About Cellular Biology DNA Energy Agriculture
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  Decomposers Natural Cycles Activities Quiz Natural Cycles We’ve learned in earlier sections of the module that all ecosystems require a constant supply of energy, which must be obtained outside the system. Within the self-sustaining ecosystems materials need to be cycled among the living organisms and the abiotic environment. In this way materials can be used and reused by different living organisms. The organisms incorporate certain substances (obtained from the environment) into their bodies. When they die, decomposers break down their bodies, and the simpler substances are returned to the environment. If this did not happen, sufficient chemical substances would not be available. The cycles in nature between the biotic communities and the physical environment are called biogeochemical cycles. The substances involved are oxygen, carbon, water, and nitrogen. We will discuss each of the following cycles: water, carbon and oxygen, and nitrogen.