An ecosystem is the functional unit of ecology — a community of organisms plus their environment, linked by energy flow and nutrient cycling. CBSE Class 12 and NEET both test productivity, pyramids and decomposition. Expect one to two questions a year on this chapter.
What makes an ecosystem work? Two processes: energy flows through it (one-way, from sun to heat) and nutrients cycle within it (recycled endlessly). If you understand these two flows, every ecosystem concept — productivity, pyramids, decomposition, succession — falls into place as a consequence.
Core Concepts
Structure of an ecosystem
Two components — biotic (living) and abiotic (non-living). Biotic includes producers, consumers and decomposers arranged in trophic levels. Abiotic includes sunlight, temperature, water and mineral nutrients. The interaction between these two components determines the character of the ecosystem.
Productivity
Gross Primary Productivity (GPP) is the total rate of energy fixation by producers through photosynthesis. Measured in g/m/year or kcal/m/year.
Net Primary Productivity (NPP) is GPP minus the energy used by producers in their own respiration:
where = plant respiration. NPP is what is available to consumers and decomposers — the actual ‘income’ of the ecosystem.
Net Ecosystem Productivity (NEP) = NPP minus respiration of all heterotrophs (consumers + decomposers). This is the net accumulation of organic matter in the ecosystem.
Productivity rankings:
- Highest NPP: Tropical rainforests (~2000 g/m/year) and coral reefs
- Moderate: Temperate forests, grasslands, agricultural land
- Lowest per unit area: Open ocean (~125 g/m/year) — but the open ocean is so vast that its total NPP is the largest of any ecosystem
- Near zero: Desert, deep ocean
Decomposition
The breakdown of dead organic matter (detritus) into inorganic nutrients. Carried out mainly by bacteria and fungi, aided by detritivores (earthworms, dung beetles) that fragment the detritus.
Five stages of decomposition:
- Fragmentation — detritivores break large pieces into smaller ones, increasing surface area
- Leaching — water-soluble nutrients dissolve and percolate into the soil
- Catabolism — microbial enzymes break down detritus into simpler molecules
- Humification — resistant dark-coloured amorphous material (humus) accumulates; decomposes very slowly
- Mineralisation — humus is further degraded into inorganic nutrients (CO, NH, PO) available for plant uptake
Factors affecting decomposition rate:
- Temperature — faster in warm climates (tropical forests decompose litter in weeks; tundra takes years)
- Moisture — optimal at moderate levels; too dry or waterlogged slows decomposition
- Chemical composition — high lignin and chitin slow decomposition; high nitrogen speeds it
- Oxygen — aerobic decomposition is faster; anaerobic (in waterlogged soils) is slower and produces methane
Energy flow
Energy enters as sunlight. Only about 1-5% of incident solar radiation is captured by photosynthesis. This energy flows through trophic levels with roughly 10% efficiency at each transfer (Lindeman’s 10% rule).
Energy flow is unidirectional — it cannot be recycled. At each trophic level, most energy is lost as heat through respiration. This is a fundamental constraint imposed by the second law of thermodynamics.
The consequence: food chains are short (4-5 levels maximum) because there is not enough energy to support further levels. Vegetarian diets are more energy-efficient than meat-based diets because they eliminate one trophic transfer.
Ecological pyramids
Three types:
Pyramid of Numbers — counts individuals at each trophic level.
- Usually upright in grasslands (many grasses, fewer insects, even fewer birds)
- Inverted in a tree ecosystem (one tree supports many herbivorous insects which support fewer parasites — but the tree is just one individual)
Pyramid of Biomass — measures total dry weight at each level.
- Usually upright on land
- Inverted in aquatic ecosystems — phytoplankton biomass at any moment is less than zooplankton biomass because phytoplankton reproduce so fast (high turnover rate)
Pyramid of Energy — measures total energy at each level.
- Always upright — energy always decreases at higher levels. This is a thermodynamic necessity.
NEET regularly asks: “Which pyramid is always upright?” Answer: pyramid of energy. Also common: “Which pyramid can be inverted in an aquatic ecosystem?” Answer: pyramid of biomass (due to rapid phytoplankton turnover).
