Biological Associations.”

THE “web of life”​—how fitting that expression is, for life truly is a network of interconnected and interdependent organisms! Humans are very much a part of that web. For evidence, you need not look any further than your own body. Quietly at work in your digestive tract, an army of friendly bacteria help you to stay healthy by destroying harmful invaders and by aiding in digestion and in the production of essential vitamins. In return, you, the host, provide the bacteria with food and a supportive environment.

Similar alliances occur in the animal kingdom, especially with ruminants​—animals that chew the cud—​such as cattle, deer, and sheep. Their rumen, the first part of their multichambered stomach, hosts a veritable ecosystem of bacteria, funguses, and protozoa. By means of fermentation, these microbes break down cellulose, a fibrous carbohydrate found in vegetation, into various nutrients. Even certain insects that eat cellulose, including members of the beetle, roach, silverfish, termite, and wasp families, employ bacteria in the digestive process.

Such close cooperation among dissimilar organisms is called symbiosis, which means “living together.”a “Such alliances are fundamental to the development of every living system,” says Tom Wakeford in his book Liaisons of Life.