How Does Energy Flow Through Ecosystems?
Energy enters ecosystems through photosynthesis (sunlight → chemical energy in plants) and flows through food chains. At each step, ~90% of energy is lost as heat and movement, leaving only ~10% to transfer to the next level.
Energy flows: sun → producers (photosynthesis) → consumers → decomposers. Only ~10% is transferred at each trophic level; 90% is lost as heat, respiration and movement.
Step-by-step worked examples
A field produces 100,000 kcal/year. How much energy is available to herbivores?
Energy to producers (plants) = 100,000 kcal/year Energy to primary consumers (herbivores) = 100,000 × 10% = 10,000 kcal/year
If herbivores have 10,000 kcal, how much energy reaches carnivores?
Energy to herbivores = 10,000 kcal Energy to secondary consumers (carnivores) = 10,000 × 10% = 1,000 kcal
Why can't we have long food chains with many trophic levels?
Each level loses 90% of energy. After 4–5 levels, remaining energy is too small to support any organisms. Most ecosystems have 3–4 trophic levels maximum.
Flashcards
Quick quiz
Q1.If plants in an ecosystem contain 50,000 kcal, approximately how much reaches herbivores?
Q2.Why are there always more plants than herbivores in an ecosystem?
Q3.In a four-level food chain (plants → herbivores → small carnivores → large carnivores), which has the least total biomass?
Q4.What happens to the 90% of energy lost at each trophic level?
The full card deck, worked steps and AI-tutor support for “How Does Energy Flow Through Ecosystems?” are in Notek — study by hand before your exam.
Common mistakes
Energy 'cycles' through ecosystems like water and nutrients do. — Correct: Energy flows one way: sun → ecosystem → heat. It is not cycled back.
All the energy in food is available to the next consumer. — Correct: Only ~10% of consumed energy is stored as body tissue. 90% is spent on respiration and movement.
Longer food chains are more efficient. — Correct: Longer chains lose more energy. Short food chains (3–4 levels) are most efficient.
Plants get their energy from the soil. — Correct: Plants get energy from the sun via photosynthesis. Soil provides nutrients (N, P, K), not energy.
FAQ
What is energy flow in ecosystems?
The movement of energy from the sun through photosynthesis into plants, then through food chains to consumers and decomposers.
What is the 10% rule?
Approximately 10% of energy is transferred from one trophic level to the next. The remaining 90% is lost as heat and through respiration.
Why is energy lost between trophic levels?
Organisms spend energy on movement, growth, respiration, and heat production. Only the energy stored in body tissue transfers to the next level.
Why can't we support top predators with a small plant area?
Top predators need 1,000s of times more plant biomass due to the 10% rule applied repeatedly through multiple trophic levels.