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Ecosystem
Biology · Class 12NCERT pp. 205–2142 sets × 25 questions
Key facts and formulas for Ecosystem, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.
Key facts for this chapter
| Idea | Rule or fact |
|---|
| Structure vs function | Structure: species composition, stratification. Function: productivity, decomposition, energy flow, nutrient cycling. |
| NPP | NPP = GPP − R. Productivity is a rate (per unit area per time); production is the amount. |
| Energy flow | One-way, with heat lost at each step. 10 per cent law (Lindeman): energy at level n = producers × (1/10)^(n−1). |
| PAR | PAR is less than 50 per cent of solar radiation. Plants capture 2 to 10 per cent of the PAR. |
| Decomposition order | Fragmentation, leaching, catabolism, humification, mineralisation. Cold and anaerobic soil slows it; lignin and chitin slow it, nitrogen and sugars speed it. |
| Pyramids | Energy pyramid is always upright. Numbers and biomass can be inverted (one tree and insects; sea phytoplankton). Pyramids ignore saprophytes and food webs. |
| Standing crop vs standing state | Standing crop = mass of living organisms. Standing state = amount of nutrients in soil. |
| Nitrogen cycle | Fixation: Rhizobium, Azotobacter. Nitrification: Nitrosomonas then Nitrobacter. Denitrification: Pseudomonas, Thiobacillus. |
| Succession | Pioneers on rock: lichens. Secondary succession starts where soil exists. Hydrarch and xerarch both reach mesic climax. |
| Names | Haeckel: ecology. Tansley: ecosystem (1935). Lindeman: 10 per cent law (1942). |
Three sample questions
Q1. Which of the following is a man-made ecosystem?
- A wetland
- A crop field
- A desert
- A river
Hint and solution
Hint: Think of an ecosystem that humans create and manage on purpose, not one that formed naturally.
- Ecosystems: natural vs man-made — Ecosystems are grouped as natural (forest, desert, pond, river) and man-made (crop field, aquarium).
- Man-made = created by humans — A man-made ecosystem is designed, planted and maintained by people.
- Wetland, desert, river are natural — These three formed by natural processes without human planning, so they are eliminated.
- Crop field is man-made — So the answer is the crop field. Option B.
Answer: B. A crop field and an aquarium are the standard man-made ecosystems.
Q2. Match the pond components.
Column I: a. Phytoplankton; b. Zooplankton; c. Fungi and bacteria at the bottom; d. Nutrient-rich soil
Column II: (i) Consumer; (ii) Autotroph; (iii) Abiotic component; (iv) Decomposer
- a-(i), b-(ii), c-(iv), d-(iii)
- a-(ii), b-(i), c-(iii), d-(iv)
- a-(iv), b-(i), c-(ii), d-(iii)
- a-(ii), b-(i), c-(iv), d-(iii)
Hint and solution
Hint: Ask for each item: does it make food, eat food, break down dead matter, or is it non-living?
- Autotroph = makes food by photosynthesis — Phytoplankton are tiny floating plants that photosynthesise, so a matches (ii).
- Consumer = eats other organisms — Zooplankton feed on phytoplankton, so b matches (i).
- Decomposer = fungi and bacteria; soil = abiotic — Fungi and bacteria at the bottom break down dead matter, so c is (iv). Nutrient-rich soil is non-living, so d is (iii).
- a-(ii), b-(i), c-(iv), d-(iii) — This combination is given only in option D. Option D.
Answer: D. Phytoplankton make food, zooplankton eat them, fungi and bacteria decompose, and soil is abiotic.
Q3. Which of these is a structural, not a functional, feature of an ecosystem?
- Stratification
- Productivity
- Energy flow
- Nutrient cycling
Hint and solution
Hint: Structure describes how the ecosystem is arranged (layers, species). Function describes processes like flow and cycling.
- Structure: species composition, stratification — Structural features describe what is present and how it is arranged in space.
- Function: productivity, decomposition, energy flow, cycling — Functional features are the processes that run in the ecosystem.
- Check each option — Productivity, energy flow and nutrient cycling are all processes. Stratification is the vertical layering of plants and animals.
- Answer: stratification — So the structural feature is stratification. Option A.
Answer: A. Stratification and species composition describe structure. Productivity, decomposition, energy flow and nutrient cycling are functions.
How to use this chapter
Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.