What is Energy Flow in Food Webs?
A food web shows how energy moves through an ecosystem as organisms eat one another, from producers to top predators. At each step, most energy is lost as heat through metabolism, so only a fraction passes to the next trophic level — roughly 10% on average. This 'energy pyramid' explains why ecosystems can support far fewer top predators than plants.
Energy flows through a food web from producers to consumers, but only about 10% of the energy at one trophic level is available to the next, because the rest is lost as heat, movement, and waste.
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Step-by-step worked examples
A field of grass captures 10,000 kcal of energy. How much energy is available to the grasshoppers (primary consumers) that eat it, assuming 10% ecological efficiency?
E_next = E_current × 0.10 E_next = 10,000 × 0.10 = 1,000 kcal
Grasshoppers pass on their 1,000 kcal to frogs (secondary consumers). How much energy reaches the frogs?
E_next = 1,000 × 0.10 = 100 kcal
If a snake (tertiary consumer) eats the frogs, how much of the original 10,000 kcal of grass energy remains available at the snake's level?
Frogs → Snake: 100 × 0.10 = 10 kcal Only 10 kcal of the original 10,000 kcal (0.1%) reaches the snake — energy loss compounds at each level
Flashcards
Quick quiz
Q1.If producers have 5,000 kcal of energy, how much is available to primary consumers at 10% efficiency?
Q2.What happens to most of the energy at each trophic level?
Q3.Which trophic level usually has the most available energy?
Q4.A food web differs from a food chain because it…
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Common mistakes
Thinking all energy passes to the next trophic level. — Correct: Only about 10% transfers on average; the rest is lost as heat and waste.
Confusing food chain and food web. — Correct: A food chain is one linear path; a food web is many interconnected chains.
Assuming top predators have the most energy available. — Correct: Producers have the most energy; it decreases at each higher trophic level.
Believing energy cycles back to producers. — Correct: Energy flows one way through an ecosystem; only matter (nutrients) cycles.
FAQ
What is energy flow in food webs?
It's the one-way movement of energy from producers through consumers, decreasing at each trophic level.
What is the food web energy flow formula?
E_next = E_current × (efficiency/100), where efficiency is typically about 10%.
What are examples of energy flow in food webs?
Sunlight captured by grass, passed partly to grasshoppers, then partly to frogs and snakes, are classic examples.
How do you calculate energy transfer between trophic levels?
Multiply the energy at one level by the ecological efficiency (commonly ~10%) to estimate what reaches the next level.