Food Chains, Energy Flow & Ecological Pyramids
How energy and matter move through an ecosystem — food chains and webs, trophic levels, the 10% law of energy transfer, and ecological pyramids.
Key Takeaways
- A food chain is a sequence of who-eats-whom; interlinked food chains form a food web.
- Energy flow is unidirectional — only about 10% passes to the next trophic level (Lindeman's 10% law).
- The pyramid of energy is always upright.
Core concept
Within an ecosystem, energy flows in a single direction — from the Sun to producers and up through trophic levels — while matter (nutrients) cycles round and round. A food chain traces this flow of energy from one organism to the next.
Static foundation — food chains and webs
- Grazing food chain: starts with producers (grass → deer → tiger).
- Detritus food chain: starts with dead organic matter (leaf litter → earthworm → bird).
- A food web is many interconnected food chains — giving the ecosystem stability.
The Pyramid of Energy (the 10% Law)
Lindeman's 10% law
At each transfer between trophic levels, roughly 90% of energy is lost as heat (respiration) and only about 10% is stored in the next level. This is why food chains are short (usually 4–5 links) and why top predators are so few. Because so much energy is lost, the pyramid of energy is always upright.
The Three Ecological Pyramids
| Pyramid | Shows | Can it be inverted? |
|---|---|---|
| Pyramid of Number | Number of organisms at each level | Yes (e.g., one tree supporting many insects) |
| Pyramid of Biomass | Total dry weight at each level | Yes (e.g., an aquatic ecosystem — tiny phytoplankton) |
| Pyramid of Energy | Energy content at each level | NO — always upright |
Bioaccumulation & biomagnification
Persistent toxins (like DDT or mercury) build up in an organism over time (bioaccumulation) and become more concentrated at higher trophic levels (biomagnification) — which is why top predators (eagles, big fish, and humans) are most at risk. This drove the ban on DDT and the concern over the mercury 'Minamata' disease.
Prelims trap zones
- Only the pyramid of ENERGY is always upright; number and biomass pyramids can be inverted.
- Energy flows (one-way); nutrients cycle (round and round) — don't confuse the two.
- Biomagnification increases toxin concentration UP the food chain — top predators suffer most.
Knowledge Check
2 questions · check your understanding
1. According to the 10% law, if producers have 10,000 units of energy, how much reaches the secondary consumers?
2. Which ecological pyramid is always upright?
Prelims Pointers
- The grazing food chain starts with producers; the detritus food chain starts with dead organic matter.
- Trophic levels: producers (T1), herbivores (T2), carnivores (T3), top carnivores (T4).
- Ecological pyramids are of three kinds — number, biomass and energy.
- The pyramid of energy is always upright; those of number and biomass can be inverted.
Mains Angle
- 'Energy flows but nutrients cycle.' Explain this fundamental principle of ecology.
- Why are food chains generally limited to four or five trophic levels?
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