Nutrient cycling
Gaseous cycles — the reservoir is in the atmosphere. Examples: carbon cycle, nitrogen cycle. These move globally and rapidly.
Sedimentary cycles — the reservoir is in rocks and soil. Examples: phosphorus cycle, sulphur cycle. These move locally and slowly.
Carbon cycle: CO fixed by photosynthesis → moves through food chains → returned by respiration, decomposition, combustion. Human activities (fossil fuel burning, deforestation) have increased atmospheric CO from ~280 ppm (pre-industrial) to ~420 ppm, driving climate change.
Nitrogen cycle: Atmospheric N is fixed by nitrogen-fixing bacteria (Rhizobium, Azotobacter, cyanobacteria) → ammonification → nitrification → absorbed by plants → moves through food chains → denitrification returns N to atmosphere.
Phosphorus cycle: No significant atmospheric phase. Released from rocks by weathering → absorbed by plants as phosphate → moves through food chains → returned to soil by decomposition. Often the limiting nutrient in freshwater ecosystems.
Ecological succession
The gradual replacement of one community by another over time, leading to a stable climax community.
Primary succession — begins on bare substrate with no soil (lava, bare rock, new sand dunes). Pioneer species: lichens and mosses → herbs → shrubs → trees → climax forest. Takes hundreds of years because soil must develop from scratch.
Secondary succession — begins on substrate where soil and seeds already exist (after fire, flood, abandoned farmland). Pioneer species: grasses and herbs → shrubs → trees → climax. Takes 50-200 years. Much faster because soil nutrients and seed banks are already present.
Hydrarch succession — in water bodies, starting from aquatic and moving toward a terrestrial climax. Xerarch succession — on dry substrate, starting from xeric conditions and moving toward a mesic climax.
Both types of succession converge toward a mesophytic climax community (moderate moisture conditions).
Worked Examples
Phytoplankton are tiny and reproduce extremely fast — their generation time is hours to days. They are consumed by larger zooplankton almost as fast as they reproduce. At any given moment, the standing biomass of phytoplankton may be less than that of zooplankton, even though total productivity is higher. This gives an inverted biomass pyramid. The pyramid of energy, however, is still upright because phytoplankton fix more total energy over time.
A grassland ecosystem has GPP = 10,000 kJ/m/year. Plant respiration uses 4,000 kJ. Consumer and decomposer respiration uses 3,000 kJ.
NPP = GPP - plant respiration = 10,000 - 4,000 = 6,000 kJ/m/year (available to herbivores and decomposers).
NEP = NPP - heterotroph respiration = 6,000 - 3,000 = 3,000 kJ/m/year (net accumulation in the ecosystem).
Humus is a complex mixture of large organic molecules that are resistant to microbial enzymes. It has a very high carbon-to-nitrogen ratio and contains aromatic compounds derived from lignin. These structural features make it difficult for decomposers to break down. However, this slow decomposition is ecologically valuable — humus improves soil structure, water retention and nutrient-holding capacity.
Atmospheric CO is fixed by grass (photosynthesis) → glucose in grass → eaten by a grasshopper → carbon incorporated into grasshopper biomass → some lost as CO through grasshopper respiration → grasshopper eaten by frog → carbon enters frog biomass → frog dies → decomposers break down the body → CO returned to atmosphere. The carbon atom has been recycled; the energy it carried has been lost as heat at each step.
Fresh lava has no soil and no life. Lichens colonise the bare rock first — they produce acids that slowly weather the rock surface. Dead lichen matter and rock particles accumulate as a thin layer of soil. Mosses colonise this soil, adding more organic matter. Over decades, enough soil develops for herbs and grasses. Shrubs follow, then small trees, then larger trees. After centuries, a climax forest may establish. Each stage modifies the environment to make it suitable for the next.
Common Mistakes
Saying the pyramid of energy can be inverted. It cannot — energy always decreases with trophic level due to the second law of thermodynamics. Only pyramids of number and biomass can be inverted.
