Ecosystem
Welcome to HSLC Guru! This page provides complete English-medium notes and question-answers for Class 12 Biology Chapter 14 — Ecosystem as per the ASSEB (Assam State School Education Board) syllabus. The chapter explains the ecosystem as a functional unit of nature, its components, productivity, decomposition, energy flow, nutrient cycling, ecological pyramids and ecological succession. The notes are written in simple English so that students can revise quickly and answer board questions confidently.
Summary
An ecosystem is a functional unit of nature where living organisms (biotic components) interact with one another and with their non-living (abiotic) environment. Ecosystems are broadly classified into terrestrial ecosystems such as forest, grassland and desert, and aquatic ecosystems such as pond, lake, river and ocean. Each ecosystem shows variations in its physical structure (vertical stratification), species composition and the interactions among its members. Man-made ecosystems include crop field and aquarium.
The structure of an ecosystem includes its species composition and stratification, while its function involves productivity, decomposition, energy flow and nutrient cycling. Productivity is the rate of biomass production. Gross Primary Productivity (GPP) is the total rate of organic matter production by producers through photosynthesis, while Net Primary Productivity (NPP) = GPP − Respiration losses (R). NPP represents the biomass available to consumers (herbivores and decomposers). Secondary productivity is the rate of formation of new organic matter by consumers.
Decomposition is the breakdown of complex organic matter into simple inorganic substances (CO2, water and nutrients) by decomposers. The important steps are fragmentation (by detritivores), leaching (water-soluble nutrients go into soil), catabolism (enzymatic conversion of detritus into simpler compounds), humification (formation of humus) and mineralisation (release of inorganic nutrients). Energy flow is unidirectional and follows the 10% law of Lindeman — only 10% of energy is transferred from one trophic level to the next. Energy enters through the grazing food chain (GFC) starting with producers and through the detritus food chain (DFC) starting with dead organic matter. The interconnection of food chains forms a food web.
Nutrient cycling (biogeochemical cycle) maintains the supply of nutrients. Gaseous cycles (e.g., nitrogen, carbon) have their reservoir in the atmosphere, whereas sedimentary cycles (e.g., phosphorus, sulphur) have reservoirs in the Earth’s crust. Ecological pyramids represent the relationship between trophic levels in terms of number, biomass or energy. The pyramid of energy is always upright; pyramids of number and biomass may be inverted in some ecosystems. Ecological succession is the orderly and predictable change in community structure over time. Primary succession begins on a bare area (e.g., xerarch on dry rock, hydrarch on water body) where the pioneer community develops first, and through seral stages reaches the stable climax community. Secondary succession occurs on land that lost its community due to fire, flood, etc., and is faster.
Very Short Answer Type Questions (1 Mark)
Q1. What is an ecosystem?
Answer: An ecosystem is a functional unit of nature in which living organisms interact with one another and with their physical environment.
Q2. Define Gross Primary Productivity (GPP).
Answer: GPP is the total rate of production of organic matter by producers per unit area in unit time during photosynthesis.
Q3. Write the formula for Net Primary Productivity (NPP).
Answer: NPP = GPP − Respiratory losses (R).
Q4. Who proposed the 10% law of energy transfer?
Answer: Lindeman (1942) proposed the 10% law of energy transfer in ecosystems.
Q5. Name the two types of food chains found in an ecosystem.
Answer: (i) Grazing Food Chain (GFC) and (ii) Detritus Food Chain (DFC).
Q6. Which ecological pyramid is always upright?
Answer: The pyramid of energy is always upright.
Q7. What is humification?
Answer: Humification is the process of accumulation of dark-coloured, amorphous, partially decomposed organic matter called humus in the soil.
Q8. Define ecological succession.
Answer: Ecological succession is the orderly and gradual change in the species composition of a community over time, leading to a stable climax community.
Q9. What is a pioneer community?
