Ecosystem - Class 12 Biology

Class 12 Biology | Unit X — Ecology

Chapter 14: Ecosystem

Structure • Productivity • Energy Flow • Ecological Pyramids • Nutrient Cycling • Ecosystem Services

1. Structure and Function of Ecosystem

Definition: An ecosystem is a functional unit of nature where living organisms interact with each other and with their physical environment. Term coined by A.G. Tansley (1935).

Types: Natural (forest, pond, ocean) and Man-made (crop field, aquarium). Ponds and forests are classic NCERT examples.

1.1 Components of an Ecosystem

Abiotic Components	Biotic Components
Temperature, Light, Water, Humidity, pH, Minerals, Soil	Producers (Autotrophs), Consumers (Herbivores, Carnivores, Omnivores), Decomposers (Reducers)

1.2 Types of Organisms

Producers (Autotrophs): Fix solar energy via photosynthesis (or chemosynthesis). Green plants on land; phytoplankton in water.
Consumers (Heterotrophs):
- Primary consumers (Herbivores): Eat producers. Example: Grasshopper, rabbit, deer.
- Secondary consumers (Primary carnivores): Eat herbivores. Example: Frog, fox.
- Tertiary consumers (Secondary carnivores): Example: Snake, hawk.
Decomposers (Reducers): Break down dead organic matter into inorganic substances. Examples: Bacteria and Fungi. Also called saprotrophs/saprobes. Products: CO₂, H₂O, inorganic nutrients.

⚠️ NEET Focus: Decomposers are also called reducers. They perform catabolism. Products of decomposition = inorganic nutrients + CO₂ + H₂O. This is called mineralization. Term “Ecosystem” coined by A.G. Tansley (1935).

2. Productivity

2.1 Primary Productivity

Definition: The rate of production of organic matter (biomass) by producers per unit area per unit time. Expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹.

Gross Primary Productivity (GPP): Total photosynthetic production (total fixation of solar energy into organic matter). GPP includes the organic matter used in plant respiration.
Net Primary Productivity (NPP): NPP = GPP − Respiration (R). The biomass available to consumers. NPP is the actual matter stored.
Net Primary Productivity of the Biosphere: ~170 Pg (170 × 10⁹ tonnes) of dry organic matter per year.

Ecosystem Type	GPP (kcal m⁻² yr⁻¹)	Notes
Tropical Rainforest	~9,000	Highest productivity on land
Temperate Forest	~6,000	Moderate
Grassland	~2,500	—
Desert	~200	Lowest on land
Open Ocean	~500	Low per unit area (but vast area)
Coral Reefs / Estuaries	~20,000	Highest aquatic productivity

2.2 Secondary Productivity

Rate of formation of new organic matter by consumers. Energy locked in organic matter consumed by heterotrophs minus respiratory losses.

⚠️ NEET Focus (2015, 2018, 2021): NPP = GPP − R. Highest productivity: Tropical rainforest on land; Coral reefs/Estuaries in aquatic systems. Open ocean has low productivity per unit area but contributes most globally due to its vast area.

3. Decomposition

Definition: Decomposition is the process by which decomposers (bacteria and fungi) break down complex organic matter (detritus) into simpler inorganic substances. It is essentially a catabolic process.

Raw Material: Detritus = Dead plant parts (leaves, bark, flowers, fallen fruit), dead animals, exuviae (shed skin/feathers), fecal matter.

3.1 Steps of Decomposition

Fragmentation: Breaking of detritus into smaller particles by detritivores (e.g., earthworms, millipedes, woodlice). Increases surface area for microbial action.
Leaching: Water-soluble inorganic nutrients leak into soil layers and get precipitated as unavailable salts.
Catabolism: Enzymatic degradation of detritus into simpler organic molecules by bacteria and fungi. Key step.
Humification: Formation of a dark-coloured amorphous substance called humus. Humus is highly resistant to decomposition (slow decomposition). Nutrient reserve for the soil.
Mineralisation: Humus is further degraded to release inorganic nutrients (mineral ions). These become available for plant uptake again.

📋 Factors affecting rate of decomposition:
• Temperature: Warm temperatures accelerate decomposition (enzymes work better).
• Soil moisture: Moist conditions favour decomposers.
• Chemical composition: Lignin and chitin-rich detritus decompose slowly. Nitrogen and water-soluble substances decompose fast.
• O₂ availability: Aerobic conditions promote faster decomposition.

