Ecosystem – Structure, Functions and Types

Ecosystem: Structure, Functions and Types (UPSC Prelims + Mains)

Think of a village pond in India. In the morning, you may see green algae on the surface, small insects skating on water, fish eating larvae, birds catching fish, and fallen leaves slowly decomposing at the bottom. This is not "just nature." It is a working system where living components and non-living components interact continuously. In Environmental Ecology, UPSC expects you to understand this system clearly and apply it to issues like wetlands, forests, climate change, biodiversity loss, invasive species, and conservation.

1) Levels of Organisation: Where does "Ecosystem" fit?

To understand ecosystem properly, we must place it in the correct ecological hierarchy.

2) Ecosystem Structure: What an ecosystem is "made of"

The structure of an ecosystem means the arrangement of its components and their relationships. It has two broad parts: (A) Abiotic structure and (B) Biotic structure.

2.1 Abiotic Structure

• Inorganic substances: water, carbon dioxide, oxygen, nitrogen, phosphorus, minerals.
• Organic substances: carbohydrates, proteins, humus, leaf litter, dead organic matter.
• Physical factors: sunlight, rainfall, wind, temperature, humidity, soil type, pH, salinity.

2.2 Biotic Structure

Biotic components are usually grouped by their role in food and energy relationships.

3) Food Chain and Food Web: Who eats whom?

Food relationships show how energy moves from one organism to another.

3.1 Food Chain

A food chain is a linear pathway of feeding. It is simple but does not show the full reality (because organisms usually have multiple food options).

Two common types:

• Grazing food chain: starts with living green plants (grass → herbivore → carnivore).
• Detritus food chain: starts with dead organic matter (leaf litter → detritivore → predators → decomposers).

3.2 Food Web

A food web is a network of interconnected food chains. It increases stability because if one prey declines, predators can shift to another food source.

UPSC angle: Food webs explain why ecosystems can tolerate some shocks (like drought or seasonal changes) better than a single chain.

4) Ecological Pyramids: How energy and biomass are distributed

Ecological pyramids help us compare quantities across trophic levels.

4.1 Pyramid of Numbers

• Usually upright (many grasses support fewer deer, fewer tigers).
• Can be inverted (one big tree can support many insects and birds).

4.2 Pyramid of Biomass

• Usually upright in terrestrial ecosystems.
• Often inverted in aquatic ecosystems: phytoplankton biomass at a moment may be small, but it reproduces very fast and supports large biomass of zooplankton and fish.

4.3 Pyramid of Energy

Always upright because energy is lost at every trophic level as heat during respiration and metabolism.

5) Ecosystem Functions: How ecosystems "work"

Ecosystem functions are the processes that keep the system running. The three most important functions for UPSC are:

• Productivity (biomass production)
• Energy flow
• Nutrient cycling (biogeochemical cycles)

5.1 Productivity: GPP and NPP

• High productivity ecosystems: tropical rainforests, wetlands, estuaries, coral reefs (because conditions support fast growth).
• Low productivity ecosystems: deserts, cold regions (limited water or low temperature).

5.2 Energy Flow: One-way movement

Energy enters ecosystems mainly as sunlight. Producers capture it. Consumers get it by eating producers/other consumers. In each transfer, large energy is lost as heat. Therefore:

• Energy flow is unidirectional (sun → producers → consumers → decomposers → heat loss).
• Energy is not recycled, but nutrients are recycled.
• Because energy reduces at each step, food chains are usually short (often 3–5 trophic levels).

5.3 Decomposition: Nature's recycling factory

Decomposition converts complex organic matter into simpler inorganic nutrients, which again become available to plants.

Main steps (remember this sequence for Prelims):

• Fragmentation: detritivores break litter into smaller pieces.
• Leaching: water dissolves and carries soluble substances into soil.
• Catabolism: microbes break down complex molecules.
• Humification: formation of humus (dark, stable organic matter).
• Mineralisation: release of inorganic nutrients (like nitrates, phosphates).

Factors affecting decomposition:

• Climate: warm and moist conditions increase decomposition (faster in tropical forests than cold regions).
• Litter quality: high lignin and tannins slow decomposition (some evergreen leaves decompose slowly).

6) Nutrient Cycling: Biogeochemical cycles

Nutrients move in cycles between living organisms and the physical environment.

