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"

Vocabulary and Key Terms