Ecosystem: structure and function
An ecosystem is a functional unit of nature where living organisms (biotic components) interact among themselves and with the surrounding physical environment (abiotic components), exchanging materials and energy. Ecosystems can range from a small pond or a patch of forest to an entire ocean or a continent.
Components of an ecosystem
- Abiotic components: temperature, water, sunlight, soil, wind, inorganic salts, and atmospheric gases. These provide the physical and chemical environment.
- Biotic components:
Functions of an ecosystem
Key distinction for NEET
Energy flow is UNIDIRECTIONAL and NON-CYCLIC (energy flows from sun to producers to consumers to decomposers; lost as heat, it cannot be recycled). Nutrient flow is CYCLIC (materials circulate between biotic and abiotic components). This distinction is tested repeatedly in NEET.
Productivity: GPP, NPP, and secondary productivity
Productivity in an ecosystem refers to the rate of production of organic matter (biomass) per unit area per unit time. It is usually expressed in units of g/m2/yr (dry weight) or kcal/m2/yr.
Primary productivity
Primary productivity is the rate at which producers (autotrophs) fix solar energy into organic molecules through photosynthesis or chemosynthesis.
Definitions you must know
- Gross Primary Productivity (GPP): the total rate of organic matter production by autotrophs via photosynthesis. It includes ALL energy fixed, even what the plant uses for its own respiration.
- Net Primary Productivity (NPP): NPP = GPP minus R (respiration by producers). This is the energy actually stored in plant biomass and available to consumers (herbivores).
Global productivity data (NEET numbers)
- NPP of the whole biosphere: approximately 170 billion tonnes (dry weight) per year
- Terrestrial ecosystems contribute about 55% and marine/aquatic ecosystems about 45%
- Highest terrestrial NPP per unit area: tropical rainforests (estuaries and swamps also very high)
- Lowest NPP: open oceans (vast area but sparse nutrients and phytoplankton)
Standing crop and standing state
Standing crop is the total amount of living organic matter in an ecosystem at a particular time. Standing state is the amount of inorganic nutrients present in the soil and water at any given time. Both change with season, age of ecosystem, and disturbance.
Decomposition: steps and factors
Decomposition is the process by which decomposers (bacteria and fungi) and detritivores break down dead organic matter (detritus) into simpler inorganic substances. This releases nutrients stored in dead organisms back into the environment, making them available for producers. Without decomposition, nutrients would remain locked up in dead matter and ecosystems would stop functioning.
What is detritus?
Detritus is dead organic matter consisting of fallen leaves, twigs, animal dung, dead animals, excretory products, and other organic debris. It is the starting material for the detritus food chain and decomposition.
Steps of decomposition (in order)
Mnemonic: F-L-C-H-M
1. Fragmentation
Agent: Detritivores (earthworms, millipedes, woodlice)
Physical breakdown of detritus into smaller pieces. Increases surface area available for microbial attack.
2. Leaching
Agent: Water
Water-soluble nutrients (nitrates, phosphates, sugars) move downward into the soil. These are lost from the detritus and enter the soil solution where they can be absorbed by plant roots.
3. Catabolism
Agent: Bacteria and fungi (secrete hydrolytic and oxidative enzymes)
Enzymatic degradation of detritus into simple organic molecules (sugars, amino acids, fatty acids) and then into inorganic substances. Exoenzymatic action.
4. Humification
Agent: Microorganisms
Formation of humus, a dark-coloured colloidal material that is resistant to further microbial decomposition. Humus improves soil water-holding capacity, fertility, and texture. It is the most stable form of soil organic matter.
5. Mineralisation
Agent: Microorganisms (specific bacteria)
Further degradation of humus to release inorganic nutrients (NH4+, NO3-, PO4 3-, SO4 2-, K+, Ca2+). These are now in forms that plants can directly absorb.
Factors affecting decomposition rate
- Temperature: higher temperature increases enzymatic activity of decomposers. Decomposition is fastest in warm tropical climates.
- Moisture: water is essential for microbial metabolism. Decomposition is rapid in moist environments; slow in dry deserts or waterlogged anaerobic soils.
- Chemical composition of detritus: detritus rich in nitrogen and simple sugars decomposes rapidly. Detritus rich in chitin, lignin, and cellulose decomposes very slowly (woody material takes years).
