Energy Flow

All the energy used by living things comes ultimately from the sun. Energy enters living systems as a result of photosynthesis by plants and some bacteria and protists. Less than 4% of the incident sunlight is captured. More than half of the energy captured by plants is used in respiration for maintenance. Energy used in respiration is lost as heat and therefore unavailable to other organisms. The other half is converted to plant tissues. There are 2 types of organisms that have direct access to the energy in plant tissues, herbivores, that feed on the plant while it is alive, and decomposers, that feed on the plant after it is dead. In most ecosystems, the majority of the energy goes to the decomposers. In a grassland, for example, only 10% of the energy in plants is taken by grazing animals such as buffalo. Herbivores use almost all of their energy intake on respiration for body maintenance; the rest goes to herbivore biomass. Much of the energy in herbivore biomass is taken by carnivores, such as wolves, while some goes to decomposers. Almost all of the energy taken in by carnivores goes to maintenance. The decomposers, which receive most of the plant energy, use up over half of it in maintenance. The rest may be locked up in soil organic material or taken by organisms that feed on decomposers. Ultimately, all of the energy originally captured by plants is transformed and lost as heat; energy is not recycled.


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