Ecological pyramids and the ten per cent law

Core facts

An ecological pyramid shows the relationship between trophic levels in terms of number, biomass or energy.
The base is always the producers (first trophic level); the apex is the top carnivore.
Three types: pyramid of numbers, pyramid of biomass, pyramid of energy.
The pyramid idea (first as the pyramid of numbers) was introduced by Charles Elton (1927), hence the name "Eltonian pyramids". Energy pyramids came later, from trophic-dynamic work.
Ten per cent law: only about 10% of the energy at one trophic level is passed on to the next. It came from Lindeman's work (1942).
The pyramid of energy is always upright. It can never be inverted.
The pyramid of biomass in the sea is generally inverted.
The pyramid of numbers for a single tree with many insects on it is inverted.
Biomass is measured more accurately as dry weight.

Explained

The three pyramids

Pyramid	Measures	Upright or inverted
Numbers	Count of individuals at each level	Upright in grassland; inverted for a tree
Biomass	Dry weight of living matter at each level	Upright on land; generally inverted in the sea
Energy	Energy at each level per unit area per unit time	Always upright

NCERT's grassland pyramid of numbers

Primary producers: about 5.8 million plants (5,842,424).
Primary consumers: 708,624.
Secondary consumers: 354,904.
Top carnivores: 3.
So nearly 6 million plants support just 3 top carnivores.

NCERT's ideal pyramid of energy

Of 1,000,000 J of sunlight, producers convert about 1% (10,000 J) into NPP.
Then about 10% passes on at each step: 10,000 J → 1,000 J → 100 J → 10 J.

Why the pyramid of energy is always upright

At each transfer, energy is lost as heat in respiration, in undigested matter and in uneaten parts.
A higher level can never have more energy than the level that feeds it.

Why the pyramid of biomass is inverted in the sea

Phytoplankton have a small standing crop at any moment, because they are eaten quickly and reproduce quickly.
The biomass of fish feeding on them is larger at any one time.
The pyramid of energy in the same sea is still upright, because the phytoplankton's yearly production is far larger than their standing crop.

Why the pyramid of numbers is inverted for a tree

One large tree (producer) supports many insects, which in turn support fewer birds.
The base is narrow and the middle is broad. Parasite chains (one host, many parasites) are also inverted.

Common traps

✗ "The pyramid of biomass is always upright." ✓ It is generally inverted in the sea. Only the energy pyramid is always upright.
✗ "The pyramid of numbers is always upright." ✓ It is inverted for a tree with many insects, and for parasite chains.
✗ "The ten per cent law means 10% of energy is lost at each level." ✓ About 10% is passed on; about 90% is lost.
✗ "The base of a pyramid is the top carnivore." ✓ The base is the producers (first trophic level).
✗ "Pyramids take full account of decomposers and food webs." ✓ They leave out saprophytes and cannot show food webs.

Quick revision

Three pyramids: numbers, biomass, energy.
Base = producers; apex = top carnivore.
Energy pyramid: always upright.
Biomass in sea: generally inverted.
Numbers for a tree: inverted.
Ten per cent law (Lindeman, 1942): about 10% passes on.
NCERT grassland: about 5.8 million plants support 3 top carnivores.
NCERT ideal energy pyramid: producers convert about 1% of sunlight into NPP.
Limits: ignore multi-level species, food webs and saprophytes.

Go deeper: Deeper, Mains writing (beyond what APPSC Paper I usually asks)

Deeper Advanced

Limitations of ecological pyramids (NCERT)

They do not take into account the same species belonging to two or more trophic levels (the sparrow eating both seeds and insects).
They assume a simple food chain, which almost never exists in nature.
They do not accommodate a food web.
Saprophytes (decomposers) are not given any place, although they play a vital role in the ecosystem.

The ten per cent law in detail

Transfer efficiency is not exactly 10%. It usually falls in a range of about 5–20%, and 10% is the rule of thumb.
Each level loses energy:
- in respiration (the largest loss)
- as food not eaten
- as food eaten but not absorbed (faeces)
This explains:
- why food chains rarely have more than four or five levels
- why top carnivores are few, and need large territories
- why top carnivores are most at risk from habitat loss and biomagnification

Numbers and biomass can mislead; energy does not. Numbers ignore size (a tree counts as one). Biomass is a snapshot (standing crop) and ignores turnover rate. Energy measures flow over time, so it reflects how the ecosystem works.

Mains writing Mains writing

Frame: the shape of the pyramids explains why apex predators are rare and vulnerable, and why "eating lower on the food chain" is a sustainability argument.
Analysis points:
1. Apex predators such as the tiger need a large prey base and so large habitats. Saving the tiger protects every level below it (an umbrella species).
2. Food security: with about 90% loss at each step, grain fed to animals feeds far fewer people than grain eaten directly.
3. Fisheries: "fishing down the food web" removes top fish first. Catches then shift to smaller species lower in the pyramid.
4. Biomagnification works in the opposite direction to energy: pollutant concentration rises as energy falls.
Way forward:
- protect prey base and habitat, not only the apex species
- support diets and farming systems that use energy efficiently
- ecosystem-based fisheries management
Example: tiger reserves under Project Tiger protect the whole pyramid: grassland and forest producers, deer and other prey, and the tiger at the top.

Medha AI writes its own notes and checks them against the sources listed. Medha AI is an independent preparation service. It is not affiliated with, endorsed by or connected to APPSC, TSPSC, MPSC, UPSC or any other commission or government body.

7.1.1.4 Ecological pyramids and the ten per cent law

Download this PDF