NEET exam tip
Humus (humification step) is specifically described as "resistant to further decomposition" in NCERT. This phrase appears exactly in NEET questions. Humus acts as a nutrient reservoir that releases nutrients slowly, preventing nutrient leaching from soil.
Energy flow and the 10% law
Energy flows through ecosystems in one direction, from the sun through producers to consumers to decomposers. Unlike nutrients, energy cannot be recycled. At each trophic level, a large fraction of the available energy is lost as heat through metabolic processes (respiration), and only a small fraction is stored in new biomass.
Trophic levels
- T1 (First trophic level): Producers (green plants, algae, cyanobacteria). All energy ultimately enters the food chain through them.
- T2 (Second trophic level): Primary consumers (herbivores: deer, rabbit, insect). Feed directly on producers.
- T3 (Third trophic level): Secondary consumers (carnivores: frog, small fish). Feed on primary consumers.
10% Law (Lindemann, 1942)
Only 10% of the energy stored at one trophic level is transferred to the next trophic level. The remaining 90% is lost through: respiratory heat loss during metabolic reactions, energy going to decomposers via urine/feces/dead tissue, and energy not consumed.
Example: T1 = 1,000,000 kcal; T2 = 100,000 kcal; T3 = 10,000 kcal; T4 = 1,000 kcal
Why food chains are short (4 to 5 levels maximum)
Because of the 10% law, very little energy remains by the fifth trophic level. A top predator at T5 would receive only 0.001% of the energy originally fixed by producers. This makes it energetically impractical to support a fifth or sixth level carnivore in most ecosystems.
Energy flow: 10% law (Lindemann's efficiency)
Set the producer energy and transfer efficiency to see how energy diminishes at each trophic level. The 10% law means food chains rarely exceed 4 trophic levels.
Producer energy (T1): 10,000 kcal
Transfer efficiency: 10%
Producers (T1)
Plants / Algae
10,000 kcal
Primary consumers (T2)
Herbivores
1,000 kcal
10.0% of T1
Lost: 9,000 kcal
Secondary consumers (T3)
Carnivores
100 kcal
10.0% of T2
Lost: 900 kcal
Tertiary consumers (T4)
Top carnivores
10 kcal
10.0% of T3
Lost: 90 kcal
NEET key facts: energy flow
- 10% law (Lindemann, 1942): only 10% of energy at one trophic level passes to the next
- Remaining 90% is lost as heat (respiration) or goes to decomposers
- Energy flow is UNIDIRECTIONAL and NON-CYCLIC (unlike nutrients which cycle)
- Food chains rarely exceed 4-5 trophic levels because so little energy remains
- Grazing food chain: living plant → herbivore → carnivore
- Detritus food chain: dead organic matter → decomposers → detritivores
Try this
- With 10,000 kcal at T1 and 10% efficiency: T2 gets 1,000 kcal; T3 gets 100 kcal; T4 gets just 10 kcal. This is why humans eating plants can support 10x more people than humans eating cattle.
- Raise efficiency to 20%: this represents some aquatic ecosystems where transfer is more efficient. Reduce to 5% to see why some ecosystems have very short food chains.
Food chains, food webs, and trophic structure
Grazing food chain (GFC)
The grazing food chain begins with living green plants (producers) and passes through herbivores (primary consumers) and then carnivores. Example: Grass → Deer → Tiger. In most terrestrial ecosystems, a significant portion of energy flows through the GFC.
Detritus food chain (DFC)
The detritus food chain begins with dead organic matter (detritus) and passes through detritivores (earthworms, millipedes) and decomposers (bacteria, fungi) to top carnivores. Example: Leaf litter → Earthworm → Thrush → Hawk. In most forest and aquatic ecosystems, more energy flows through the DFC than the GFC.
Connecting the two food chains
Decomposers connect the grazing and detritus food chains. Dead matter from any organism in the GFC enters the DFC through decomposers. The two chains are interconnected within the same ecosystem, forming a food web rather than isolated linear chains.
Food web
A food web is an interconnected network of multiple overlapping food chains within an ecosystem. Most organisms feed on more than one species and are eaten by more than one predator, creating a complex web of energy flow. Food webs are more realistic representations of energy flow than simple linear food chains. The stability of an ecosystem depends partly on the complexity of its food web.
Ecological pyramids
An ecological pyramid is a graphical representation of the trophic structure and function of an ecosystem. The base represents the lowest trophic level (producers) and the apex represents the highest trophic level. Three types are used depending on what is being measured.
