When an organism from one trophic level of a food chain is eaten by the organism at the next level up the chain, an average of about 10% of the energy from the first organism is available for use by the next organism. Where does the other 90% go at each level?

The 90 % of energy is released in the ecosystem in the form of heat.

Explanation:

In each trophic levels of ecosystem, 90 percent of energy is lost as heat energy in the ecosystem while only 10 percent of energy is transferred from one trophic level to the next trophic level and becomes a part of that organism. For example, plants are the producer having 5000 kcal energy. When these plants are eaten by primary consumer such as cow, rabbit etc, about 500 kcal of energy is transferred to primary consumer and the rest of it is lost as heat energy.