⚠️ NEET Focus (2016, 2019, 2022): Correct order of decomposition: Fragmentation → Leaching → Catabolism → Humification → Mineralisation. Detritivores = fragmentation. Humus = resistant to decomposition = serves as nutrient reservoir. Tropical forests decompose faster (warm + moist). Tundra/Taiga decomposes slowly (cold + dry).

4. Energy Flow

Definition: Energy flow in an ecosystem is the transfer of energy from producers through successive trophic levels. It is unidirectional and non-cyclic (unlike materials which cycle).

4.1 Food Chain

A sequence of organisms where each feeds on the previous. Two main types:

Grazing Food Chain (GFC): Producers → Herbivores → Carnivores. Starts with living green plants. Example: Grass → Grasshopper → Frog → Snake → Hawk.
Detritus Food Chain (DFC): Dead organic matter (detritus) → Detritivores → Carnivores. Starts with dead organic matter. Works mainly in forests, estuaries, deep sea.

In most ecosystems, DFC carries more energy than GFC.

4.2 Food Web

Interconnected and interlinking food chains within an ecosystem. More realistic representation of feeding relationships. Greater food web complexity = greater ecosystem stability.

4.3 Ten Percent Law (Lindeman, 1942)

Lindeman's 10% Law: Only about 10% of energy at one trophic level is transferred to the next trophic level. 90% is lost as heat, respiration, excretion, and incomplete digestion. Proposed by Raymond Lindeman (1942).

Example: If producers have 1,000 kcal → Herbivores get 100 kcal → Primary carnivores get 10 kcal → Secondary carnivores get 1 kcal.

This is why food chains are typically limited to 3–4 trophic levels (insufficient energy at higher levels).

📋 Standing Crop: The total amount of living organic matter present at any trophic level at a given time. Measured as biomass or number of individuals. Standing state refers to the amount of nutrients in the abiotic environment at a given time.

⚠️ NEET Focus (2013, 2016, 2017, 2020, 2022): 10% law by Lindeman (1942). Energy flow = unidirectional; NOT cyclic. In grassland/aquatic ecosystems: GFC dominates. In forest ecosystems: DFC dominates. Food chains are rarely >4 trophic levels due to 10% rule.

5. Ecological Pyramids

Definition: An ecological pyramid is a graphical representation showing the relationship between different trophic levels in an ecosystem, based on number, biomass, or energy. Concept developed by Charles Elton (1927). Also called Eltonian Pyramids.

Type	Upright Ecosystems	Inverted Ecosystems	Never Inverted
Pyramid of Number	Grassland (grass → insects → birds → hawk)	Tree → insects → birds (parasitic: inverted). Also pond ecosystem.	—
Pyramid of Biomass	Grassland, Forest, Most terrestrial	Aquatic (open ocean/pond — phytoplankton less biomass than zooplankton)	—
Pyramid of Energy	ALL ecosystems	NEVER inverted	Always upright (energy always decreases up trophic levels)

⚠️ NEET Focus (2014, 2017, 2019, 2021): Pyramid of Energy is ALWAYS upright and NEVER inverted — most important fact! Pyramid of biomass in aquatic ecosystem = inverted (phytoplankton have low standing biomass but high turnover). Pyramid of number: tree ecosystem = inverted. Charles Elton = Eltonian pyramids.

6. Nutrient Cycling (Biogeochemical Cycles)

Definition: Nutrient cycling (biogeochemical cycling) is the cyclic movement of nutrients between biotic and abiotic components of an ecosystem. Unlike energy, nutrients are cyclic and reusable.

6.1 Carbon Cycle

Carbon reservoir = atmosphere (CO₂) and ocean. Key processes:

Photosynthesis: CO₂ → organic compounds (fixation by plants).
Respiration: Organic compounds → CO₂ (by all living organisms).
Decomposition: Dead organic matter → CO₂ (by microbes).
Combustion: Burning of fossil fuels → CO₂ to atmosphere.

About 4 × 10¹³ kg of carbon is fixed annually by terrestrial plants. ~0.03% CO₂ in atmosphere.

