"The building blocks for all life as we know it are the elements carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur. The first four elements make up about 99% of the mass of most cells. They are the basis of the proteins, carbohydrates, and fats in the bodies of living things.

In addition to them, most organisms have trace amounts of other elements: 16 elements are common to all organisms, and some have other elements, too. Because they are the foundation of all life, the cycles of these materials are of critical significance for ecologists.

Ecologists study them using information from the disciplines of biology, geography, and chemistry, so they are referred to as biogeochemical cycles. The name is long, but the idea is simple: The living and non-living worlds are inseparably linked by the flows of materials that cycle through them.

These cycles have evolved over long periods of time, sometimes hundreds of millions of years, based on the types and amounts of materials that have been available in any given area during those long periods. That is the context in which our life-support systems have developed.

If we now change the types and amounts of materials in ecosystems, then we are of changing the context for our life support and we risk destroying that support. If, for example, our pollution or waste emissions introduce materials into ecosystems that were not there before, or not at such concentrations, then we risk poisoning the organisms that are not used to such materials."

                                                       7 Perspectives On Sustainability, page 21