Why can pyramids of numbers and biomass be inverted but the pyramid of energy never is? (10% law)
Pooja Iyer12th Sample
21 Aug 2026 · 1 min read
Dramatic confession: I had the 10% law memorised, I could draw all three pyramids, and still one MCQ made me feel like I'd never seen biology before. 🎭
The question said: "Which of the following ecological pyramids is always upright?" and the options were numbers, biomass, energy, and "none". I picked biomass because in a grassland it's upright. The answer was energy.
What I know from NCERT:
- Pyramid of numbers: upright in a grassland (many grasses, fewer herbivores, fewer carnivores) but in a tree ecosystem it can be inverted, since one big tree supports many insects.
- Pyramid of biomass: usually upright, but inverted in the sea, where the biomass of fishes (consumers) at an instant is more than that of the phytoplankton.
- Pyramid of energy: always upright.
- 10% law: only about 10% of energy passes from one trophic level to the next.
Questions: (a) why is the sea biomass pyramid inverted, since the fishes can't have more energy than the producers? (b) What exactly is different about "energy" that it never inverts? (c) If producers fix 10,000 J, how much reaches the third trophic level?
Please tell me what I'm not seeing!
7 answers & replies
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[+][–]Kavya NaikDropperSample· 1mo ago
(c) is easy: 10,000 → 1,000 at 2nd level (herbivores) → 100 at 3rd level (first carnivores). So 100 J at the third trophic level.
(a)+(b) are the real question. Biomass is a snapshot: the mass of living things at one moment (standing crop). In the sea, the phytoplankton are tiny and reproduce extremely fast, so at any moment there's a small mass of them, but they're eaten and replaced very quickly. The energy that flowed through them over time is still large. Energy is measured as a rate (per unit area per unit time), so it counts everything produced across the year, not just what's alive at the moment.
[+][–]Pooja Iyer12thSample· 1mo agoAuthor
"Snapshot vs rate" is the line!! Writing it as the title of my pyramid page. Thank you.
[+][–]Nisha Verma12thSample· 1mo ago
Reason energy never inverts: energy is lost at every transfer, mostly as heat in respiration, so each level necessarily receives less than the level below. It follows from the laws of thermodynamics. The 10% law is the average idea (it isn't exactly 10% every time) and it gives the shape, i.e. a smaller bar on each higher level.
[+][–]Tanvi Deshmukh12thSample· 1mo ago
Pyramid of numbers is always upright, right? Because there are always more producers than herbivores?
[+][–]Zoya Khan12thSample· 1mo ago
Tanvi, no, that's what the tree example is about: one tree with thousands of insects feeding on it gives an inverted pyramid of numbers. Parasites are the same thing too: a single host with many parasites, and then hyperparasites on them, so it's inverted again. So the pyramid of numbers can be upright or inverted depending on the ecosystem. Only the pyramid of energy is always upright.
[+][–]Tanvi Deshmukh12thSample· 1mo ago
oh!! the tree one! yes I even have it drawn in my notes, I just forgot the point of drawing it 😅 thanks Zoya!
[+][–]Siddharth RaoDropperSample· 1mo ago
Two exam notes. First, in the pyramid of energy, the diagram is drawn with the producers as the base, and NCERT says the energy shown is for a given area per unit time, so don't mix the units with biomass (g/m² or kg/m²). Second, pyramids don't include decomposers in their representation. I remember this since one old PYQ-style option used "decomposers at the top" as a distractor.