6.1 Carbon Cycle (High UPSC relevance)

• Plants absorb CO₂ through photosynthesis and store carbon in biomass.
• Animals get carbon by eating plants/other animals.
• Respiration, decomposition, and burning release CO₂ back to atmosphere.
• Oceans also absorb and store carbon (important for climate change).

6.2 Nitrogen Cycle

• Atmospheric nitrogen is abundant but plants cannot use it directly.
• Nitrogen fixation converts nitrogen into usable forms (by bacteria like Rhizobium in legume roots, and also by lightning to some extent).
• Nitrification converts ammonia into nitrites/nitrates.
• Denitrification returns nitrogen back to atmosphere.

6.3 Phosphorus Cycle

• Phosphorus mostly comes from rocks (weathering).
• It has no significant gaseous phase, so it can become limiting in ecosystems.
• Runoff can carry phosphates into water bodies and cause eutrophication (algal blooms).

7) Ecosystem Services: Benefits humans receive from ecosystems

UPSC increasingly asks ecology in an applied way: what ecosystems do for people and development.

7.1 Major categories (easy to remember)

• Provisioning services: food, timber, fish, medicinal plants, fresh water.
• Regulating services: climate regulation, flood control, pollination, disease regulation.
• Supporting services: soil formation, nutrient cycling, primary production (these support all other services).
• Cultural services: recreation, tourism, spiritual value (sacred groves, pilgrimage landscapes).

Indian examples:

• Mangroves (e.g., Sundarbans): protect coast from cyclones and storm surges (regulating service).
• Wetlands (e.g., Keoladeo, Chilika): water purification and fishery support.
• Forests (e.g., Western Ghats): rainfall regulation, soil conservation, carbon storage.

8) Stability, Homeostasis, and Biodiversity: Why some ecosystems survive disturbances

Ecosystems face disturbances such as droughts, floods, fires, invasive species, and human pressure. Yet many ecosystems show the ability to maintain balance.

8.1 Role of biodiversity

• More species means more alternative pathways for energy and nutrients.
• If one species declines, another can perform a similar function (functional redundancy to some extent).
• Hence, biodiversity supports stability, but the relationship is not always perfectly linear.

8.2 Ecotone and edge effect (frequent Prelims concept)

9) Ecological Succession: How ecosystems change over time

Ecosystems are dynamic. Over years or decades, species composition can change in a predictable manner, especially after disturbances.

9.1 Types of succession

• Primary succession: starts on bare area without soil (new rock surface, fresh sand, lava). It is slow because soil must form first.
• Secondary succession: starts where soil already exists but community was disturbed (fire, flood, cultivation). It is faster than primary succession.

9.2 Why UPSC cares

• Restoration ecology uses succession concepts to regenerate degraded forests and grasslands.
• After floods or landslides in Himalayas, natural succession explains vegetation recovery patterns.

10) Types of Ecosystems: Classification you must remember

UPSC questions often test classification and examples. Ecosystems can be classified on different bases.

10.1 Based on origin

• Natural ecosystems: forests, grasslands, deserts, ponds, rivers, oceans.
• Artificial (man-made) ecosystems: crop fields, plantations, reservoirs, aquariums, urban ecosystems.

10.2 Based on habitat: Terrestrial and Aquatic

A) Terrestrial ecosystems

They are found on land and are strongly influenced by temperature and rainfall.

B) Aquatic ecosystems

Aquatic ecosystems are influenced by water depth, flow, salinity, temperature, and dissolved oxygen.

10.3 Based on size

• Micro-ecosystem: a small puddle, a fallen log, a tiny pond corner.
• Meso-ecosystem: a lake, a forest patch, a grassland region.
• Macro-ecosystem: a biome-level system like tropical rainforest biome.

11) Common UPSC Confusions: Quick clarity points

• Ecosystem vs Community: Community is only living organisms; ecosystem includes abiotic environment too.
• Energy flow vs Nutrient cycling: Energy flows one-way and is lost as heat; nutrients recycle.
• Food chain vs Food web: chain is linear; web is interconnected and usually more stable.
• Productivity: NPP is the energy available for consumers; that is why NPP matters more in ecology questions.
• Decomposers: they are essential because without nutrient recycling, producers will run out of minerals.

12) Relevance for India: Ecosystem approach in governance and conservation

UPSC Mains frequently expects you to connect ecosystem understanding with real policy and environmental management.

