Board exam focus — Ecosystem (CBSE & HBSE)

CBSE focuses on ecosystem components, food chains/webs, trophic levels, ecological pyramids (number, biomass, energy), 10% energy law, productivity (GPP, NPP), nutrient cycling (carbon, nitrogen cycles), decomposition, and ecosystem services. HBSE emphasises definitions, producers/consumers/decomposers, food chain examples, pyramids, carbon cycle, and nitrogen cycle.

Ecosystem Structure and Energy Flow

Ecosystem: Structure and Function

An ecosystem is a functional unit of nature where living organisms (biotic component) interact with the physical environment (abiotic component) through energy flow and nutrient cycling. The term was coined by A.G. Tansley (1935).

Components of an Ecosystem:

Abiotic Components:

• Physical factors: light, temperature, rainfall, humidity, wind, soil
• Chemical factors: mineral nutrients (NPK), CO2, O2, water, organic matter
• Inorganic substances: N2, CO2, H2O, calcium, phosphorus, etc.

Biotic Components:

1. Producers (Autotrophs):

• Synthesise organic food from inorganic materials using energy (sunlight in photosynthesis; chemical energy in chemosynthesis)
• Photoautotrophs: plants, algae, phytoplankton, cyanobacteria (use sunlight + CO2 + H2O → glucose + O2)
• Chemoautotrophs: certain bacteria (Nitrosomonas, Nitrobacter, sulphur bacteria, iron bacteria) — use chemical energy from oxidation of inorganic compounds
• Base of all food chains

1. Consumers (Heterotrophs):

• Cannot synthesise own food; depend on producers (directly or indirectly)
• Primary consumers (Herbivores): eat plants directly; rabbit, deer, cattle, caterpillar, grasshopper
• Secondary consumers (Primary carnivores): eat herbivores; fox, frog, snake (in grassland)
• Tertiary consumers (Secondary carnivores): eat primary carnivores; hawk eating snake
• Omnivores: eat both plants and animals; humans, bears, rats, cockroaches

1. Decomposers (Detritivores/Saprotrophs):

• Degrade dead organic matter (detritus = dead organic matter) into inorganic compounds
• Bacteria and fungi are the main decomposers
• Return nutrients to the soil (mineralisation) → available for producers → complete nutrient cycles
• Detritivores (physical degradation): earthworms, millipedes, woodlice, mites — fragment and shred detritus
• Saprotrophs (chemical degradation): fungi, bacteria secrete enzymes → break down complex organic molecules

Food Chains, Food Webs and Trophic Levels

Food Chain: A linear sequence of organisms through which energy and nutrients pass.

• Grazing food chain (GFC): starts with living producers; more common and important in most terrestrial and aquatic ecosystems
• Example: Grass → Grasshopper → Frog → Snake → Eagle
• Example: Phytoplankton → Zooplankton → Small fish → Large fish → Shark

• Detritus food chain (DFC): starts with dead organic matter (detritus)
• Example: Dead organic matter → Bacteria/Fungi (decomposers) → Protozoa → Nematodes → Arthropods
• More important in forest and aquatic (deep-sea) ecosystems

Food Web: Network of interconnected food chains; more realistic than a single food chain; most organisms eat multiple things and are eaten by multiple things.

Trophic Levels: Each step in a food chain = one trophic level.

• TL1: Producers (plants)
• TL2: Primary consumers (herbivores)
• TL3: Secondary consumers (primary carnivores)
• TL4: Tertiary consumers (secondary carnivores)
• Decomposers: at all trophic levels (they decompose dead matter from all levels)

Energy Flow in Ecosystems

First and Second Laws of Thermodynamics:

• First law: energy cannot be created or destroyed; only transformed
• Second law: every energy transformation results in some energy being lost as heat; entropy increases

Energy flows unidirectionally through ecosystems (cannot be recycled), unlike matter/nutrients which cycle.

10% Law (Lindeman's Law, 1942): Only about 10% of the energy (or biomass) at one trophic level is transferred to the next trophic level. The remaining ~90% is lost:

• Through respiration (metabolic heat)
• As faecal matter (detritus — not consumed)
• To decomposers
• As heat from feeding activities

Implication: food chains rarely have more than 4-5 trophic levels (energy becomes too limited to sustain higher levels). Also explains why vegetarian diet is more energy-efficient than meat-eating.

