Energy Flow in Ecosystems
4. Trophic Level
Learning outcomes
- I can define trophic level.
- I can identify organisms at different trophic levels.
- I can explain why energy decreases at higher trophic levels.
- I can compare primary, secondary, and tertiary consumers.
- I can analyze energy transfer between trophic levels.
Introduction
Every organism in an ecosystem occupies a particular position in the flow of energy. Plants capture energy from the Sun, herbivores feed on plants, carnivores feed on herbivores, and decomposers recycle nutrients after organisms die. These feeding positions are known as trophic levels.
Understanding trophic levels helps scientists explain how energy moves through ecosystems and why there are usually many more plants than large predators. As energy is transferred from one trophic level to the next, much of it is used by organisms or lost as heat, leaving less energy available for higher levels.
What Is a Trophic Level?
A trophic level is the feeding position an organism occupies in a food chain or food web.
Organisms are grouped according to how they obtain their energy.
Each step in a food chain represents a different trophic level.
For example:
Grass → Rabbit → Snake → Hawk
| Organism. | Trophic Level |
|---|---|
| Grass | Producer (1st trophic level) |
| Rabbit | Primary consumer (2nd trophic level) |
| Snake | Secondary consumer (3rd trophic level) |
| Hawk | Tertiary consumer (4th trophic level) |
Figure 1. Each organism occupies a different trophic level based on how it obtains energy.
The Main Trophic Levels
First Trophic Level – Producers
Producers form the base of every food chain.
They capture energy from the Sun through photosynthesis.
Examples include:
- Grass
- Trees
- Algae
- Phytoplankton
Producers contain the greatest amount of available energy because they receive energy directly from sunlight.
Second Trophic Level – Primary Consumers
Primary consumers are herbivores that feed on producers.
Examples include:
- Rabbits
- Deer
- Caterpillars
- Grasshoppers
- Zooplankton
They obtain energy by eating plants or algae.
Third Trophic Level – Secondary Consumers
Secondary consumers feed on primary consumers.
Examples include:
- Frogs
- Snakes
- Small fish
- Foxes
Some are carnivores, while others are omnivores.
Fourth Trophic Level – Tertiary Consumers
Tertiary consumers feed on secondary consumers.
Examples include:
- Hawks
- Eagles
- Sharks
- Lions
Many are apex predators, meaning they have few or no natural predators.
Decomposers
Although decomposers are not usually assigned a single trophic level, they play a vital role by breaking down dead organisms from all trophic levels.
Examples include:
- Fungi
- Bacteria
They recycle nutrients back into the ecosystem, allowing producers to grow again.
Figure 2. An energy pyramid illustrates the major trophic levels in an ecosystem.
Why Does Energy Decrease at Higher Trophic Levels?
When one organism eats another, not all of the energy is passed on.
Energy is used for:
- Movement
- Growth
- Reproduction
- Breathing (respiration)
- Maintaining body temperature
Much of this energy is eventually released as heat.
As a result, only a small proportion of the energy stored in one trophic level becomes available to the next.
This explains why ecosystems have:
- Many producers
- Fewer herbivores
- Even fewer carnivores
- Very few top predators
The 10% Rule
A useful guideline in ecology is the 10% Rule.
It states that, on average, only about 10% of the energy at one trophic level is transferred to the next trophic level.
The remaining energy is:
- Used for life processes.
- Lost as heat.
- Not eaten or not fully digested.
Example
| Trophic Level | Available Energy |
|---|---|
| Producers | 10,000 kJ |
| Primary consumers | 1,000 kJ |
| Secondary consumers. | 100 kJ |
| Tertiary consumers | 10 kJ |
The exact percentage varies between ecosystems, but the 10% Rule is a useful model for understanding energy transfer.
Figure 3. Only a small fraction of energy is transferred from one trophic level to the next.
Comparing Consumers
| Consumer Type | Eats | Example |
|---|---|---|
| Primary Consumer | Producers | Rabbit |
| Secondary Consumer. | Primary consumers | Snake |
| Tertiary Consumer | Secondary consumers | Hawk |
Some organisms occupy more than one trophic level.
For example:
A bear may eat:
- Berries (acting as a primary consumer)
- Fish (acting as a secondary or tertiary consumer)
This is why food webs are often more complex than simple food chains.
Analysing Energy Transfer
Scientists often compare the amount of energy available at each trophic level.
Example
Suppose an ecosystem contains:
- Producers: 20,000 kJ
- Primary consumers: 2,000 kJ
- Secondary consumers: 200 kJ
- Tertiary consumers: 20 kJ
Notice that:
- Energy decreases rapidly.
- Far fewer organisms can survive at higher trophic levels.
- Top predators require large hunting territories because so little energy reaches them.
Figure 4. Energy decreases at each trophic level because organisms use much of it for life processes.
Why Energy Loss Matters
The decrease in available energy affects ecosystem structure.
It explains why:
- Grasslands contain millions of plants but relatively few wolves.
- Oceans contain vast numbers of phytoplankton but far fewer sharks.
- Large predators require extensive habitats to find enough food.
Energy availability limits the number of organisms that each trophic level can support.
Worked Example
Question
The following organisms form a food chain:
Grass → Grasshopper → Frog → Snake
Identify the trophic level of each organism.
Solution
| Organism | Trophic Level |
|---|---|
| Grass | Producer (1st trophic level) |
| Grasshopper. | Primary consumer (2nd trophic level) |
| Frog | Secondary consumer (3rd trophic level) |
| Snake | Tertiary consumer (4th trophic level) |
If the grass contains 5,000 kJ of energy, approximately how much energy would be available to the grasshopper according to the 10% Rule?
Answer: Approximately 500 kJ.
Real-World Connection
Commercial fisheries must consider trophic levels when managing fish populations. Large predators such as tuna and sharks are naturally less abundant because much less energy reaches the highest trophic levels. Overfishing these species can disrupt marine food webs and affect many other organisms within the ecosystem.
Did You Know?
The blue whale, the largest animal ever to live on Earth, feeds mainly on tiny shrimp-like animals called krill. Because krill feed directly on phytoplankton, blue whales feed at a relatively low trophic level, allowing enough energy to support their enormous size.
Key Terms
Apex predator – A predator at the highest trophic level with few or no natural predators.
Consumer – An organism that obtains energy by eating other organisms.
Energy transfer – The movement of energy from one trophic level to another.
Primary consumer – A herbivore that feeds on producers.
Producer – An organism that captures energy from sunlight or chemicals to make its own food.
Secondary consumer – A consumer that feeds on primary consumers.
Tertiary consumer – A consumer that feeds on secondary consumers.
Trophic level – The feeding position an organism occupies in a food chain or food web.
10% Rule – The guideline that, on average, only about 10% of the energy at one trophic level is transferred to the next.
Key Takeaways
- A trophic level is an organism's feeding position in a food chain or food web.
- Producers occupy the first trophic level, followed by primary, secondary, and tertiary consumers.
- Energy decreases at each trophic level because organisms use energy for life processes and lose energy as heat.
- The 10% Rule helps explain why only a small amount of energy is passed to the next trophic level.
- The limited transfer of energy explains why ecosystems contain many producers but relatively few top predators.