Class 12 Biology Notes Chapter 14 (Ecosystem) – Biology Book
Alright class, let's delve into 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 biology are key components. Pay close attention to the definitions, processes, and cycles.
Chapter 14: Ecosystem - Detailed Notes for Government Exam Preparation
1. Ecosystem: Definition, Structure, and Function
- 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).
- Term Coined By: A.G. Tansley (1935).
- Types:
- Terrestrial: Forest, Grassland, Desert.
- Aquatic:
- Freshwater: Ponds, Lakes, Rivers, Wetlands.
- Marine: Oceans, Estuaries, Coral reefs.
- Man-made (Artificial): Crop fields, Aquariums.
- Structural Components:
- Abiotic Components: Non-living factors like temperature, water, light, soil, minerals, humidity, pH, topography.
- Biotic Components: Living organisms.
- Producers (Autotrophs): Organisms that synthesize their own food, primarily through photosynthesis (e.g., plants, algae, photosynthetic bacteria). They form the base of the food chain.
- Consumers (Heterotrophs): Organisms that obtain energy by feeding on other organisms.
- Primary Consumers (Herbivores): Feed on producers (e.g., insects, rabbits, deer).
- Secondary Consumers (Primary Carnivores): Feed on herbivores (e.g., fox, frog, predatory birds).
- Tertiary Consumers (Secondary Carnivores): Feed on secondary consumers (e.g., lion, tiger, snake).
- Omnivores: Feed on both plants and animals (e.g., humans, bears, crows).
- Decomposers (Saprotrophs): Organisms that break down dead organic matter (detritus) into simpler inorganic substances (e.g., bacteria, fungi). They play a vital role in nutrient cycling.
- Functional Aspects: Ecosystem function involves:
- Productivity
- Decomposition
- Energy Flow
- Nutrient Cycling (Biogeochemical Cycles)
2. Productivity
- Definition: The rate of biomass production per unit area over a time period. Expressed as weight (g⁻²) or energy (kcal m⁻²).
- Primary Productivity: Rate of biomass production by producers (photosynthesis).
- Gross Primary Productivity (GPP): The total rate of organic matter production during photosynthesis.
- Net Primary Productivity (NPP): The rate of storage of organic matter by producers after meeting their respiratory losses (R).
NPP = GPP - R - NPP is the available biomass for consumption by heterotrophs (herbivores, decomposers).
- Factors affecting primary productivity: Plant species, photosynthetic capacity, nutrient availability, solar radiation, temperature, soil moisture.
- Highest primary productivity: Tropical rainforests, Coral reefs, Estuaries.
- Lowest primary productivity: Deserts, Arctic regions, Deep oceans.
- Secondary Productivity: The rate of formation of new organic matter by consumers.
3. Decomposition
- Definition: The process of breakdown of complex organic matter (detritus) into simpler inorganic substances like CO₂, water, and nutrients by decomposers.
- Detritus: Dead organic matter, including dead plant remains (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). Increases surface area for microbial action.
- Leaching: Water-soluble inorganic nutrients go 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 slowly, and serves as a reservoir of nutrients.
- Mineralisation: Release of inorganic nutrients (like Ca²⁺, Mg²⁺, K⁺, NH₄⁺) from humus due to degradation by some microbes.
- Factors Affecting Decomposition:
- Oxygen: Decomposition is largely an oxygen-requiring process (aerobic). Anaerobic conditions slow down decomposition.
- 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:
- Temperature: Warm temperatures favour decomposition. Low temperatures inhibit it.
- Moisture: Optimal soil moisture favours decomposition. Too dry or waterlogged conditions inhibit it.
4. Energy Flow
- Source: The Sun is the ultimate source of energy for almost all ecosystems (except deep-sea hydrothermal vents).
- Photosynthetically Active Radiation (PAR): Only 50% of incident solar radiation is PAR (wavelength 400-700 nm). Plants capture only 2-10% of PAR for photosynthesis.
- Unidirectional Flow: Energy flows from the sun to producers, then to consumers. It is not recycled back to the sun.
- Trophic Levels: Specific functional levels occupied by organisms in a food chain based on their source of nutrition.
- Trophic Level 1 (T1): Producers (Plants)
- Trophic Level 2 (T2): Primary Consumers (Herbivores)
- Trophic Level 3 (T3): Secondary Consumers (Carnivores)
- Trophic Level 4 (T4): Tertiary Consumers (Top Carnivores)
- Food Chain: A sequence of organisms where nutrients and energy are transferred from one trophic level to the next.
