Question
Explain the concept of energy flow in an ecosystem. State the 10% law by Lindeman. Describe the three types of ecological pyramids.
(NEET 2023, similar pattern)
Solution — Step by Step
Energy enters the ecosystem as sunlight, is captured by producers (plants) through photosynthesis, and flows through the food chain: producers → primary consumers → secondary consumers → tertiary consumers.
Two critical rules of energy flow:
- Energy flow is unidirectional — it moves from lower to higher trophic levels and cannot be recycled (unlike nutrients).
- At each trophic level, a large portion of energy is lost as heat through respiration, and some is lost in undigested food and excreta.
Proposed by Raymond Lindeman (1942), the 10% law states: Only about 10% of energy at one trophic level is transferred to the next trophic level.
The remaining 90% is lost through:
- Cellular respiration (heat)
- Undigested/uneaten parts
- Excretory products
- Dead organic matter going to decomposers
Example: If producers fix 10,000 kcal of energy, primary consumers get ~1,000 kcal, secondary consumers get ~100 kcal, and tertiary consumers get ~10 kcal.
This is why food chains rarely have more than 4-5 trophic levels — there is simply not enough energy left to sustain another level.
1. Pyramid of Numbers:
- Shows the number of organisms at each trophic level.
- Usually upright (many producers, fewer herbivores, even fewer carnivores).
- Can be inverted in a tree ecosystem (1 tree supports many herbivorous insects, which support fewer parasites — but still an inverted shape at the base).
2. Pyramid of Biomass:
- Shows the total dry weight of organisms at each trophic level.
- Usually upright in terrestrial ecosystems (grass has more biomass than deer, which has more than lions).
- Inverted in aquatic ecosystems — phytoplankton (producers) have less standing biomass than zooplankton at any given time because phytoplankton reproduce rapidly and are consumed quickly.
3. Pyramid of Energy:
- Shows the energy content at each trophic level per unit time.
- Always upright — this can never be inverted because of the second law of thermodynamics (energy is always lost at each transfer).
- This is the most accurate representation of trophic structure.
Why This Works
The 10% rule is a direct consequence of the laws of thermodynamics. The second law states that energy transformations are never 100% efficient — some energy is always lost as unusable heat. Biological energy conversions (eating, digesting, metabolising) are no exception.
This progressive energy loss explains why top predators (tigers, eagles) are always rare — the ecosystem simply cannot support large numbers of organisms at the top of the food chain.
NEET repeatedly tests: “Which pyramid is always upright?” Answer: Pyramid of energy. “Which pyramid can be inverted in an aquatic ecosystem?” Answer: Pyramid of biomass. These two facts cover most questions on ecological pyramids.
Common Mistake
The most frequent error: writing that “the pyramid of biomass is always upright.” It is upright in terrestrial ecosystems but inverted in aquatic ecosystems (open ocean). The small, fast-reproducing phytoplankton maintain low standing biomass at any instant, even though they produce enormous energy over time.
Another mistake: confusing standing crop (biomass at a point in time) with productivity (energy produced over time). The pyramid of biomass measures standing crop, which can be misleading. The pyramid of energy measures productivity, which always shows a decrease from producers to top consumers.