In a typical food chain, the level with the least energy is the tertiary consumers, which are often at the top of the food chain. These include predators that feed on secondary consumers (carnivores) and often rely on a lower number of organisms for their sustenance compared to the levels below them.
The reason for this decrease in energy is due to the inefficiency of energy transfer between the different levels of the food chain. As energy moves from one trophic level to the next—starting from producers (like plants) through primary consumers (herbivores) and up to secondary and tertiary consumers—only about 10% of the energy is transferred to the next level. This phenomenon is known as the 10% rule. Therefore, by the time energy reaches the tertiary consumers, there is significantly less available energy compared to that available to primary producers.
This concept highlights the importance of producers in an ecosystem, as they support all higher levels of consumers by capturing energy from the sun and contributing to the overall energy flow within the food chain.