• Wetland conservation: wetlands act as flood buffers and water purifiers; destroying them increases urban flooding.
• Mangroves and coastal safety: mangroves reduce cyclone impacts; restoring mangroves is a nature-based solution.
• Forest ecosystem and climate goals: forests store carbon; degradation reduces carbon sink capacity.
• Invasive species: invasives can disturb food webs and nutrient cycles, harming native biodiversity.
• River ecosystems: flow alteration by dams affects fish migration, sediment transport, and estuarine productivity.

UPSC Previous Year Question Themes (PYQs) with Approach

Practice MCQs (UPSC Level) with Answers and Explanations

1. Q1. Which one of the following best describes an ecosystem?

   • A) A group of organisms of the same species living together
   • B) A community of organisms interacting only with each other
   • C) A functional unit where biotic and abiotic components interact through energy flow and nutrient cycling
   • D) A large region with uniform climate and vegetation

   Answer: C

   Explanation: Ecosystem includes both biotic and abiotic components working together as a functional unit.

2. Q2. Which statement is correct about energy flow in an ecosystem?

   • A) Energy is recycled like nutrients
   • B) Energy flow is cyclic and returns fully to producers
   • C) Energy flow is unidirectional and decreases at each trophic level
   • D) Energy increases at higher trophic levels

   Answer: C

   Explanation: Energy moves one-way (sun → producers → consumers) and is lost as heat at every step.

3. Q3. The pyramid of energy in an ecosystem is always upright because:

   • A) Producers are always more in number than consumers
   • B) Energy at each trophic level is fully converted into biomass
   • C) Energy is lost as heat during metabolic activities at each trophic level
   • D) Consumers cannot respire

   Answer: C

   Explanation: Heat loss through respiration ensures less energy is available at higher trophic levels.

4. Q4. Net Primary Productivity (NPP) is best understood as:

   • A) Total solar energy falling on an ecosystem
   • B) Total energy captured by producers before respiration
   • C) Energy stored by producers after subtracting respiration losses
   • D) Energy stored at the top trophic level

   Answer: C

   Explanation: NPP = GPP − Respiration; it is the energy available to consumers.

5. Q5. In an aquatic ecosystem, an inverted pyramid of biomass is commonly observed because:

   • A) Phytoplankton are absent
   • B) Phytoplankton have small standing biomass but very high reproduction rate
   • C) Fish do not depend on phytoplankton
   • D) Decomposers dominate the ecosystem

   Answer: B

   Explanation: Phytoplankton reproduce fast, so even with low standing biomass, they support higher consumer biomass.

6. Q6. Which of the following is the correct sequence of decomposition steps?

   • A) Mineralisation → Humification → Leaching → Fragmentation
   • B) Fragmentation → Leaching → Catabolism → Humification → Mineralisation
   • C) Catabolism → Fragmentation → Mineralisation → Humification
   • D) Humification → Leaching → Fragmentation → Mineralisation

   Answer: B

   Explanation: The usual sequence is fragmentation, leaching, microbial breakdown (catabolism), humus formation, and finally mineral release.

7. Q7. Eutrophication of a lake is most directly linked to:

   • A) Decrease in nitrogen and phosphorus
   • B) Increase in nitrogen and phosphorus leading to algal bloom
   • C) Increase in dissolved oxygen at all times
   • D) Increase in salinity due to tides

   Answer: B

   Explanation: Nutrient enrichment causes algal bloom, which later reduces oxygen and harms aquatic life.

8. Q8. The transition zone between two ecosystems is called:

   • A) Biome
   • B) Ecotone
   • C) Niche
   • D) Biosphere

   Answer: B

   Explanation: Ecotone is the boundary/transition area, often showing an edge effect with higher diversity.

9. Q9. Secondary succession differs from primary succession mainly because:

   • A) It starts without soil and is slower
   • B) It starts with soil already present and is faster
   • C) It occurs only in oceans
   • D) It always ends with grassland

   Answer: B

   Explanation: Presence of soil makes recovery quicker in secondary succession (after fire, cultivation, flood).

10. Q10. Which of the following is an example of an artificial ecosystem?

    • A) Mangrove forest
    • B) Coral reef
    • C) Paddy field
    • D) Alpine meadow

    Answer: C

    Explanation: Paddy fields are human-managed systems with external inputs, making them artificial (agroecosystems).

Conclusion: One-line UPSC takeaway

An ecosystem is not just "plants and animals." It is a functional unit driven by energy flow, maintained by nutrient cycling, and shaped over time by succession and disturbances—making it a core foundation for understanding environment, biodiversity, and sustainable development in UPSC.