Answer:
The amount of energy
Step-by-step explanation:
The percentage represents the amount of energy at each trophic level of the food chain/web.
A trophic level represents a hierarchical level in a food chain or food web in an ecosystem with each level consisting of organisms that share the same function and nutritional relationship to energy source.
The amount of energy in each trophic level decreases as we move up the food chain/web due to a loss to the environment as heat.
In the image, the first level represents the primary producer that are responsible for fixing energy directly from the sun. Hence, they posses 100% of the energy in the food web.
The second level represents the first-level consumers of primary producers. The amount of energy that gets to these consumers has reduced from 100% to 10%, meaning that 90% has been lost as heat to the environment.
The third level represents the second-level consumers. That is, those that directly feed on the first-level consumers. The amount of energy they derive has also reduced from 10% to 1% while in the third-level consumers, the amount of energy derived has reduced to 0.1%.