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The living world depends upon the energy flow and the nutrients circulation that occurs through ecosystem. Both influence the abundance of organisms, the metabolic rate at which they live, and the complexity of the ecosystem.
Energy flows through ecosystems enabling the organisms to perform various kinds of work and this energy is ultimately lost as heat forever in terms of the usefulness of the system. On the other hand, nutrients of food matter never get used up. They can be recycled again and again indefinitely.
For e.g. when we breathe we may be inhaling several million atoms of elements that may have been inhaled by our ancestors or other organisms.
Carbon, hydrogen, oxygen, nitrogen and phosphorus as elements and compounds make up 97% of the mass of our bodies and are more than 95% of the mass of all living organisms. In addition to these about 15 to 25 other elements are needed in some form for the survival and good health of plants and animals.
These elements or mineral nutrients are always in circulation moving from non-living to living and then back to the non-living components of the ecosystem in a more or less circular fashion. This circular fashion is known as biogeochemical cycling (bio for living; geo for atmosphere).