Class 12 Biology Notes Chapter 14 (Ecosystem) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 14: Ecosystem. This is a crucial chapter, not just for your board exams but also for various government competitive exams where environmental science and ecology form a significant part of the syllabus. We will cover the core concepts systematically.

Ecosystem: Structure and Function

1. Definition: An ecosystem is a functional unit of nature, where living organisms (biotic components) interact among themselves and also with the surrounding physical environment (abiotic components). The term was coined by A.G. Tansley.

2. Scope: Ecosystems vary greatly in size, from a small pond to a large forest or an ocean. Even an aquarium can be considered a man-made ecosystem.

3. Components:

   • Abiotic Components: Non-living factors like temperature, water, light, soil, air, inorganic nutrients (carbon, nitrogen, phosphorus), etc. These factors determine the habitat and influence the distribution and functioning of organisms.
   • Biotic Components: Living organisms, categorized based on their nutritional mode:
     • Producers (Autotrophs): Primarily green plants, photosynthetic bacteria, and chemosynthetic microbes that synthesize their own food (e.g., using sunlight). They form the base of the food chain.
     • Consumers (Heterotrophs): Organisms that depend on producers directly or indirectly for food.
       • Primary Consumers (Herbivores): Feed directly on producers (e.g., insects, grazing animals).
       • Secondary Consumers (Primary Carnivores): Feed on herbivores (e.g., fox, frog).
       • Tertiary Consumers (Secondary Carnivores): Feed on secondary consumers (e.g., lion, eagle).
       • Omnivores: Consume both plants and animals (e.g., humans, bears).
     • Decomposers (Saprotrophs): Primarily bacteria and fungi that break down dead organic matter (detritus) from producers and consumers, returning essential nutrients to the soil, water, and air for reuse by producers.

4. Stratification: The vertical distribution of different species occupying different levels in an ecosystem. Example: Trees occupy the top layer in a forest, shrubs the second, and herbs/grasses the bottom layer.

Ecosystem Functions

An ecosystem performs several key functions:

1. Productivity: The rate of biomass production per unit area over a time period.

   • Primary Productivity: Rate of biomass production by producers (photosynthesis).
     • Gross Primary Productivity (GPP): Total rate of energy capture or organic matter synthesis by producers.
     • Net Primary Productivity (NPP): The energy or biomass remaining after producers meet their respiratory needs (R). NPP = GPP - R. This is the biomass available to consumers.
     • Units: (kcal m⁻²) yr⁻¹ or g m⁻² yr⁻¹.
     • Factors Affecting Primary Productivity: Sunlight availability, temperature, moisture, nutrient availability, photosynthetic capacity of plants. Tropical rainforests and coral reefs exhibit high productivity.
   • Secondary Productivity: Rate of formation of new organic matter by consumers.

2. Decomposition: The breakdown of complex organic matter (detritus) into simpler inorganic substances like CO₂, water, and nutrients. This process is crucial for nutrient cycling.

   • Detritus: Dead organic matter, including dead plant parts (leaves, bark, flowers), dead animal remains, and fecal matter.
   • Steps in Decomposition:
     • Fragmentation: Breakdown of detritus into smaller particles by detritivores (e.g., earthworms, termites). This increases the surface area for microbial action.
     • Leaching: Water-soluble inorganic nutrients seep down into the soil horizon and get precipitated as unavailable salts.
     • Catabolism: Enzymatic degradation of detritus into simpler inorganic substances by bacteria and fungi.
     • Humification: Accumulation of a dark-coloured, amorphous, colloidal substance called humus. Humus is highly resistant to microbial action, decomposes very slowly, and serves as a reservoir of nutrients.
     • Mineralisation: Release of inorganic nutrients (like Ca²⁺, Mg²⁺, K⁺, NH₄⁺) from humus by further degradation by some microbes.
   • Factors Affecting Decomposition:
     • Oxygen: Decomposition is largely an oxygen-requiring process. Anaerobic conditions slow it down.
     • Chemical Composition of Detritus: Decomposition is slower if detritus is rich in lignin and chitin; quicker if rich in nitrogen and water-soluble substances like sugars.
     • Climatic Factors: Warm and moist environments favour decomposition; low temperature and anaerobiosis inhibit it.

