Ecosystems, food chains, energy flow & biogeochemical cycles
Ecosystem structure and function for UPSC: trophic levels, energy flow laws, ecological pyramids, productivity, and the carbon, nitrogen and phosphorus 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:
- Grazing food chain (GFC): begins with living producers — Grass → Grasshopper → Frog → Snake → Hawk. Dominant in aquatic ecosystems.
- 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.