Ecosystems: Structure, Energy Flow, Cycles and Succession

Prerequisites

Forest Types of India — the vegetation zones that make up India’s terrestrial ecosystems.
Climate of India — temperature and rainfall decide which ecosystem develops where.
Basic biology (photosynthesis, respiration) from Biology and Health.

The Hook: One Mahua Tree in Bastar

Connect

Walk into a sal forest in Bastar in March. A mahua (महुआ) tree is dropping its flowers. Villagers collect them, deer and bears eat them, langurs feed in the canopy, a leopard waits for the langur, and termites and fungi break down every fallen leaf into soil that feeds the sal again.

Nobody planned this system, yet it runs every year on sunlight alone. How does energy move through such a system, why does it get “lost” at each step, and why do nutrients go round in circles while energy does not? That is what the science of ecosystems explains.

Why it matters: Prelims asks exact concepts (10% law, which pyramid can be inverted, who fixes nitrogen, pioneer species, biomagnification). Mains GS-II (Paper IV) asks you to explain ecosystem services and why degrading forests, wetlands or mangroves has economic costs. Chhattisgarh, with forests on about two-fifths of its area, is the natural example for every answer.

Basic Terms of Ecology (पारिस्थितिकी)

The word ecology was coined by Ernst Haeckel (1866). The term ecosystem (पारिस्थितिकी तंत्र) was introduced by A.G. Tansley (1935).

Term	Meaning	Example
Habitat	The physical place where an organism lives	Wild buffalo in the grasslands and sal forests of Udanti
Niche	The organism’s functional role: what it eats, when it is active, what it needs	Vulture as a carcass scavenger
Population	Individuals of one species in an area	Tigers of Achanakmar
Community	Populations of all species living together	All plants and animals of Kanger Valley
Ecosystem	Community + its abiotic environment, interacting as a unit	A pond, a forest, a reservoir
Biome	A large regional ecosystem defined by climate and dominant vegetation	Tropical deciduous forest, desert, tundra
Ecotone	A transition zone between two ecosystems	Mangrove (land–sea), forest edge (forest–grassland)
Edge effect	Ecotones often have more species (and sometimes more density) than either neighbour	Bird diversity at a forest–field boundary
Biosphere	All parts of Earth where life exists	—

Niche vs habitat

Habitat is the address; niche is the profession. Two species can share a habitat, but the competitive exclusion principle (Gause) says two species cannot occupy exactly the same niche for long; one is excluded.

Structure of an Ecosystem

graph TD
    E["Ecosystem"] --> A["Abiotic (non-living)"]
    E --> B["Biotic (living)"]
    A --> A1["Inorganic: water, CO2, N, P, minerals"]
    A --> A2["Organic: proteins, humus"]
    A --> A3["Climatic: light, temperature, rainfall"]
    B --> P["Producers (autotrophs)
green plants, phytoplankton"]
    B --> C["Consumers (heterotrophs)
herbivores, carnivores, omnivores"]
    B --> D["Decomposers
bacteria, fungi"]

Producers (उत्पादक) convert solar energy into chemical energy by photosynthesis. Some bacteria near deep-sea vents are chemosynthetic producers.
Consumers (उपभोक्ता): primary (herbivores), secondary (carnivores eating herbivores), tertiary (top carnivores).
Decomposers (अपघटक) break down dead matter and return nutrients to the soil. Detritivores such as earthworms and termites physically break detritus first.

Functions of an Ecosystem

1. Productivity

Term	Meaning
Gross Primary Productivity (GPP)	Total rate of photosynthesis (energy fixed by producers)
Net Primary Productivity (NPP)	GPP minus energy used by plants in respiration; this is what is available to consumers
Secondary productivity	Rate at which consumers form new organic matter

Among natural ecosystems, tropical rainforests, estuaries, coral reefs and swamps/marshes have very high productivity per unit area. Open oceans have low productivity per unit area but, because of their huge area, contribute a large share of global production. Deserts and the open sea are the least productive per unit area.

2. Energy flow: food chains and food webs

A food chain is a linear sequence: grass → deer → tiger.
Grazing food chain (GFC): starts with living green plants.
Detritus food chain (DFC): starts with dead organic matter (leaf litter → earthworm → bird). In a forest, more energy flows through the detritus chain than through grazing.
A food web is the network of interlinked chains. More interlinking means a more stable ecosystem.
Trophic level (पोषण स्तर): the feeding position (producers = level 1).

