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Each consumer is consuming for a number of reasons, only one of them being physical growth. Calories consumed are turned into body heat, healing, reproduction, and motion, all of which are necessary life processes, but none of which retain captured calories in the food chain. An animal must expend calories in order to obtain food. Those expended calories are not available to be eaten by members of the next trophic level, so they in essence leave the food chain.
Ecologists estimate that plants, at the base of most food chains, convert only about 1% of the sunlight they receive into plant material which is then available as food. For animals, an average of about 10% of the energy a creature consumes is retained as additional body mass, which is the only thing that can be passed up the food chain to the next trophic level.
Foraging efficiency is the term for how animals maximize calorie intake versus calories burned to attain food; it is a very interesting ecological topic. Check out the link below for an example of how it works in a grazing species, the bighorn sheep of western North America.
We must remember that energy cannot be created or destroyed, it can only be imperfectly (meaning that part of it is lost in the process of conversion) converted from one form to another. Energy transfer from producers to primary consumers is the least efficient, with only 1-10% of the energy transferred in the form of biomass. This is because of several reasons:
- Plants contain high levels of indigestible cellulose and lignin
- Leaves of plants may contain poisonous substances such as glycosides and phenolics and are hence, not consumed by animals.
- Certain parts of the plant (e.g. roots which are deeply buried in the soil) are inaccessible to herbivores.
- Some plants die before they can be eaten.
- Also, some of the energy is first expended by the plant for key cellular processes and to make food; there is the inevitable loss of heat energy in the reactions of respiration.
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