APES Notes 1.9 & 1.10 - Trophic Levels & The 10% Rule

Scientific concepts and phenomena presented

Trophic levels and energy flow through ecosystems

• Energy moves from producers → consumers at higher trophic levels.
• As energy transfers upward, the available energy decreases because organisms use much of it for metabolism/respiration and the remainder is lost as heat.

Conservation of matter

• Matter is not created or destroyed, only transformed.
• Example: when a tree dies, its carbon, nitrogen, water, and phosphorus return to the soil and/or atmosphere through decomposition, then are reused by other organisms.

Conservation of energy & thermodynamics

• Energy is not created or destroyed, only converted between forms.
• Example: photosynthesis converts light energy into chemical energy (glucose) while conserving atoms (e.g., carbon, oxygen, and hydrogen).
• Named principle: First Law of Thermodynamics.

Biogeochemical cycles

• The water cycle, carbon cycle, nitrogen cycle, and phosphorus cycle show conservation of matter across ecosystems and time.
• Example mentioned: nitrogen in animal biomass can be transformed (e.g., into ammonia) and reused for plant growth.

Energy loss between trophic levels (heat loss)

• During energy transfer, organisms lose energy as heat: it is not destroyed, but becomes dispersed and no longer usable.
• Analogies used to illustrate inefficiency:
  • Power plant (coal → electricity): ~35% converted to electricity; ~65% lost as heat.
  • Transmission and home use: additional losses occur (e.g., ~10% lost during transmission).
  • Light bulb: ~95% lost as heat; ~5% becomes light.

The 10% Rule

• Only about 10% of energy (and thus ~10% of biomass) transfers to the next trophic level.
• As a result, trophic pyramids narrow toward the top because each higher level receives less usable energy.

Biomass and trophic pyramid

• Biomass = total mass of living organisms at a trophic level.
• Since growth requires energy and only ~10% passes upward, biomass supported at each level declines by about 10× each step.
• Example described:
  • Starting with 1000 kg producers
  • → 100 kg primary consumers
  • → 10 kg secondary consumers
  • → 1 kg tertiary consumer

Trophic level categories (names and roles)

• Producers: plants that convert light energy to chemical energy (glucose).
• Primary consumers (herbivores): eat producers.
• Secondary consumers: eat primary consumers (can be carnivores or omnivores).
• Tertiary consumers: top predators/apex predators that feed on secondary consumers.

Calculations and method using the 10% rule

• To estimate energy at the next trophic level: divide by 10 (equivalently, move the decimal one place left).
• Example given:
  • If producers have 95,000 joules
    • Primary consumers: ~9,500 joules
    • Secondary consumers: ~950 joules
    • Tertiary consumers: ~95 joules
• For biomass, the same ~10% passes upward logic applies—divide by 10 each level upward.
• Practice directions include:
  • Move the decimal left to go up the pyramid
  • Move the decimal right to go down (as described for biomass exercises)

Researchers or sources featured

• None explicitly named.