Topic 1.9 & 1.10 Trophic Levels and the 10% Rule Notes

Trophic Levels and the 10% Rule

Matter is never created or destroyed: It only changes forms.
- Example: A dead tree decomposes, returning carbon, nitrogen, water, and phosphorus to the soil, atmosphere, or decomposers.
Energy is never created or destroyed: It is only transformed (First Law of Thermodynamics).
- Example: Photosynthesis
  - Sun's rays (light energy) are converted into glucose (chemical energy) by plants.
  - CO2 and H2O are converted into glucose and oxygen.
Biogeochemical cycles (water, carbon, nitrogen, phosphorus) demonstrate the conservation of matter.
- When an animal dies, the nitrogen in its body is transformed into ammonia and returns to the soil.
Food webs demonstrate the conservation of energy.
- A rabbit eats a leaf: the energy in the leaf is transferred to the rabbit, fueling growth and movement.

Each time energy transfers from one form to another, some is lost as heat.
Example: Electricity Generation
- Coal-fired power plant: potential energy in coal --> electricity.
  - Only 35% of the energy in coal becomes electricity; 65% is lost as heat.
  - Transmission wires: 10% energy loss; 90% reaches homes.
  - Light bulb: 95% energy loss as heat; only 5% as light.
Each time energy is transferred from one organism to another, the amount of available energy decreases.
Organisms use most of the energy for movement, development, and cellular respiration.
Example: Ecosystem
- Producers receive 1000 joules of light energy.
  - Use 990 joules.
  - 10 joules available to elk.
- Elk eats the grass.
  - Gets 10 joules of energy.
  - Uses 9 joules for development and cellular respiration (lost as heat).
- Lion eats the elk. Only 1 joule of energy left.

Only about 10% of the energy from one trophic level makes it to the next.
The other 90% is lost as heat during metabolic processes.
A pyramid shape represents the decreasing available energy at each level.
The base is the widest, representing the most energy (producers).
Each level above gets narrower, representing less available energy.
10% Rule: Only about 10% of the energy from one trophic level transfers to the next. The other 90% is lost as heat.

Producers: Plants that convert light energy into chemical energy (glucose).
Primary Consumers: Herbivores that eat plants to get energy.
Secondary Consumers: Carnivores or omnivores that eat primary consumers (and sometimes producers).
Tertiary Consumers: Apex predators that feed on secondary consumers.

Biomass: The total mass of living things at a trophic level.
Only about 10% of the biomass can be supported at each level due to energy constraints.
Example:
- 1000 kg of producers --> 100 kg of primary consumers.
- 100 kg of primary consumers --> 10 kg of secondary consumers.
- 10 kg of secondary consumers --> 1 kg of tertiary consumers.
In ecosystems, there are far more plants than animals because only 10% of the producer biomass can support the primary consumer level.

To calculate energy available at the next trophic level: move the decimal one place to the left (divide by 10).
Example:
- Producers: 95,000 joules --> Primary Consumers: 9,500 joules
- Primary Consumers: 9,500 joules --> Secondary Consumers: 950 joules
- Secondary Consumers: 950 joules --> Tertiary Consumers: 95 joules
The same calculation applies to biomass.
To go down the pyramid use multiplication.

Why can a relatively large forest only support a small number of wolves?
Calculate the amount of energy available to a tertiary consumer in an ecosystem where plants produce 100,000 joules of energy.