1.10 Energy Flow and the 10% Rule

Energy Flow in Ecosystems

  • Energy enters ecosystems as sunlight

  • Producers convert light energy into chemical energy via photosynthesis

  • Energy flows one way:
    Sun → producers → consumers → decomposers → heat

  • Energy is not recycled (unlike matter)

The 10% Rule

  • Only about 10% of energy at one trophic level is transferred to the next

  • About 90% is lost at each transfer

  • Example:

    • Producers: 10,000 kcal

    • Primary consumers: 1,000 kcal

    • Secondary consumers: 100 kcal

    • Tertiary consumers: 10 kcal

Why Energy Is Lost

Energy transfer is inefficient because:

  • Cellular respiration releases energy as heat

  • Movement and metabolism use energy

  • Incomplete consumption (not all organisms are eaten)

  • Incomplete digestion (indigestible parts are excreted)

  • Second Law of Thermodynamics (energy becomes less usable)

Laws of Thermodynamics (APES Focus)

First Law (Conservation of Energy)

  • Energy cannot be created or destroyed

  • Energy only changes form

  • In ecosystems:
    light → chemical → kinetic → heat

Second Law (Entropy)

  • Every energy transfer increases disorder

  • Energy transfers are never 100% efficient

  • Explains why energy decreases at higher trophic levels

Ecological Consequences of the 10% Rule

  • Food chains are short (usually 4–5 levels)

  • Higher trophic levels have:

    • Less energy

    • Less biomass

    • Smaller populations

  • Top predators are rare

  • Explains pyramid shapes in ecosystems

Ecological Pyramids

Pyramid of Numbers

  • Shows number of organisms at each trophic level

  • Can be upright or inverted

  • Example inversion:
    one oak tree → many insects

Pyramid of Biomass

  • Shows dry mass of organisms

  • Usually pyramid-shaped

  • Can be inverted in aquatic ecosystems due to:

    • Fast reproduction of phytoplankton

    • High productivity but low standing biomass

Pyramid of Energy (Productivity)

  • Shows energy flow per unit area per unit time

  • Always upright

  • Best representation of energy transfer

  • Follows the 10% Rule strictly

Efficiency of Photosynthesis

  • Only about 1% of incoming sunlight is captured by producers

  • Limits the total energy available to all trophic levels

Human Application

  • Eating lower on the food chain is more energy-efficient

  • ~10 kg of grain → ~1 kg of beef

  • Explains why plant-based diets use less energy and land

Exam Must-Knows

  • Energy flows one direction only

  • Only ~10% transfers between trophic levels

  • Energy pyramids are never inverted

  • Second Law explains energy loss

  • Use ×0.1 or ÷10 for energy calculations