Samacheer Kalvi 12th Biology Chapter 6 Exercise Solutions

An ecosystem is a structural and functional unit of the biosphere where living (biotic) and non-living (abiotic) components interact with each other. Biotic components include producers (green plants), consumers (herbivores and carnivores), and decomposers (bacteria and fungi). Abiotic components include light, temperature, water, soil, minerals, and pH.

A grazing food chain begins with live green plants (producers) and goes through herbivores to carnivores — e.g., Grass → Rabbit → Fox. A detritus food chain begins with dead organic matter (detritus) and goes through decomposers and detritivores — e.g., Leaf litter → Earthworm → Frog. In terrestrial ecosystems, the detritus food chain is the major pathway of energy flow.

The energy pyramid is always upright because energy decreases at each successive trophic level. According to Lindeman's 10% law, only about 10% of energy is transferred from one level to the next; the remaining 90% is lost as heat during respiration and metabolism. Since energy can never increase up the chain, the pyramid must narrow at each level and can never be inverted.

GPP (Gross Primary Productivity) is the total amount of energy fixed by photosynthesis per unit area per unit time. NPP (Net Primary Productivity) is the energy remaining after subtracting respiration losses from GPP. The relationship is: NPP = GPP − R (where R = respiration). NPP represents the energy actually stored in plant biomass and available to consumers.

• Rhizobium — symbiotic nitrogen fixation in legume root nodules (N₂ → NH₃)
• Azotobacter — free-living nitrogen fixation in soil (N₂ → NH₃)
• Nitrosomonas — converts ammonium to nitrite (NH₄⁺ → NO₂⁻)
• Nitrobacter — converts nitrite to nitrate (NO₂⁻ → NO₃⁻)
• Pseudomonas — denitrification, converts nitrate to nitrogen gas (NO₃⁻ → N₂)

Ecological succession is the sequential development of biotic communities in an area over time. Primary succession begins on a bare, lifeless surface (e.g., bare rock) where no soil exists — it is slow and starts with pioneer species like lichens. Secondary succession begins in an area where a previous community was destroyed (e.g., after fire, flood) but soil remains — it is faster because soil and some organisms are already present.

Three types of ecological pyramids:

1. Pyramid of Number: Shows the number of individual organisms at each trophic level.

• Can be upright (grassland: many grasses → fewer rabbits → few foxes) or inverted (single tree → many insects → many parasites).
• Diagram description: Draw trophic levels as stacked bars showing count of organisms.

2. Pyramid of Biomass: Shows the total biomass (dry weight) at each trophic level.

• Can be upright (terrestrial ecosystems) or inverted (aquatic ecosystems).
• Inverted in aquatic ecosystems because phytoplankton (producers) reproduce rapidly and are eaten quickly, so their standing biomass is less than that of zooplankton (consumers) at any given time.
• Diagram description: Draw upright pyramid for forest and inverted pyramid for lake.

3. Pyramid of Energy: Shows the energy content at each trophic level.

• Always upright because energy decreases at each level (10% law).
• Can never be inverted because energy cannot increase up the food chain.

Why biomass pyramid can be inverted: In aquatic ecosystems, phytoplankton have a very high turnover rate — they are eaten as fast as they reproduce. So their standing crop (biomass at any moment) is less than the biomass of zooplankton that feed on them. This makes the pyramid inverted.

The nitrogen cycle involves five key processes, each mediated by specific microorganisms:

1. Nitrogen fixation (N₂ → NH₃/NH₄⁺):

• Symbiotic: Rhizobium in legume root nodules
• Free-living: Azotobacter, Clostridium in soil
• Atmospheric: Lightning and industrial (Haber process)
• Converts inert N₂ gas into biologically usable ammonia

2. Nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻):

• Step 1: Nitrosomonas converts ammonium to nitrite (NH₄⁺ → NO₂⁻)
• Step 2: Nitrobacter converts nitrite to nitrate (NO₂⁻ → NO₃⁻)
• Nitrates are the most usable form of nitrogen for plants

3. Assimilation:

• Plants absorb nitrates through roots and incorporate nitrogen into amino acids and proteins
• Animals obtain nitrogen by consuming plants or other animals

4. Ammonification (Organic N → NH₃):

• Decomposers (bacteria and fungi) break down dead organisms and waste
• Convert organic nitrogen (proteins, urea) back into ammonia (NH₃)

5. Denitrification (NO₃⁻ → N₂):

• Pseudomonas denitrificans converts nitrates back to N₂ gas
• Returns nitrogen to the atmosphere, completing the cycle

Diagram description: Draw a circular flow with arrows: Atmosphere N₂ → Nitrogen fixation → NH₃ → Nitrification → NO₂⁻ → NO₃⁻ → Plant uptake → Animal consumption → Death/waste → Ammonification → NH₃ → back to nitrification; also NO₃⁻ → Denitrification → N₂ atmosphere.

Energy flow in an ecosystem:

• Sun is the ultimate source of energy for all ecosystems
• Producers (plants) capture solar energy through photosynthesis → stored as chemical energy (GPP)
• Energy flows from producers → primary consumers → secondary consumers → tertiary consumers
• At each transfer, only about 10% of energy passes to the next level (Lindeman's 10% law)
• The remaining 90% is lost as:
• Heat during respiration and metabolism
• Energy in dead matter and waste
• Energy used for life processes (movement, digestion)

Example calculation:

• If producers capture 10,000 J of solar energy
• Primary consumers receive: 10,000 × 10% = 1,000 J
• Secondary consumers receive: 1,000 × 10% = 100 J
• Tertiary consumers receive: 100 × 10% = 10 J
• Quaternary consumers would receive only 1 J — insufficient for survival

Why food chains are limited to 3–4 levels:

• At each trophic level, 90% of energy is lost
• After 3–4 transfers, too little energy remains to sustain another level
• This explains why apex predators are rare and ecosystems with long food chains are uncommon
• Also explains why meat production requires much more energy than crop production

Ecological succession is the orderly and sequential development of biotic communities in an area over time.

Primary succession — begins on a bare, lifeless surface with no soil: 1. Pioneer stage: Lichens colonise bare rock → secrete acids that slowly weather rock 2. Moss stage: Mosses grow in accumulated soil matter → add more organic material 3. Herb stage: Grasses and herbs establish → deeper soil forms 4. Shrub stage: Shrubs grow → provide shade and more organic matter 5. Forest stage (climax): Trees establish → stable, self-sustaining community

• Very slow process (hundreds to thousands of years)
• Example: Colonisation of lava flow after volcanic eruption

Secondary succession — begins where a previous community was destroyed but soil remains: 1. Pioneer stage: Hardy grasses and weeds colonise disturbed area 2. Herb stage: Perennial herbs establish 3. Shrub stage: Shrubs and bushes grow 4. Forest stage (climax): Trees dominate → stable community

• Much faster than primary succession (decades rather than centuries)
• Example: Recovery of a forest after a fire or abandoned farmland

Key terms:

• Sere: Entire sequence of communities during succession
• Seral stage: Each intermediate community in the sere
• Climax community: Final, stable community in equilibrium with the environment