Ecosystem Class 12 Chapter Notes | CBSE Biology

Key Ecological Terms

Ecosystem

A community of living organisms (biotic components) interacting with the non-living components (abiotic components) of their environment as a system.

Stratification

The vertical distribution of different species occupying different levels in an ecosystem (e.g., trees at the top, shrubs in the middle, herbs and grasses at the bottom).

Productivity

The rate of biomass or organic matter accumulation per unit area per unit time.

Gross Primary Productivity (GPP)

The total rate of organic matter production during photosynthesis by producers in an ecosystem.

Net Primary Productivity (NPP)

The amount of biomass available to heterotrophs, calculated as GPP minus respiration (R) losses by producers (NPP = GPP - R).

Decomposition

The process by which decomposers break down complex organic matter into simpler inorganic substances like CO₂, water, and nutrients.

Humification

The accumulation of a dark-coloured, amorphous substance called humus, which is highly resistant to microbial action and undergoes decomposition at a very slow rate.

Mineralisation

The process by which humus is further degraded by microbes, releasing inorganic nutrients into the soil.

Standing Crop

The mass of living organisms (biomass) at a particular trophic level at a particular time.

Standing State (Standing Quality)

The amount of inorganic nutrients (e.g., C, N, P, Ca) present in the soil at any given time.

Ecological Succession

The gradual and predictable change in the species composition of an area over time.

Climax Community

The final, stable, and self-perpetuating community that develops at the end of ecological succession, in equilibrium with the environment.

Ecosystem: Structure and Function

An ecosystem is a functional unit where living organisms interact among themselves and with the surrounding physical environment. It can be as small as a pond or as large as an ocean. The structure of an ecosystem is characterized by the species composition and stratification. Species composition refers to the identification and enumeration of plant and animal species found in the ecosystem. Stratification is the vertical distribution of different species occupying different levels (e.g., in a forest, trees occupy top vertical strata or layers, shrubs the second, and herbs and grasses the bottom layers).

The components of an ecosystem are broadly divided into abiotic (non-living) and biotic (living). Abiotic components include climatic factors (temperature, light, rainfall), edaphic factors (soil, minerals), and inorganic substances (water, oxygen, carbon dioxide). Biotic components include producers (autotrophs like plants, algae), consumers (heterotrophs like herbivores, carnivores, omnivores), and decomposers (saprotrophs like bacteria and fungi).

Functional aspects or processes that operate within an ecosystem are:

1. Productivity: The rate of biomass production.
2. Decomposition: The breakdown of complex organic matter.
3. Energy Flow: The movement of energy through different trophic levels.
4. Nutrient Cycling: The circulation of essential elements between biotic and abiotic components.

Ecosystem Productivity

Decomposition: The Breakdown Process

1. Fragmentation — Detritivores, like earthworms, break down detritus into smaller particles, increasing the surface area for microbial action.
2. Leaching — Water-soluble inorganic nutrients seep down into the soil horizon and get precipitated as unavailable salts, a process called leaching.
3. Catabolism — Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
4. Humification — Formation of humus – a dark-coloured, amorphous, highly resistant colloidal substance, which is a reservoir of nutrients and prevents leaching.
5. Mineralisation — Further degradation of humus by microbes, releasing inorganic nutrients back into the soil (e.g., CO₂, H₂O, mineral salts).

Energy Flow in Ecosystems

Ecological Pyramids

Ecological Succession

Nutrient Cycling (Biogeochemical Cycles)

Key Points to Remember

• An ecosystem is a structural and functional unit of nature, comprising biotic and abiotic components.
• The four major functional components of an ecosystem are productivity, decomposition, energy flow, and nutrient cycling.
• NPP (Net Primary Productivity) = GPP (Gross Primary Productivity) - R (Respiration loss). NPP is the available biomass for heterotrophs.
• Decomposition involves fragmentation, leaching, catabolism, humification, and mineralisation.
• Energy flow in an ecosystem is always unidirectional and follows the 10% law, meaning only 10% of energy is transferred to the next trophic level.
• Ecological pyramids can be of number, biomass, or energy. The pyramid of energy is always upright.
• Ecological succession is the gradual, predictable change in species composition over time, leading to a stable climax community.
• Primary succession occurs on barren land, while secondary succession occurs in disturbed areas with existing soil.
• Nutrient cycles (biogeochemical cycles) involve the circulation of elements like carbon and phosphorus. Carbon is a gaseous cycle, Phosphorus is a sedimentary cycle.

Worked Example: Energy Transfer

• Example 1: Ten Percent Law Application If producers in an ecosystem fix 10,000 J of solar energy, how much energy will be available to the primary carnivores? Solution: Producers: 10,000 J Primary Consumers (Herbivores): 10% of 10,000 J = 1,000 J * Secondary Consumers (Primary Carnivores): 10% of 1,000 J = 100 J

Section 11

When answering questions on Ecosystems, always:

• Define key terms clearly (e.g., GPP, NPP, Detritus, Humus, Succession).
• Draw and label diagrams for food chains, food webs, and ecological pyramids if asked. Remember the always upright nature of the pyramid of energy.
• Provide examples where appropriate, especially for different types of food chains or successional stages.
• Explain the 'Why': For instance, why is the pyramid of energy always upright? (Because energy is lost as heat at each trophic level).
• Distinguish carefully between primary and secondary succession, and gaseous vs. sedimentary cycles, highlighting key differences in pioneer species, time taken, and main reservoirs.

Practice Questions with Solutions

• Q: Differentiate between GPP and NPP. Which one represents the available energy for heterotrophs? A: GPP (Gross Primary Productivity) is the total rate of organic matter production during photosynthesis. NPP (Net Primary Productivity) is GPP minus respiration losses (R), representing the biomass available for heterotrophs.
• Q: What are the main steps involved in the process of decomposition? A: The main steps are fragmentation, leaching, catabolism, humification, and mineralisation.
• Q: Why is the pyramid of energy always upright? A: The pyramid of energy is always upright because energy is lost as heat at each successive trophic level, meaning less energy is available to higher trophic levels according to the 10% law.
• Q: Give one major difference between primary and secondary succession. A: Primary succession occurs in areas where no life existed before or no soil is present (e.g., bare rock), while secondary succession occurs in areas where a community has been disturbed but soil remains intact (e.g., abandoned farmland).

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