Ecosystem Functions
How ecosystems transform energy, cycle matter, and organise themselves over time.
2.1 Energy Flow & Lindeman's 10% Law +2 Marks 2026 Hot
All ecosystem functions are ultimately driven by energy. Unlike nutrients, energy flows through ecosystems in one direction — it is not recycled. Solar energy enters via photosynthesis, is stored in organic molecules, and is progressively lost as heat at each trophic transfer, following the second law of thermodynamics.
Formula: Ecological Efficiency = (Energy at Tn+1 / Energy at Tn) × 100 ≈ 10%
Each upward step transfers only 10% of energy · Energy flow is unidirectional and non-recyclable · Heat lost is irreversible (2nd Law of Thermodynamics)
The GPP (Gross Primary Productivity) is the total rate of photosynthesis, including the respiration used by producers. NPP (Net Primary Productivity) = GPP − Respiration. NPP represents the energy actually available to herbivores and is the most ecologically important productivity measure. Tropical rainforests have the highest terrestrial NPP; open oceans have low per-unit NPP but contribute enormously due to their vast area.
2.2 Food Chains and Food Webs +2 Marks
A food chain is a linear sequence of organisms through which nutrients and energy pass as each organism eats the next. A food web is a complex, realistic network of interconnected food chains. In nature, food webs are the rule — the greater the web complexity, the more stable the ecosystem (more alternative pathways when one species declines).
2.3 Ecological Pyramids +1 Mark Visual Concept
An ecological pyramid is a graphical representation of the relationship between different trophic levels in an ecosystem, expressed in terms of numbers, biomass, or energy. The base always represents producers; successive levels represent consumers. Pyramids may be upright (decreasing toward apex) or inverted (increasing toward apex) depending on the type and ecosystem.
Inverted in tree-insect systems
(1 tree → thousands of insects)
Always upright for energy
Phytoplankton (tiny, fast-cycling)
supports larger zooplankton biomass
Upright in terrestrial ecosystems
Energy can never increase up
a food chain (2nd Law)
Most important pyramid type
2.4 Nutrient Cycling (Biogeochemical Cycles) +4 Marks Highest Weightage
Unlike energy, nutrients cycle continuously between living organisms and the abiotic environment. These are called biogeochemical cycles (bio = living; geo = earth; chemical = elements). There are two fundamental types based on the primary reservoir of the nutrient:
| Parameter | Gaseous Cycles | Sedimentary Cycles |
|---|---|---|
| Reservoir | Atmosphere (gas form) | Earth's crust / rocks / soil |
| Cycling Speed | Fast — complete cycles in days to years | Slow — cycles over millions of years |
| Self-regulating? | Yes — atmosphere acts as buffer | Less so — depends on weathering |
| Examples | Carbon (C), Nitrogen (N), Oxygen (O) | Phosphorus (P), Sulphur (S), Calcium |
| Limiting Factor? | CO₂ for photosynthesis; N₂ for protein | Phosphorus = key limiting nutrient in water bodies |
A. The Carbon Cycle
Carbon is the backbone of all organic molecules. The atmosphere (CO₂) and oceans (dissolved CO₂, carbonates) serve as the primary reservoirs. The carbon cycle is deeply entwined with climate change — anthropogenic disruption of this cycle is the central driver of global warming.
- Photosynthesis: CO₂ + H₂O + sunlight → glucose + O₂. Removes ~120 PgC/year from atmosphere (terrestrial + marine).
- Respiration: All organisms release CO₂ through cellular respiration. Terrestrial ecosystems respire ~60 PgC/year back.
- Decomposition: Microbial breakdown of dead organic matter releases CO₂ and CH₄ (methane) — especially significant in wetlands and permafrost.
- Combustion: Burning fossil fuels + deforestation releases ~10 PgC/year — the primary anthropogenic flux disrupting the cycle.
- Ocean-Atmosphere Exchange: Oceans absorb ~26% of anthropogenic CO₂. Increased CO₂ → ocean acidification (pH↓ → carbonate dissolution → coral bleaching).
- Long-term sequestration: Marine organisms incorporate carbon into shells (CaCO₃); burial creates limestone over geological timescales.
B. The Nitrogen Cycle — The Most Complex Gaseous Cycle
Nitrogen makes up 78% of the atmosphere as N₂, but most organisms cannot use it directly. The cycle converts N₂ into biologically usable forms through a set of microbially-mediated transformations — making it arguably the most ecologically critical nutrient cycle.
Fig 2.1 — The Nitrogen Cycle: Fixation → Nitrification → Assimilation → Ammonification → Denitrification · Zeluno ©
- Nitrogen Fixation: Conversion of atmospheric N₂ → NH₃/NH₄⁺. By: Rhizobium (symbiotic, in legume root nodules), Azotobacter, Anabaena, Nostoc (free-living). Also by lightning (non-biological) and industrial Haber-Bosch process.