6.2 Phosphorus Cycle

Phosphorus main reservoir = rocks (unlike N or C where reservoir is atmosphere / ocean). Sedimentary cycle (no gaseous phase). P released by weathering of rocks → soil → plants → animals → decomposers → soil. Lost to ocean bottom (very slow return via geological uplift).

Phosphorus is often the limiting nutrient in freshwater lakes (controls algal growth; key in eutrophication).

⚠️ NEET Focus (2015, 2018, 2020): Carbon cycle reservoir = atmosphere + ocean (gaseous cycle). Phosphorus cycle reservoir = rocks (sedimentary cycle — NO gaseous phase). Phosphorus = limiting nutrient in freshwater lakes. Globally, C fixed = 4 × 10¹³ kg/yr.

6.3 Ecosystem Services

📋 Ecosystem Services (Robert Constanza et al., 1997): Functions that ecosystems perform for free:
• Production of O₂ and purification of air.
• Pollination of crops (bees, birds, bats).
• Mitigation of drought and flood.
• Generation and preservation of soil.
• Cycling of nutrients.
• Control of agricultural pests by natural predators.
• Biodiversity maintenance.
Estimated value: US$ 33 trillion/year globally.

🎓 Key NEET Questions (Previous Years)

Q1. [NEET 2022] Which of the following ecological pyramids is always upright and can never be inverted?

(a) Pyramid of biomass (b) Pyramid of number (c) Pyramid of energy (d) Both (a) and (b)

Answer: (c) Pyramid of energy is always upright in all ecosystems. Energy always decreases at each successive trophic level due to the 10% law (losses as heat and respiration).

Q2. [NEET 2020] In an open-ocean ecosystem, the pyramid of biomass is:

(a) Always upright (b) Inverted (c) Spindle-shaped (d) Irregular

Answer: (b) In open ocean, phytoplankton (producers) have very low standing biomass but high turnover rate. Zooplankton accumulate more biomass. Hence pyramid of biomass is inverted in aquatic ecosystems.

Q3. [NEET 2019] Which of the following correctly represents the flow of energy in an ecosystem?

(a) Producers → Consumers → Decomposers (b) Decomposers → Producers → Consumers (c) Consumers → Producers → Decomposers (d) Producers → Decomposers → Consumers

Answer: (a) Energy flows from Producers → Consumers → Decomposers. At each step ~90% energy is lost. This flow is unidirectional and non-cyclic.

Q4. [NEET 2017] Productivity of which ecosystem is highest?

(a) Open ocean (b) Tropical rain forest (c) Savanna (d) Coral reef and estuaries

Answer: (d) Coral reefs and estuaries have the highest productivity (~20,000 kcal m⁻² yr⁻¹) in the world. Among terrestrial ecosystems, tropical rain forest has highest productivity.

Q5. [NEET 2016] The correct sequence of steps in decomposition is:

(a) Leaching → Fragmentation → Humification → Catabolism → Mineralisation
(b) Fragmentation → Leaching → Catabolism → Humification → Mineralisation
(c) Humification → Fragmentation → Catabolism → Leaching → Mineralisation
(d) Catabolism → Leaching → Fragmentation → Humification → Mineralisation

Answer: (b) Correct order: Fragmentation → Leaching → Catabolism → Humification → Mineralisation.

Q6. [NEET 2013] 10% law of energy transfer was given by:

(a) Elton (b) Tansley (c) Lindeman (d) Odum

Answer: (c) The 10% law (Lindeman's efficiency) was proposed by Raymond Lindeman in 1942. Only ~10% of energy passes to the next trophic level.

💡 Rapid Revision — Key Facts

“Ecosystem” coined by: A.G. Tansley (1935)
10% law: Raymond Lindeman (1942)
Ecological pyramids: Charles Elton (1927)
NPP = GPP − Respiration
Pyramid of ENERGY = ALWAYS upright, never inverted
Pyramid of biomass inverted in: Aquatic (ocean/pond)
Highest productivity: Coral reefs/Estuaries (~20,000 kcal m⁻² yr⁻¹)
DFC dominates in: Forest ecosystems | GFC dominates in: Grassland/Aquatic
Decomposition steps: F-L-C-H-M (Frag, Leach, Catabol, Humif, Mineral)
Carbon cycle reservoir: Atmosphere + Ocean | Phosphorus: Rocks
Ecosystem services — estimated at: US$ 33 trillion/year

NCERT Solutions - Ecosystem - Class 12

CLASS 12 BIOLOGY | NCERT SOLUTIONS

Chapter 14 — Ecosystem

All NCERT Exercise Questions with Detailed Solutions

📋 Note: All questions from NCERT Class 12 Biology Chapter 14 Exercise. Answers as per NCERT and CBSE marking scheme.

