Ecology Fundamentals — Ecosystems, Food Chains & Biogeochemical Cycles

What is Ecology?

The term ecology was coined by the German zoologist Ernst Haeckel in 1866, derived from the Greek words oikos (house/habitat) and logos (study). Ecology is the scientific study of the interactions between organisms and their environment, including both biotic (living) and abiotic (non-living) components.

1.1 Levels of Ecological Organisation

Ecology operates across a hierarchy of increasingly complex levels:

1.2 Branches of Ecology

Ecosystem

The term ecosystem was coined by A.G. Tansley in 1935. An ecosystem is a functional unit of nature where living organisms interact among themselves and with their surrounding physical environment.

2.1 Structure of an Ecosystem

Every ecosystem has two fundamental components:

A. Abiotic Components (Non-living)

• Climatic factors -- temperature, light, humidity, wind, rainfall
• Edaphic factors -- soil type, pH, mineral content
• Topographic factors -- altitude, slope, aspect
• Inorganic substances -- carbon, nitrogen, phosphorus, water
• Organic substances -- proteins, carbohydrates, lipids, humus

B. Biotic Components (Living)

2.2 Types of Ecosystems

Food Chain and Food Web

3.1 Food Chain

A food chain is a linear sequence showing the transfer of food energy from producers through a series of organisms, each feeding on the preceding one.

Types of Food Chains:

In most ecosystems, the detritus food chain contributes more energy flow than the grazing food chain. In a forest ecosystem, as much as 80% of energy flows through the detritus pathway.

3.2 Food Web

A food web is an interconnected network of multiple food chains in an ecosystem. In nature, organisms rarely follow a single food chain; most organisms feed on multiple species and are eaten by multiple predators. Food webs provide greater stability to ecosystems because the removal of one species can be compensated by other available food sources.

Ecological Pyramids

An ecological pyramid is a graphical representation of the trophic structure of an ecosystem, first described by Charles Elton in 1927 (hence also called Eltonian pyramids). The base represents producers, and successive tiers represent higher trophic levels.

4.1 Types of Ecological Pyramids

4.2 Lindeman's 10% Law

Raymond Lindeman (1942) proposed the 10 percent law of energy transfer, published in his landmark paper "The Trophic-Dynamic Aspect of Ecology." According to this law:

• Only about 10% of the energy at one trophic level is transferred to the next trophic level.
• The remaining 90% is lost as heat through respiration, used in metabolic activities, or lost through incomplete digestion and excretion.
• This limits most food chains to 4-5 trophic levels.

Worked Example: If producers fix 10,000 kcal of energy:

Ecological Succession

Ecological succession is the orderly and predictable process by which a biological community changes over time until a stable climax community is established.

5.1 Types of Succession

5.2 Seral Stages

The entire sequence of communities that successively change in a given area is called a sere. Each transitional community in the sere is called a seral stage or seral community.

5.3 Hydrosere (Succession in Water Bodies)

Succession beginning in a freshwater body (pond or lake) and ending as a terrestrial climax forest:

1. Phytoplankton Stage -- pioneer colonisers (algae, diatoms)
2. Submerged Plant Stage -- rooted submerged plants (Hydrilla, Vallisneria)
3. Floating Plant Stage -- rooted floating-leaved plants (Nymphaea, Nelumbo)
4. Reed Swamp Stage -- emergent vegetation (Typha, Scirpus)
5. Sedge Meadow Stage -- grasses and sedges colonise as water recedes
6. Woodland Stage -- shrubs and shade-intolerant trees
7. Climax Forest -- stable, self-sustaining forest community

5.4 Xerosere (Succession on Bare Rock)

Succession beginning on dry, bare rock:

1. Crustose Lichen Stage -- pioneer species; secrete acids that weather rock
2. Foliose Lichen Stage -- larger lichens trap dust and organic matter
3. Moss Stage -- mosses colonise thin soil layer
4. Herb Stage -- annual grasses and herbs
5. Shrub Stage -- perennial shrubs
6. Climax Forest -- stable tree community

Productivity in Ecosystems

6.1 Key Definitions

6.2 Productivity Across Ecosystems

Tropical rainforests and estuaries are among the most productive ecosystems on Earth. Although the open ocean has low productivity per unit area, its vast area makes it the largest contributor to total global productivity.

Nutrient Cycling

Nutrient cycling (biogeochemical cycling) is the movement and exchange of organic and inorganic matter back into the production of living matter. Nutrients are never lost from an ecosystem; they are recycled.

7.1 Types of Nutrient Cycles

Biogeochemical Cycles

8.1 Carbon Cycle

Carbon makes up approximately 49% of the dry weight of organisms and is the backbone of all organic molecules.

