Last reviewed 4 Nov 2026 · 2 min read
1. Energy flow
- The sun is the source of energy; plants capture only about 1–2% of the sunlight falling on them (photosynthetically active radiation) through photosynthesis.
- Energy flow is unidirectional (sun → producers → consumers) — it is not recycled, unlike nutrients.
- It obeys the laws of thermodynamics: energy is not created or destroyed (1st law), and at each transfer much is lost as heat (2nd law).
FormulaLindeman's 10% law (1942)
Only about 10% of the energy at one trophic level is passed on to the next; the rest is used in respiration and lost as heat.
Worked ExampleEnergy reaching a tertiary consumer
Producers fix 10,000 kJ. Primary consumers get ≈ 1,000 kJ, secondary ≈ 100 kJ, tertiary ≈ 10 kJ. This is why food chains rarely have more than 4–5 trophic levels.
2. Trophic levels
| Level | Organisms |
|---|---|
| T1 — Producers | Green plants, phytoplankton |
| T2 — Primary consumers | Herbivores (deer, grasshopper, zooplankton) |
| T3 — Secondary consumers | Carnivores feeding on herbivores (frog, small fish) |
| T4 — Tertiary consumers | Top carnivores (snake, hawk, large fish) |
3. Food chains and food webs
| Type | Starts with | Example |
|---|---|---|
| Grazing food chain | Living green plants | Grass → grasshopper → frog → snake → hawk |
| Detritus food chain | Dead organic matter | Leaf litter → earthworm → bird; dominant in forests |
| Parasitic chain | Host → parasite | Tree → bird → lice |
- Food web — interlocking food chains. More complex webs make an ecosystem more stable, because consumers have alternative food sources.
4. Ecological pyramids (Charles Elton, 1927)
| Pyramid | What it shows | Shape |
|---|---|---|
| Number | Number of individuals at each level | Upright in grassland; inverted in a tree ecosystem (one tree, many insects) and parasitic chains |
| Biomass | Dry weight of living matter | Upright on land; inverted in a pond/ocean (small phytoplankton biomass supports larger zooplankton/fish because phytoplankton reproduce fast) |
| Energy | Energy flow per unit area per unit time | Always upright — never inverted |
RememberEasy way to recall
Energy pyramid — never inverted. Number pyramid inverts in a tree; biomass pyramid inverts in the sea.
5. Ecological efficiency
- Ecological efficiency = energy at level n+1 ÷ energy at level n (typically 5–20%, about 10% on average).
- Production efficiency and assimilation efficiency describe how much of the food eaten becomes body mass.
Revision SummaryQuick recap
- Energy flow is one-way; nutrients cycle.
- 10% law (Lindeman 1942) → limits food chains to 4–5 levels.
- Grazing vs detritus food chains; food webs give stability.
- Pyramids (Elton): energy always upright; numbers inverted in trees; biomass inverted in water.