Lindeman's 10% law (1942)

Only about 10% of the energy at one trophic level is passed on to the next. The rest is lost mainly as heat in respiration, plus undigested matter.

10,000 J (plants) → 1,000 J (deer) → 100 J (leopard)

Consequences: food chains are short (usually 3–5 levels), top carnivores are few, and energy flow is one-way (unidirectional). It is never recycled.

3. Ecological pyramids (Charles Elton)

Pyramid	Can it be inverted?	Example of inversion
Pyramid of number	Yes	One tree supports many insects, which support even more parasites (inverted / spindle)
Pyramid of biomass	Yes	In the open sea, the standing biomass of phytoplankton is less than that of zooplankton or fish because phytoplankton reproduce very fast
Pyramid of energy	Never	Always upright, because energy is lost at every step (10% law)

4. Biogeochemical (nutrient) cycles

Unlike energy, nutrients cycle. Cycles are of two kinds:

Gaseous cycles (reservoir in the atmosphere): carbon, nitrogen, oxygen.
Sedimentary cycles (reservoir in the Earth’s crust): phosphorus, sulphur, calcium.

Carbon cycle (कार्बन चक्र): Photosynthesis removes CO₂; respiration, decomposition, burning of fossil fuels and forest fires return it. The oceans are the largest active carbon reservoir, and forests and soils are major land sinks. Human disturbance of this cycle is the core of climate change.

Nitrogen cycle (नाइट्रोजन चक्र): Atmospheric N₂ (about 78% of air) cannot be used directly by most plants.

flowchart LR
    A["Atmospheric N2"] -->|"Nitrogen fixation
Rhizobium (legume roots),
Azotobacter, cyanobacteria,
lightning, Haber process"| B["Ammonia / ammonium"]
    B -->|"Nitrification
Nitrosomonas then Nitrobacter"| C["Nitrites then Nitrates"]
    C -->|"Assimilation by plants"| D["Plant and animal proteins"]
    D -->|"Ammonification
by decomposers"| B
    C -->|"Denitrification
Pseudomonas, Thiobacillus"| A

Phosphorus cycle: has no significant atmospheric phase. Phosphorus comes from weathering of rocks. Excess phosphates and nitrates from fertilisers cause eutrophication of lakes and reservoirs (algal blooms, oxygen depletion).

Ecological Succession (पारिस्थितिक अनुक्रमण)

Succession is the gradual, predictable change in species composition of an area until a stable climax community is reached.

Type	Starts on	Speed	Example
Primary succession	Bare area with no soil (bare rock, new volcanic island, sand dune)	Very slow	Lichens on rock
Secondary succession	Area where life existed and soil remains (abandoned field, burnt or felled forest)	Faster	Shifting-cultivation fallow returning to forest

Pioneer species: first colonisers. Lichens on bare rock (xerarch succession, in dry habitats); phytoplankton in water (hydrarch succession).
The series of transitional communities is called a sere (each stage is a seral stage).
Both hydrarch and xerarch successions move towards medium-water (mesic) conditions.
The climax depends on climate. In most of Chhattisgarh the climax is tropical dry or moist deciduous forest dominated by sal or teak.

Ecological Concepts Examiners Love

Concept	Meaning	Example
Keystone species	Has an effect on the ecosystem far larger than its numbers suggest; removing it changes the whole system	Fig trees (food for many birds and mammals in lean seasons), top predators like the tiger
Flagship species	Charismatic species used to win public support for conservation	Tiger, elephant, Asiatic lion
Indicator species	Its presence or absence signals environmental quality	Lichens are sensitive to SO₂ pollution; certain aquatic insects indicate clean water
Umbrella species	Protecting it protects many others in its large range	Tiger, elephant
Invasive alien species	Introduced species that spreads and harms native ecosystems	Lantana camara in central Indian forests, water hyacinth in wetlands, Parthenium
Bioaccumulation	Build-up of a persistent chemical within one organism over its lifetime	Mercury in a single fish
Biomagnification	Increase in concentration of a persistent chemical at each higher trophic level	DDT in fish-eating birds (thin eggshells); mercury in Minamata Bay, Japan