Example:

• If grass stores 1,000,000 kJ of energy...
• Grasshoppers (TL2) get 100,000 kJ (10%)
• Frogs (TL3) get 10,000 kJ (10% of TL2)
• Snakes (TL4) get 1,000 kJ (10% of TL3)
• Eagles (TL5) get 100 kJ (10% of TL4)

Ecological Pyramids

Ecological pyramids represent graphically the number, biomass, or energy content at successive trophic levels.

1. Pyramid of Numbers:

• Represents number of organisms at each trophic level
• Usually upright in grassland (many grass plants → fewer grasshoppers → fewer frogs → fewer snakes → 1 eagle)
• Inverted in forest: 1 large tree supports thousands of insects supports fewer birds
• Spindle-shaped in parasitic food chains: many parasites on one host

2. Pyramid of Biomass:

• Represents total dry weight (biomass) of organisms at each trophic level
• Usually upright in terrestrial ecosystems
• Inverted in aquatic ecosystems (especially open ocean): phytoplankton (small biomass but high turnover rate) supports larger biomass of zooplankton. Because biomass is measured at one instant — phytoplankton divides rapidly, so zooplankton can have more biomass at any moment despite lower energy input.

3. Pyramid of Energy:

• Represents amount of energy at each trophic level (kJ/m²/year)
• ALWAYS UPRIGHT — energy is always lost at each level; energy at higher levels can never exceed lower levels
• Most accurate representation of ecosystem function
• At broader base = more energy; narrows at top

Productivity, Decomposition and Nutrient Cycling

Productivity

Primary Production: The amount of organic matter (biomass) produced per unit area per unit time by producers through photosynthesis or chemosynthesis.

Gross Primary Production (GPP): The total organic matter synthesised by producers in a given time (total photosynthesis).

Net Primary Production (NPP): GPP minus the amount used in plant respiration (R). NPP = GPP - R

• GPP represents all energy fixed by producers
• NPP is the energy available to consumers (herbivores and decomposers)
• NPP is the most ecologically important measure (what's available to the rest of the ecosystem)

Secondary Production: Rate of formation of new organic matter by consumers (heterotrophs); energy available to next higher consumer level.

Standing Crop: The amount of living material (biomass) in an ecosystem at a given time. The ratio of NPP to standing crop gives productivity (turnover rate).

Units: g/m²/day (dry weight), kJ/m²/day, or t/ha/year

Comparison by ecosystem type:

Decomposition

Decomposition is the process by which decomposers (bacteria and fungi) break down dead organic matter (detritus) into inorganic forms. It is essential for nutrient cycling — making minerals available for producers.

Steps of Decomposition:

1. Fragmentation: detritivores (earthworms, mites, millipedes) physically break up dead plant matter into smaller pieces → increases surface area
2. Leaching: water-soluble inorganic nutrients leach into soil
3. Catabolism: bacteria and fungi secrete hydrolytic enzymes (cellulases, amylases, proteases, lipases) → break down complex organic molecules (cellulose, lignin, proteins, fats) into simpler compounds
4. Humification: resistant organic residues (lignin, waxes, humic acids) form dark, complex humus; humus resists decomposition; improves soil structure and water retention
5. Mineralisation: complete breakdown of organic matter to inorganic forms (CO2, H2O, nitrate, phosphate, sulphate, etc.) → returned to the environment for use by plants

Factors affecting decomposition rate:

• Temperature: higher temperature (up to ~30-35°C) → faster enzymatic reactions → faster decomposition
• Moisture: decomposition slows in dry conditions (decomposers need water); waterlogged conditions (anaerobic) slow aerobic decomposition
• Soil pH: affects microbial activity; most decomposers prefer neutral pH
• Chemical composition of detritus: low C:N ratio (fresh leaves, manure) → fast decomposition; high C:N ratio (wood, lignin-rich material) → slow decomposition
• Oxygen availability: aerobic decomposition much faster than anaerobic

Nutrient Cycling (Biogeochemical Cycles)

Nutrients cycle between the living (biotic) and non-living (abiotic) components of the ecosystem. Unlike energy (which flows unidirectionally and is lost as heat), nutrients are recycled.

