The correct explanation for why each level of the energy pyramid is smaller than the level directly below it is generally summarized by the 10% rule of energy transfer. This rule states that, on average, only about 10% of the energy from one trophic level is transferred to the next level.
Here’s a breakdown of the concepts involved:
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Energy Loss: When energy is transferred from one trophic level to the next, significant energy is lost at each level primarily due to metabolic processes, movement, and heat. Typically, about 90% of the energy is used by living organisms for various life processes (growth, reproduction, maintenance) or lost as heat, leaving only about 10% of the energy available to the next trophic level.
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Smaller Biomass: Because only a fraction of energy is passed up the pyramid, less biomass can be supported at higher trophic levels; thus, the sizes of each level decrease as you move up the pyramid.
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Energy Efficiency: The efficiency of energy transfer between trophic levels affects the population sizes and biomass at each level, resulting in a pyramid shape where producers (at the bottom) have the most energy and support the largest biomass, while apex predators (at the top) have the least energy and biomass.
So, the most accurate statement would be: "Because 90% of the energy at each level moves to the next level and some additional energy is converted to heat." This explains why the energy pyramid gets smaller at higher trophic levels.
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