Ecosystems: Structure, Energy Flow and Nutrient Cycles

Prerequisites: Ecology and Ecosystems — overview · Biogeography and Soils

The Hook: What Happens When the Wolves Come Back?

The Core Concept

An ecosystem is a functional unit of nature in which living organisms interact with each other and with their physical environment. The term was coined by the British ecologist A.G. Tansley in 1935. Every ecosystem has a structure (what it is made of) and functions (what it does).

Structure: biotic and abiotic components

Each step in the feeding sequence is a trophic level: producers form the first, herbivores the second, and so on.

Function 1: Productivity

• Gross primary productivity (GPP) is the rate at which producers fix organic matter during photosynthesis.
• Net primary productivity (NPP) = GPP minus respiration losses (R). NPP is what is available to herbivores and decomposers.
• Secondary productivity is the rate at which consumers form new organic matter.

Only a small share of sunlight becomes plant matter. NCERT Biology (Class XII) notes that less than 50% of incident solar radiation is photosynthetically active radiation (PAR), and plants capture only about 2–10% of the PAR.

Productivity depends on light, temperature, water and nutrients. Per unit area, tropical rainforests, coral reefs, estuaries and marshes are highly productive; deserts and the open ocean are among the least. NCERT notes that oceans, despite covering about 70% of the Earth’s surface, contribute only around a third of the biosphere’s annual net primary productivity.

Function 2: Decomposition

Decomposition breaks complex organic matter (detritus) into inorganic substances. NCERT lists five steps:

1. Fragmentation — detritivores such as earthworms break detritus into smaller particles.
2. Leaching — water-soluble nutrients move down into the soil and may form unavailable salts.
3. Catabolism — bacterial and fungal enzymes break detritus into simpler inorganic substances.
4. Humification — formation of humus, a dark, amorphous substance highly resistant to microbial action, which decomposes very slowly.
5. Mineralisation — humus is further degraded and inorganic nutrients are released.

Decomposition is fastest in warm, moist, oxygen-rich conditions with detritus rich in nitrogen and sugars; it slows in cold, dry or waterlogged conditions and with detritus rich in lignin and chitin. That is why organic matter accumulates in bogs and peatlands.

Function 3: Energy flow and the ten per cent law

Energy flows one way: sun → producers → consumers → decomposers, and leaves the system as heat. It is never recycled. The American ecologist Raymond Lindeman (1942) showed that only about 10% of the energy at one trophic level is transferred to the next; the rest is lost mainly through respiration as heat. This is why food chains rarely exceed four or five levels.

Food chains interlink to form a food web, which makes an ecosystem more stable: if one prey species declines, a predator can switch to another.

Function 4: Ecological pyramids

Pyramids ignore species that occupy more than one trophic level, and they do not include decomposers, even though decomposers handle much of the energy.

Function 5: Nutrient cycling (biogeochemical cycles)

Unlike energy, nutrients are recycled. Cycles are of two types:

• Gaseous cycles — the reservoir is the atmosphere (and ocean): carbon, nitrogen, oxygen.
• Sedimentary cycles — the reservoir is the Earth’s crust: phosphorus, sulphur, calcium. Phosphorus has no significant gaseous phase.

Carbon: NCERT notes that about 71% of carbon is dissolved in the oceans, which regulate atmospheric carbon dioxide. Photosynthesis removes carbon dioxide; respiration, decomposition, burning of fossil fuels and forest fires return it.

Nitrogen is the cycle Prelims tests most:

graph LR
    A[Atmospheric nitrogen] -->|Fixation: Rhizobium, cyanobacteria, lightning| B[Ammonia]
    B -->|Nitrosomonas| C[Nitrite]
    C -->|Nitrobacter| D[Nitrate]
    D -->|Absorbed by plants| E[Plant and animal proteins]
    E -->|Ammonification by decomposers| B
    D -->|Denitrification: Pseudomonas| A

Ecological succession

Succession is the gradual, fairly predictable change in species composition of an area until a stable climax community develops. The series of stages is a sere; each stage is a seral stage.

• Xerarch succession starts in dry areas; the pioneers on rock are usually lichens, which secrete acids that help weather rock into soil, followed by mosses, herbs, shrubs and trees.
• Hydrarch succession starts in water; pioneers are phytoplankton, followed by submerged and floating plants, reed swamp, marsh meadow, shrubs and trees.
• Both move towards mesic (medium-moisture) conditions.

Species interactions

Gause’s competitive exclusion principle: two closely related species competing for the same limiting resource cannot coexist indefinitely; the weaker one is eliminated. In nature, species often avoid this through resource partitioning.

Concepts that look alike

• Habitat — the physical place where an organism lives (its “address”).
• Ecological niche — the functional role of a species: what it eats, when it is active, how it uses resources (its “profession”).
• Ecotone — a transition zone between two ecosystems, such as a mangrove (land and sea), an estuary (river and sea) or a forest–grassland boundary.
• Edge effect — the tendency for an ecotone to hold more species, and sometimes higher density, than either adjoining ecosystem. Species found mainly at edges are edge species.
• Keystone species — a species whose impact on its community is far larger than its abundance suggests. The idea comes from Robert Paine’s experiments with the starfish Pisaster on the Pacific coast of the United States in the 1960s: removing the predator let mussels take over and diversity fell.
• Indicator species — species whose presence or absence signals environmental conditions; lichens are sensitive to sulphur dioxide and are used as air-pollution indicators.
• Flagship species — charismatic species used to rally public support (the tiger). Umbrella species — species whose protection protects many others sharing its habitat.

Population growth

• Exponential (J-shaped) growth — when resources are unlimited.
• Logistic (S-shaped) growth — growth slows as the population approaches the carrying capacity (K) of the environment. NCERT calls this the Verhulst–Pearl logistic growth.
• r-selected species produce many small offspring with little parental care (insects, many fish); K-selected species produce few offspring with high parental care (elephants, humans).

Memory Anchors

Five steps of decomposition: “Five Lazy Cows Hate Mud” → Fragmentation · Leaching · Catabolism · Humification · Mineralisation

Nitrification order: “So Must Be” — Nitrosomonas comes before Nitrobacter (ammonia → nitrite → nitrate).

Pyramids: “Energy never stands on its head.” Number and biomass can be inverted; energy cannot.

Comparison table — the three look-alikes

One-line Mains hook: “Energy flows, nutrients cycle” — the two laws of every ecosystem, and the reason pollution that blocks a cycle or a species that breaks a chain has system-wide effects.

For Prelims

For Mains

• Dimensions: ecological (cascades, resilience), economic (ecosystem services such as pollination, water purification, soil formation), social (forest-dependent livelihoods), policy (landscape-level planning rather than species-by-species protection).
• Keywords: trophic cascade, ecosystem services, resilience, carrying capacity, keystone species, edge effect, habitat fragmentation, succession, nutrient loading.
• Value-addition: the Yellowstone wolf reintroduction (1995) as an illustration of top-down control; the Millennium Ecosystem Assessment (2005) classification of ecosystem services into provisioning, regulating, cultural and supporting services; fragmentation creating artificial edges that favour generalists over interior species.

Common Traps

Practice

Prelims

Mains

Quick Revision

Key Takeaways

Related Topics

• Ecology and Ecosystems — quick revision
• Ecology and Ecosystems — practice questions
• Biodiversity and Conservation
• Environmental Pollution
• Biogeography and Soils
• Environment & Ecology