Carbon Cycle:

Carbon reservoirs:

• Atmosphere: ~800 Gt C (as CO2) — rapidly exchanged
• Ocean: ~38,000 Gt C (dissolved CO2, HCO3- — largest reservoir in rapid cycle)
• Terrestrial biota: ~550 Gt C
• Soil organic matter: ~1,500 Gt C
• Fossil fuels: ~3,700 Gt C (mostly unavailable until burnt)

Carbon fluxes:

1. Photosynthesis: atmosphere → biota; CO2 + H2O → (CH2O)n + O2 (~120 Gt C/year by terrestrial plants)
2. Respiration: biota → atmosphere; (CH2O)n + O2 → CO2 + H2O (~60 Gt C/year plant respiration; ~60 Gt by animals and decomposers)
3. Decomposition: dead organic matter → CO2 (via microbial respiration)
4. Ocean exchange: CO2 absorbed/released at ocean surface
5. Combustion (fossil fuels): burning coal, oil, gas → releases stored carbon → ~8.1 Gt C/year anthropogenic emission
6. Volcanic eruptions: release CO2 from magma
7. Ocean sedimentation: marine organisms die → sink → form calcium carbonate sediments → limestone → subducted → volcanic eruption → long-term geological cycle

Greenhouse effect: CO2, CH4, N2O, water vapour, CFCs absorb infrared radiation re-emitted from Earth's surface → retain heat → global warming. Anthropogenic increase in CO2 (~280 ppm pre-industrial to ~420 ppm today) → enhanced greenhouse effect.

Nitrogen Cycle:

Nitrogen is essential for amino acids, nucleotides, chlorophyll, ATP. N2 is 78% of atmosphere but most organisms cannot use it directly.

Steps:

1. Nitrogen fixation: N2 → NH3 (ammonia)

• Biological nitrogen fixation (BNF): most important; enzyme nitrogenase (Fe-Mo protein; oxygen-sensitive) in:
• Rhizobium (symbiotic with legumes, in root nodules)
• Azotobacter, Clostridium (free-living in soil)
• Anabaena, Nostoc (cyanobacteria — in heterocysts)
• Frankia (symbiotic with non-legumes, e.g., alder trees)
• Atmospheric nitrogen fixation: lightning (~10 Tg N/year); high temperature + pressure → NO (nitric oxide) → HNO3 → carried by rain to soil
• Industrial nitrogen fixation (Haber-Bosch process): N2 + 3H2 → 2NH3 at 450°C, 150-300 atm, Fe catalyst → fertiliser production

1. Nitrification: NH4+ → NO2- → NO3- (ammonium → nitrite → nitrate)

• Nitrosomonas/Nitrosospira (NH4+ → NO2-, releasing energy)
• Nitrobacter (NO2- → NO3-, releasing energy)
• Nitrate (NO3-) is the form most easily absorbed by plant roots

1. Assimilation: NO3- (or NH4+) absorbed by plant roots → incorporated into amino acids, proteins, nucleic acids

1. Ammonification (Mineralisation): dead organic matter → NH4+ by bacteria and fungi (enzymatic degradation of proteins → amino acids → NH3/NH4+); also called ammonification

1. Denitrification: NO3- → N2 (or N2O) released back to atmosphere

• Pseudomonas denitrificans, Paracoccus denitrificans, Thiobacillus denitrificans
• Anaerobic conditions (waterlogged soils, deep ocean) favour denitrification
• Returns fixed nitrogen to atmosphere — completes cycle

Ecosystem Services and Ecological Succession

Ecosystem Services

Ecosystem services are the benefits that humans and other organisms derive from properly functioning ecosystems. The concept emphasises the economic and social value of biodiversity and healthy ecosystems.