Key Steps:

1. Photosynthesis -- Plants absorb atmospheric CO2 and convert it into organic compounds (glucose) using sunlight
2. Respiration -- All living organisms release CO2 back into the atmosphere through cellular respiration
3. Decomposition -- Decomposers break down dead organic matter, releasing CO2
4. Combustion -- Burning of fossil fuels and biomass releases stored carbon as CO2
5. Ocean Absorption -- Oceans absorb large quantities of CO2 from the atmosphere; marine organisms use it to form calcium carbonate shells
6. Fossil Formation -- Over millions of years, dead organisms get buried and compressed into fossil fuels (coal, petroleum, natural gas), locking carbon in the lithosphere

UPSC Relevance: The burning of fossil fuels has increased atmospheric CO2 from approximately 280 ppm (pre-industrial) to over 420 ppm (2024), driving global warming and climate change.

8.2 Nitrogen Cycle

Nitrogen constitutes 78% of the atmosphere by volume but is unavailable to most organisms in its gaseous form (N2). It must be converted (fixed) into usable forms.

Key Processes:

8.3 Phosphorus Cycle

Phosphorus is essential for DNA, RNA, ATP, and cell membranes. Unlike carbon and nitrogen, phosphorus has no gaseous phase -- it cycles through rock, water, soil, and organisms (a sedimentary cycle).

Key Steps:

1. Weathering -- Phosphate is released from rocks through weathering and erosion
2. Absorption by Plants -- Plants absorb dissolved phosphate (PO4^3-) from soil through roots
3. Transfer Through Food Chain -- Herbivores obtain phosphorus from plants; carnivores from herbivores
4. Decomposition -- Decomposers return phosphorus to the soil from dead organisms and excreta
5. Sedimentation -- Phosphorus washed into rivers and oceans settles on continental shelves as sediment; after millions of years, geological uplift returns it to land
6. Guano -- Seabird and bat droppings (guano) return marine phosphorus to terrestrial ecosystems

8.4 Sulphur Cycle

Sulphur is a component of amino acids (cysteine, methionine), vitamins, and coenzymes.

Key Steps:

1. Weathering -- Sulphur is released from rocks as sulphate (SO4^2-)
2. Plant Uptake -- Plants absorb sulphate from soil
3. Decomposition -- Dead organisms release hydrogen sulphide (H2S) through anaerobic decomposition
4. Oxidation -- H2S is oxidised to sulphate by chemosynthetic bacteria (Thiobacillus)
5. Atmospheric Component -- Burning of fossil fuels and volcanic eruptions release SO2 into the atmosphere, which combines with water to form sulphuric acid (acid rain)

8.5 Water (Hydrological) Cycle

Key Processes:

1. Evaporation -- Water from oceans, rivers, and lakes evaporates due to solar energy
2. Transpiration -- Plants release water vapour from leaves (evapotranspiration = evaporation + transpiration)
3. Condensation -- Water vapour cools and condenses to form clouds
4. Precipitation -- Water returns to the surface as rain, snow, sleet, or hail
5. Runoff and Infiltration -- Water flows over the surface (runoff) or seeps into the ground (infiltration) to replenish groundwater

Biomes of the World

A biome is a large-scale community of organisms defined primarily by the dominant vegetation pattern and climate of a region.

The taiga is the largest terrestrial biome in the world by area.

Important for UPSC

Prelims Focus

• Lindeman's 10% law -- energy transfer between trophic levels
• Difference between GPP, NPP, and standing crop
• Inverted pyramids: biomass (ocean/aquatic), numbers (tree/parasite ecosystem); energy pyramid is always upright
• Hydrosere vs xerosere -- pioneer species and sequence of seral stages
• Nitrogen cycle bacteria: Rhizobium (fixation), Nitrosomonas and Nitrobacter (nitrification), Pseudomonas (denitrification)
• Phosphorus cycle has no gaseous phase (sedimentary cycle)
• Biome-specific questions: largest biome (taiga), most biodiverse (tropical rainforest)
• Food chain types: grazing vs detritus; detritus chain dominates in forest ecosystems
• Ecosystem coined by A.G. Tansley (1935); ecology coined by Ernst Haeckel (1866)

Mains Dimensions

• GS3 -- Environment: Role of biogeochemical cycles in maintaining ecological balance; human disruption of the carbon and nitrogen cycles through fossil fuel burning and excessive fertiliser use
• GS3 -- Biodiversity: How understanding food webs and ecological succession helps in conservation planning and habitat restoration
• GS3 -- Climate Change: Carbon cycle disruption and its link to global warming; nitrogen cycle disruption leading to eutrophication and dead zones
• Essay: Themes linking ecological principles to sustainable development -- "Man is just one strand in the web of life"

Recent Developments (2024–2026)

India State of Forest Report 2023 — Forest and Tree Cover at 25.17%

The Forest Survey of India (FSI) released the 18th India State of Forest Report (ISFR) in 2023, presenting the most comprehensive biennial assessment of forest and tree cover. India's total forest and tree cover stands at 8,27,357 sq km, constituting 25.17% of the country's geographical area — comprising 7,15,343 sq km (21.76%) as forest cover and 1,12,014 sq km (3.41%) as tree cover.

