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Energy flow, food chains, food webs, and biogeochemical cycles
Learn step-by-step with practice exercises built right in.
Ecosystem: Community + abiotic environment
Producers (Autotrophs):
Consumers (Heterotrophs):
Decomposers (Detritivores):
Explain the 10% rule (energy transfer efficiency) in ecosystems. Why is energy transfer between trophic levels so inefficient?
The 10% Rule: Approximately 10% of energy from one trophic level is transferred to the next level. About 90% is lost at each step.
Energy Flow: Sun → Producers (100%) → Primary Consumers (~10%) → Secondary Consumers (~1%) → Tertiary Consumers (~0.1%)
Example: • Producers capture: 10,000 kcal from sun • Primary consumers get: ~1,000 kcal (10%) • Secondary consumers get: ~100 kcal (1% of original) • Tertiary consumers get: ~10 kcal (0.1% of original)
Why Is Transfer So Inefficient?
CELLULAR RESPIRATION (largest loss) • Organisms use energy for life processes • Cellular respiration converts glucose to ATP • Energy lost as heat (2nd law of thermodynamics) • ~60-90% of consumed energy used for metabolism • Only ~10-40% goes to growth/biomass
| Section | Format | Questions | Time | Weight | Calculator |
|---|---|---|---|---|---|
| Multiple Choice | MCQ | 60 | 90 min | 50% | 🚫 |
| Free Response (Long) | FRQ | 2 | 50 min | 30% | 🚫 |
| Free Response (Short) | FRQ | 4 | 40 min | 20% | 🚫 |
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Direction: One-way through ecosystem (enters as light, exits as heat)
10% Rule:
Energy pyramid:
Biomass pyramid:
Food chain: Linear energy transfer (A → B → C)
Food web: Interconnected food chains
Processes:
Processes:
Human impact:
Nitrogen fixation:
Nitrification:
Assimilation:
Ammonification:
Denitrification:
Human impact:
Benefits from ecosystems:
NOT ALL BIOMASS CONSUMED • Herbivores don't eat all plants (roots, bark, etc.) • Carnivores don't eat all prey (bones, hair, etc.) • Woody tissue, shells often indigestible • ~50% of plant biomass never consumed
NOT ALL CONSUMED IS ASSIMILATED • Some material passes through as feces • Cellulose (plant cell walls) hard to digest • Chitin (insect exoskeletons) indigestible • ~20-50% lost in feces
ENERGY IN WASTE PRODUCTS • Urine, feces contain energy • Urea, ammonia have chemical energy • Not available to consumer
Breakdown of Energy Flow: • 100 units consumed • ~50 units lost as feces (undigested) • ~30-40 units lost as heat (respiration) • ~10 units stored as biomass (growth)
Consequences:
Biomass Pyramids • Less biomass at each higher level • Top predators rarest • Cannot support many trophic levels (usually 4-5 max)
Limited Food Chain Length • Not enough energy to support more levels • Top predators must be efficient hunters • Large territories needed
Human Diet Implications • Eating plants (primary consumers) more efficient • Eating meat requires 10× more plant energy • Cattle eat grain → we eat cattle (two steps) • Direct grain consumption more efficient
Variations: • Actual efficiency varies: 5-20% typical range • Aquatic systems sometimes more efficient (~15-20%) • Endotherms (mammals/birds) less efficient (high metabolism) • Ectotherms (reptiles/fish) more efficient (lower metabolism)
Key Principle: Energy FLOWS through ecosystems (one direction), it is not recycled! Each transfer loses energy as heat, limiting food chain length.
Define and distinguish between gross primary productivity (GPP), net primary productivity (NPP), and net ecosystem productivity (NEP).
