Ecosystems, food chains, energy flow & biogeochemical cycles

What an Ecosystem Is

An ecosystem is a self-regulating community of organisms (biotic component) interacting with the physical environment (abiotic component) through flows of energy and cycles of matter. The term was coined by British ecologist Arthur Tansley in 1935. Every ecosystem has two structural attributes: species composition (the kinds and number of organisms) and stratification (vertical layering, e.g. the canopy, understorey, shrub and ground layers of a forest).

Functionally, ecosystems run on four processes: productivity, decomposition, energy flow and nutrient cycling. The biotic structure is organised into:

• Producers (autotrophs): green plants, algae, cyanobacteria that fix solar energy via photosynthesis.
• Consumers (heterotrophs): primary (herbivores), secondary and tertiary (carnivores), and omnivores.
• Decomposers (saprotrophs): bacteria and fungi that break down detritus, releasing inorganic nutrients back to the abiotic pool.

Food Chains and Food Webs

Energy moves from one organism to another along a food chain, a sequence of who-eats-whom. Two basic types exist:

1. Grazing food chain (GFC): begins with living producers — Grass → Grasshopper → Frog → Snake → Hawk. Dominant in aquatic ecosystems.
2. Detritus food chain (DFC): begins with dead organic matter — detritus → detritivores (earthworms, fungi) → predators. Dominant in terrestrial ecosystems, where the bulk of net primary production is not grazed but decomposed.

Each step is a trophic level. In nature, single chains rarely exist in isolation; they interlink to form a food web, which confers stability — if one species declines, predators switch to alternative prey. Loss of a keystone species (e.g. the sea otter, the Asian elephant as an ecosystem engineer) can collapse the web disproportionately to its biomass.

Productivity

Gross Primary Productivity (GPP) is the total rate of organic matter produced by photosynthesis. Net Primary Productivity (NPP) = GPP − Respiration (R); NPP is the biomass available to consumers. Oceans, despite covering 70% of Earth, contribute only about a third of global NPP because of nutrient limitation; tropical rainforests and estuaries are the most productive terrestrial and coastal systems respectively. Secondary productivity is the rate of new biomass formation by consumers.

These definitions are repeatedly tested. Candidates must hold the equation NPP = GPP − R and remember that NPP, not GPP, sets the carrying capacity for heterotrophs. The standing crop (biomass present at a given moment) is distinct from productivity (a rate). Confusing the two is a common Prelims trap, as is mistaking the detritus food chain for a minor pathway when it actually processes most terrestrial energy.

Laws Governing Energy Flow

Energy flow in ecosystems is unidirectional and obeys the laws of thermodynamics. The First Law (energy is neither created nor destroyed) explains why the sun is the ultimate source and why energy is merely transformed. The Second Law (every transfer dissipates energy as heat) explains why energy decreases at each trophic step and why no ecosystem can recycle energy the way it recycles matter.

Lindeman's Ten Percent Law (Raymond Lindeman, 1942) states that only about 10% of energy at one trophic level is transferred to the next; the remaining ~90% is lost as metabolic heat and through respiration. This is why food chains rarely exceed four or five links — energy at the top is too scarce to support further levels.

Ecological Pyramids

A pyramid (concept introduced by Charles Elton, 1927) graphically represents trophic structure. Three types:

• Pyramid of numbers: can be upright (grassland) or inverted (a single tree supporting many insects and birds).
• Pyramid of biomass: usually upright on land, but inverted in oceans, where small, fast-reproducing phytoplankton (low standing biomass, high turnover) support a larger biomass of zooplankton and fish.
• Pyramid of energy: always upright, because of the Second Law and the ten percent rule — energy must diminish at each successive level. This is the most reliable pyramid and a frequent answer in MCQs.

Note two pyramid pitfalls: pyramids ignore decomposers, and a given species may occupy several trophic levels (an omnivore), which pyramids cannot capture.

Why This Matters for the Exam

This topic is foundational to GS Paper III environment and is among the most reliably tested in Prelims. UPSC has repeatedly framed questions on the detritus vs grazing food chain, the ten percent law, why the energy pyramid is always upright, and the inverted biomass pyramid in oceans. The 2013, 2014 and 2019 Prelims sets carried items on ecosystem productivity, food chains and biogeochemical cycling.

In Mains, examiners weave these fundamentals into applied questions — for instance, how biomagnification of DDT or mercury (Minamata, 1956) up trophic levels justifies the Stockholm Convention on POPs (2001) and the Minamata Convention on Mercury (2013, in force 2017). A candidate who can connect energy-flow theory to bioaccumulation, eutrophication and carbon sequestration writes answers with analytical depth rather than rote recall. High-yield retention: NPP = GPP − R; 10% law; energy pyramid always upright; oceanic biomass pyramid inverted; DFC dominant on land.

Nutrient Cycling: Gaseous vs Sedimentary

While energy flows through ecosystems and is lost, nutrients cycle repeatedly between the biotic and abiotic compartments — these are biogeochemical cycles. They divide into gaseous cycles (reservoir is the atmosphere or hydrosphere: carbon, nitrogen, oxygen, water) and sedimentary cycles (reservoir is the Earth's crust: phosphorus, sulphur). Gaseous cycles are relatively fast and self-adjusting; sedimentary cycles are slow because the rock reservoir releases nutrients only by weathering over geological time.

The Carbon Cycle

Carbon moves via photosynthesis (CO₂ fixed into organic matter) and respiration, decomposition and combustion (CO₂ returned). Only about 1% of total carbon is actively cycling; the rest is locked in fossil fuels, sediments and the deep ocean. Burning fossil fuels and deforestation have raised atmospheric CO₂ from ~280 ppm pre-industrial to over 420 ppm (2023), the driver of anthropogenic climate change addressed by the UNFCCC (1992) and the Paris Agreement (2015). Oceans absorb roughly a quarter of emitted CO₂, causing ocean acidification.

The Nitrogen Cycle

Atmospheric N₂ (78% of air) is inert and must be fixed. Stages:

1. Nitrogen fixation — by Rhizobium (in legume root nodules), free-living Azotobacter, cyanobacteria (Anabaena, Nostoc), lightning, and industrially via the Haber–Bosch process (1909).
2. Ammonification — decomposers convert organic N to ammonia.
3. Nitrification — Nitrosomonas (NH₃ → nitrite) then Nitrobacter (nitrite → nitrate).
4. Denitrification — Pseudomonas returns nitrate to atmospheric N₂.

Human synthetic fertiliser use has more than doubled reactive nitrogen inputs, causing eutrophication, nitrate pollution and N₂O emissions (a greenhouse gas ~273× CO₂ over 100 years). India's Nitrogen Assessment and neem-coated urea policy target this excess.

The Phosphorus Cycle

Phosphorus is sedimentary and has no significant gaseous phase. Its reservoir is phosphate rock; weathering releases phosphate to soil and water, where it is the commonest limiting nutrient for plant growth. Run-off of phosphate fertiliser and detergents triggers algal blooms and eutrophication of lakes (e.g. recurring blooms tied to nutrient loading). Because there is no atmospheric reservoir to buffer it, phosphorus lost to deep ocean sediments is effectively removed from the cycle for millions of years — a key reason phosphate rock is a strategic, finite resource. Retain the contrast: carbon and nitrogen are gaseous (atmospheric reservoir); phosphorus and sulphur are sedimentary (crustal reservoir).