πΏ Chapter 01 Β· Topic 03 Β· Ecological Pyramids
Ecological Pyramids β Numbers, Biomass & Energy
Graphical representation of trophic structure; upright and inverted pyramids; Lindeman’s 10% Law of energy transfer.
What are Ecological Pyramids?
An ecological pyramid is a graphical representation of the trophic structure and function of an ecosystem. It shows the relationship between producers and consumers at different trophic levels in terms of numbers, biomass, or energy.
- Concept introduced by Charles Elton (1927) β also called Eltonian Pyramids
- Producers always form the base; top carnivores form the apex
- Three types: Pyramid of Numbers, Pyramid of Biomass, Pyramid of Energy
Shapes: Upright (base wider than apex), Inverted (apex wider than base), Spindle-shaped (middle wider than both ends)
Pyramid of Numbers
Represents the number of individual organisms at each trophic level.
| Shape | Ecosystem | Reason |
|---|---|---|
| Upright | Grassland ecosystem | Millions of grass plants β thousands of insects β hundreds of frogs β few snakes β one eagle |
| Inverted | Tree ecosystem (single tree) | One tree supports thousands of insects β many birds β few top predators |
| Spindle-shaped | Parasitic food chain | One host β many parasites β more hyperparasites (e.g., one dog β many fleas β many bacteria) |
Limitation: Does not account for the size of organisms. A single large tree and a single small herb are both counted as “1” β making this the least accurate pyramid.
Pyramid of Biomass
Represents the total dry weight (biomass) of organisms at each trophic level, measured in g/mΒ² or kg/mΒ².
| Shape | Ecosystem | Reason |
|---|---|---|
| Upright | Terrestrial ecosystem (forest, grassland) | Large biomass of plants β less biomass of herbivores β even less of carnivores |
| Inverted | Aquatic/Pond ecosystem | Small biomass of phytoplankton (short-lived, rapid turnover) supports large biomass of zooplankton and fish |
Why inverted in aquatic? Phytoplankton reproduce very rapidly (high turnover rate), so at any given moment their standing biomass is less than the consumers they support. The total production over time is still high.
Pyramid of Energy
Represents the amount of energy at each trophic level, measured in kcal/mΒ²/year or J/mΒ²/year.
- Always upright β energy always decreases from lower to higher trophic levels
- Never inverted β energy cannot be created; it is always lost as heat at each level
- Most accurate and most useful representation of ecosystem function
- Based on Lindeman’s 10% Law (1942)
Why always upright? Due to the Second Law of Thermodynamics β energy is lost as heat at every trophic level. Energy can never flow upward in greater amounts than it enters from below.
Lindeman’s 10% Law
Proposed by Raymond Lindeman (1942) based on his study of Cedar Bog Lake, Minnesota.
Only 10% of energy at one trophic level is transferred to the next trophic level. The remaining 90% is lost as heat through respiration, movement, and metabolic activities.
Example calculation:
- T1 (Producers) = 10,000 kcal
- T2 (Primary consumers) = 1,000 kcal (10% of 10,000)
- T3 (Secondary consumers) = 100 kcal (10% of 1,000)
- T4 (Tertiary consumers) = 10 kcal (10% of 100)
Implication: Eating lower in the food chain is more energy-efficient. A vegetarian diet uses 10x less energy than a meat-based diet β important for understanding food security and sustainability.
Comparison of All Three Pyramids
| Feature | Pyramid of Numbers | Pyramid of Biomass | Pyramid of Energy |
|---|---|---|---|
| Measures | Number of organisms | Dry weight of organisms | Energy content |
| Unit | Number/mΒ² | g/mΒ² or kg/mΒ² | kcal/mΒ²/yr |
| Can be inverted? | Yes (tree, parasitic) | Yes (aquatic) | Never |
| Accuracy | Least accurate | More accurate | Most accurate |
| Upright example | Grassland | Terrestrial/forest | All ecosystems |
| Inverted example | Tree ecosystem | Aquatic/pond | None |
| Limitation | Ignores organism size | Ignores energy content | Difficult to measure |
Revision Checklist
β
Ecological pyramids introduced by Charles Elton (1927)
β Three types: Numbers, Biomass, Energy
β Pyramid of Numbers β upright in grassland; inverted in tree ecosystem; spindle in parasitic
β Pyramid of Biomass β upright in terrestrial; inverted in aquatic (pond)
β Pyramid of Energy β ALWAYS upright; NEVER inverted
β Pyramid of Energy = most accurate representation
β Pyramid of Numbers = least accurate (ignores organism size)
β Lindeman’s 10% Law (1942) β only 10% energy transferred per trophic level
β 90% energy lost as heat at each trophic level
β Aquatic biomass pyramid inverted due to rapid phytoplankton turnover
β Energy pyramid always upright due to Second Law of Thermodynamics
β Three types: Numbers, Biomass, Energy
β Pyramid of Numbers β upright in grassland; inverted in tree ecosystem; spindle in parasitic
β Pyramid of Biomass β upright in terrestrial; inverted in aquatic (pond)
β Pyramid of Energy β ALWAYS upright; NEVER inverted
β Pyramid of Energy = most accurate representation
β Pyramid of Numbers = least accurate (ignores organism size)
β Lindeman’s 10% Law (1942) β only 10% energy transferred per trophic level
β 90% energy lost as heat at each trophic level
β Aquatic biomass pyramid inverted due to rapid phytoplankton turnover
β Energy pyramid always upright due to Second Law of Thermodynamics