Biology, stuff on chemical cycling.
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Well, I am not quite sure what you exactly need as an answer. The major things that I can think of that occur in an ecosystem is the concept of photsythesis, respiration, water cycle, carbon cycle, and the nitrogen cycle. Plants give us oxygen that they make through photosynthesis and we give them carbon dioxide from respiration that we do. It's a cycle that our products are plants reactants. Water moves through the ecosystem in many ways including runoff, transpiration, condenstion, precipation, evaporation, etc. And carbon and nitrogen get passed on in many ways as well. Google search those topics if you need anymore info. Hope that helps.