Primary Productivity Definitions:
GROSS PRIMARY PRODUCTIVITY (GPP): • TOTAL energy captured by photosynthesis • All organic molecules produced by autotrophs • Before any energy is used by plants themselves • Units: energy/area/time (e.g., kcal/m²/year) or biomass (g/m²/year)
Equation: GPP = Total photosynthesis
NET PRIMARY PRODUCTIVITY (NPP): • Energy stored in plant biomass AFTER cellular respiration • Energy available to herbivores/decomposers • GPP minus plant respiration • Represents actual plant growth • What we can measure as biomass accumulation
Equation: NPP = GPP - R_plants
where R_plants = plant cellular respiration
Typically: NPP ≈ 50-60% of GPP (40-50% lost to plant respiration)
NET ECOSYSTEM PRODUCTIVITY (NEP): • Net carbon accumulation in ecosystem • NPP minus ALL heterotroph respiration • Includes consumers and decomposers • Can be positive (carbon sink) or negative (carbon source)
Equation: NEP = NPP - R_heterotrophs
OR
NEP = GPP - R_total (all organisms)
Relationships: GPP > NPP > NEP
GPP (100%) → NPP (50%) → NEP (variable, often ~10%)
Example with Numbers:
Tropical Rainforest: • GPP = 20,000 kcal/m²/year • Plant respiration = 10,000 kcal/m²/year • NPP = 10,000 kcal/m²/year • Heterotroph respiration = 9,000 kcal/m²/year • NEP = 1,000 kcal/m²/year (carbon sink)
Factors Affecting Productivity:
Light availability • More light → higher GPP • Tropical > temperate > polar
Temperature • Affects enzyme activity • Warm (optimal) > cold
Water availability • Rainforest > desert • Essential for photosynthesis
Nutrients • Nitrogen, phosphorus limiting • Fertilization increases NPP
Growing season length • Longer season → more production • Tropical (year-round) > temperate (seasonal)
Ecosystem Comparisons:
HIGHEST NPP: • Tropical rainforests • Swamps/marshes • Coral reefs • Estuaries
LOWEST NPP: • Deserts • Tundra • Open ocean (low nutrients)
NEP Significance:
Positive NEP (NEP > 0): • Carbon sink • Accumulating organic matter • Young forest, growing • Peat bogs
Negative NEP (NEP < 0): • Carbon source • Releasing CO₂ to atmosphere • Disturbed forest, decomposing • After clear-cutting
Zero NEP (NEP = 0): • Steady state • Mature forest (climax) • Production = decomposition • No net carbon accumulation
Measurement Methods:
Harvest method • Measure biomass accumulation • Destructive • Direct measurement of NPP
Gas exchange • Measure O₂ production or CO₂ uptake • Chamber or tower measurements • Can separate GPP and respiration
Remote sensing • Satellite data (NDVI) • Large-scale estimates • Correlates with chlorophyll
Human Impacts:
• Agriculture: Maximize NPP for crops • Deforestation: Reduces GPP, can make NEP negative • Climate change: Affects all productivity measures • Nutrient pollution: Can increase NPP (eutrophication)
Key Distinctions Summary:
GPP = Energy IN (photosynthesis) NPP = Energy stored by PLANTS (after plant respiration) NEP = Energy stored by ECOSYSTEM (after all respiration)
GPP - R_plants = NPP NPP - R_heterotrophs = NEP
Describe the carbon cycle, including major reservoirs and fluxes. How have human activities altered the global carbon cycle?
The Carbon Cycle: Movement of carbon through Earth's systems (atmosphere, biosphere, hydrosphere, lithosphere).