3. Energy Flow: Energy flows unidirectionally through an ecosystem, from the sun to producers, then to consumers, and eventually dissipates as heat. It does not cycle back.

   • Source: The sun is the ultimate source of energy for almost all ecosystems (except deep-sea hydrothermal vent ecosystems).
   • Capture: Producers capture only a small fraction (typically 1-5% of incident solar radiation, specifically of the Photosynthetically Active Radiation - PAR) to produce organic matter.
   • Trophic Levels: Specific positions in the food chain occupied by organisms based on their feeding habits (e.g., Producer level, Primary Consumer level, etc.).
   • Food Chain: The sequence of organisms where energy is transferred from producers through a series of consumers.
     • Grazing Food Chain (GFC): Starts with producers (plants) -> Herbivores -> Carnivores. This is the major conduit for energy flow in aquatic ecosystems.
     • Detritus Food Chain (DFC): Starts with dead organic matter (detritus) -> Detritivores -> Predators. A large amount of energy flows through the DFC in terrestrial ecosystems. DFC may be connected to GFC at some levels.
   • Food Web: An interconnected network of several food chains. Food webs provide stability to the ecosystem as organisms often have alternative food sources.
   • Ten Percent Law (Lindeman's Law): Only about 10% of the energy from one trophic level is transferred to the next trophic level. The rest is lost during metabolic processes (respiration) or remains unconsumed. This limits the number of trophic levels in a food chain (usually 3-5).

4. Nutrient Cycling (Biogeochemical Cycles): The movement of nutrient elements (e.g., carbon, nitrogen, phosphorus, calcium) through the various biotic and abiotic components of the ecosystem. Nutrients are constantly recycled, unlike energy.

   • Types:
     • Gaseous Cycles: Reservoir is in the atmosphere or hydrosphere (e.g., Carbon cycle, Nitrogen cycle).
     • Sedimentary Cycles: Reservoir is in the Earth's crust (rocks) (e.g., Phosphorus cycle, Sulphur cycle).
   • Carbon Cycle:
     • Reservoir: Atmosphere (CO₂), oceans (dissolved CO₂), fossil fuels, rocks (carbonates).
     • Processes: Photosynthesis (uptake by producers), Respiration (release by organisms), Decomposition (release by decomposers), Combustion (burning of fossil fuels, wood), Volcanic activity. Oceans regulate atmospheric CO₂ levels significantly. Human activities (deforestation, burning fossil fuels) have significantly increased atmospheric CO₂.
   • Phosphorus Cycle:
     • Reservoir: Rocks (phosphate minerals).
     • Processes: Weathering of rocks releases phosphates into soil and water -> Uptake by producers -> Transfer through food chain -> Decomposition returns phosphorus to soil/water.
     • Key Features: Primarily sedimentary; atmospheric input is negligible. Natural release is very slow. Human activities (fertilizers, detergents) can accelerate the cycle, often leading to eutrophication in aquatic systems.

Ecological Pyramids

Graphical representation of the relationship between different trophic levels in terms of number, biomass, or energy. The base always represents producers (first trophic level), and the apex represents top consumers.

1. Pyramid of Numbers: Represents the number of individuals at each trophic level.
   • Upright: Usually, in grassland and pond ecosystems (Producers > Herbivores > Carnivores).
   • Inverted: In a tree ecosystem (One large tree supports many insects, which support fewer birds). Also possible in parasitic food chains.
2. Pyramid of Biomass: Represents the total dry weight (biomass) of organisms at each trophic level.
   • Upright: Generally, for terrestrial ecosystems (Biomass of producers > herbivores > carnivores).
   • Inverted: Common in aquatic ecosystems (e.g., ocean), where the biomass of producers (phytoplankton) at any given time is less than that of primary consumers (zooplankton) due to the rapid turnover rate of phytoplankton.
3. Pyramid of Energy: Represents the rate of energy flow (productivity) at successive trophic levels.
   • Always Upright: Because energy transfer is inefficient (10% law), energy always decreases at successive trophic levels. There are no exceptions. This is the most fundamental and accurate pyramid.

Limitations of Ecological Pyramids:

• Do not account for species belonging to two or more trophic levels (e.g., omnivores).
• Assume simple food chains, not complex food webs.
• Decomposers are not given any place in ecological pyramids.

