Pyramids of Biomass

Biomass

• Definition: The mass of living tissue (proteins, lipids, carbohydrates) in organisms.

• Measured as dry biomass (organisms dried and weighed).

  • More reliable than wet biomass (moisture levels vary).

• Scientists usually use published data rather than killing animals.

Pyramids of Biomass

• Show the biomass at each trophic level in a food chain.

• Shape: always a pyramid (biomass decreases at higher levels).

• Trophic levels:

  • 1 = producers (plants/algae).

  • 2 = primary consumers (herbivores).

  • 3 = secondary consumers.

  • 4 = tertiary consumers.

Why biomass decreases at each level

• Only ~10% of biomass is transferred to the next trophic level.

• Reasons:

  • Not all ingested material absorbed → some egested as faeces.

  • Some absorbed biomass excreted as waste (e.g., urea in urine).

  • Respiration uses biomass to release energy:

    • Movement.

    • Maintaining body temperature (especially in mammals & birds).

    • Produces CO₂ and water as waste.

Efficiency of Biomass Transfer

• Equation:

  Efficiency (%) = (Biomass gained ÷ Biomass intake) × 100

• Example:

  • Cow eats 300 kg grass biomass.

  • Gains 15 kg body mass.

  • Efficiency = 15 ÷ 300 × 100 = 5%.

✅ Key Points:

• Biomass decreases up the food chain.

• Energy losses → mainly through waste & respiration.

• Efficiency is usually low (~10%), limiting the length of food chains.