Compared to the 2021 assessment, there has been a net increase of 1,445 sq km in combined forest and tree cover, with forest cover itself increasing by 156 sq km and tree cover increasing by 1,289 sq km. Chhattisgarh (684 sq km), Uttar Pradesh (559 sq km), Odisha (559 sq km), and Rajasthan (394 sq km) recorded the maximum increase. Total carbon stock in India's forests is estimated at 7,285.5 million tonnes — an increase of 81.5 million tonnes since the previous assessment.

Mangrove cover increased to 4,992 sq km, and bamboo-bearing area expanded to 1,54,670 sq km. These gains reflect the impact of afforestation programs such as the Green India Mission and compensatory afforestation under CAMPA.

UPSC angle: ISFR data is frequently cited in Mains GS-3 for environmental conservation questions; forest cover percentage (21.76% vs the 33% national target) is a recurring Prelims fact.

Ecosystem Tipping Points — IPCC Sixth Assessment Report Findings

The IPCC Sixth Assessment Report (AR6, 2021–2023) identified 16 major climate tipping points that could be triggered between 1.5°C and 2°C of global warming, including collapse of the West Antarctic Ice Sheet, dieback of the Amazon rainforest, and large-scale permafrost thaw. These are of direct relevance to ecosystem fundamentals because tipping points alter biogeochemical cycles irreversibly.

For India, the AR6 highlighted that the monsoon system is showing increased variability, coral bleaching events in the Indian Ocean are intensifying, and the Hindu Kush Himalayan (HKH) cryosphere is retreating at an accelerated rate. The WMO Global Climate Report 2024 confirmed that 2024 was the warmest year on record globally, breaching the 1.5°C warming threshold for a calendar year for the first time.

These findings reinforce the ecological concept of feedback loops: reduced forest cover raises albedo and decreases evapotranspiration, further amplifying regional warming — a classic ecosystem-level cascading effect.

UPSC angle: Prelims may ask about IPCC assessment cycle or tipping point terminology; Mains GS-3 essays on climate change benefit from citing AR6 findings with specific warming thresholds.

Biogeochemical Cycles Under Stress — Nitrogen and Carbon Dynamics 2024

The Global Nutrient Cycle Assessment 2024 (published by the International Nitrogen Initiative) confirmed that reactive nitrogen pollution now exceeds planetary boundaries in South Asian agricultural systems. India's excessive use of synthetic nitrogenous fertilisers results in nitrous oxide (N₂O) emissions — a greenhouse gas 265 times more potent than CO₂ over 100 years — contributing to both ozone depletion and climate change.

On the carbon side, India's terrestrial carbon sink (forests and soils) absorbs approximately 2.03 billion tonnes of CO₂ per year, which partially offsets the country's gross emissions. The updated National Biodiversity Strategy and Action Plan (NBSAP) launched at CBD COP16 in Cali (October–November 2024) committed India to ecosystem-based approaches that enhance natural carbon sequestration while protecting biodiversity.

UPSC angle: Biogeochemical cycle disruptions link GS-3 environment topics to agriculture and climate change; the nitrogen cycle and nitrous oxide connection to greenhouse gases is high-yield for Prelims.

Ecological Succession and Rewilding — Emerging Conservation Paradigm

Rewilding — allowing natural ecological succession to proceed with minimal human intervention — has gained traction globally and in India after 2024. The reintroduction of African cheetahs at Kuno National Park (Madhya Pradesh) since 2022 and the potential wolf and dhol reintroduction discussions represent applied succession ecology: reintroducing apex predators to trigger trophic cascades that restore grassland ecosystems.

India's National Biodiversity Strategies and Action Plan (NBSAP), submitted at CBD COP16 in October 2024, specifically includes targets for ecosystem restoration aligned with the Kunming-Montreal Global Biodiversity Framework's 30×30 goal (conserving 30% of land and ocean by 2030). Currently, approximately 5.3% of India's land area is formally protected under the Protected Area Network.

UPSC angle: Rewilding, trophic cascades, and the 30×30 biodiversity target are emerging Mains themes; Prelims may test the distinction between primary and secondary succession in specific ecosystems.

Biome Shifts Under Climate Change — Himalayan Treeline Migration

Scientific studies published in 2024 documented upward migration of the Himalayan treeline by 20–30 metres per decade due to rising temperatures, fundamentally altering alpine grassland (bugyals) and subalpine ecosystems. Species such as oak and rhododendron are encroaching on areas previously dominated by sedge-grass meadows, threatening endemic species adapted to high-altitude cold conditions.

Similar biome-boundary shifts have been observed in the Western Ghats (expansion of dry deciduous zones into evergreen forest margins) and in Rajasthan (northward expansion of thorny scrub). These shifts disrupt established food webs and biogeochemical cycles, with implications for water availability in downstream river basins.

UPSC angle: Biome shifts are a Mains GS-3 topic linking climate change to biodiversity loss; Prelims questions on endemic species, altitude zonation, and indicator species have appeared.

Vocabulary and Key Terms