MAJOR CARBON RESERVOIRS:
ATMOSPHERE (~800 Gt C) • CO₂ gas • CH₄ (methane) • Smallest reservoir but critical • Fast turnover
TERRESTRIAL BIOSPHERE (~2,000 Gt C) • Living biomass (plants, animals) • Soil organic matter (~1,500 Gt) • Plant biomass (~500 Gt)
OCEANS (~38,000 Gt C) • Largest active reservoir • Dissolved CO₂ • Marine organisms • Carbonate sediments • Deep ocean vs. surface ocean
FOSSIL FUELS (~4,000 Gt C) • Coal, oil, natural gas • Ancient organic matter • Locked underground
SEDIMENTARY ROCKS (>50,000,000 Gt C) • Limestone, chalk • Largest total reservoir • Very slow turnover (millions of years)
MAJOR CARBON FLUXES (Gt C/year):
NATURAL Processes:
Photosynthesis (~120 Gt/yr) • Atmosphere → Terrestrial biosphere • CO₂ + H₂O → glucose + O₂ • Removes CO₂ from atmosphere
Cellular Respiration (~120 Gt/yr) • Terrestrial biosphere → Atmosphere • Glucose + O₂ → CO₂ + H₂O • Returns CO₂ to atmosphere • Nearly balances photosynthesis
Ocean-Atmosphere Exchange (~90 Gt/yr each direction) • CO₂ dissolves in ocean • CO₂ released from ocean • Temperature-dependent • Cold water absorbs more CO₂
Ocean Photosynthesis (~50 Gt/yr) • Phytoplankton fix CO₂ • Base of marine food web
Ocean Respiration/Decomposition (~50 Gt/yr) • Returns CO₂ to ocean
Sedimentation (~0.2 Gt/yr) • Dead organisms sink • Form sediments • Very slow removal from cycle
Volcanic Emissions (~0.1 Gt/yr) • CO₂ released from Earth's interior • Slow addition to cycle
Weathering (~0.3 Gt/yr) • Chemical breakdown of rocks • CO₂ consumed • Forms carbonates
HUMAN Alterations:
FOSSIL FUEL COMBUSTION (~9 Gt C/yr) • Burning coal, oil, gas • Transfers ancient carbon to atmosphere • NEW input (not balanced) • Largest human impact
Process: Fossil fuels → Atmosphere C (solid/liquid) + O₂ → CO₂
DEFORESTATION (~1-2 Gt C/yr) • Reduces photosynthesis • Releases stored carbon • Soil carbon exposed to decomposition • Double impact: less uptake + more release
LAND USE CHANGES • Agriculture replaces forests • Reduces carbon storage • Soil degradation
CEMENT PRODUCTION (~0.5 Gt C/yr) • Heating limestone releases CO₂ • CaCO₃ → CaO + CO₂
OCEAN ACIDIFICATION • Ocean absorbs ~2.5 Gt C/yr of human emissions • CO₂ + H₂O → H₂CO₃ (carbonic acid) • Decreases pH • Harms coral reefs, shellfish
Net Result: • Human activities add ~10 Gt C/yr to atmosphere • Ocean absorbs ~2.5 Gt C/yr • Terrestrial biosphere absorbs ~2.5 Gt C/yr • Atmosphere accumulates ~5 Gt C/yr • CO₂ concentration rising (~420 ppm in 2024, was ~280 ppm pre-industrial)
Consequences:
Climate Change • CO₂ is greenhouse gas • Global warming • Weather pattern changes
Ocean Acidification • Threatens marine life • Coral bleaching • Shell formation impaired
Altered Ecosystem Function • Photosynthesis rates change • Plant growth patterns shift • Migration of species
Positive Feedbacks • Warming → permafrost thaw → CH₄ release • Warming → less ocean CO₂ uptake • Warming → increased respiration
Natural Carbon Sinks: • Oceans (largest) • Forests (especially tropical) • Soil organic matter • Peatlands
Carbon Cycle Time Scales: • Fast cycle: Atmosphere ↔ Biosphere (years to decades) • Slow cycle: Sediments ↔ Rocks (millions of years) • Human activities have accelerated the fast cycle dramatically
Key Insight: Natural carbon cycle was roughly balanced. Human activities (fossil fuels, deforestation) have unbalanced it, causing atmospheric CO₂ to rise and driving climate change.
Compare and contrast energy flow and nutrient cycling in ecosystems. Why does energy flow one-way while nutrients cycle?