- Grazing Food Chain (GFC): Starts with producers (plants) -> Herbivores -> Carnivores. Major conduit for energy flow in aquatic ecosystems.
- Detritus Food Chain (DFC): Starts with dead organic matter (detritus) -> Detritivores -> Predators. Major conduit for energy flow in terrestrial ecosystems. DFC may be connected to GFC at some levels.
- Food Web: An interconnected network of several food chains. Provides stability to the ecosystem as organisms often feed at multiple trophic levels (e.g., omnivores).
- Ten Percent Law (Lindeman's Law): Only about 10% of the energy is transferred from one trophic level to the next successive trophic level. The rest is lost during transfer, respiration, and other metabolic activities as heat. This limits the number of trophic levels in a food chain (usually 3-5).
5. Ecological Pyramids
- Definition: Graphical representation of the relationship between different trophic levels in terms of number, biomass, or energy. The base represents producers (T1), and successive tiers represent higher trophic levels.
- Types:
- Pyramid of Numbers: Represents the number of individuals at each trophic level.
- Upright: In most ecosystems (e.g., grassland, pond) - number of producers is maximum, decreasing at successive levels.
- Inverted: In a tree ecosystem - one large tree (producer) supports many insects (primary consumers), which support fewer birds (secondary consumers).
- Spindle-shaped: Can occur in some forest ecosystems.
- Pyramid of Biomass: Represents the total dry weight (biomass) of organisms at each trophic level.
- Upright: In most terrestrial ecosystems (e.g., forest, grassland) - biomass of producers is maximum.
- Inverted: In aquatic ecosystems (e.g., ocean, lake) - biomass of phytoplankton (producers) is less than that of zooplankton (primary consumers) at any given point in time due to the short lifespan and high turnover rate of phytoplankton.
- Pyramid of Energy: Represents the amount of energy flow at successive trophic levels.
- Always Upright: Because energy always decreases at each successive trophic level according to the 10% law. Energy cannot be created, and some is always lost as heat at each transfer. This is the most fundamental and accurate pyramid.
- Pyramid of Numbers: Represents the number of individuals at each trophic level.
- Limitations of Ecological Pyramids:
- Does not account for the same species belonging to two or more trophic levels (food web complexity).
- Assumes simple food chains.
- Does not accommodate decomposers/saprophytes, even though they play a vital role.
6. Ecological Succession
- Definition: The gradual and fairly predictable change in the species composition of a given area over time, leading eventually to a stable climax community.
- Sere: The entire sequence of communities that successively change in a given area.
- Seral Stages/Seral Communities: The individual transitional communities during succession.
- Pioneer Species: The first species to colonize a bare area (e.g., lichens on rocks, phytoplankton in water).
- Climax Community: The final, relatively stable community that is in near equilibrium with the environment.
- Types of Succession:
- Primary Succession: Occurs in areas where no living organisms ever existed (e.g., bare rock, newly cooled lava, newly created pond/reservoir). It is a very slow process as soil formation takes time.
- Secondary Succession: Occurs in areas where existing life was destroyed (e.g., abandoned farmlands, burned or cut forests, flooded lands). It is faster than primary succession because soil or sediment is already present.
- Succession in Plants:
- Hydrarch Succession: Starts in wet areas (e.g., ponds, lakes) and progresses towards mesic (medium water) conditions.
- Stages: Phytoplankton -> Rooted-submerged plants -> Rooted-floating angiosperms -> 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 (Pioneer) -> Mosses -> Annual grasses -> Perennial grasses -> Shrubs -> Forest (Climax).
- Hydrarch Succession: Starts in wet areas (e.g., ponds, lakes) and progresses towards mesic (medium water) conditions.
- Trends during Succession:
- Increase in species diversity.
- Increase in biomass and humus content of soil.
- Shift from r-selected species (pioneers) to K-selected species (climax).
- Increased complexity of food webs.
- Increased nutrient conservation.
7. Nutrient Cycling (Biogeochemical Cycles)
- Definition: The movement of nutrient elements through the various components (biotic and abiotic) of an ecosystem. Also called biogeochemical cycles (Bio = living organisms, Geo = rocks, air, water).
- Types:
- Gaseous Cycles: Reservoir exists in the atmosphere (e.g., Carbon cycle, Nitrogen cycle).
- Sedimentary Cycles: Reservoir exists in the Earth's crust (e.g., Phosphorus cycle, Sulphur cycle).