Answer: The pioneer community is the first group of organisms (usually lichens or mosses) that colonise a bare or barren area during primary succession.
Q10. Give one example each of gaseous and sedimentary nutrient cycles.
Answer: Gaseous cycle — Nitrogen cycle / Carbon cycle. Sedimentary cycle — Phosphorus cycle / Sulphur cycle.
Short Answer Type Questions (2-3 Marks)
Q1. Differentiate between terrestrial and aquatic ecosystems with examples.
Answer: Terrestrial ecosystems are land-based ecosystems where soil is the substratum and water availability is the limiting factor. Examples — forest, grassland and desert. Aquatic ecosystems are water-based ecosystems in which water is the medium of life and dissolved oxygen is often a limiting factor. Examples — pond, lake, river and ocean. Terrestrial ecosystems generally show greater productivity per unit area than freshwater ecosystems.
Q2. Distinguish between Gross Primary Productivity and Net Primary Productivity.
Answer: Gross Primary Productivity (GPP) is the total amount of organic matter produced by green plants through photosynthesis in unit area in unit time. Net Primary Productivity (NPP) is the biomass left after producers lose a portion of GPP in respiration. Thus, NPP = GPP − R. NPP represents the biomass available to consumers, while GPP indicates the total photosynthetic output of an ecosystem.
Q3. Write the steps of decomposition in proper sequence.
Answer: The major steps of decomposition are: (i) Fragmentation — breakdown of detritus into smaller particles by detritivores like earthworms; (ii) Leaching — water-soluble inorganic nutrients percolate into soil and become unavailable salts; (iii) Catabolism — bacterial and fungal enzymes degrade detritus into simpler inorganic substances; (iv) Humification — accumulation of dark-coloured humus that is highly resistant to microbial action; and (v) Mineralisation — humus is further degraded to release inorganic nutrients.
Q4. Explain Lindeman’s 10% law of energy flow.
Answer: According to Lindeman (1942), only about 10% of the energy stored at one trophic level is transferred to the next higher trophic level in a food chain. The remaining 90% is lost as heat in respiration, used for metabolic activities, or remains as undigested matter. This law explains why food chains usually have only 4-5 trophic levels.
Q5. What are food chains and food webs? Give one difference.
Answer: A food chain is a linear sequence of organisms in which each one feeds on the previous and is eaten by the next, e.g., grass → grasshopper → frog → snake. A food web is the network of interconnected food chains in an ecosystem. Difference — a food chain shows a single linear pathway of energy, whereas a food web shows multiple alternative pathways and is more stable ecologically.
Q6. Differentiate between primary and secondary succession.
Answer: Primary succession begins in an area where no community has existed before, e.g., bare rock (xerarch) or newly formed water body (hydrarch); it is a slow process and takes hundreds of years. Secondary succession begins in an area where a community already existed but was destroyed by fire, flood, deforestation, etc.; soil and some organisms remain, so it is much faster than primary succession.
Long Answer Type Questions (5-7 Marks)
Q1. Describe the structure and function of an ecosystem.
Answer: An ecosystem is a functional unit of nature consisting of biotic and abiotic components interacting with each other.
Structure: (i) Species composition — identification and enumeration of plant and animal species present. (ii) Stratification — vertical distribution of organisms; e.g., in a forest, trees occupy the top stratum, shrubs the middle and herbs the bottom. (iii) Abiotic components — sunlight, temperature, water, soil, air. (iv) Biotic components — producers, consumers (primary, secondary, tertiary) and decomposers.
Function: The four main functional aspects are — (a) Productivity (rate of biomass production), (b) Decomposition (breakdown of dead organic matter), (c) Energy flow (unidirectional transfer through trophic levels) and (d) Nutrient cycling (biogeochemical cycles of N, C, P, S, etc.). Together these processes maintain the dynamic equilibrium of an ecosystem.
Q2. Explain the process of decomposition with the help of a flow diagram (steps).
