A trophic level describes an organism’s position in a food chain or food web. This hierarchy illustrates the flow of energy from one group of organisms to the next as they consume each other. Ecological communities are typically limited to three to five trophic levels, a number determined by fundamental constraints related to energy transfer.
Establishing the Ecological Hierarchy
The first trophic level is occupied by producers (autotrophs). These organisms, like plants and algae, create their own food, usually through photosynthesis, by converting light energy into chemical energy. This level is the source of energy for nearly all other life forms in the ecosystem.
The second level is designated for primary consumers, which are the herbivores that feed directly on the producers. Examples include deer grazing on plants or zooplankton consuming phytoplankton in the ocean.
Moving up, the third trophic level contains secondary consumers, typically carnivores that prey on primary consumers. The fourth level, occupied by tertiary consumers, consists of carnivores that consume other carnivores. A fifth trophic level may sometimes be present, occupied by quaternary consumers or apex predators.
The Energetic Constraint That Limits the Number
The primary reason ecological communities rarely extend beyond four or five levels is the inefficiency of energy transfer between them. Energy is transferred when one organism consumes another, taking in the energy-rich molecules stored in its prey’s body. However, this transfer is far from perfect, resulting in a significant loss of usable energy at each step.
This phenomenon is often described by the “10% rule,” which estimates that, on average, only about 10% of the energy from one trophic level is transferred and incorporated into the biomass of the next. The vast majority of the remaining energy, approximately 90%, is lost to the environment. This lost energy is primarily used by the organisms for metabolic processes, such as movement, growth, and cellular respiration, with a large portion dissipating as heat.
This massive loss of energy explains why the ecological community is visually represented as a pyramid, with a broad base and a narrow top. The base (producers) must contain a proportionally large amount of biomass and stored energy to support the small number of organisms at the top. For instance, if producers capture 10,000 units of energy, the primary consumers only receive about 1,000 units, the secondary consumers receive 100 units, and the tertiary consumers only 10 units.
By the time the fourth or fifth level is reached, the amount of energy available is so severely diminished that it can no longer support a viable population of consumers. This constraint is a fundamental consequence of the laws of thermodynamics, which dictate that energy transformations are always accompanied by some loss of usable energy. The resulting lack of sufficient energy and biomass acts as a hard limit on the possible length of food chains in a community.
Real-World Complexity and Variation
While the simple food chain model is helpful for understanding energy flow, real-world communities are more accurately represented by complex food webs. A food web shows multiple interconnected food chains, accounting for the fact that most organisms consume more than one type of prey. This interwoven structure means that a single species can occupy multiple trophic levels simultaneously depending on what it is eating.
Omnivores, for example, feed on both plants and animals, making them primary consumers when they eat a plant and secondary or tertiary consumers when they eat meat. A human eating vegetables is a primary consumer, but a secondary consumer when eating beef, and potentially a tertiary consumer when eating a predatory fish. This flexible feeding behavior blurs the distinct lines between the integer-based trophic levels.
There can also be slight differences in the typical length of food chains across different environments. Aquatic ecosystems, particularly open-ocean environments, sometimes exhibit slightly longer food chains than terrestrial ones. This difference is thought to be partly due to the small size and high turnover rate of primary producers like phytoplankton, which may allow for an additional step in the energy transfer before the energetic limit is reached.