Herbivores occupy the second trophic level because producers constitute the first. Primary consumers obtain carbon and energy directly from plants, algae, or other autotrophs; predators that eat herbivores occupy the third level. Trophic level is determined by feeding position, not taxonomy, so an omnivorous species can operate at more than one fractional trophic level. Detritivores are often treated separately because their food integrates material derived from several levels, although their energy ultimately originated in primary production. Quantitative interpretation requires explicit syst
Ref:
Fundamentals of Ecology, Odum & Barrett, 5th Ed., Ch. 3