Plants are categorized as primary producers.
Explanation: In the ecological hierarchy, plants play a fundamental role as primary producers. They have the unique ability to convert sunlight into energy through photosynthesis, creating organic compounds from carbon dioxide and water. This process forms the base of the food chain, allowing them to produce the energy that is essential for all other organisms in the ecosystem. Herbivores, as primary consumers, feed on these plants, while omnivores consume both plants and animals. Therefore, while plants are not herbivores or omnivores, they are vital as the primary source of energy for all other trophic levels.