- Standing State: The amount of nutrients (e.g., carbon, nitrogen, phosphorus) present in the soil at any given time. Varies seasonally and ecosystem-wise.
- Carbon Cycle (Gaseous):
- Reservoir: Atmosphere (CO₂), Oceans (dissolved CO₂), Fossil fuels, Rocks (carbonates).
- Process:
- Photosynthesis: Plants take up atmospheric CO₂.
- Food Chain: Carbon transferred to consumers.
- Respiration: Organisms release CO₂.
- Decomposition: Decomposers release CO₂ from dead organic matter.
- Combustion: Burning of fossil fuels, wood releases CO₂.
- Volcanic activity also releases CO₂.
- Oceans play a major role in regulating atmospheric CO₂.
- Human Impact: Deforestation and burning of fossil fuels have significantly increased atmospheric CO₂ levels, leading to global warming.
- Phosphorus Cycle (Sedimentary):
- Reservoir: Rocks (phosphate rocks). Atmospheric input is negligible.
- Process:
- Weathering: Rocks release phosphates into the soil.
- Plant Uptake: Plants absorb phosphates from the soil.
- Food Chain: Phosphorus transferred to consumers.
- Decomposition: Decomposers return phosphorus to the soil from dead organic matter.
- Runoff: Can carry phosphorus to aquatic ecosystems, potentially causing eutrophication.
- Significant differences from Carbon Cycle: No significant respiratory release of phosphorus; atmospheric inputs are very small; cycling is generally slower.
8. Ecosystem Services
- Definition: The products and benefits obtained from ecosystems that sustain human life.
- Examples:
- Purification of air and water (forests, wetlands).
- Mitigation of droughts and floods.
- Nutrient cycling.
- Generation of fertile soils.
- Providing wildlife habitat.
- Maintenance of biodiversity.
- Pollination of crops.
- Providing aesthetic, cultural, and spiritual values.
- Climate regulation (carbon storage).
- Valuation: Robert Constanza and colleagues tried to put price tags on nature's life-support services, estimated at US $33 trillion a year (1997), nearly twice the value of the global GNP. This highlights the immense value of ecosystems.
Multiple Choice Questions (MCQs)
-
Which of the following represents the correct sequence in the process of decomposition?
(a) Catabolism -> Fragmentation -> Leaching -> Humification -> Mineralisation
(b) Fragmentation -> Leaching -> Catabolism -> Humification -> Mineralisation
(c) Fragmentation -> Humification -> Catabolism -> Leaching -> Mineralisation
(d) Leaching -> Fragmentation -> Humification -> Catabolism -> Mineralisation -
The pyramid of biomass in a sea/ocean ecosystem is generally:
(a) Upright
(b) Inverted
(c) Spindle-shaped
(d) Cannot be determined -
Net Primary Productivity (NPP) is given by the equation:
(a) NPP = GPP + R
(b) NPP = GPP / R
(c) NPP = GPP x R
(d) NPP = GPP - R -
Which ecological pyramid is always upright?
(a) Pyramid of Numbers
(b) Pyramid of Biomass
(c) Pyramid of Energy
(d) Both (a) and (b) -
The pioneer species in Xerarch succession are typically:
(a) Mosses
(b) Lichens
(c) Phytoplankton
(d) Annual grasses -
Which of the following is a sedimentary biogeochemical cycle?
(a) Carbon cycle
(b) Nitrogen cycle
(c) Oxygen cycle
(d) Phosphorus cycle -
The term 'Detritus' refers to:
(a) Living plant material
(b) Dead organic matter
(c) Primary consumers
(d) Inorganic nutrients in the soil -
According to Lindeman's Ten Percent Law, the energy transferred from one trophic level to the next higher trophic level is approximately:
(a) 1%
(b) 10%
(c) 50%
(d) 90% -
Which of the following ecosystems is expected to have the highest primary productivity per unit area?
(a) Temperate forest
(b) Tropical rainforest
(c) Desert
(d) Deep sea -
Humification, a step in decomposition, leads to the accumulation of:
(a) Simple inorganic nutrients
(b) Water-soluble salts
(c) A dark, amorphous substance resistant to microbial action
(d) Small detritus particles
Answer Key for MCQs:
- (b)
- (b)
- (d)
- (c)
- (b)
- (d)
- (b)
- (b)
- (b)
- (c)
Remember to revise these concepts thoroughly. Focus on the definitions, the sequence of processes like decomposition and succession, the factors affecting these processes, and the characteristics of different cycles and pyramids. Good luck with your preparation!