Lecture Notes on Trophic Levels and the 10% Rule

Key Topics

• Trophic Levels
• The 10% Rule
• Energy and Matter Flow in Ecosystems
• Conservation of Energy and Matter

Objectives

1. Explain energy and matter flow through trophic levels.
2. Determine how energy decreases as it flows through an ecosystem.

Skills to Practice

• Explaining an environmental concept or process.
• Calculating accurate answers with units.

Conservation of Matter and Energy

• Law of Conservation: Matter is never created or destroyed, only changes forms.
  • Example: Decomposition of a tree – carbon, nitrogen, phosphorus, and water are conserved.
• Photosynthesis: Demonstrates conservation of matter and energy.
  • Sun’s light energy is converted to chemical energy (glucose).
  • Atoms (carbon, oxygen, hydrogen) are conserved.
• First Law of Thermodynamics: Energy is not created or destroyed, only transformed.

Biogeochemical Cycles

• Water, carbon, nitrogen, and phosphorus cycles show conservation of matter.

Energy Flow in Food Webs

• Energy is passed along food webs from producers to consumers.
  • Example: Rabbit eating a leaf retains energy from the leaf.
• Energy not destroyed but converted to usable forms.

Energy Transfer Between Trophic Levels

• Energy Loss as Heat: Each energy transfer involves loss of energy as heat.
  • Example: Coal-fired power plant loses 65% energy as heat.
• Ecosystem Example:
  • 1,000 Joules of light energy to producers; only 10 Joules to elk, 1 Joule to lion.

The 10% Rule

• Only 10% of energy from one trophic level is transferred to the next.
• Trophic Pyramid:
  • Widest at the base (most energy) and narrows upward (less energy).
  • 10% of energy moves to rabbit level, then to snakes, and finally top predators.

Trophic Levels

• Producers: Plants converting light energy to glucose.
• Primary Consumers: Herbivores eating plants.
• Secondary Consumers: Carnivores or omnivores eating primary consumers.
• Tertiary Consumers: Apex predators eating secondary consumers.

Biomass and the 10% Rule

• Biomass: Total mass of living things at a trophic level.
• Decrease in biomass correlates with energy transfer.
  • Example: 1,000 kg of producers supports 100 kg of primary consumers, 10 kg secondary, 1 kg tertiary.

Calculations of Energy and Biomass

• Calculate energy by moving decimal one place left (divide by 10).
  • Example: 95,000 Joules at producer level = 9,500 Joules at primary consumer.
• Practicing Calculations:
  • Deduce energy at tertiary consumers from given energy at producers.
  • Determine biomass at each level by moving decimal.

Practice Questions

1. Explain why large forests support few wolves.
2. Calculate energy available to tertiary consumers where plants produce 100,000 Joules.