- Nitrification: NH₄⁺ → NO₂⁻ (by Nitrosomonas) → NO₃⁻ (by Nitrobacter). Makes nitrogen available to most plants as nitrate.
- Assimilation: Plants absorb NO₃⁻ via roots → incorporate into amino acids, proteins, nucleic acids. Animals obtain nitrogen by consuming plants/other animals.
- Ammonification: Decomposers (bacteria, fungi) break down dead organic matter → release NH₃/NH₄⁺ back to soil.
- Denitrification: NO₃⁻ → N₂ gas. By anaerobic bacteria (Pseudomonas, Thiobacillus) in waterlogged, oxygen-poor soils. Returns nitrogen to atmosphere — closes the cycle.
C. The Phosphorus Cycle — A Sedimentary Cycle
Phosphorus has no atmospheric reservoir — it cycles entirely through the lithosphere, soil, water, and organisms. This makes it a non-gaseous/sedimentary cycle. Phosphorus is the primary limiting nutrient in most freshwater ecosystems and many terrestrial systems. It enters the cycle through the weathering of phosphate rocks (apatite).
- Weathering: Phosphate rocks (Ca₃(PO₄)₂) → phosphate ions (PO₄³⁻) released into soil and water by mechanical and chemical weathering.
- Uptake: Plants absorb PO₄³⁻ through roots → incorporated into ATP, DNA, RNA, phospholipids — essential for energy transfer and cell membranes.
- Return to soil: Decomposers mineralise organic phosphorus → inorganic phosphate back in soil. Slow cycling — no gas phase shortcut.
- Eutrophication: Excess PO₄³⁻ (from fertiliser runoff) → explosive algal growth → dissolved oxygen depletion → aquatic dead zones. Critical UPSC topic.
D. The Sulphur Cycle — Mixed Gaseous-Sedimentary
Sulphur cycles through both atmosphere (SO₂, H₂S, DMS) and lithosphere (sulphate minerals). Its atmospheric leg is most relevant to UPSC due to the acid rain connection.
- Reservoir: Mostly in rocks and soils as sulphate (SO₄²⁻) and pyrite (FeS₂). Small atmospheric pool as SO₂ and H₂S.
- Natural release: Volcanic eruptions (SO₂), microbial decomposition (H₂S), ocean algae release DMS (dimethyl sulphide) — important climate feedback.
- Anthropogenic release: Burning of fossil fuels (coal, petroleum) → SO₂ → reacts with water → H₂SO₄ → acid rain (pH < 5.6). Major UPSC policy linkage.
- Biotic uptake: Plants absorb SO₄²⁻; essential for proteins (cysteine, methionine amino acids).
Sedimentary cycles (P, S, Ca) have lithosphere as reservoir — slower, less self-correcting.
Phosphorus: only major nutrient with NO atmospheric phase → most vulnerable to disruption → primary cause of freshwater eutrophication.
Nitrogen: most complex; involves 5 distinct microbial transformations — most likely to appear in 10-mark Mains question.
Sulphur: atmosphere linkage = acid rain; DMS from oceans = cloud seeding (climate regulation) — links to GS-1 and GS-3.
2.5 Ecological Succession Core Concept
Ecological succession is the directional, predictable process of change in the species composition and community structure of an ecosystem over time. Each set of organisms modifies the environment, making it suitable for a new set — until a climax community (relatively stable, self-perpetuating state) is reached.
Each stage (sere/seral community) modifies the abiotic environment → enables next seral stage → hundreds to thousands of years for primary succession
- Primary Succession: Occurs on a completely lifeless substrate with no soil — bare rock, new volcanic island, glacial moraine, sand dunes. Very slow (centuries to millennia). Pioneer species: crustose lichens (acid-producing, start soil formation).
- Secondary Succession: Occurs on a previously vegetated area that has been disturbed (fire, flood, farming abandonment, clear-cutting) but retains soil, seeds, and spores. Much faster (decades) because soil and propagules already exist. E.g., an abandoned agricultural field reverting to forest.
- Autogenic Succession: Driven by the organisms themselves modifying the habitat (self-generated). The typical succession described above is autogenic — each seral stage alters soil, light, and moisture.
- Allogenic Succession: Driven by external environmental forces (floods, fires, sea-level change, climate change). Not initiated or controlled by the organisms themselves.
- Sere / Seral Stage: Each intermediate community in the succession sequence. The complete series from pioneer to climax = a sere. Types: Lithosere (rock), Hydrosere (water), Halosere (saltwater/mangrove).
- Climax Community: The final stable, self-perpetuating community. In equilibrium with the regional climate. MonoClimax theory (Clements): one climax per climate zone. Polyclimax theory (Tansley): multiple stable endpoints possible.
Facilitation (Clements): Each species makes conditions more favourable for the next — the classic view.
Tolerance (Connell & Slatyer): Later species can establish from the start but grow slowly; they overtop early colonists over time.
Inhibition: Each species inhibits invasion by later species; succession only proceeds when a resident dies. Explains why succession can be non-linear.