Types of ecological pyramids
- Pyramid of number: shows the number of individual organisms at each trophic level. Can be upright (grassland: millions of grass plants support thousands of insects) or inverted (parasitic food chain: one tree supports thousands of insects which support millions of parasites).
- Pyramid of biomass: shows the total mass of living matter (dry weight per unit area) at each trophic level. Usually upright in terrestrial ecosystems (plant biomass far exceeds animal biomass). Inverted in marine ecosystems: phytoplankton have low standing biomass at any moment (they reproduce so fast and are consumed so quickly) while zooplankton accumulate more biomass.
Critical NEET fact
The pyramid of energy is ALWAYS upright. It is the only pyramid that gives a true picture of ecosystem functioning because it accounts for the rate of energy flow (not just standing stock). All other pyramids (number, biomass) can be inverted depending on the ecosystem type.
Ecological pyramids: number, biomass, and energy
Switch between pyramid type and ecosystem to see which pyramids can be inverted and why. The energy pyramid is always upright.
Pyramid type:
Ecosystem:
T1
Grasses (millions)
T2
Insects (thousands)
T3
Frogs (hundreds)
T4
Hawks (few)
Grassland: upright pyramid of number. Producers (grasses) are most numerous.
NEET key: which pyramids can be inverted?
| Pyramid type | Can be inverted? | Example |
|---|---|---|
| Number | YES | Parasitic chain (tree → insects → parasites) |
| Biomass | YES | Marine (phytoplankton < zooplankton standing crop) |
| Energy | NEVER | Always upright; 10% law ensures energy decreases |
Try this
- Select "Pyramid of energy" — then try all three ecosystems. It stays upright every time. This is the most-tested fact in NEET about ecological pyramids.
- Select "Pyramid of biomass" and "Marine": phytoplankton have LOW standing biomass because they are consumed faster than they accumulate — hence the inverted pyramid.
Carbon cycle
Carbon is the backbone of all organic molecules and is cycled between the biotic and abiotic components of ecosystems through the carbon cycle. It is a gaseous cycle with the atmosphere as its main reservoir.
Main processes in the carbon cycle
- Photosynthesis (Carbon fixation): autotrophs (plants, algae, cyanobacteria) fix atmospheric CO2 into organic carbon compounds. About 4 x 10^13 kg CO2 per year is fixed globally.
- Respiration: all living organisms release CO2 via cellular respiration. This is the primary biotic return pathway.
- Decomposition: dead organic matter is broken down by decomposers, releasing CO2. This is a major pathway returning carbon from organisms to atmosphere.
- Combustion: burning fossil fuels (coal, petroleum, natural gas) releases ancient stored carbon as CO2. Forest fires also release carbon. Human combustion is the primary cause of rising atmospheric CO2 (from pre-industrial 280 ppm to current ~420 ppm).
Reservoirs of carbon
Phosphorus cycle
Phosphorus is an essential element in ATP, ADP, nucleic acids (DNA, RNA), phospholipids in cell membranes, and bone (calcium phosphate). Unlike carbon and nitrogen, phosphorus has NO significant gaseous phase, making it a sedimentary cycle. The main reservoir is phosphate rock (apatite) in the Earth's crust.
Steps in the phosphorus cycle
- Weathering: phosphate rocks are slowly dissolved by rain and chemical weathering, releasing inorganic phosphate ions (H2PO4-, HPO4 2-) into soil and water. This is the primary entry point of phosphorus into ecosystems.
- Plant uptake: plants absorb inorganic phosphate from soil through root hairs. Mycorrhizal fungi greatly enhance phosphate absorption by extending the effective root surface. Algae in aquatic systems absorb dissolved phosphate directly.
- Consumer transfer: herbivores acquire phosphorus by eating plants; carnivores acquire it from prey. Phosphorus is stored in bones (calcium phosphate), teeth, shells, and within every cell (ATP, nucleic acids).
- Decomposition (mineralisation): dead organisms and wastes are broken down by decomposers using phosphatase enzymes. Organic phosphorus is converted to inorganic phosphate and returned to soil.
Key NEET point
Phosphorus cycle = sedimentary cycle (no atmospheric phase). The phosphorus cycle is the SLOWEST of all nutrient cycles because of the long geological time required to form phosphate rocks. This contrasts with the carbon cycle (gaseous; rapid atmospheric reservoir) and nitrogen cycle (gaseous; N2 in atmosphere is the main reservoir).