Major Ecosystems and Biomes

Terrestrial

Biome	Key features	India example
Tropical rainforest (evergreen)	Rainfall above 200 cm, multi-layered canopy, highest diversity	Western Ghats, North-East, Andaman
Tropical deciduous forest	Shed leaves in the dry season; India’s most widespread type	Chhattisgarh, Madhya Pradesh, Odisha, Jharkhand
Grasslands	Too dry for forest, maintained by grazing and fire	Banni (Gujarat), Terai grasslands, Deccan grasslands
Desert	Rainfall below about 25 cm, xerophytic plants	Thar
Alpine / tundra-like	Cold, short growing season	Ladakh, high Himalaya

Aquatic

Freshwater: lentic (still: ponds, lakes, reservoirs) and lotic (flowing: rivers, streams). Lakes show zonation (littoral, limnetic, profundal) and seasonal thermal stratification in temperate regions.
Wetlands (आर्द्रभूमि): land where water covers the soil or is near the surface for much of the year. They purify water, recharge groundwater, buffer floods and support migratory birds. India designates wetlands of international importance under the Ramsar Convention. Chhattisgarh’s first Ramsar site is the Kopra reservoir (Bilaspur), designated in 2025. See Lakes of India for the current count.
Estuaries: where rivers meet the sea; brackish water, very high productivity, nurseries for fish.
Mangroves (मैंग्रोव): salt-tolerant (halophytic) trees of tropical intertidal zones, with pneumatophores (breathing roots) and vivipary (seeds germinate on the tree). They shield coasts from cyclones and storm surges. The Sundarbans (West Bengal, shared with Bangladesh) is the largest mangrove forest; Bhitarkanika (Odisha) and the Andaman and Nicobar Islands are other major areas.
Coral reefs: built by coral polyps living in symbiosis with zooxanthellae (algae). They need warm (roughly 20–28°C), clear, shallow, saline water. India’s four major reef areas: Gulf of Mannar, Gulf of Kachchh, Lakshadweep, Andaman and Nicobar Islands. Rising sea temperature causes coral bleaching (zooxanthellae are expelled).

Chhattisgarh angle: an inland state of forests and reservoirs

Chhattisgarh has no coast, mangroves or reefs, so its ecosystem examples are tropical deciduous (sal and teak) forests, grasslands within forests (habitat of the wild buffalo), rivers (Mahanadi, Indravati, Hasdeo) and man-made reservoirs that function as wetlands (Kopra, and large reservoirs such as Gangrel and Hasdeo Bango). For protected areas, see Forests and Wildlife of Chhattisgarh.

Ecosystem Services (Mains)

The Millennium Ecosystem Assessment (2005) grouped the benefits people get from ecosystems into four types:

Service	Examples from Chhattisgarh
Provisioning	Timber, tendu leaves, mahua, sal seeds, lac, fish, water
Regulating	Climate regulation (carbon storage), flood control, water purification, pollination
Cultural	Sacred groves (devgudi), tourism at Chitrakote and Kanger Valley, tribal identity
Supporting	Soil formation, nutrient cycling, primary production

Analysis for Mains: Market prices capture provisioning services but ignore regulating and supporting ones, so forests and wetlands get converted for short-term gain. Tools to correct this include payment for ecosystem services, valuation in environmental impact assessment (EIA), and community rights over forests (Forest Rights Act, 2006).

Memory Tricks

Pyramids: 'Energy Never Inverts'

Energy pyramid is Never inverted. Number pyramid inverts on a tree (one tree, many insects). Biomass pyramid inverts in the Blue sea (phytoplankton).

Nitrogen cycle order: 'Fix, Nitrify, Assimilate, Ammonify, Denitrify' — FNAAD

Nitrosomonas comes before Nitrobacter alphabetically and in the cycle: Nitroso makes nitrite, Nitro-bacter makes nitrate. Pseudomonas “pseudo-returns” nitrogen to the air (denitrification).

Coral reefs of India: 'MAKL'

Mannar, Andaman–Nicobar, Kachchh, Lakshadweep.

