An ecosystem is a functional unit of nature where living organisms interact among themselves and with the surrounding physical (abiotic) environment. It includes both biotic (living) and abiotic (non-living) components that interact through energy flow and nutrient cycling.
Examples of ecosystems: a pond, a forest, a grassland, an ocean, even a small aquarium.
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Structure of an Ecosystem
- Biotic Components:
- Producers (Autotrophs): Green plants, algae, cyanobacteria — synthesise organic matter through photosynthesis or chemosynthesis.
- Consumers (Heterotrophs):
- Primary consumers (herbivores): eat plants (e.g., insects, rabbits, deer)
- Secondary consumers: eat herbivores (frogs, small fish)
- Tertiary consumers: eat secondary consumers (snakes, large fish)
- Decomposers (Saprotrophs): Bacteria and fungi — break down dead organic matter, releasing nutrients back into the environment. They are also called reducers.
- Abiotic Components:
- Climatic factors: sunlight, temperature, rainfall
- Edaphic factors: soil type, pH, mineral content
- Inorganic chemicals: CO2, O2, water, minerals
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Energy Flow in an Ecosystem
The Sun is the primary source of energy for most ecosystems (except hydrothermal vent ecosystems where chemosynthesis drives energy).
Photosynthetically Active Radiation (PAR): Only about 50% of total incident solar radiation is PAR; of that, plants capture only 2-10% for photosynthesis.
Trophic Levels:
Producers form trophic level 1 (T1), primary consumers form T2, secondary consumers T3, and so on.
Gross Primary Productivity (GPP): Total rate of photosynthesis (biomass production) by producers.
Net Primary Productivity (NPP): NPP = GPP - Respiration. This is the biomass available to consumers.
Secondary Productivity: Rate of formation of new organic matter by consumers.
Lindeman's 10% Law (1942):
Only about 10% of energy is transferred from one trophic level to the next. The remaining ~90% is lost as heat (respiration), excretion, and in unconsumed biomass.
Key formulas
This energy loss explains why food chains rarely have more than 4-5 trophic levels.
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Ecological Pyramids
1. Pyramid of Numbers: represents the number of individuals at each trophic level. Usually upright; can be inverted (e.g., a single large tree supporting many insects, many birds).
- 2. Pyramid of Biomass: represents dry weight (biomass) of organisms at each trophic level.
- Upright in most terrestrial ecosystems
- Inverted in aquatic ecosystems (phytoplankton have less biomass than zooplankton they support — due to rapid turnover rate)
3. Pyramid of Energy (productivity): Always upright — energy decreases at each successive trophic level. This pyramid is the most accurate representation of ecosystem function.
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Decomposition
Decomposition is the breakdown of dead organic matter (detritus) into simpler inorganic substances.
- 1.Steps of decomposition:
- 2.Fragmentation: Detritivores (earthworms, millipedes) break large detritus into smaller pieces
- 3.Leaching: Water soluble substances leach into soil
- 4.Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances
- 5.Humification: Accumulation of dark-coloured amorphous substance called humus (resistant to decomposition — slow decomposition)
- 6.Mineralisation: Humus is further broken down to release inorganic nutrients
- Factors affecting decomposition:
- Temperature: Low temperature (cold) and waterlogging slow decomposition significantly (tropical forests decompose faster than arctic or waterlogged forests)
- Oxygen availability: Aerobic decomposition is faster; anaerobic decomposition is slow
- Chemical composition of detritus: High lignin or cellulose content → slow decomposition; high N and soluble sugars → fast decomposition
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Nutrient Cycling (Biogeochemical Cycles)
Nutrients cycle between biotic and abiotic components. These are of two types:
