Understanding Pyramids of Numbers and Biomass 

 

What Are Pyramids of Numbers and Biomass?

Pyramids of numbers and biomass are diagrams that help us understand how organisms and energy are distributed at each level of a food chain. They represent the number or mass of organisms at different trophic levels.

 

Pyramid Type	What It Shows	Example
Pyramid of Numbers	The number of individual organisms at each trophic level.	A single oak tree supports thousands of caterpillars, which in turn feed a few birds.
Pyramid of Biomass	The total mass of organisms (living material) at each trophic level.	Grasses (large biomass) support a smaller mass of herbivores, which support even fewer carnivores.

 

Pyramid of Numbers: Counting Organisms

• Definition: Shows the number of organisms at each trophic level in a food chain.
• Typical Shape: Usually a pyramid, but sometimes not!
  • Example: A single tree (producer) may support thousands of insects, making the base smaller than the level above.
• Example: $$\text{Tree (1)} \rightarrow \text{Caterpillars (500)} \rightarrow \text{Birds (50)} \rightarrow \text{Hawk (1)}$$

 

Advantages:

• Simple to create.
• Highlights ecosystem structures.

Disadvantages:

• Doesn’t account for the size or mass of organisms.
• One tree and one insect are counted equally, even though their biomass differs greatly.

 

Pyramid of Biomass: Measuring Mass

• Definition: Shows the total dry mass (biomass) of organisms at each trophic level.
• Pyramids of biomatter are always pyramid shapes because biomatter measures how much living matter is present once water is removed i.e dry mass. 
• Typical Shape: Always pyramid-shaped due to energy loss as you move up the food chain.
• Biomass decreases because:
  1. Energy is used for life processes (e.g., movement, respiration).
  2. Energy is lost as heat.
  3. Some material is indigestible or excreted.

Example of Biomass (Grassland Ecosystem):

Trophic Level	Organism	Biomass (kg)
1st (Producer)	Grass	1000
2nd (Primary Consumer)	Grasshopper	100
3rd (Secondary Consumer)	Frog	10
4th (Tertiary Consumer)	Snake	1

 

 

Why Use Biomass Over Numbers?

Pyramid of Numbers	Pyramid of Biomass
May not always form a pyramid shape.	Always forms a pyramid shape.
Counts organisms without considering size.	Considers the actual energy or mass each level holds.
Easy to count and visualize.	More accurate but harder to measure (requires drying organisms).

 

Step-by-Step Example: Drawing a Pyramid of Biomass

Scenario:

• A field contains$500 \text{kg}$ of grass (producers).
• The grass feeds$50\text{kg}$ of grasshoppers (primary consumers).
• The grasshoppers feed $5\text{kg}$> of frogs (secondary consumers).
• The frogs feed $0.5\text{kg}$ of snakes (tertiary consumers).

Steps:

1. Draw a large base for the producers (grass: $500\text{kg}$).
2. Each level above gets smaller:
   • Grasshoppers: $50\text{kg}$.
   • Frogs:$5\text{kg}$.
   • Snakes: $0.5\text{kg}$.
3. Label each level with the organism and biomass.