Final answer:
The statement about omnivores receiving 10% of their energy from the level below them is false as the 10% transfer is a general rule for all trophic levels and is not precise. Omnivores can function at multiple trophic levels, not just above herbivores. Energy loss due to metabolic processes limits the number of trophic levels.
Step-by-step explanation:
The correct answer to the question is option d: A is false but R is true. The assertion that omnivores receive 10% of their energy from the trophic level below them is a simplification of the ten percent law, which states that in general, only about 10% of the energy from one trophic level is passed to the next trophic level. However, this percentage can vary widely and is not specific to omnivores alone, as it applies to all trophic levels and is not a precise number. The reason that an omnivore is always in the trophic level just above herbivores is partially true but misleading, as omnivores can function at multiple trophic levels, not solely just above herbivores.
Furthermore, it's important to understand that the loss of energy at each trophic level is primarily due to metabolic processes and energy conversion inefficiencies. When an herbivore eats a plant, around 90% of the energy it receives is used for the herbivore's metabolic needs and is dissipated as heat, with only about 10% of the energy transferred to the next trophic level when the herbivore is eaten by a predator. Therefore, there are a limited number of trophic levels because the energy available decreases sharply with each successive level.