Nutrient cycles: carbon (gaseous) vs phosphorus (sedimentary)
Click each step of the carbon or phosphorus cycle to see the process, equation, and agents involved. Understand why carbon is a gaseous cycle and phosphorus is a sedimentary cycle.
Gaseous cycle (atmospheric reservoir)
Atmospheric CO2 is the reservoir. Photosynthesis, respiration, decomposition, and combustion are the main processes. Oceans are the largest carbon sink.
Click a step to explore:
NEET comparison: carbon vs phosphorus
| Feature | Carbon cycle | Phosphorus cycle |
|---|---|---|
| Type | Gaseous cycle | Sedimentary cycle |
| Main reservoir | Atmosphere (CO2), oceans | Phosphate rocks, soil |
| Atmospheric phase | YES (CO2, CH4) | NO (no gaseous form) |
| Key process in | Photosynthesis, respiration | Weathering, decomposition |
| Speed | Relatively fast | Slowest of nutrient cycles |
| Human impact | Fossil fuel CO2 rise | Mining, fertiliser use, eutrophication |
Try this
- The key NEET distinction: carbon cycle = gaseous (has atmospheric CO2); phosphorus cycle = sedimentary (no atmospheric form). Always asked in NEET.
- Click "Fossilisation" in the carbon cycle. Fossil fuels are ancient stored carbon. Burning them releases this carbon in decades, not millions of years, causing the greenhouse effect.
Ecosystem services
Ecosystem services are the benefits that human societies obtain from functioning ecosystems. These are often unpriced and taken for granted but are essential for human well-being and survival. The estimated economic value of all ecosystem services globally is over USD 33 trillion per year.
Provisioning services
- Food (crops, livestock, fish, wild plants)
- Fresh water
- Timber and fibre
- Medicinal plants and drugs
- Genetic resources
Regulating services
- Climate regulation (carbon sequestration by forests)
- Flood control (wetlands, mangroves)
- Disease regulation (predators control vectors)
- Water purification (wetlands filter pollutants)
- Pollination by insects and other animals
Cultural services
- Recreation and ecotourism
- Aesthetic values (scenic beauty)
- Spiritual and religious significance
- Educational and scientific value
Supporting services
- Nutrient cycling (N, P, C cycles)
- Soil formation (decomposition, weathering)
- Primary production (foundation of all food chains)
- Water cycle maintenance (evapotranspiration)
- Oxygen production (photosynthesis)
Worked problems
NEET-style problem · 10% law calculation
Question
Food chain: Grass (T1) → Rabbit (T2) → Fox (T3) → Eagle (T4)
Solution
Apply the 10% law (Lindemann efficiency) at each trophic level transfer:
T2 (Rabbit) = 10% of 80,000 = 8,000 kcal
T3 (Fox) = 10% of 8,000 = 800 kcal
T4 (Eagle) = 10% of 800 = 80 kcal
NEET-style problem · NPP calculation
Question
Solution
NPP = GPP minus Respiration (R)
NEET-style problem · Ecological pyramid identification
Question
Solution
This is a classic marine ecosystem example from NCERT:
- Pyramid of biomass (standing crop): Phytoplankton (5 g/m2) is LESS than Zooplankton (21 g/m2). This is an inverted pyramid of biomass. Reason: Phytoplankton reproduce very rapidly (short generation time) and are consumed continuously by zooplankton. At any snapshot in time, the standing biomass of phytoplankton is low even though their annual productivity is high.
Key insight: the pyramid of energy gives a true picture; the pyramid of biomass can mislead when production rate (not standing crop) is the relevant quantity.
Ecosystem NEET quiz: 12 questions
12-question scored quiz covering ecological pyramids, 10% law, GPP vs NPP, decomposition steps, energy flow, nutrient cycling, and ecosystem services.
Question 1 of 12
Score: 0
Which of the following ecological pyramids is ALWAYS upright, without exception?
