Food Chains, Food Webs, and Energy Pyramids
Read food-web pathways, track energy across trophic levels, and predict population and contaminant changes from ecosystem evidence.
Food Chains, Food Webs, and Energy Flow
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Arrows show where energy goes—not which organism is chasing another
In a food chain or food web, an arrow points from the organism being eaten toward the organism receiving its energy. Thus, grass → rabbit → fox means the rabbit obtains energy from grass and the fox obtains energy from the rabbit.
Producers form the first trophic level. Primary consumers eat producers; secondary consumers eat primary consumers. An omnivore may occupy different trophic levels in different pathways.
Energy flows; matter cycles
Energy enters mainly as sunlight and eventually leaves as heat. Nutrients return through waste and decomposition and can be reused.
A food web supports conditional predictions
A population change can affect its food, predators, and competitors. Follow arrows one connection at a time rather than claiming the entire ecosystem changes in one fixed way.
Read a food web in three passes
Pass 1—Food: Arrows entering an organism show its listed energy sources.
Pass 2—Predators: Arrows leaving an organism show which listed consumers eat it.
Pass 3—Change: If a population falls, its consumers lose food while the organisms it ate may face less feeding pressure.
Then check for alternative links before predicting how large the effect will be.
Why it works
The three-pass method prevents the two most common errors: reading arrows backward and ignoring alternative food sources.
Five forms you should recognize
Chain: rice → grasshopper → frog → snake.
Interpretation: Rice is the producer, grasshopper the primary consumer, frog the secondary consumer, and snake a higher-level consumer.
Web links: rat → snake and frog → snake.
Interpretation: The snake has two listed food sources. A decline in rats may be partly buffered if frogs remain available.
Problem: Producers store 50,000 kJ. Estimate energy after two 10% transfers.
50,000 → 5,000 → 500 kJChange: A predator is removed.
Possible chain: Herbivores increase → plants experience heavier feeding. The conclusion depends on the actual web and other controls.
Observation: Toxin concentration rises from algae to zooplankton to fish to fish-eating birds.
Reasoning: Persistent chemicals accumulate as predators consume many contaminated prey.
Check before you commit
- Reading arrows from predator to prey
- Calling every consumer at one fixed trophic level
- Saying energy is recycled like nutrients
- Ignoring alternative prey in a food web
- Assuming every population response is immediate and unlimited
- Confusing bioaccumulation within one organism with biomagnification across trophic levels
Do you need the lesson-or just practice?
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Foundations
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Core Practice
Use mixed forms with less scaffolding.
UPCAT-Style Transfer
Apply the competency in unfamiliar representations.
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Food Chains, Food Webs, and Energy Pyramids FAQ
Why is the 10% rule only approximate?
Transfer efficiency varies among organisms and ecosystems; 10% is a useful model, not a universal constant.
Can a hawk occupy different trophic levels?
Yes. Its position depends on whether it eats a primary, secondary, or higher-level consumer in a particular pathway.
Why are decomposers not simply the last link?
They act on dead material and waste from organisms at many trophic levels.
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