Trophic Levels
Trophic levels describe an organism's position in a food chain or food web based on how it obtains energy. Understanding trophic levels helps explain energy flow, ecological pyramids, and the relationships among producers and consumers.
What Is a Trophic Level?
A trophic level is a feeding position within a food chain or food web. It describes how an organism obtains energy.
Producers occupy the first trophic level because they capture energy and make organic food. Consumers occupy higher trophic levels because they obtain energy by eating producers or other consumers.
A trophic level is about how an organism gets its energy, not simply whether the organism is a plant, animal, predator, or prey.
The Major Trophic Levels
Producers
Producers make their own organic food and provide the energy foundation for most ecosystems.
Primary Consumers
Primary consumers obtain energy by eating producers. They are often herbivores.
Secondary Consumers
Secondary consumers obtain energy by eating primary consumers.
Higher-Level Consumers
These consumers obtain energy by eating organisms from lower consumer levels.
Producers: The First Trophic Level
Producers form the first trophic level because they introduce usable energy into most ecosystems.
Plants, algae, and phytoplankton are examples of producers. Through photosynthesis, they use light energy to build organic molecules that contain chemical energy.
Because producers are the foundation of the food chain, changes in producer populations can affect organisms at higher trophic levels.
Primary Consumers
Primary consumers obtain their energy directly from producers.
Many primary consumers are herbivores, such as rabbits, deer, grasshoppers, and many zooplankton. However, the important characteristic is not the label herbivore; it is the fact that the organism gets its energy by consuming producers.
Secondary Consumers
Secondary consumers obtain energy by consuming primary consumers.
Depending on the ecosystem, secondary consumers can include carnivores or omnivores. What matters is the trophic relationship represented in the particular food chain.
Higher-Level Consumers
A food chain may continue beyond secondary consumers. A consumer that eats a secondary consumer is often called a tertiary consumer.
Some food chains contain additional levels, including quaternary consumers or other higher-level predators. There is no universal rule that every food chain must contain the same number of trophic levels.
Trophic levels are determined by feeding relationships. An organism's trophic level depends on what it is eating in that particular food chain or food web.
Putting the Trophic Levels Together
Consider this food chain:
In this example, grass is the first trophic level, the grasshopper is the second, the frog is the third, and the snake is the fourth.
Energy and Trophic Levels
Trophic levels are closely connected to the amount of energy available in an ecosystem.
Producers generally contain the greatest amount of energy available to the food chain. As energy moves from one trophic level to another, only a fraction becomes available to the next level.
The 10% Rule
The 10% rule is a useful ecological approximation stating that, on average, about 10% of the energy available at one trophic level is transferred to the next trophic level.
The remaining energy is used in the organism's life processes, such as movement, growth, cellular respiration, reproduction, and maintaining body functions. Energy is ultimately released as heat.
producer
energy units
primary consumer
energy units
secondary consumer
energy units
tertiary consumer
energy units
The 10% value is an approximation, not a fixed amount that is identical in every ecosystem or every trophic transfer.
Omnivores and Multiple Trophic Levels
Not every organism fits neatly into one trophic level. Omnivores eat organisms from different trophic levels.
Eating Plants
If an omnivore eats a producer, it is acting as a primary consumer in that feeding relationship.
Example: plant → omnivore
Eating an Animal
If that same omnivore eats a primary consumer, it is acting as a secondary consumer in that feeding relationship.
Example: plant → herbivore → omnivore
If a question asks for an organism's trophic level, look at what it is eating in the diagram rather than relying on the organism's common name.
Where Do Decomposers Fit?
Decomposers, including many bacteria and fungi, break down dead organisms and organic waste.
Because decomposers obtain material from organisms at different trophic levels, they do not fit neatly into one simple trophic level in a food chain.
Decomposers connect to every level.
Organic material from producers and consumers can eventually become available to decomposers.
Common Misconceptions
If You See This on a Test, Think This
Quick Practice
1. What is a trophic level?
2. Which trophic level contains producers?
3. What trophic level does a rabbit occupy if it eats grass?
4. In grass → grasshopper → frog → snake, what trophic level is the frog?
5. Why is there generally less available energy at higher trophic levels?
6. Can an omnivore occupy more than one trophic level?
Key Takeaways
- A trophic level describes an organism's feeding position in an ecosystem.
- Producers occupy the first trophic level.
- Primary consumers eat producers.
- Secondary consumers eat primary consumers.
- Tertiary and higher-level consumers eat organisms from lower consumer levels.
- Available energy generally decreases as you move upward through trophic levels.
- The 10% rule is an approximate model of energy transfer.
- Omnivores can occupy different trophic levels depending on what they eat.
- Decomposers obtain energy from dead material and waste originating from organisms at different trophic levels.
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