NCERT Exercise Questions & Solutions

1 MarkQ1. Fill in the blanks: (a) Plants are called ______ because they fix carbon dioxide. (b) In an ecosystem dominated by trees, the pyramid of numbers would be ______.

✓ Answer
(a) Plants are called producers (autotrophs) because they fix carbon dioxide via photosynthesis, converting solar energy into organic matter.
(b) In an ecosystem dominated by trees (tree ecosystem), the pyramid of numbers would be inverted because a single large tree (producer) supports thousands of insects/parasites.

2 MarksQ2. Define ecological pyramids and describe with examples, pyramids of number and biomass.

✓ Answer
Ecological Pyramid: A graphical representation of the relationship between different trophic levels of an ecosystem, showing the number, biomass, or energy at each level. Concept by Charles Elton (1927).

Pyramid of Number: Represents the total number of organisms at each trophic level.
• Upright — Grassland: Grass (millions) > Insects (thousands) > Frogs (hundreds) > Snakes > Hawks.
• Inverted — Tree ecosystem: 1 tree supports many insects which support fewer birds.

Pyramid of Biomass: Represents total dry weight (biomass) of organisms at each level.
• Upright — Grassland/Forest (producers have most biomass).
• Inverted — Aquatic/Open ocean (phytoplankton have lower standing biomass than zooplankton they support, due to rapid turnover).

3 MarksQ3. What is primary productivity? Give brief description of factors that affect primary productivity.

✓ Answer
Primary Productivity: The rate of production of organic matter (biomass) by producers per unit area per unit time. Expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹.
• GPP = Total rate of photosynthesis.
• NPP = GPP − Respiration (R). Biomass available to consumers.

Factors affecting primary productivity:

Light availability: More light = more photosynthesis = higher productivity. Tropical regions receive more light year-round.
Water / Moisture: Essential for photosynthesis. Aquatic ecosystems and rainforests have high productivity.
Temperature: Optimal temperatures (20–35°C) enhance enzyme activity and photosynthetic rate.
Nutrient availability: Nitrogen, phosphorus (limiting nutrients) affect plant growth and productivity. Estuaries and coral reefs (high nutrients) have highest productivity.
CO₂ concentration: Higher CO₂ can increase photosynthesis up to a point (CO₂ fertilization effect).

3 MarksQ4. Define decomposition and describe the process and products of decomposition.

✓ Answer
Decomposition: The catabolic process by which decomposers (bacteria and fungi) break down complex organic matter (detritus) into simpler inorganic substances.

Process (5 steps in order):

Fragmentation: Physical breakdown of detritus into smaller particles by detritivores (earthworms, mites, millipedes) increasing surface area.
Leaching: Water-soluble nutrients leach down into soil layers and get deposited as unavailable salts.
Catabolism: Enzymatic degradation of detritus by bacteria and fungi into simpler organic molecules.
Humification: Accumulation of dark, amorphous, colloidal substance called humus which is resistant to microbial action.
Mineralisation: Slow conversion of humus into inorganic nutrients (mineral ions: N, P, K) making them available for plant uptake.

Products of decomposition: CO₂, H₂O, inorganic mineral salts, and humus.

5 MarksQ5. Give an account of energy flow in an ecosystem.

✓ Answer
Energy flow in an ecosystem is unidirectional and non-cyclic. Solar energy is the ultimate source.

Pathway:

Sun → Producers (photosynthesis): Only ~1–5% of incoming solar radiation is used for photosynthesis; rest reflected or used for heating.
Producers → Primary Consumers (Herbivores): Only 10% of producer energy is transferred.
Herbivores → Primary Carnivores: 10% of herbivore energy transferred.
Primary → Secondary → Tertiary Carnivores: continues with 10% transfer each time.

Lindeman's 10% Law (1942): About 90% energy is lost as heat, respiration, excretion, and undigested food at each trophic level. Only 10% passes to the next level. Hence food chains are limited to 3–4 trophic levels.

