10% of available energy flows from one trophic level to the next. what happens to the other 90%?

Primary consumers only obtain a fraction of the total solar energy—about 10%—captured by the producers they eat. The other 90% is used by the producer for growth, reproduction, and survival, or it is lost as heat. You can probably see where this is going. Primary consumers are eaten by secondary consumers. An example would be birds that eat bugs that eat leaves. Secondary consumers are eaten by tertiary consumers. Cats that eat birds that eat bugs that eat leaves, for instance.

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