Energy Flow

Direction: One-way through ecosystem (enters as light, exits as heat)

10% Rule:

• Only ~10% of energy transferred to next level
• 90% lost as heat (metabolism, movement)
• Limits food chain length (~4-5 levels)

Energy pyramid:

• Producers (largest energy)
• Primary consumers
• Secondary consumers
• Tertiary consumers (smallest energy)

Biomass pyramid:

• Usually same shape as energy pyramid
• Total mass of organisms at each level

Food Chains and Webs

Food chain: Linear energy transfer (A → B → C)

Food web: Interconnected food chains

• More realistic
• Shows multiple feeding relationships
• More stable (redundancy)

Biogeochemical Cycles

Water Cycle

Processes:

• Evaporation: liquid → gas
• Transpiration: plants release water
• Condensation: gas → liquid (clouds)
• Precipitation: rain, snow
• Runoff: water flows to ocean

Carbon Cycle

Processes:

• Photosynthesis: CO₂ → organic compounds
• Cellular respiration: organic → CO₂
• Combustion: burning releases CO₂
• Decomposition: releases CO₂
• Fossilization: long-term storage

Human impact:

• Burning fossil fuels
• Deforestation
• Increased atmospheric CO₂
• Climate change

Nitrogen Cycle

Nitrogen fixation:

• N₂ (atmosphere) → NH₃ or NO₃⁻
• Bacteria in soil or root nodules
• Lightning

Nitrification:

• NH₃ → NO₂⁻ → NO₃⁻
• Bacteria convert

Assimilation:

• Plants absorb NO₃⁻
• Incorporate into proteins, DNA

Ammonification:

• Decomposers convert organic N → NH₃

Denitrification:

• Bacteria convert NO₃⁻ → N₂
• Returns N to atmosphere

Phosphorus Cycle

• No atmospheric component
• Weathering releases from rocks
• Plants absorb from soil
• Passed through food web
• Returns via decomposition
• Runoff to ocean (sediments)

Human impact:

• Fertilizers cause eutrophication
• Algal blooms
• Oxygen depletion

Ecosystem Services

Benefits from ecosystems:

1. Provisioning: Food, water, timber, fiber
2. Regulating: Climate, water purification, pollination
3. Supporting: Nutrient cycling, soil formation
4. Cultural: Recreation, aesthetic, spiritual

Key Concepts

1. Energy flows one-way through ecosystems (enters as light, exits as heat)
2. 10% rule: Only ~10% energy transferred between trophic levels
3. Producers form base; decomposers recycle nutrients
4. Food webs show interconnected feeding relationships
5. Carbon cycle: photosynthesis removes CO₂, respiration releases
6. Nitrogen cycle: bacteria fix N₂, plants assimilate, denitrification returns
7. Human activities alter biogeochemical cycles (climate change, eutrophication)