Example: 1,000 kcal (Producers) → 100 kcal (Herbivores) → 10 kcal (Primary carnivores) → 1 kcal (Secondary carnivores).

Types of Food chains:

GFC: Starts with living plants. Dominates in grassland and aquatic ecosystems.
DFC: Starts with dead organic matter. Dominates in forest ecosystems. Carries more energy overall in most ecosystems.

3 MarksQ6. Describe briefly nutrient cycling in nature and explain the carbon cycle.

✓ Answer
Nutrient Cycling (Biogeochemical Cycle): The cyclic movement of nutrients between biotic and abiotic components of an ecosystem. Unlike energy, nutrients are reusable and cyclic. Two main types: Gaseous (atmospheric reservoir) and Sedimentary (earth's crust/soil reservoir).

Carbon Cycle:

Carbon reservoir: Atmosphere (0.03% CO₂) and ocean.
Fixation: Plants absorb CO₂ via photosynthesis → stored as organic molecules (~4×10¹³ kg/yr fixed).
Return to atmosphere: by respiration (plants, animals, decomposers), decomposition of dead matter, combustion of fossil fuels, volcanism.
Ocean as reservoir: CO₂ dissolves in seawater → bicarbonate ions → shells of marine organisms → limestone rocks.
Human impact: Burning fossil fuels + deforestation → increased atmospheric CO₂ → enhanced greenhouse effect → global warming.

3 MarksQ7. What are ecosystem services? Give examples.

✓ Answer
Ecosystem Services: All the benefits provided by ecosystems to human societies and other organisms, for free, as a result of natural functioning. Estimated value: US$ 33 trillion/year globally (Constanza et al., 1997).

Examples:

Production and maintenance of oxygen in the atmosphere (photosynthesis).
Pollination of crops by bees, birds, bats — essential for food production.
Nutrient cycling — making nutrients available to plants and other organisms.
Water purification by wetlands and riparian forests.
Flood and drought mitigation by forests and wetlands.
Pest control by natural predators of agricultural pests.
Soil formation and fertility maintenance by decomposers.
Climate regulation — forests absorb CO₂, reduce greenhouse effect.

Facts Capsule - Ecosystem - Class 12

CLASS 12 BIOLOGY | NEET RAPID CAPSULE

Facts & High-Yield Points

Chapter 14 — Ecosystem | 28 Key Facts for NEET

🌿 Structure

FACT #01 — Ecosystem Term

Coined by A.G. Tansley (1935). A functional unit where biotic + abiotic components interact. Natural (forest, pond) and Man-made (crop field, aquarium).

FACT #02 — Decomposers

Also called reducers / saprotrophs. Examples: bacteria + fungi. They break detritus by catabolism. Products = CO₂ + H₂O + inorganic nutrients (mineralisation).

FACT #03 — Detritus

Dead plant parts (leaves, bark, flowers, fruit), dead animals, exuviae (shed skin), fecal matter = detritus. Raw material for decomposition.

☀️ Productivity

FACT #04 — GPP vs NPP

GPP = total photosynthesis. NPP = GPP − R (Respiration). NPP = biomass available to consumers. Net biosphere productivity = ~170 Pg dry matter/year.

FACT #05 — Highest Productivity

Coral reefs & Estuaries: ~20,000 kcal m⁻² yr⁻¹ (highest anywhere). Tropical rainforest: ~9,000 (highest land). Open ocean: ~500 (low/unit area).

FACT #06 — Factors Affecting Productivity

Light, Water, Temperature, Nutrient availability (N, P), CO₂ concentration. P = limiting nutrient in freshwater. N = limiting in marine systems.

🟢 Decomposition Steps

FACT #07 — Sequence (F-L-C-H-M)

Fragmentation → Leaching → Catabolism → Humification → Mineralisation. Only this order is correct for NEET.

FACT #08 — Humification

Humus = dark, amorphous, colloidal, resistant to decomposition. Acts as nutrient reservoir. Formed by humification. Not rapidly decomposed.

FACT #09 — Factors Affecting Rate

Slow: cold, dry, lignin/chitin-rich material. Fast: warm, moist, N-rich material. Tropical forest = fast decomp. Tundra/Taiga = peat forms (very slow = cold + dry).

