Ecosystem: Structure and Function
An ecosystem is the functional unit of nature where biotic and abiotic components interact through productivity, decomposition, energy flow and nutrient cycling.
Ecosystem functioning is the conceptual backbone of the Environment & Ecology section and yields direct Prelims MCQs almost every year on GPP vs NPP, the 10% law, ecological pyramids and decomposition steps. In Mains GS-III it underpins biodiversity, conservation, soil health and EIA answers. It also links to the constitutional environment mandate (Article 48A and Article 51A(g)) that frames most ecology questions.
Understand the chapter
Ecosystem as a Functional Unit
An ecosystem is a functional unit of nature where living organisms interact among themselves and with the surrounding physical environment. It varies in size from a small pond to a large forest or sea, and the entire biosphere is regarded as one global ecosystem composed of all local ecosystems. Because the biosphere is too vast to study at once, it is divided into terrestrial and aquatic categories.
- Terrestrial ecosystems: forest, grassland, desert
- Aquatic ecosystems: pond, lake, wetland, river, estuary
- Man-made ecosystems: crop fields and an aquarium
- Biosphere = composite of all local ecosystems = the global ecosystem
Structure: Species Composition and Stratification
Interaction of biotic and abiotic components gives each ecosystem a characteristic physical structure. Its two structural features are species composition (identification and enumeration of plant and animal species) and stratification (vertical distribution of species at different levels). A pond illustrates all components and functions of an ecosystem in a self-sustaining unit.
- Stratification example: trees (top), shrubs (middle), herbs/grasses (bottom)
- Abiotic: water, dissolved inorganic/organic substances, bottom soil, solar input
- Biotic: autotrophs (phytoplankton, algae, plants), consumers (zooplankton), decomposers (fungi, bacteria, flagellates)
- Four functions: productivity, decomposition, energy flow, nutrient cycling
Productivity (Input)
Primary production is the amount of biomass or organic matter produced per unit area over time by plants during photosynthesis, expressed in weight (g/m2) or energy (kcal/m2). The rate of biomass production is productivity (g/m2/yr). Gross Primary Productivity (GPP) is the rate of organic matter production in photosynthesis; subtracting respiration losses gives Net Primary Productivity available to heterotrophs.
- GPP - R = NPP; NPP is the biomass available to herbivores and decomposers
- Secondary productivity = rate of formation of new organic matter by consumers
- Annual NPP of biosphere ~170 billion tons (dry weight)
- Oceans yield only ~55 billion tons despite covering ~70% of the surface
Decomposition
Decomposers break complex organic matter (detritus) into inorganic substances like carbon dioxide, water and nutrients. Detritus, the raw material, comprises dead leaves, bark, flowers, animal remains and faecal matter. The steps operate simultaneously and decomposition is largely an oxygen-requiring (aerobic) process.
- Five steps: fragmentation, leaching, catabolism, humification, mineralisation
- Humus: dark, amorphous, colloidal, resistant to microbes, a nutrient reservoir
- Slow if detritus is rich in lignin/chitin; fast if rich in nitrogen and sugars
- Warm + moist favours decomposition; low temperature and anaerobiosis inhibit it
Energy Flow, Food Chains and Trophic Levels
The Sun is the only energy source for all ecosystems except deep-sea hydrothermal ones. Less than 50% of incident solar radiation is photosynthetically active radiation (PAR), and plants capture only 2-10% of PAR. Energy flow is unidirectional, obeying the laws of thermodynamics, and moves from producers to consumers to decomposers.
- Grazing Food Chain (GFC): Grass to Goat to Man
- Detritus Food Chain (DFC) begins with dead organic matter, run by saprotrophs (fungi, bacteria)
- GFC dominates aquatic ecosystems; DFC dominates terrestrial ecosystems
- 10% law: only 10% of energy is transferred to each higher trophic level, limiting chain length
Ecological Pyramids
Ecological pyramids express the number, biomass or energy relationship between trophic levels, with producers at the broad base and top consumers at the apex. Each calculation must include all organisms at a trophic level, which is a functional level, not a species. Most pyramids are upright, but exceptions exist.
- Three types: pyramid of number, biomass and energy
- Pyramid of biomass in the sea is inverted (fish biomass exceeds phytoplankton)
- Pyramid of energy is ALWAYS upright, never inverted (energy lost as heat each step)
- A species may occupy more than one trophic level (sparrow: primary on seeds, secondary on insects)
Key terms
- Ecosystem
- A functional unit of nature where biotic and abiotic components interact and exchange energy and matter.
- Stratification
- Vertical distribution of species at different levels of an ecosystem (e.g., trees, shrubs, herbs in a forest).
- Gross Primary Productivity (GPP)
- Rate of total organic matter production by producers during photosynthesis.
- Net Primary Productivity (NPP)
- GPP minus respiration losses; the biomass actually available to heterotrophs.
