A Pyramid of Biomass represents the total dry mass of all organisms at each trophic level, usually measured in or .
Dry mass is used because water content varies between organisms and does not contain chemical energy that can be transferred.
In most terrestrial ecosystems, the pyramid is upright because producers must have more mass to support the consumers above them.
In some aquatic ecosystems, the pyramid can be inverted because the producers (phytoplankton) reproduce and are consumed so quickly that their standing crop biomass at any one time is less than that of the consumers (zooplankton).
A Pyramid of Energy shows the rate of energy flow through each trophic level over a period of time, typically measured in .
These pyramids are always upright because energy is lost as heat at every transfer, following the laws of thermodynamics.
The 10% Rule suggests that only approximately of the energy from one level is incorporated into the biomass of the next level.
The remaining is lost through metabolic heat during respiration, incomplete digestion (egestion), and excretion of waste products.
| Feature | Pyramid of Numbers | Pyramid of Biomass | Pyramid of Energy |
|---|---|---|---|
| Unit | Number of individuals | (Dry mass) | |
| Shape | Can be inverted | Usually upright (can be inverted) | Always upright |
| Focus | Population count | Standing crop mass | Energy flow over time |
| Accuracy | Lowest (ignores size) | Medium (ignores energy content) | Highest (shows productivity) |
Check the Units: Always look for units like to identify a pyramid of energy versus a pyramid of biomass.
Identify Inversions: If a question describes a single large producer (like a tree) or a parasite-heavy chain, expect a non-standard pyramid of numbers.
The 10% Rule: Use this rule to estimate values; if producers have , primary consumers will likely have roughly .
Decomposers: Note that decomposers and detritivores are often omitted from these diagrams despite their critical role in energy cycling.
Scale Matters: When drawing, ensure the bars are centered and the width is proportional to the values provided in the data.