ENERGY FLOW vs. NUTRIENT CYCLING:
ENERGY FLOW:
Pattern: ONE-WAY, LINEAR • Sun → Producers → Consumers → Lost as heat • Does NOT cycle • Continuously input from sun • Continuously lost as heat • Must have constant external source
Characteristics:
Laws of Thermodynamics: • 1st Law: Energy cannot be created or destroyed (only converted) • 2nd Law: Energy conversions increase entropy (disorder) → Some energy always lost as heat → Heat is most disordered form → Cannot be converted back to useful form
Visual: SUN → Plants → Herbivores → Carnivores ↓ ↓ ↓ ↓ HEAT HEAT HEAT HEAT (end) (end) (end) (end)
NUTRIENT CYCLING:
Pattern: CYCLICAL, CIRCULAR • Nutrients move between biotic and abiotic components • Continuously recycled • Same atoms used over and over • No new input needed (except small additions) • Limited supply must be shared
Characteristics:
Visual: Plants ← Soil ↓ ↑ Animals → Decomposers ↓ ↑ Waste → (recycle) ↑_______↓
KEY DIFFERENCES:
| Feature | Energy | Nutrients |
|---|---|---|
| Pattern | One-way flow | Cyclical |
| Source | Sun (external) | Earth (internal) |
| Can be recycled? | NO | YES |
| Losses | Heat (all lost) | Minimal (locked in sediments) |
| Limiting? | Rarely | Often (N, P) |
| Must be renewed? | Constantly |
WHY ENERGY FLOWS ONE-WAY:
Thermodynamics • 2nd Law: Energy quality decreases • Heat is lowest quality (high entropy) • Cannot spontaneously increase quality • Like water flowing downhill - can't flow back up
Chemical Nature • Energy stored in chemical bonds • When bonds broken, energy released as heat • Heat disperses to environment • Too diffuse to recapture
Biological Reality • No organism can use heat energy • Photosynthesis requires light, not heat • Each metabolic process loses energy as heat • Cumulative losses prevent cycling
WHY NUTRIENTS CYCLE:
Conservation of Matter • Atoms are not destroyed • Only change form/location • Carbon atom in your body was once in dinosaur, tree, etc.
Limited Supply • Fixed amount of elements on Earth • Must be reused • Otherwise life would run out
Decomposer Role • Break down dead organic matter • Release nutrients back to environment • Makes nutrients available again • Essential link in cycle
Chemical Transformations • Nutrients change chemical form • But same elements present • Example: N₂ → NH₃ → NO₃⁻ → protein → NH₃ → N₂ • Nitrogen atom conserved through cycle
IMPLICATIONS:
Ecosystem Sustainability • Energy: Need constant sun input (not sustainable without) • Nutrients: Can sustain indefinitely if cycles intact
Human Impact • Energy: Fossil fuels (one-time use, becomes heat) • Nutrients: Pollution disrupts cycles (eutrophication) • Must maintain nutrient cycles
Trophic Structure • Energy limits food chain length (runs out) • Nutrients don't limit length (cycle back)
Ecosystem Productivity • Energy: Usually abundant (from sun) • Nutrients: Often limiting (fixed supply) • Nutrient addition can increase productivity
Evolution • Organisms evolved to capture energy efficiently • Organisms evolved to recycle nutrients • Decomposers critical for nutrient cycling
EXCEPTIONS/QUALIFICATIONS:
Energy can be "stored" temporarily • Biomass, fossil fuels • But eventually all becomes heat • Not true cycling
Nutrients can be "lost" • Sedimentation (geological time) • Leaching from ecosystem • But eventually return (weathering, uplift) • Much slower than biological cycling
ANALOGY: • Energy like money you spend (gone forever) • Nutrients like money in circulation (changes hands but exists)
Key Principle: Energy and nutrients both essential, but behave fundamentally differently due to laws of thermodynamics (energy) vs. conservation of matter (nutrients)!
Explain the nitrogen cycle in detail. Why is nitrogen often a limiting nutrient despite being 78% of the atmosphere?
The Nitrogen Cycle: Movement of nitrogen through atmosphere, soil, water, and organisms.
Why Nitrogen is Limiting (Paradox): • 78% of atmosphere is N₂ gas • BUT most organisms CANNOT use N₂ directly • N≡N triple bond extremely strong • Requires huge energy to break • Only certain bacteria can fix N₂ • "Plenty" in air but "unavailable"
Usable Forms: • NH₃/NH₄⁺ (ammonia/ammonium) • NO₃⁻ (nitrate) • Organic nitrogen (proteins, nucleic acids)
Stages of Nitrogen Cycle:
NITROGEN FIXATION Converting atmospheric N₂ to usable forms
Biological fixation: • Nitrogen-fixing bacteria • N₂ → NH₃ (ammonia) • Enzyme: nitrogenase • Requires lots of ATP (energy expensive)
Types of nitrogen fixers: a) Free-living soil bacteria • Azotobacter, Clostridium • Live independently in soil
b) Symbiotic bacteria • Rhizobium in legume root nodules • Peas, beans, clover, alfalfa • Mutualism: bacteria get sugars, plant gets nitrogen
c) Cyanobacteria • Aquatic environments • Some rice paddies (Anabaena)
Abiotic fixation: • Lightning: N₂ → NOₓ • Industrial (Haber process): N₂ + H₂ → NH₃ (fertilizer) • ~50% of N fixation now human-caused!