Don’t Confuse

This	vs	That
Habitat — where it lives	vs	Niche — what role it plays
Population — one species	vs	Community — all species
Bioaccumulation — within one organism over time	vs	Biomagnification — across trophic levels
Primary succession — no soil, starts with lichens	vs	Secondary succession — soil present, faster
GPP — total fixed	vs	NPP — GPP minus plant respiration
Gaseous cycles — C, N, O	vs	Sedimentary cycles — P, S
Nitrification — ammonia to nitrate (Nitrosomonas, Nitrobacter)	vs	Denitrification — nitrate to N₂ (Pseudomonas)

Common Mistakes

MCQ traps

“Energy flow in an ecosystem is cyclic.” Wrong. Energy flows one way; only nutrients cycle.
“The pyramid of biomass is always upright.” Wrong. It is inverted in the open sea.
“Phosphorus cycle has a large atmospheric reservoir.” Wrong. It is a sedimentary cycle.
“The 10% law means 90% of energy is stored.” Wrong. Most of it is lost as heat in respiration.
“Ecotones have fewer species than adjoining ecosystems.” Usually the opposite: the edge effect.
“Tansley coined ’ecology’.” No. Haeckel coined ecology (1866); Tansley coined ecosystem (1935).

Practice

All questions below are original practice questions written in CGPSC style. They are not previous-year questions.

Level 1: Foundation

The term “ecosystem” was coined by ______ in ______.
Which ecological pyramid is always upright?
The pioneer species in primary succession on bare rock are usually ______.

Answers

A.G. Tansley, 1935.
The pyramid of energy.
Lichens.

Level 2: Prelims MCQs

Q1. Consider the following statements:

The flow of energy in an ecosystem is unidirectional.
The pyramid of biomass in an open-sea ecosystem may be inverted.
The phosphorus cycle is a gaseous cycle.

Which of the statements given above is/are correct?

(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

Answer and explanation

(b) 1 and 2 only. Energy enters as sunlight and leaves as heat, so it flows one way. In the open sea, fast-reproducing phytoplankton have a small standing biomass compared with the zooplankton and fish they support, so the biomass pyramid can be inverted. Statement 3 is wrong: phosphorus has no significant atmospheric phase, so its cycle is sedimentary.

Q2. Match List I (Organism) with List II (Role in the nitrogen cycle):

List I	List II
A. Rhizobium	1. Converts nitrite to nitrate
B. Nitrosomonas	2. Returns nitrogen to the atmosphere
C. Nitrobacter	3. Fixes atmospheric nitrogen in legume roots
D. Pseudomonas	4. Converts ammonia to nitrite

(a) A-3, B-4, C-1, D-2
(b) A-3, B-1, C-4, D-2
(c) A-4, B-3, C-1, D-2
(d) A-2, B-4, C-1, D-3

Answer and explanation

(a) A-3, B-4, C-1, D-2. Rhizobium lives in root nodules of legumes and fixes N₂. Nitrification happens in two steps: Nitrosomonas oxidises ammonia to nitrite, and Nitrobacter oxidises nitrite to nitrate. Pseudomonas is a denitrifier that releases N₂ back to the air.

Q3. If producers in a forest fix 20,000 J of energy, approximately how much energy will reach a secondary consumer, according to the 10% law?

(a) 2,000 J
(b) 200 J
(c) 20 J
(d) 2 J

Answer and explanation

(b) 200 J. Producers (20,000 J) → primary consumers (10% = 2,000 J) → secondary consumers (10% = 200 J). Option (a) is the trap: it is the energy at the primary consumer level.

Q4. The increase in concentration of DDT in fish-eating birds compared with the water of a lake is an example of:

(a) Eutrophication
(b) Bioaccumulation only
(c) Biomagnification
(d) Ecological succession

Answer and explanation

(c) Biomagnification. DDT is persistent and fat-soluble, so its concentration rises at each step of the food chain (water → plankton → fish → bird). Bioaccumulation refers to build-up within a single organism; the comparison across trophic levels makes this biomagnification.

Q5. Which one of the following is an example of an ecotone?

(a) A sal forest in Bastar
(b) A mangrove forest at a river mouth
(c) The deep open ocean
(d) The interior of a desert

Answer and explanation

(b) A mangrove forest. Mangroves occupy the transition zone between land and sea, between fresh and salt water. Ecotones often show the edge effect, with more species than the neighbouring systems.

Level 3: Mains Answer Writing

Q1. (60 words) Distinguish between primary and secondary ecological succession with one example each.