Ecological Succession

The gradual and fairly predictable change in the species composition of a given area over time.

1. Sere: The entire sequence of communities that successively change in a given area.
2. Seral Stages/Seral Communities: The individual transitional communities during succession.
3. Pioneer Species: The first species to colonise a bare area (e.g., lichens on rocks, phytoplankton in water). They are hardy and often modify the environment, making it suitable for subsequent species.
4. Climax Community: The final, relatively stable community that is in near equilibrium with the environment. It changes very slowly.
5. Types of Succession:
   • Primary Succession: Occurs in areas where no living organisms ever existed (e.g., bare rock, newly cooled lava, newly created pond or reservoir). It is a very slow process as soil formation is needed.
   • Secondary Succession: Occurs in areas where existing life was destroyed by disturbances (e.g., abandoned farmland, burned or cut forests, flooded lands). It is faster than primary succession because soil and some propagules (seeds, roots) are already present.
6. Succession in Plants:
   • Hydrarch Succession: Starts in wet areas (e.g., ponds, lakes) and progresses towards mesic (medium water) conditions. Stages: Phytoplankton -> Submerged plants -> Submerged-free floating plants -> Reed-swamp -> Marsh-meadow -> Scrub -> Forest (climax).
   • Xerarch Succession: Starts in dry areas (e.g., bare rock) and progresses towards mesic conditions. Stages: Lichens (pioneers) -> Mosses -> Annual grasses -> Perennial grasses -> Shrubs -> Forest (climax).
   • Note: Both hydrarch and xerarch succession lead to mesic climax communities (forests, grasslands depending on climate).

Ecosystem Services

The products and processes of ecosystems that benefit humans. Robert Constanza and colleagues tried to put price tags on nature's life-support services. Examples include:

• Purification of air and water
• Mitigation of droughts and floods
• Nutrient cycling
• Generation of fertile soils
• Providing wildlife habitat
• Pollination of crops
• Climate regulation (e.g., carbon storage)
• Aesthetic, cultural, and spiritual values

Multiple Choice Questions (MCQs)

1. Net Primary Productivity (NPP) is given by the equation:
   a) NPP = GPP + R
   b) NPP = GPP / R
   c) NPP = GPP - R
   d) NPP = R - GPP
   Answer: c) NPP = GPP - R

2. Which step in the decomposition process involves the enzymatic degradation of detritus into simpler inorganic substances by microbes?
   a) Fragmentation
   b) Leaching
   c) Catabolism
   d) Humification
   Answer: c) Catabolism

3. According to Lindeman's Ten Percent Law, approximately what percentage of energy is transferred from one trophic level to the next?
   a) 1%
   b) 10%
   c) 50%
   d) 100%
   Answer: b) 10%

4. Which ecological pyramid can never be inverted?
   a) Pyramid of Numbers
   b) Pyramid of Biomass
   c) Pyramid of Energy
   d) Both a and b
   Answer: c) Pyramid of Energy

5. The main reservoir for the phosphorus cycle is:
   a) Atmosphere
   b) Oceans
   c) Rocks
   d) Living organisms
   Answer: c) Rocks

6. Which of the following biogeochemical cycles is primarily sedimentary?
   a) Carbon cycle
   b) Nitrogen cycle
   c) Phosphorus cycle
   d) Water cycle
   Answer: c) Phosphorus cycle

7. The first organisms to colonise a bare rock during primary succession are typically:
   a) Mosses
   b) Grasses
   c) Lichens
   d) Shrubs
   Answer: c) Lichens

8. The Detritus Food Chain (DFC) begins with:
   a) Green plants
   b) Sunlight
   c) Dead organic matter
   d) Primary consumers
   Answer: c) Dead organic matter

9. The vertical distribution of different species occupying different levels in a forest is called:
   a) Stratification
   b) Succession
   c) Trophic structure
   d) Species composition
   Answer: a) Stratification

10. Which of the following represents an ecosystem service?
    a) Production of timber
    b) Pollination of crops
    c) Cycling of nutrients
    d) All of the above
    Answer: d) All of the above

Study these notes carefully. Focus on definitions, processes, examples, and the interconnections between different ecosystem components and functions. Understanding these fundamentals is key for tackling questions in competitive exams. Good luck!