The primary energy transfer mechanism in food webs is through trophic levels, where energy moves from primary producers (autotrophs) to various levels of consumers (herbivores, carnivores, omnivores). Energy transfer is governed by the 10% rule, indicating that only about 10% of energy is passed to the next trophic level, with the rest lost as heat or used for metabolic processes. This efficiency underlines ecosystem dynamics and limits the number of trophic levels.