Confusing GPP and NPP. NPP = GPP minus plant respiration. A common exam trap is giving GPP and asking for NPP without explicitly saying “subtract respiration.” Always check what the question is actually asking.
Writing that decomposers are only bacteria. Fungi are equally important (and often more important in terrestrial ecosystems). The two work together — bacteria excel at breaking down simple compounds; fungi can tackle complex ones like lignin and cellulose.
Calling the nitrogen cycle a sedimentary cycle. It is gaseous — the main reservoir is atmospheric N. The phosphorus cycle is sedimentary (reservoir in rocks).
Thinking succession ends with a grassland. Unless the climate only supports grassland, succession proceeds all the way to a forest climax. The climax community is determined by the climate of the region.
Exam Weightage and Strategy
The Ecosystem chapter carries 6-8 marks in CBSE Class 12 boards and 2-3 NEET questions per year. The high-frequency PYQ topics are: (1) GPP vs NPP, (2) pyramid of energy always upright, (3) decomposition stages, (4) types of nutrient cycles, (5) primary vs secondary succession.
Learn the five steps of decomposition (FLCHM — Fragmentation, Leaching, Catabolism, Humification, Mineralisation) and the three pyramid types as one table. Add the productivity formulas. That covers most PYQs. For succession, remember: primary = bare rock, secondary = soil already exists.
Practice Questions
Q1. Why is the pyramid of energy always upright?
At each trophic transfer, about 90% of energy is lost as heat through respiration (second law of thermodynamics). The next level always has less total energy than the one below. This is an inviolable physical law — no biological mechanism can reverse it. Pyramids of number and biomass can be inverted due to size differences or turnover rates, but energy pyramids cannot.
Q2. Distinguish between hydrarch and xerarch succession.
Hydrarch succession begins in water (pond, lake) and moves toward a drier, terrestrial climax. Stages: phytoplankton → submerged plants → floating plants → reed swamp → sedge meadow → woodland → climax forest. Xerarch succession begins on dry bare rock and moves toward a wetter, mesic climax. Stages: lichen → moss → herbs → shrubs → trees → climax forest. Both converge on a mesophytic climax community.
Q3. What are the factors that affect the rate of decomposition?
(1) Temperature — warm temperatures accelerate microbial activity. (2) Moisture — moderate moisture is optimal; extremes slow decomposition. (3) Oxygen — aerobic decomposition is faster. (4) Chemical quality of detritus — high N, low lignin = fast; low N, high lignin = slow. (5) Soil pH — slightly acidic to neutral is optimal for most decomposers.
Q4. Explain why vegetarian diets are more energy-efficient.
Each trophic transfer loses about 90% of energy. When humans eat plants directly (T1 → T2), they access 10% of plant energy. When humans eat herbivores like cattle (T1 → T2 → T3), they access only 1% of the original plant energy. To produce 1 kg of beef requires about 7-10 kg of grain. Vegetarian diets skip a trophic level, making food production more efficient per unit of land and energy.
FAQs
What is the difference between a food chain and a food web?
A food chain is a single linear pathway of energy transfer (grass → grasshopper → frog → snake). A food web is a network of interconnected food chains showing all feeding relationships in an ecosystem. Real ecosystems always have food webs because most organisms eat multiple things and are eaten by multiple predators.
Why is the open ocean low in productivity per unit area but high in total productivity?
The open ocean has low nutrient concentrations (limiting phytoplankton growth) and sunlight only penetrates the top 200 m. So productivity per m is low (~125 g/m/year). But the ocean covers 71% of Earth’s surface — such a huge area that the total NPP exceeds that of any single terrestrial biome.
What happens if decomposers are removed from an ecosystem?
Dead matter would accumulate without being broken down. Nutrients locked in dead organisms would not return to the soil. Plants would eventually run out of minerals like nitrogen and phosphorus. The ecosystem would collapse because the nutrient cycle would be broken, even though the energy flow (from sunlight) would continue.
An ecosystem is a machine — inputs, throughput and outputs. Track the flow of energy and the cycling of nutrients and the rest follows.