Recycling of chemical nutrients and the flow of energy.  

The most basic chemicals necessary for life - carbon dioxide, oxygen, water, and various minerals - flow from the air and soil to plants, to animals and decomposers, and back to the air and soil.

Energy flows into the ecosystem when photosynthesizers absorb light energy from the sun and convert it to the chemical energy of sugars and other complex molecules.  Chemical energy is then passed through a series of consumers and, eventually decomposer, powering each organism in turn.  In the process of these energy conversions between and within organisms, some energy is converted to heat.