Pyramid of number
Pyramid of biomass
Pyramid of energy
Both biomass and number
Quick-recall cheat sheet
Productivity
- GPP = total photosynthesis; NPP = GPP minus R; NPP available to consumers
- NPP of biosphere = 170 billion tonnes/yr
- Highest terrestrial NPP: tropical rainforests
- Secondary productivity = energy stored by consumers
Decomposition (F-L-C-H-M)
- Fragmentation by detritivores (earthworms)
- Leaching by water (nutrients into soil)
- Catabolism by microbial enzymes
- Humification forms dark, resistant humus
- Mineralisation releases inorganic nutrients
- Fastest: warm and moist; slowest: cold and dry
Energy flow
- Unidirectional and non-cyclic (unlike nutrients)
- 10% law (Lindemann, 1942): only 10% transferred up
- Grazing food chain: starts with living plants
- Detritus food chain: starts with dead organic matter
- Food chains limited to 4 to 5 levels due to energy loss
Ecological pyramids
- Pyramid of energy: ALWAYS UPRIGHT
- Pyramid of number: can be inverted (parasitic chain)
- Pyramid of biomass: can be inverted (marine)
- Marine inverted biomass: phytoplankton < zooplankton standing crop
- Energy pyramid is the most accurate representation
Carbon vs phosphorus cycle
- Carbon: gaseous cycle; atmospheric CO2 is reservoir
- Phosphorus: sedimentary cycle; NO atmospheric phase
- Carbon released by: respiration, decomposition, combustion
- Phosphorus enters via: rock weathering
- Phosphorus cycle: SLOWEST of all nutrient cycles
Ecosystem services
- Provisioning: food, water, timber, medicine
- Regulating: climate, flood, disease, pollination
- Cultural: recreation, aesthetic, spiritual
- Supporting: nutrient cycling, soil formation, O2 production
- Global value: >USD 33 trillion per year
Frequently asked questions
How often does Ecosystem appear in NEET?
Ecosystem appears in almost every NEET paper, contributing 3 to 5 questions. High-yield topics are ecological pyramids (especially which pyramid can be inverted), the 10% law, GPP vs NPP, decomposition steps, and nutrient cycling. NEET 2023 had 4 questions from this chapter.
What is the difference between GPP and NPP?
Gross Primary Productivity (GPP) is the total rate of organic matter production by autotrophs through photosynthesis. Net Primary Productivity (NPP) is the energy available to consumers after the producers have used some for their own respiration. NPP = GPP minus Respiration (R). NPP represents the actual biomass available to herbivores. Tropical rainforests have the highest NPP on land; open ocean has the lowest per unit area.
What are the steps of decomposition in the correct order?
Decomposition has 5 steps: (1) Fragmentation: detritivores (earthworms, millipedes, fungi) break detritus into smaller pieces, increasing surface area. (2) Leaching: water-soluble nutrients move into the soil. (3) Catabolism: bacteria and fungi secrete enzymes to degrade detritus into simpler molecules. (4) Humification: formation of dark, colloidal humus that is resistant to further microbial breakdown. (5) Mineralisation: further degradation of humus releases inorganic nutrients into the soil. Remember: F-L-C-H-M (Fragmentation, Leaching, Catabolism, Humification, Mineralisation).
Why is the pyramid of energy always upright but pyramids of biomass and number can be inverted?
The pyramid of energy is always upright because energy always decreases from one trophic level to the next due to the 10% law. You cannot get more energy at a higher level than was present at the lower level. Pyramids of biomass and number can be inverted in specific ecosystems. Example: In a tree-insect-parasites food chain, one large tree (low number, high biomass) supports thousands of insects (high number), which support even more parasites. So the pyramid of number becomes inverted. In a marine ecosystem, phytoplankton (reproduce rapidly, low standing biomass at any moment) support zooplankton (higher standing biomass), giving an inverted pyramid of biomass.
What is the 10% law and who proposed it?
The 10% law was proposed by Raymond Lindemann (1942). It states that only 10% of energy at one trophic level is transferred to the next trophic level. The remaining 90% is lost as heat, used in respiration, or goes to decomposers. Example: If producers have 1000 kcal, primary consumers get 100 kcal, secondary consumers get 10 kcal, tertiary consumers get 1 kcal. This is why food chains are typically limited to 4 to 5 trophic levels.
Why does the phosphorus cycle have no atmospheric phase but the carbon cycle does?
Phosphorus exists as phosphate ions in soil and rock, not in a volatile gaseous form. So its cycle is called a sedimentary cycle: phosphate is released by rock weathering, absorbed by plants, passed through food chains, released by decomposition, and eventually returns to rocks/sediments. Carbon, on the other hand, exists as CO2 in the atmosphere, and plants fix CO2 via photosynthesis while all organisms release it via respiration, making it a gaseous cycle with an atmospheric reservoir.
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