FACT #10 — Detritivores

Earthworms, millipedes, mites, woodlice = detritivores. They do fragmentation (physical breakdown), not chemical. Bacteria / fungi do chemical catabolism.

⚡ Energy Flow

FACT #11 — 10% Law

Lindeman's 10% law (1942): Only 10% energy transferred to next trophic level. 90% lost as heat, respiration, excretion, egestion. Food chains: max 3–4 trophic levels.

FACT #12 — Energy Direction

Energy flow = UNIDIRECTIONAL (Sun → Producers → Consumers → Decomposers). NOT cyclic. Nutrients are cyclic; energy is NOT. Key NEET distinction.

FACT #13 — GFC vs DFC

GFC: starts with living plants. Dominant in grassland & aquatic ecosystems. DFC: starts with dead matter. Dominant in forest ecosystems. DFC carries more energy in most ecosystems.

FACT #14 — Solar Energy Use

Only 1–5% of total solar radiation reaching earth is used in photosynthesis. Rest reflected or used for heating. Of this, only ~10% passes to herbivores.

📈 Ecological Pyramids

FACT #15 — Eltonian Pyramids

Concept by Charles Elton (1927). Three types: Number, Biomass, Energy. Most useful = Energy pyramid (overcomes limitations of others).

FACT #16 — Energy Pyramid

ALWAYS upright, NEVER inverted in ANY ecosystem. The most important pyramid fact for NEET. Represents energy AT each trophic level (kcal m⁻² yr⁻¹).

FACT #17 — Biomass Pyramid

Upright in grassland + forest. Inverted in open ocean/aquatic: phytoplankton (low standing biomass, high turnover) < zooplankton (higher accumulated biomass).

FACT #18 — Number Pyramid

Upright in grassland. Inverted in tree ecosystem (1 tree → many insects → fewer birds). Also inverted in some parasitic food chains.

♻️ Nutrient Cycling

FACT #19 — Carbon Cycle

Reservoir = atmosphere + ocean (gaseous cycle). ~4 × 10¹³ kg CO₂ fixed/year. CO₂ in atmosphere = 0.03%. Ocean = major carbon sink (as bicarbonates).

FACT #20 — Phosphorus Cycle

Reservoir = rocks (sedimentary cycle, NO gaseous phase). Released by weathering. Limiting nutrient in freshwater lakes. Key in eutrophication. No atmospheric form.

FACT #21 — Nitrogen Cycle

Reservoir = atmosphere (78% N₂). Fixed by Rhizobium, Azotobacter, Anabaena + lightning. Denitrification by Pseudomonas returns N₂ to atmosphere. Gaseous cycle.

FACT #22 — Standing Crop vs State

Standing crop = total amount of living matter at a trophic level at a given time (as biomass or individuals). Standing state = amount of nutrients (abiotic) at a given time.

🧠 Mnemonics — Remember Fast

Decomposition Steps: “Funny Lions Can Hunt Mice” Fragmentation, Leaching, Catabolism, Humification, Mineralisation.

Pyramid of Energy: “Energy is EMpowered” Energy pyramid is Always Upright in Every (EM = Every) ecosystem. NEVER inverted. Others can be inverted.

Nutrient Cycles: “CNP Reservoirs” Carbon = Atmosphere + Ocean. Nitrogen = Atmosphere. Phosphorus = Rocks (no gas).

Pioneers of Ecosystem: “TLE” Tansley = Ecosystem (1935). Lindeman = 10% law (1942). Elton = Pyramids (1927).

📊 Ecological Pyramids — Summary

Pyramid Type	Grassland	Forest/Tree	Aquatic/Ocean
Number	Upright	Inverted (tree→insects)	Upright (usually)
Biomass	Upright	Upright	Inverted
Energy	Upright	Upright	Upright

🔢 Critical Numbers — Never Forget

10% — Lindeman's energy transfer 1942 — Lindeman's 10% law 1935 — Tansley coins “Ecosystem” 1927 — Elton's ecological pyramids 1–5% — solar energy used in photosynthesis 3–4 — typical food chain trophic levels ~20,000 — kcal/m²/yr (coral reef, highest) ~9,000 — kcal/m²/yr (tropical rainforest) 0.03% — CO₂ in atmosphere 4 × 10¹³ kg — C fixed/year 170 Pg — net biosphere productivity/yr US$ 33 trillion — ecosystem services/yr

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