1. Gaseous Cycles (reservoir: atmosphere): Carbon cycle, Nitrogen cycle. Rapid cycling.
2. Sedimentary Cycles (reservoir: earth's crust): Phosphorus cycle, Sulphur cycle. Slower cycling.
- Carbon Cycle:
- CO2 is fixed by photosynthesis (producers)
- Respiration by producers, consumers, decomposers returns CO2 to atmosphere
- Burning of fossil fuels adds CO2 to atmosphere (greenhouse effect)
- Reservoir of carbon: atmosphere, oceans (dissolved CO2), fossil fuels, living organisms
- Phosphorus Cycle:
- Phosphorus has no atmospheric form (unlike C and N)
- Rocks are the main reservoir; weathering releases phosphate (PO43-)
- Plants absorb phosphates from soil; transferred to consumers; decomposers release it back
- Leaching into water bodies → sedimentation (slow return)
- Human input: fertilisers cause phosphorus enrichment and eutrophication
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Ecosystem Services
- Services provided by ecosystems:
- Purification of air and water
- Flood and drought mitigation
- Cycling of nutrients
- Generation and preservation of soils
- Pollination of crops
- Aesthetic, cultural, and recreational benefits
The economic value of natural pollination (by insects) is estimated at over USD 160 billion per year globally.
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If producers in a grassland fix 10,000 kcal, how much energy is available to secondary consumers?
Using Lindeman's 10% law: T1 = 10,000 kcal; T2 (primary consumers) = 10% of 10,000 = 1,000 kcal; T3 (secondary consumers) = 10% of 1,000 = 100 kcal.
Explain why the pyramid of biomass is inverted in aquatic ecosystems.
In open ocean, phytoplankton (T1) have a very high turnover rate — they reproduce rapidly but are consumed just as fast, so standing biomass at any one time is LOW. Zooplankton (T2) accumulate more biomass than phytoplankton at any given moment, creating an inverted pyramid of biomass.
Why does food chain length rarely exceed 4-5 trophic levels?
Due to the 10% energy transfer rule, energy decreases exponentially from one trophic level to the next. By trophic level 4 or 5, the energy available is so small (0.01% of original productivity) that it cannot support a viable population of top predators. Energy limitation constrains chain length.
Distinguish between GPP and NPP.
GPP (Gross Primary Productivity) is the total rate of photosynthesis — total biomass synthesised per unit time per unit area. NPP (Net Primary Productivity) = GPP - Respiration losses. NPP represents the biomass actually available to primary consumers (herbivores).
What factors make decomposition faster in tropical forests than in temperate or arctic forests?
Tropical forests have high temperature (optimum for decomposer enzymes) and high moisture. Both accelerate microbial and fungal enzyme activity. Arctic/cold environments have low temperatures that slow enzyme activity; waterlogged conditions limit oxygen availability, further retarding aerobic decomposition.
What happens when phosphorus is applied in excess to agricultural fields?
Excess phosphorus leaches from fields into water bodies (rivers, lakes). This causes eutrophication — a sudden growth of algae (algal bloom) that depletes dissolved oxygen when the algae die and decompose, killing aquatic life.
Calculate NPP if GPP = 8,000 g/m2/year and respiration losses = 2,500 g/m2/year.
NPP = GPP - Respiration = 8,000 - 2,500 = 5,500 g/m2/year. This means 5,500 g of dry organic matter per square metre per year is available for consumers.
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- Key Formulas
- NPP = GPP - Respiration
- Lindeman's 10% Law: Energy at Tn+1 = 10% × Energy at Tn
- Ecological efficiency = (Energy at Tn+1 / Energy at Tn) × 100
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Common mistakes
Pyramid of energy is ALWAYS upright — never inverted. Pyramid of biomass can be inverted (aquatic). Pyramid of numbers can be inverted (tree-insect-bird). Do not confuse GPP with NPP. Decomposers (bacteria, fungi) are NOT consumers — they are a separate functional category. Phosphorus cycle has NO gaseous phase — unlike carbon and nitrogen cycles.
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Summary
Ecosystems are structured around energy flow (Sun → producers → consumers → decomposers) and nutrient cycling (biogeochemical cycles). Energy flows unidirectionally and decreases at each trophic level (~10% efficiency). Nutrients cycle between biotic and abiotic components. Decomposition by microbes closes the nutrient loop, maintaining ecosystem productivity.