Ecosystem Class 12 CBSE Notes: Structure, Function & Pyramids

Key Terms in Ecosystem

Ecosystem

A functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.

Stratification

The vertical distribution of different species occupying different levels in an ecosystem, e.g., trees at the top, followed by shrubs, herbs, and grasses in a forest.

Primary Productivity

The rate at which biomass or organic matter is produced by producers (photosynthesis) per unit area over a time period. Expressed in units of weight (g⁻²) or energy (kcal m⁻²).

Decomposition

The process by which decomposers break down complex organic matter into simpler inorganic substances like carbon dioxide, water, and nutrients.

Detritus

Dead organic matter, including fallen leaves, remains of animals, and fecal matter, which serves as the raw material for decomposition.

Food Chain

The sequence of organisms through which energy is transferred, starting from a producer and moving to successive consumers.

Food Web

A network of interconnected food chains within an ecological community, representing all the feeding relationships.

Standing Crop

The total mass of living material (biomass) in a specific trophic level at a particular time. It is measured as mass or number in a unit area.

Ecological Pyramids

A graphical representation of the relationship between different organisms in an ecosystem at different trophic levels, based on number, biomass, or energy.

Ecological Succession

The gradual and fairly predictable change in the species composition of a given area over time, leading towards a stable climax community.

Structure and Function of an Ecosystem

An ecosystem's structure is defined by its biotic and abiotic components. Biotic components include all living organisms, categorized by their trophic level:

1. Producers: Autotrophs, primarily photosynthetic organisms like plants and algae, that produce their own food.
2. Consumers: Heterotrophs that depend on producers for food. They are further divided into Primary Consumers (herbivores), Secondary Consumers (carnivores that eat herbivores), and Tertiary Consumers (carnivores that eat other carnivores).
3. Decomposers: Saprotrophs, like fungi and bacteria, that break down dead organic matter (detritus) and release nutrients back into the ecosystem.

Abiotic components are the non-living chemical and physical parts of the environment, such as sunlight, temperature, water, soil, and inorganic nutrients (carbon, nitrogen).

The function of an ecosystem is understood through four key processes:

1. Productivity: The rate of biomass generation. Gross Primary Productivity (GPP) is the total rate of photosynthesis. After accounting for energy used in respiration (R), the remaining energy is Net Primary Productivity (NPP). Thus, NPP = GPP - R. NPP is the energy available to consumers.
2. Decomposition: The essential process of nutrient recycling driven by decomposers.
3. Energy Flow: The unidirectional transfer of energy from the sun to producers and then through various consumer levels.
4. Nutrient Cycling: The cyclical movement of nutrients (e.g., carbon, nitrogen, phosphorus) between biotic and abiotic components, also known as biogeochemical cycles.

The 5 Steps of Decomposition

1. —
2. —
3. —
4. —
5. —

Ecological Pyramids

Must-Remember Facts for Ecosystems

• Energy flow in an ecosystem is always unidirectional, from the sun to producers to consumers. It is never cyclic.
• Nutrient flow is cyclic, moving from the environment to organisms and back to the environment.
• The 10% Law (proposed by Lindeman) states that only about 10% of the energy from one trophic level is transferred to the next.
• The Pyramid of Energy is always upright and is the most accurate representation of trophic structure and ecosystem function.
• Net Primary Productivity (NPP) is the biomass available for consumption by herbivores and decomposers. The formula is NPP = GPP - R.
• Decomposition is an oxygen-requiring process. Its rate is controlled by the chemical composition of detritus (lignin/chitin slow it down) and climatic factors (warmth/moisture speed it up).
• A pioneer species is the first species to colonize a barren area during primary succession (e.g., lichens on rocks).
• A climax community is the stable, self-perpetuating final stage of ecological succession.
• In aquatic ecosystems, light and nutrients (like nitrogen and phosphorus) are often the primary limiting factors for productivity.

CBSE Exam Traps & Tips

Examiners love to test the exceptions and conditions. Be very clear about when pyramids of number and biomass can be inverted (tree ecosystem for number, aquatic for biomass). Always be ready to justify why. The most important trap is the Pyramid of Energy. If an MCQ asks which pyramid can never be inverted, the answer is always the Pyramid of Energy. Justify this with the 10% law and the second law of thermodynamics (energy is lost as heat at each transfer). Also, questions on the factors affecting the rate of decomposition are common. Remember: warm and moist conditions accelerate it, while low temperature, anaerobiosis, and detritus rich in lignin/chitin inhibit it.

Practice Questions with Solutions

• Why is the pyramid of energy always upright? Because energy is lost as heat at each successive trophic level according to the 10% law. The total energy available at a higher level can never be more than the energy at the level below it.
• What is the difference between GPP and NPP? Gross Primary Productivity (GPP) is the total rate of organic matter synthesis by producers. Net Primary Productivity (NPP) is the energy that remains after the producers have used some for their own respiration (R). Formula: NPP = GPP - R.
• Give an example of an ecosystem with an inverted pyramid of biomass and explain why. An aquatic ecosystem (e.g., a pond or ocean). The biomass of producers (phytoplankton) at any given time is less than the biomass of primary consumers (zooplankton). This is because phytoplankton have a high turnover rate (rapid reproduction and short lifespan).
• What are the two main types of nutrient cycles? Give an example for each. 1. Gaseous cycles, which have their reservoir in the atmosphere (e.g., Carbon cycle, Nitrogen cycle). 2. Sedimentary cycles, which have their reservoir in the Earth's crust (e.g., Phosphorus cycle, Sulphur cycle).

Frequently Asked Questions

Frequently Asked Questions