Answer: Decomposition is the breakdown of detritus (dead plants, animals, faecal matter) into simpler inorganic substances by decomposers. The process is influenced by temperature, soil moisture, oxygen and the chemical quality of detritus.
Steps: Detritus → Fragmentation by detritivores (earthworms) → Leaching of water-soluble nutrients into soil → Catabolism by bacterial and fungal enzymes converting detritus to simpler substances → Humification producing dark, resistant humus → Mineralisation releasing inorganic nutrients (CO2, water, NH3, etc.) back to the environment.
Decomposition is faster when detritus is rich in nitrogen and water-soluble sugars, and slower when it is rich in lignin and chitin. Warm and moist conditions favour decomposition, while low temperature and anaerobic conditions retard it.
Q3. Describe energy flow in an ecosystem with the help of food chains.
Answer: The Sun is the ultimate source of energy. Producers fix solar energy through photosynthesis; only about 1% of incident PAR (Photosynthetically Active Radiation) is captured by green plants. Energy then flows from producers to consumers through food chains.
Grazing Food Chain (GFC): Begins with producers; e.g., Grass → Goat → Man. Detritus Food Chain (DFC): Begins with dead organic matter and is dominated by saprotrophs (fungi, bacteria); e.g., Dead leaves → Earthworm → Bird. In aquatic ecosystems, GFC is the major channel; in terrestrial ecosystems, DFC may transfer more energy.
Energy flow is unidirectional (does not return to the Sun) and follows the 10% law of Lindeman — at each trophic level, only 10% of the energy is transferred to the next level; the rest is lost as heat. This limits the length of food chains to 4-5 trophic levels.
Q4. What are ecological pyramids? Describe the three types with reasons for their shape.
Answer: An ecological pyramid is a graphical representation of the relationship between organisms at different trophic levels in terms of number, biomass or energy. Producers form the base and successive consumers occupy higher tiers.
(i) Pyramid of Number: Shows number of individuals at each trophic level. Usually upright (grassland) but inverted in a tree ecosystem (one tree supports many insects, birds, parasites).
(ii) Pyramid of Biomass: Shows total dry weight of organisms at each trophic level. Usually upright on land. Inverted in a pond ecosystem because the standing crop of phytoplankton (producers) is small compared with that of fishes (consumers).
(iii) Pyramid of Energy: Shows the rate of energy flow at each trophic level. It is always upright because energy decreases at each successive level due to the 10% rule, so producers always have the maximum energy and top consumers the least.
Q5. Describe the process of ecological succession in a water body (hydrarch succession).
Answer: Hydrarch succession is the primary succession that begins in a fresh water body (pond/lake) and ends in a mesic (moderate moisture) climax community.
The seral stages are: (i) Phytoplankton stage — pioneer community of algae and diatoms; (ii) Submerged plant stage — Hydrilla, Vallisneria; (iii) Submerged free-floating stage — Wolffia, Lemna; (iv) Reed swamp / amphibious stage — Typha, Sagittaria; (v) Sedge meadow / marsh-meadow stage — Carex, Juncus; (vi) Woodland stage — shrubs and small trees; (vii) Climax community — forest of large trees in equilibrium with the climate.
Each seral stage modifies the environment, making it less suitable for itself and more suitable for the next community, until a stable climax is reached. Hydrarch succession converts a water body into a land community.
Multiple Choice Questions (MCQ)
Q1. The 10% law of energy transfer was given by —
(a) Odum (b) Tansley (c) Lindeman (d) Elton
Answer: (c) Lindeman.
Q2. Which of the following pyramids is always upright?
(a) Pyramid of number (b) Pyramid of biomass (c) Pyramid of energy (d) None
Answer: (c) Pyramid of energy.
Q3. NPP is equal to —
(a) GPP + R (b) GPP − R (c) GPP × R (d) GPP / R
Answer: (b) GPP − R.
Q4. Which of the following is a sedimentary cycle?