NITRIFICATION Conversion of ammonia to nitrate (two steps)
Step 1: Ammonia → Nitrite • By Nitrosomonas bacteria • NH₄⁺ → NO₂⁻ (nitrite) • Aerobic process
Step 2: Nitrite → Nitrate • By Nitrobacter bacteria • NO₂⁻ → NO₃⁻ (nitrate) • Most usable form for plants
Both steps are chemosynthesis (bacteria get energy from oxidation)
ASSIMILATION Uptake of nitrogen by organisms
• Plants absorb NH₄⁺ and NO₃⁻ from soil • Synthesize amino acids, proteins, nucleic acids • Animals eat plants (or other animals) • Incorporate nitrogen into biomolecules • Nitrogen becomes organic nitrogen
AMMONIFICATION (Decomposition) Return of nitrogen to soil
• Decomposers break down dead organisms, waste • Proteins, nucleic acids → NH₃/NH₄⁺ • By bacteria and fungi • Releases ammonia back to soil • Can be nitrified again or used directly
DENITRIFICATION Return of nitrogen to atmosphere
• Denitrifying bacteria (Pseudomonas, Clostridium) • NO₃⁻ → NO₂⁻ → NO → N₂O → N₂ • Use nitrate as electron acceptor (anaerobic respiration) • Occurs in waterlogged, oxygen-poor soils • Completes the cycle • Farmers try to prevent this (lose nitrogen)
Cycle Summary: N₂ (atmosphere) ↓ fixation NH₃/NH₄⁺ (soil) ↓ nitrification NO₃⁻ (soil) ↓ assimilation Organic N (organisms) ↓ ammonification NH₃/NH₄⁺ (soil) ↓ denitrification N₂ (atmosphere)
Human Impacts:
EXCESS Nitrogen (Major Problem)
a) Fertilizer Production • Haber-Bosch process • Doubled available nitrogen on Earth • Runoff into waterways
b) Fossil Fuel Combustion • NOₓ emissions • Acid rain • Air pollution
c) Legume Cultivation • Increased biological fixation
Consequences of Excess:
a) Eutrophication • Excess nutrients in water • Algal blooms • Oxygen depletion (dead zones) • Fish kills
b) Groundwater Contamination • High nitrate in drinking water • Health hazard (blue baby syndrome)
c) Air Pollution • NOₓ → smog • N₂O greenhouse gas (300× stronger than CO₂)
d) Soil Acidification • Nitrification produces H⁺ • Lowers soil pH
e) Loss of Biodiversity • Favors fast-growing species • Outcompete native plants • Simplifies ecosystems
Ecological Importance:
• Nitrogen in proteins (enzymes, structural) • Nitrogen in nucleic acids (DNA, RNA) • Nitrogen in ATP, chlorophyll • Essential for all life • Often THE limiting nutrient in ecosystems
Adaptations to Low Nitrogen:
Carnivorous plants • Venus flytrap, pitcher plants • Get nitrogen from insects • Live in nutrient-poor bogs
Mycorrhizae • Fungi help plants absorb nutrients • Include nitrogen
Legume-Rhizobium symbiosis • Plants "farm" nitrogen-fixing bacteria
Agricultural Applications:
• Crop rotation with legumes • Adds nitrogen to soil naturally • Reduces fertilizer need • Example: Corn → soybeans → corn
Key Insight: Nitrogen is abundant but "locked up" in N₂. Life depends on nitrogen-fixing bacteria to convert it to usable forms. Humans have dramatically altered the nitrogen cycle through fertilizer production and fossil fuel use, causing environmental problems.
| Recycled naturally |