Our Environment · Lesson 3 of 8
Food Chains And Webs
“Energy moves up the food chain, but most of it quietly leaves before reaching the top.”
• Construct and interpret food chains from feeding relationships. • Identify trophic levels from producers to tertiary consumers. • Apply the approximate ten percent transfer rule to energy or organic matter between trophic levels. • Explain why food chains are usually short and why energy flow is unidirectional. • Distinguish a food chain from a food web and interpret interconnected feeding relationships.
A grassland may contain grass, insects, goats, snakes, birds and larger predators. If we ask who eats whom, a sequence begins to appear. But when we ask the same question for every organism, the simple sequence becomes a network. Food chains and food webs are ways of representing these feeding relationships and the movement of energy through an ecosystem.
Food Chain
A Food Chain is a sequence of organisms in which each organism is eaten by the next organism in the sequence, showing a pathway of food and energy transfer.
A food chain is read from the organism that is eaten toward the organism that eats it. The examples shown for forest, grassland and pond ecosystems all begin with producers and continue through consumers.
Trophic Levels
A Trophic Level is each feeding step or level in a food chain.
| Trophic Position | Typical Organisms | Role |
|---|---|---|
| First trophic level | Producers | Capture solar energy and make food |
| Second trophic level | Primary consumers | Usually herbivores |
| Third trophic level | Secondary consumers | Small carnivores |
| Fourth trophic level | Tertiary consumers | Larger carnivores |
Flow Of Energy
The Sun is the main source of energy for most ecosystems. Producers capture some of the energy in sunlight and convert it into chemical energy stored in food. In a terrestrial ecosystem, green plants capture about one percent of the sunlight energy falling on their leaves and convert it into food energy.
Ten Percent Rule
When producers are eaten by primary consumers, much of the food energy does not become new consumer body material. Energy is lost as heat, used in digestion and work, and used in growth and reproduction. On average, about ten percent of the organic matter at one trophic level becomes available to the next trophic level.
The rule is applied repeatedly. If producers contain 10,000 units of food energy, primary consumers receive about 1,000 units, secondary consumers about 100 units and tertiary consumers about 10 units.
Why Food Chains Are Usually Short
Every trophic transfer leaves much less energy available to the next level. After several transfers, very little usable energy remains. This is why food chains generally contain only three or four major trophic steps.
A Strategy For Energy-transfer Problems
Identify the trophic levels in order. Decide where the given energy belongs. For each step upward, multiply by 0.10. If working backward toward a lower trophic level, divide by 0.10 for each step. Finally, check that energy decreases upward.
Problem
Producers contain 5,000 kJ of food energy. Estimate the energy available to primary consumers.
- 1.Given: 5,000 kJ at the producer level.
- 2.Primary consumers are one step above producers.
- 3.Calculate 0.10 × 5,000 = 500 kJ.
- 4.Answer: about 500 kJ.
- 5.Check: the value is smaller than the producer energy.
Problem
A producer level contains 20,000 kJ. Estimate the energy available to secondary consumers.
- 1.Primary consumers receive about 2,000 kJ.
- 2.Secondary consumers receive about 200 kJ.
- 3.Answer: about 200 kJ.
- 4.Two trophic transfers reduce the starting energy to about one hundredth.
Problem
A tertiary-consumer level receives about 8 kJ. Estimate producer-level energy.
- 1.Tertiary consumers are three transfers above producers.
- 2.Secondary level ≈ 8 ÷ 0.10 = 80 kJ.
- 3.Primary level ≈ 80 ÷ 0.10 = 800 kJ.
- 4.Producer level ≈ 800 ÷ 0.10 = 8,000 kJ.
- 5.Check by moving upward again: 800, 80, 8 kJ.
Unidirectional Energy Flow
Energy captured by producers does not return to the Sun, and energy transferred to herbivores does not return to producers as the same usable food energy. As energy moves through trophic levels, the amount available progressively decreases.
Food Web
A Food Web is a network of interconnected food chains showing multiple feeding relationships within an ecosystem.
In nature, an organism may eat more than one kind of food and may itself be eaten by more than one predator. The food-web diagram therefore represents real feeding relationships more completely than one straight chain.
Food Chain Versus Food Web
| Feature | Food Chain | Food Web |
|---|---|---|
| Structure | Single feeding pathway | Many interconnected pathways |
| Use | Shows one route of food and energy transfer | Shows broader feeding relationships |
| Complexity | Simple | More realistic and complex |
There are generally more individuals at lower trophic levels, with producers forming the greatest number in many ecosystems. This pattern is consistent with progressively lower energy availability at higher levels.
The ten percent relationship does not mean that exactly ten percent of every organism is eaten. It is an approximate ecological relationship describing average transfer between trophic levels.
Quiz
Which organisms occupy the first trophic level?
If producers contain 9,000 kJ, about how much is available to primary consumers?
Why are long food chains uncommon?
Which statement correctly describes energy flow?
How does a food web differ from a food chain?
Practice Problems
- A grassland food chain is grass → grasshopper → frog → snake. Identify the trophic level of each organism.
- Producers contain 30,000 kJ. Estimate the energy available to primary, secondary and tertiary consumers.
- A secondary-consumer level receives about 60 kJ. Work backward to estimate the primary-consumer and producer energy levels.
- Explain why energy flow is unidirectional even though decomposers return materials to the environment.
- Construct a simple food web using grass, grasshopper, rabbit, frog, snake and hawk. Describe at least three food chains within it.
Key Takeaways
• A food chain shows a feeding pathway, while a food web shows many interconnected feeding pathways. • Each step in a food chain is a trophic level, beginning with producers. • Terrestrial green plants capture about one percent of the sunlight energy falling on their leaves. • About ten percent of organic matter or energy at one trophic level is available to the next on average. • Large energy losses explain why food chains are generally limited to a few trophic levels. • Energy flow is unidirectional and becomes progressively smaller at higher trophic levels.