Ecosystems and Energy Flow
Explore how energy moves through ecosystems via trophic levels, food webs, and energy pyramids -- and why only about 10% of energy transfers between levels.
Trophic Levels
A trophic level describes the position an organism occupies in a food chain. Energy enters ecosystems through producers (autotrophs) that convert sunlight into chemical energy via photosynthesis. This energy is then passed to consumers at successive levels.
Energy Pyramid -- Trophic Levels
Tertiary Consumers
Apex predators
0.1% energy
Secondary Consumers
Carnivores
1% energy
Primary Consumers
Herbivores
10% energy
Producers
Plants, algae, cyanobacteria
100% of captured energy
Energy decreases at each trophic level. The pyramid shape reflects this energy loss.
Important: Decomposers (bacteria and fungi) operate across all trophic levels, breaking down dead organisms and recycling nutrients back into the ecosystem. They are essential for nutrient cycling.
Food Webs and the 10% Rule
A food web is a network of interconnected food chains showing the complex feeding relationships within an ecosystem. Unlike a simple food chain, a food web shows that most organisms eat (and are eaten by) multiple species. The 10% rule states that only approximately 10% of the energy at one trophic level is transferred to the next. The remaining 90% is lost as heat through cellular respiration, used for life processes, or remains in undigested material.
Australian Bushland Food Web
Wedge-tailed Eagle
Tertiary consumer
Kookaburra
Secondary consumer
Goanna
Secondary consumer
Quoll
Secondary consumer
Grasshopper
Primary consumer
Kangaroo
Primary consumer
Wombat
Primary consumer
Eucalyptus
Producer
Native Grasses
Producer
Wattle
Producer
The 10% Rule in Action
Producers
10,000 kJ
Primary
1,000 kJ
Secondary
100 kJ
Tertiary
10 kJ
At each level, ~90% of energy is lost as heat through cellular respiration.
Ecosystem Stability and Disruption
Healthy ecosystems maintain a balance between energy input and energy use. However, removing or adding organisms can cause trophic cascades -- chain reactions that affect multiple trophic levels. Understanding these dynamics is critical for conservation biology.
Top-Down Control
Predators regulate the population of prey below them. Removing an apex predator causes prey populations to explode, overgrazing producers and destabilising the ecosystem.
Bottom-Up Control
Producer availability limits the rest of the food web. A drought reducing plant growth will decrease herbivore populations, which in turn reduces carnivore numbers.
Australian Case Study: Dingo Removal
When dingoes are removed from Australian ecosystems, kangaroo and feral cat populations increase. This leads to overgrazing of native vegetation and increased predation on small native mammals, demonstrating a trophic cascade.
Key Vocabulary
Trophic Level
The feeding position of an organism in a food chain or food web (e.g. producer, primary consumer, secondary consumer).
Biomass
The total mass of living organisms at a particular trophic level, usually measured in grams per square metre (g/m²).
Trophic Cascade
A chain reaction of ecological changes triggered by the addition or removal of a top predator, affecting multiple trophic levels.
Autotroph
An organism that produces its own food from inorganic substances using light energy (photosynthesis) or chemical energy (chemosynthesis).
Worked Examples
If producers in an ecosystem capture 50,000 kJ of energy, how much energy is available to secondary consumers?
Step 1: Apply the 10% rule. Primary consumers receive 10% of 50,000 kJ = 5,000 kJ.
Step 2: Secondary consumers receive 10% of 5,000 kJ = 500 kJ.
Answer: Approximately 500 kJ of energy is available to secondary consumers.
Explain why food chains rarely have more than four or five trophic levels.
Step 1: At each trophic level, approximately 90% of energy is lost as heat through cellular respiration.
Step 2: By the fourth or fifth level, so little energy remains that it cannot support a viable population of organisms.
Conclusion: The exponential energy loss at each level limits food chain length. There is simply not enough energy to sustain additional trophic levels.
In a food web, a disease kills most of the primary consumers. Predict the effects on other trophic levels.
Producers: Would likely increase, as fewer herbivores means less grazing pressure.
Secondary consumers: Would decrease due to reduced food supply (fewer primary consumers to eat).
Tertiary consumers: Would also decrease as secondary consumer numbers fall. This demonstrates how disruption at one level cascades through the entire food web.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
According to the 10% rule, if producers contain 20,000 kJ of energy, how much energy is available to primary consumers?
Question 2
What happens to the approximately 90% of energy that is NOT transferred to the next trophic level?
Question 3
Which organisms form the base of virtually every food web?
Question 4
A hawk eats a snake, which eats a frog, which eats insects, which eat plants. At which trophic level is the hawk?
Question 5
What is a trophic cascade?
Key Concepts Summary
- ●Trophic levels describe feeding positions: producers → primary consumers → secondary consumers → tertiary consumers.
- ●The 10% rule states that only ~10% of energy is transferred between trophic levels; ~90% is lost as heat.
- ●Food webs show complex interconnected feeding relationships within an ecosystem.
- ●Energy loss at each level explains why food chains rarely exceed four or five trophic levels.
- ●Removing or adding species can trigger trophic cascades that affect the entire ecosystem.