(a) Carbon cycle (b) Nitrogen cycle (c) Phosphorus cycle (d) Oxygen cycle
Answer: (c) Phosphorus cycle.
Q5. The first step of decomposition is —
(a) Mineralisation (b) Catabolism (c) Fragmentation (d) Humification
Answer: (c) Fragmentation.
Q6. Pioneer species in a hydrarch succession are —
(a) Lichens (b) Mosses (c) Phytoplankton (d) Grasses
Answer: (c) Phytoplankton.
Q7. Which of the following is an example of an aquatic ecosystem?
(a) Forest (b) Grassland (c) Desert (d) Pond
Answer: (d) Pond.
Q8. The grazing food chain begins with —
(a) Detritus (b) Producers (c) Decomposers (d) Carnivores
Answer: (b) Producers.
Q9. Pioneer species on bare rock are —
(a) Mosses (b) Ferns (c) Lichens (d) Trees
Answer: (c) Lichens.
Q10. The stable end community of a succession is called —
(a) Pioneer community (b) Seral community (c) Climax community (d) Sub-climax
Answer: (c) Climax community.
Fill in the Blanks
Q1. The amount of biomass available to consumers is called __________ Primary Productivity.
Answer: Net.
Q2. The accumulation of dark-coloured organic matter in soil during decomposition is known as __________.
Answer: Humification.
Q3. __________ cycle has its reservoir in the Earth’s crust.
Answer: Sedimentary (Phosphorus).
Q4. Succession that begins on a bare rock is called __________ succession.
Answer: Xerarch (primary).
Q5. The pyramid of __________ is always upright.
Answer: Energy.
True / False
Q1. Energy flow in an ecosystem is bidirectional.
Answer: False. (Energy flow is unidirectional.)
Q2. The pyramid of biomass in a pond ecosystem is inverted.
Answer: True.
Q3. Phosphorus cycle is a gaseous cycle.
Answer: False. (It is a sedimentary cycle.)
Q4. Lichens are pioneers on bare rock.
Answer: True.
Q5. Secondary succession is slower than primary succession.
Answer: False. (Secondary succession is faster because soil already exists.)
Glossary
| Term | Meaning |
|---|---|
| Ecosystem | Functional unit of nature where biotic and abiotic components interact. |
| Productivity | Rate of biomass production per unit area per unit time. |
| GPP | Gross Primary Productivity — total photosynthetic output of producers. |
| NPP | Net Primary Productivity = GPP − Respiration; biomass available to consumers. |
| Decomposition | Breakdown of dead organic matter into simpler inorganic substances. |
| Fragmentation | Breakdown of detritus into smaller particles by detritivores. |
| Leaching | Percolation of water-soluble nutrients into deeper soil layers. |
| Catabolism | Enzymatic breakdown of detritus by bacteria and fungi. |
| Humification | Formation of dark, amorphous, partially decomposed humus. |
| Mineralisation | Release of inorganic nutrients from humus. |
| Food Chain | Linear sequence of organisms through which food energy flows. |
| Food Web | Network of interconnected food chains. |
| GFC | Grazing Food Chain — starts with producers. |
| DFC | Detritus Food Chain — starts with dead organic matter. |
| 10% Law | Lindeman’s law — only 10% of energy is transferred to next trophic level. |
| Ecological Pyramid | Graphical representation of trophic structure (number, biomass, energy). |
| Gaseous Cycle | Nutrient cycle with reservoir in atmosphere (N, C). |
| Sedimentary Cycle | Nutrient cycle with reservoir in Earth’s crust (P, S). |
| Succession | Gradual and orderly change in community over time. |
| Xerarch | Primary succession on dry rock/land. |
| Hydrarch | Primary succession in a water body. |
| Pioneer Community | First colonisers of a bare area (lichens, phytoplankton). |
| Climax Community | Stable, self-perpetuating final community in equilibrium with climate. |
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