Natural Selection Image Map

Ever hear the saying "What goes around comes around?" This idea applies to the natural world, where many things go around in cycles. For example, cycles preserve a given chemical element or molecule so that it can be used again and again and not lost. Some of the most important ecosystem cycles are for carbon, nitrogen, oxygen, water, and energy. These cycles were introduced in Toxic or NotHere we extend the idea of cycles to show how ecosystems can be sustained.  We will also see that disturbing these cycles can devastate ecosystems.

This costal-water brittle star has died. The water had a low level of dissolved oxygen. See

Why do cycles work to sustain ecosystems? Basically, by re-cycling materials that are crucial to life these materials can be used over and over again by different animals and plants at different times. There is a limited supply of matter.  So to sustain life, there have to be ways to pass the material around in the environment for reuse.   The air you breath this moment may contain some oxygen molecules that were made by plants in the ocean long before land plants existed or may have been breathed by a dinosaur.

We humans have discovered recycling too.  By re-cycling paper, cans, and plastic, we help to conserve our natural resources. 

After completing this lesson, each student should be able to:

  • Explain the steps in the major cycles for carbon, oxygen, nitrogen, and water
  • State what photosynthesis does and explain why it is important
  • State what is meant by "nitrogen fixation" and explain why that is important


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