Classification of Ecosystem Services (Millennium Ecosystem Assessment, 2005):

1. Provisioning Services (Products obtained from ecosystems):

• Food (crops, fish, wild fruits, game meat)
• Fresh water (rivers, groundwater recharge by forests)
• Timber and fibre
• Fuel (firewood, biogas from wetlands)
• Genetic resources (wild relatives of crops for breeding)
• Natural medicines (plants used in traditional/modern medicine)

2. Regulating Services (Benefits from regulation of ecosystem processes):

• Climate regulation: forests absorb CO2 (carbon sequestration); evapotranspiration affects regional precipitation
• Water regulation: forests regulate water flow, prevent floods and droughts
• Pollination: ~75% of the world's food crop species depend on animal pollination (bees, butterflies, birds, bats); estimated value >$577 billion/year globally
• Pest control: natural enemies regulate pest populations (reduces need for pesticides)
• Water purification: wetlands and riparian vegetation filter pollutants
• Erosion control: plant roots bind soil
• Disease regulation: biodiversity dilutes pathogens (dilution effect)

3. Cultural Services (Non-material benefits):

• Aesthetic beauty (wildlife tourism, ecotourism — $77 billion/year in developing countries)
• Recreation (hiking, fishing, birdwatching)
• Spiritual and religious values
• Cultural heritage and identity
• Education and science (ecosystem as natural laboratory)

4. Supporting Services (Underlying processes that support all other services):

• Soil formation (pedogenesis) by decomposers, fungi, and physical weathering
• Nutrient cycling (nitrogen, carbon, phosphorus cycles)
• Primary production (photosynthesis — base of all food chains)
• Habitat provision for species

Economic Valuation:

• Forest ecosystem services in India estimated at Rs 2.6 lakh crore/year
• Amazon rainforest provides >$8 trillion in ecosystem services/year globally
• Wetlands provide water purification, flood control, groundwater recharge services worth billions

Ecological Succession

Succession is the sequential, directional, and predictable change in the species composition and community structure of an ecosystem over time. It is driven by species modifying the physical environment, making it more suitable for the next community and less suitable for themselves.

Types of Succession:

1. Primary Succession:

• Occurs in a bare area devoid of soil and any life (no previous biological community)
• Examples: bare rock after glacier retreat, volcanic island, landslide area
• Very slow: hundreds to thousands of years
• Pioneer community: lichens (on rock); mosses (primary succession on rock: lichen → moss → fern → herbaceous plants → shrubs → trees)
• As organisms grow, die and accumulate → humus forms → soil develops → new species establish

2. Secondary Succession:

• Occurs in an area where a community existed before but was destroyed
• Soil and some organisms remain
• Much faster than primary succession (years to decades)
• Examples: after forest fire, flood, clear-cutting, agricultural land abandonment
• Example sequence (old field succession, eastern USA): annual weeds → perennial grasses → shrubs → pioneer trees (light-demanding) → climax forest

Seral Stages (Seres): Each transitional community in succession is called a seral stage. A complete sequence of communities from first colonisers to climax = a sere (hydrosere in water, lithosere on rock, psammosere on sand dunes).

Climax Community: The final, stable community that develops when ecosystem reaches equilibrium with the regional climate. Self-replacing; relatively stable over long periods. Composition depends on climate (tropical rainforest in humid tropics, grassland in semi-arid zones, tundra in Arctic).

Facilitation model: Early species modify the environment → facilitates establishment of later species (e.g., lichens on rock release acids that weather rock and accumulate organic matter → facilitates moss establishment).

Hydrosere (Aquatic Succession): Open water → rooted hydrophytes (Hydrilla) → sedges/reeds → marsh → meadow → woodland → forest; water body gradually fills with sediment → terrestrial community eventually

Mechanisms driving succession:

• Facilitation: pioneer species create conditions favouring next community
• Competition: better-adapted species outcompete earlier species
• Tolerance: various species can establish at any stage; competitive hierarchy determines outcome
• Inhibition: established species inhibit colonisation by other species (maintains seral stage longer)

Characteristics of successional change:

• Species diversity generally increases (initially) then may decrease in late stages
• Biomass increases
• Net primary production increases in early stages
• Respiration increases; P/R ratio → 1 (balanced) in climax
• Food web complexity increases
• Soil development (increasing depth, organic matter, structure)

Frequently asked questions

Are these Ecosystem notes free?

Yes — the Ecosystem notes for Biology (Class 12) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Ecosystem notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Ecosystem chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Ecosystem.