Ecosystem Class 12 Notes

Ecosystem Structure and Components

• Abiotic components — physical factors like temperature, water, light, soil and air.
• Producers (autotrophs) — plants, algae and some bacteria that fix energy via photosynthesis; the base of every ecosystem.
• Consumers (heterotrophs) — herbivores (primary), carnivores (secondary and tertiary) that eat producers or other consumers.
• Decomposers — bacteria and fungi that break down dead organic matter and return nutrients to the soil (saprophytes).
• Examples of ecosystems: forest, grassland, pond, marine and desert; man-made ones include croplands and aquariums.

Productivity — GPP, NPP and R

• Secondary productivity is the rate at which consumers convert ingested food into their own biomass.
• NPP is highest in tropical rainforests (about 2.2 kg C m⁻² yr⁻¹) and lowest in deserts and deep seas.
• Productivity can be measured as kilocalories, biomass or carbon fixed per unit area per unit time.

Energy Flow and the 10% Law

Energy flows through an ecosystem in a single direction — from the sun into producers, then to consumers — and is never recycled. The producers capture solar energy that enters the ecosystem, but this energy does not flow back to the sun and cannot be reused because energy is lost as heat at each step.

• Food chain — a linear sequence: grass → grasshopper → frog → snake → hawk.
• Food web — many interconnected food chains in an ecosystem; a web is more stable than a single chain.
• Trophic levels — the positions an organism occupies in a food chain (producer, primary consumer, etc.).
• Detritus food chain starts with dead organic matter (detritivores and decomposers); grazing food chain starts with green plants.

Ecological Pyramids

• Pyramid of numbers — usually upright, but inverted in some cases (e.g. a single tree supporting many herbivore insects).
• Pyramid of biomass — upright in most terrestrial ecosystems, but inverted in the sea where a large mass of phytoplankton supports smaller zooplankton and fish.
• Pyramid of energy — always upright, because energy decreases at each successive trophic level and can never be greater than the level below.

Decomposition

• Fragmentation — detritus is broken into smaller pieces by detritivores such as earthworms.
• Leaching — water-soluble inorganic nutrients drain into the soil (downward).
• Catabolism — fungi and bacteria digest detritus and release inorganic substances; enzymes break down complex molecules.
• Humification — accumulation of a dark coloured, amorphous humus layer that is resistant to further microbial action.
• Mineralisation — the release of inorganic minerals (e.g. nitrates, phosphates) from the humus.

Nutrient Cycling

• Carbon cycle — carbon moves between the atmosphere (CO₂), producers (photosynthesis) and consumers, and returns via respiration, decomposition and combustion.
• Nitrogen cycle — nitrogen fixation (by Rhizobium, Azotobacter, cyanobacteria), nitrification (Nitrosomonas, Nitrobacter), assimilation, ammonification and denitrification (Pseudomonas).
• Phosphorus cycle — a sedimentary cycle; phosphorus moves through rocks, soil, water and organisms, with no atmospheric phase, so it is slower and can limit productivity.

Ecological Succession and Ecosystem Services

• Primary succession — starts from bare rock or barren land with no soil (e.g. a newly exposed lava or glacier), taking very long to form soil.
• Secondary succession — starts where soil already exists (e.g. after a forest fire or abandoned farmland) and is faster than primary succession.
• Hydrarch succession — begins in water (ponds, marshes) and leads to a terrestrial climax.
• Xerarch succession — begins in dry, bare conditions (e.g. bare rock or sand) and leads to a terrestrial climax.
• Seral stages — the intermediate communities in succession; the final stable community is the climax community.

Ecosystem services are the benefits humans receive from ecosystems — purification of air and water, nutrient cycling, crop pollination, climate regulation and the prevention of soil erosion. These services have enormous economic value and underscore why ecosystems must be conserved.

Solved Examples

Example: An ecosystem has a Gross Primary Productivity (GPP) of 10,000 kcal/m²/year and respiration (R) of 3,000 kcal/m²/year. What is its Net Primary Productivity (NPP), and what does it mean?

Solution: NPP = GPP − R = 10,000 − 3,000 = 7,000 kcal/m²/year. This is the energy stored by producers after their own respiration, and it is the amount actually available to consumers and decomposers in the ecosystem.

Example: If a producer stores 10,000 kcal and the 10% law applies, how much energy reaches the third trophic level?

Solution: Only 10% transfers at each step. Primary consumer receives 10% of 10,000 = 1,000 kcal; secondary (third trophic level) receives 10% of 1,000 = 100 kcal. This rapid loss explains why food chains are short and why the energy pyramid is always upright.