Model answer outline

Primary succession: begins on a surface with no soil (bare rock, cooled lava, sand dune). Pioneers are lichens; soil must form first, so it takes centuries. Example: lichens and mosses on exposed rock.
Secondary succession: begins where a community was removed but soil remains (abandoned field, burnt forest). Faster, because seeds and nutrients are present. Example: a shifting-cultivation fallow in Abujhmarh returning to sal forest.
Conclusion: both proceed towards a climate-determined climax.

Q2. (150–200 words) “Forests are worth more standing than felled.” Examine this statement with reference to ecosystem services, using examples from Chhattisgarh.

Model answer outline

Intro: Ecosystem services are the benefits people derive from ecosystems (Millennium Ecosystem Assessment, 2005): provisioning, regulating, cultural and supporting.
Body:
1. Provisioning: non-timber forest produce (tendu leaves, mahua, sal seeds, lac, honey) gives recurring income to forest-dwelling households, while timber is a one-time gain.
2. Regulating: forests in the catchments of the Mahanadi, Hasdeo and Indravati regulate stream flow, reduce siltation of reservoirs and store carbon.
3. Supporting: soil formation and nutrient cycling sustain rainfed paddy in the plains.
4. Cultural: sacred groves and tribal identity; eco-tourism in Kanger Valley and Chitrakote.
5. Counterpoint: mining (coal, iron ore) brings revenue, jobs and energy security. The real question is valuation: market prices miss regulating and supporting services, so clearance looks cheaper than it is.
Way forward: include ecosystem-service valuation in EIA, respect Forest Rights Act and PESA processes, use compensatory afforestation well, and enforce no-go areas in dense, biodiverse forests.
Conclusion: when all services are counted, a standing forest usually yields more value over time, especially to the communities that depend on it.

Quick Revision Table

Fact	Detail
“Ecology” coined by	Ernst Haeckel, 1866
“Ecosystem” coined by	A.G. Tansley, 1935
10% law	Lindeman, 1942
Pyramid always upright	Energy
Biomass pyramid inverted	Open sea (phytoplankton)
Number pyramid inverted	Tree with insects and parasites
Gaseous cycles	Carbon, nitrogen, oxygen
Sedimentary cycles	Phosphorus, sulphur
Nitrogen fixers	Rhizobium (symbiotic), Azotobacter, cyanobacteria
Nitrification	Nitrosomonas (to nitrite) → Nitrobacter (to nitrate)
Denitrification	Pseudomonas, Thiobacillus
Pioneers	Lichens (rock); phytoplankton (water)
SO₂ indicator	Lichens
India’s coral reef areas	Gulf of Mannar, Gulf of Kachchh, Lakshadweep, Andaman and Nicobar
Chhattisgarh’s first Ramsar site	Kopra reservoir, Bilaspur (2025)

Key Takeaways

An ecosystem is a community plus its abiotic environment; producers, consumers and decomposers are its living parts.
Energy flows one way and about 10% passes to each higher level; nutrients cycle.
Only the pyramid of energy can never be inverted.
Carbon and nitrogen have gaseous cycles; phosphorus and sulphur have sedimentary cycles.
Succession runs from pioneers (lichens, phytoplankton) to a climate-determined climax; in Chhattisgarh that is deciduous sal or teak forest.
Ecosystem services give Mains answers an economic argument for conservation.

What’s Next

Biodiversity: Levels, Hotspots and Threats — the variety of life within these ecosystems
Climate Change — what happens when the carbon cycle is disturbed
Biodiversity and Conservation — protected areas, in-situ and ex-situ methods in detail
Pollution and Waste Management — eutrophication, pollutants and biomagnification in practice

Prerequisites

The Hook: One Mahua Tree in Bastar

Basic Terms of Ecology (पारिस्थितिकी)

Structure of an Ecosystem

Functions of an Ecosystem

1. Productivity

2. Energy flow: food chains and food webs

3. Ecological pyramids (Charles Elton)

4. Biogeochemical (nutrient) cycles

Ecological Succession (पारिस्थितिक अनुक्रमण)

Ecological Concepts Examiners Love

Major Ecosystems and Biomes

Terrestrial

Aquatic

Ecosystem Services (Mains)

Memory Tricks

Don’t Confuse

Common Mistakes

Practice

Level 1: Foundation

Level 2: Prelims MCQs

Level 3: Mains Answer Writing

Quick Revision Table

Key Takeaways

What’s Next

Keep going in this chapter