By Fatskills Exam Guides Team — the exam nerds behind 28,500+ quizzes and 2.1M practice questions across 500+ global exams.
Energy flow in an ecosystem is unidirectional and non-cyclic, starting from solar radiation to producers and ending with top carnivores. Example: Sun → Grass → Grasshopper → Frog → Snake → Hawk.
Only 10% of energy is transferred from one trophic level to the next; the rest is lost as heat during metabolic processes (Lindeman’s 10% law, 1942).
Producers (autotrophs) fix 2–10% of solar energy through photosynthesis; the rest is reflected or lost as heat.
Gross Primary Productivity (GPP) is the total organic matter synthesized by producers; Net Primary Productivity (NPP) = GPP – R (where R is respiratory losses).
NPP of terrestrial ecosystems averages 170 billion tons/year; oceans contribute about 55 billion tons/year (verify from NCERT).
Detritus food chain begins with dead organic matter (detritus) and ends with decomposers like fungi and bacteria; example: fallen leaves → earthworm → bird.
Grazing food chain starts with living green plants; example: phytoplankton → zooplankton → fish.
Food web increases ecosystem stability by providing alternative pathways for energy flow; example: a single insect may be eaten by multiple predators.
Ecological pyramids represent trophic structure: pyramid of energy is always upright; pyramid of biomass in aquatic systems may be inverted (e.g., phytoplankton → fish).
Pyramid of numbers in a tree ecosystem can be inverted: one tree supports numerous birds and insects.
Detritivores (e.g., earthworms, millipedes) break down detritus into smaller particles; decomposers (e.g., bacteria, fungi) mineralize it.
In sedimentary cycles (e.g., phosphorus cycle), the reservoir is the Earth’s crust; in gaseous cycles (e.g., carbon cycle), it is the atmosphere.
Phosphorus cycle lacks a gaseous phase; phosphates are released from rocks by weathering and enter soil and water.
Carbon cycle reservoirs: atmosphere (CO₂), oceans (dissolved CO₂), fossil fuels, and biomass; human activities like combustion increase atmospheric CO₂.
Nitrogen fixation is carried out by free-living bacteria (e.g., Azotobacter) and symbiotic bacteria (e.g., Rhizobium in root nodules of legumes).
Ammonification is the conversion of organic nitrogen into ammonia by decomposers; example: protein → amino acids → NH₃.
Nitrification involves two steps: Nitrosomonas converts NH₄⁺ to NO₂⁻; Nitrobacter converts NO₂⁻ to NO₃⁻.
Denitrification by Pseudomonas and Thiobacillus converts nitrates back to atmospheric N₂.
Upright pyramid of biomass in grassland: producers (grasses) > primary consumers (herbivores) > secondary consumers (carnivores).
Energy flow follows the second law of thermodynamics: energy transformations are inefficient and result in entropy increase.
Intermediate — Requires understanding of interlinked concepts like energy transfer efficiency, pyramid variations, and cycle types, but all based on NCERT-defined examples and laws.
Trap: Assuming all ecological pyramids are upright. Avoid: Remember that pyramid of biomass in aquatic ecosystems (e.g., pond) is often inverted due to low producer biomass but high consumer turnover.
Trap: Confusing detritus food chain with grazing food chain origin. Avoid: Detritus food chain starts with dead organic matter; grazing food chain starts with living producers.
Trap: Thinking phosphorus cycle has a significant atmospheric component. Avoid: Phosphorus cycle is sedimentary with no major gaseous phase; phosphates enter ecosystems via rock weathering.
Q1. Which of the following represents the correct sequence in a grazing food chain? A. Snake → Frog → Insect → Grass B. Grass → Insect → Frog → Snake C. Frog → Insect → Grass → Snake D. Insect → Grass → Snake → Frog
Answer: B Explanation: Grazing food chain starts with producers (grass), followed by primary (insect), secondary (frog), and tertiary (snake) consumers. Why others fail: Option A reverses the entire chain, a common mistake when not identifying the producer.
Q2. In a pond ecosystem, the pyramid of biomass is typically: A. Upright B. Inverted C. Spindle-shaped D. Absent
Answer: B Explanation: In ponds, phytoplankton (producers) have short life spans and low standing biomass compared to longer-lived fish (consumers), leading to an inverted pyramid. Why others fail: Students often assume all biomass pyramids are upright, ignoring aquatic exceptions.
Q3. What percentage of energy is typically transferred from one trophic level to the next? A. 1% B. 10% C. 50% D. 90%
Answer: B Explanation: Lindeman’s 10% law states that only about 10% of energy is transferred between successive trophic levels. Why others fail: Option D (90%) is the energy lost, not transferred—confusing transfer with loss is a frequent error.
Q4. Which bacteria convert nitrite (NO₂⁻) to nitrate (NO₃⁻) in the nitrogen cycle? A. Rhizobium B. Azotobacter C. Nitrosomonas D. Nitrobacter
Answer: D Explanation: Nitrobacter performs the second step of nitrification by oxidizing NO₂⁻ to NO₃⁻. Why others fail: Nitrosomonas (option C) converts NH₄⁺ to NO₂⁻, so students often mix up the two nitrifying bacteria.
Q5. Which of the following cycles is sedimentary and lacks a gaseous phase? A. Carbon cycle B. Nitrogen cycle C. Water cycle D. Phosphorus cycle
Answer: D Explanation: The phosphorus cycle is sedimentary with no significant atmospheric phase; phosphorus moves through soil, water, and living organisms. Why others fail: The nitrogen cycle (option B) has a large gaseous phase, making it tempting but incorrect.
Join 4M+ learners. Unlock unlimited quizzes, wrong-answer tracking, flashcards + reminders, study guides, and 1-on-1 challenges.