The rectangles in the graphic represent the amount of energy at every trophic level of an energy pyramid. why does the total amount of energy decrease at each level?

Due to the inefficiencies of passing energy from one level to another

Explanation:

Approximately 10% of the energy that is passed to one level from another is turned to biomass. The rest is lost in energy forms such as heat due to natural inefficiencies. An example is when we feed on plants, only a small fraction is absorbed into our bodies and turned into the tissue. Most are lost in the process either through excretion, or heat during cellular respiration.

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