89. Pyramids of Biomass

Pyramids of biomass represent the total mass of living material at each trophic level in a food chain. Unlike pyramids of numbers, which show the quantity of individual organisms, these diagrams always have a pyramid shape because biomass is lost at each stage.

1. Understanding Biomass

• Definition: Biomass is the total mass of living organisms in a particular area or volume.

• Trophic Levels: In a typical pyramid:

  • The bottom bar is Trophic Level 1 (Producers, e.g., grass).

  • The next is Trophic Level 2 (Primary Consumers, e.g., rabbits).

  • Followed by Trophic Level 3 (Secondary Consumers, e.g., snakes).

  • The top is Trophic Level 4 (Tertiary Consumers, e.g., hawks).

• Mass vs. Numbers: Even if a predator (like a snake) weighs more than its prey (like a rabbit), there are far fewer predators in the ecosystem. Therefore, the total mass of the predator population is always lower than that of the prey population.

2. Why is Biomass Lost?

Only about 10% of the biomass from one trophic level is transferred to the next. The three main reasons for this loss are:

1. Uneaten Parts: Consumers do not usually eat every part of an organism (e.g., bones, teeth, or woody stems).

2. Incomplete Absorption: Not everything eaten can be digested. Undigested material is passed out of the body as feces (egestion).

3. Metabolic Processes (Respiration): Most of the biomass an organism consumes is used to release energy through respiration for movement and maintaining body temperature. This produces waste products like carbon dioxide and water, and in animals, urea is lost in urine. Only a small fraction is used for growth to create new biomass.

3. Calculating Efficiency

To find out how much biomass is successfully transferred between levels, use the following formula:

\text{Efficiency} = \left( \frac{\text{Biomass transferred to next level}}{\text{Biomass available at previous level}} \right) \times 100

Example Calculation: If rabbits have a total biomass of 144 kg and the snakes that eat them have a total biomass of 15 kg:

• Efficiency = (15 / 144) x100

• Efficiency = 10.4%

Summary Table: Biomass Loss Factors