Overview
This lecture covers how energy and matter flow through ecosystems, focusing on trophic levels and the 10% rule to illustrate decreasing energy availability up the food chain.
Conservation of Matter and Energy
- Matter cannot be created or destroyed; it only changes forms (conservation of matter).
- Energy is not destroyed but is transformed (conservation of energy), as seen in photosynthesis.
- The first law of thermodynamics states that energy can change form but cannot be created or destroyed.
Energy Flow in Ecosystems
- In food webs, energy from the sun is converted by plants (producers) into chemical energy (glucose).
- When animals eat plants, the energy transfers but is used for growth, movement, and lost as heat.
- Every time energy moves between organisms, a portion is lost as heat and becomes unavailable for further transfer.
The 10% Rule
- Only about 10% of the energy at one trophic level is passed to the next level.
- The remaining 90% is lost as heat or used by the organism for metabolism and movement.
- The 10% rule explains why energy pyramids are widest at the base and narrow at the top.
Trophic Levels and Biomass
- Producers (plants) form the base of the trophic pyramid by converting light energy into chemical energy.
- Primary consumers (herbivores) eat producers; secondary consumers (carnivores/omnivores) eat primary consumers; tertiary consumers (apex predators) eat secondary consumers.
- Only 10% of the biomass at one level can support the next level up due to the 10% rule.
- This results in many more producers than consumers at each successive trophic level.
Calculating Energy and Biomass Transfer
- To find energy at the next trophic level, divide by 10 or move the decimal one place left.
- Example: 95,000 J at producer level → 9,500 J at primary consumer → 950 J secondary → 95 J tertiary.
- The same calculation applies to biomass, but move the decimal right as you go down the pyramid.
Key Terms & Definitions
- Trophic Level — each step in a food chain, representing positions organisms occupy (producer, primary consumer, etc.).
- 10% Rule — only 10% of energy is transferred to the next trophic level; 90% is lost as heat/work.
- Biomass — total mass of living matter at a given trophic level.
- Conservation of Matter/Energy — principle that matter/energy is not created or destroyed, only transformed.
Action Items / Next Steps
- Explain why a large forest can only support a small number of wolves.
- Calculate the energy available to a tertiary consumer if plants produce 100,000 J of energy.