Net production efficiency (NPE) is the efficiency at which organisms assimilate energy into biomass for the next trophic level. Due to low metabolic rates and less energy spent on thermoregulatory processes, the NPE of ectotherms (cold-blooded animals) is 10 times higher than endotherms (warm-blooded animals).
Energy flows through ecosystems, from one organism to the next. However, only the energy stored in an organism as biomass is available as food for the next trophic level. The rest of the energy is lost over time as heat as a byproduct of metabolic processes and excreted wastes. The efficiency with which organisms assimilate this usable energy into biomass is called net production efficiency (NPE), or the percentage of energy stored in biomass that is not used for respiration. For example, in a study of a desert scrub ecosystem, it was found that only 0.016% of the energy produced by primary producers was then assimilated into small herbivore mammal tissue and available for carnivores in this system.
Endotherms like birds and mammals typically have low production efficiencies due to the larger quantities of energy spent maintaining constant high body temperatures, and high metabolic rates. On the other hand, the NPE for ectotherms is an order of magnitude higher due to their lower metabolic rates and thermoregulatory behaviors. Therefore, a mammal must consume more energy to assimilate the same amount of biomass that a reptile would.
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