- Secondary productivity
- Rate of formation of new organic matter by consumers.
- Detritus
- Dead plant and animal remains and faecal matter that form the raw material for decomposition.
- Humus
- Dark, amorphous, colloidal substance resistant to microbial action that acts as a soil nutrient reservoir.
- Standing crop
- Mass of living material (biomass or number) at a trophic level at a given time, measured best as dry weight.
- PAR
- Photosynthetically Active Radiation; less than 50% of incident solar radiation, of which plants capture only 2-10%.
- Trophic level
- A functional feeding level in a food chain (producers first, herbivores second, carnivores third).
Must-know facts exam-ready
- Ecosystem is the structural and functional unit of nature; the biosphere is the global ecosystem (term 'ecosystem' coined by A.G. Tansley, 1935).
- Two structural features: species composition and stratification.
- Four functional features: productivity, decomposition, energy flow and nutrient cycling.
- NPP = GPP - R; NPP (not GPP) is the biomass available to heterotrophs.
- Secondary productivity = rate of formation of new organic matter by consumers.
- Annual NPP of the biosphere ~170 billion tons (dry weight); oceans only ~55 billion tons despite ~70% surface area.
- Five steps of decomposition: fragmentation, leaching, catabolism, humification, mineralisation; decomposition is aerobic.
- Lignin/chitin-rich detritus decomposes slowly; nitrogen/sugar-rich detritus fast; temperature and soil moisture are key controls.
- PAR is less than 50% of incident solar radiation; plants capture only 2-10% of PAR; producers convert ~1% of available sunlight into NPP.
- 10% law (Lindeman): only 10% of energy passes to the next trophic level, restricting food chain length.
- GFC dominates aquatic ecosystems; DFC dominates terrestrial ecosystems.
- Pyramids of number and biomass can be inverted (e.g., sea biomass), but the pyramid of energy is always upright.
Memory tricks remember it for good
Traps to avoid
- GPP vs NPP: NPP = GPP minus respiration, and it is NPP (not GPP) that is available to heterotrophs.
- Production vs productivity: production is an amount (biomass); productivity is a rate (per unit time).
- Pyramid of energy can NEVER be inverted, but pyramids of number and biomass CAN be inverted (sea, big tree).
- Decomposition is aerobic: anaerobiosis and low temperature INHIBIT it; warm and moist conditions favour it.
- Lignin and chitin SLOW decomposition while nitrogen and sugars SPEED it up (students often reverse this).
- Terrestrial ecosystems are DFC-dominated and aquatic ecosystems are GFC-dominated (commonly swapped); trophic level is a functional level, not a species.
Exam focus
🧠 Prelims angles
- GPP, NPP and secondary productivity definitions and the NPP = GPP - R relationship.
- 10% law of energy transfer and its effect on the number of trophic levels.
- Which ecological pyramids can be inverted (number, biomass) versus always upright (energy).
- Correct sequence and definitions of decomposition steps (humus, leaching, mineralisation).
- PAR and capture facts: <50% of solar radiation is PAR, plants fix only 2-10%.
- Biosphere NPP (~170 billion tons) versus low ocean productivity (~55 billion tons) despite 70% area.
✍️ Mains angles GS-III
- Why is energy flow in an ecosystem unidirectional, and how does it limit food chain length?Link to laws of thermodynamics, the 10% law and continuous heat loss at each trophic level.
- Decomposition is central to nutrient cycling and soil fertility - discuss with its climatic controls.Use the five steps plus temperature/moisture controls and lignin/chitin vs nitrogen/sugar effects; tie to soil health and agriculture.
- Despite covering 70% of Earth, oceans show low productivity - examine the paradox.Contrast biosphere NPP (170 bn tons) with ocean (55 bn tons); discuss light/nutrient limitation and conservation relevance.
Last-minute revision tick as you recall
- Ecosystem = biotic + abiotic functional unit; biosphere = global ecosystem.
- Structure = species composition + stratification; functions = Productivity, Decomposition, Energy flow, Nutrient cycling.
- NPP = GPP - R; NPP feeds heterotrophs; secondary productivity = consumers' rate.
- Biosphere NPP ~170 bn tons; oceans only ~55 bn tons despite ~70% area.
- Decomposition order F-L-C-H-M; aerobic; warm+moist fast; lignin/chitin slow.
- Energy flow unidirectional; PAR <50% of solar, plants fix 2-10%; ~1% becomes NPP.
- 10% law restricts trophic levels; GFC aquatic-dominant, DFC terrestrial-dominant.
- Pyramids of number and biomass can invert; pyramid of energy always upright.
- Trophic level is a functional level, not a species (sparrow example).
Distilled from NCERT Class 12 · Biology (Class 12) for UPSC. Always cross-check facts with the original NCERT.