Environment & Ecology
Ecology and Ecosystems
GS Paper III
Ecology and Ecosystems — Quick Revision
One-page UPSC revision of ecology: ecosystem components, productivity, food chains, ecological pyramids, nutrient cycles, succession and species interactions.
SyllabusGS III · General issues on environmental ecology, biodiversity and climate change that do not require subject specialisation
A last-week revision sheet for ecology concepts. Prelims frames almost every line below as a true-or-false statement.
Ecosystem at a Glance
| Item | Key fact |
|---|---|
| Term “ecosystem” | A.G. Tansley, 1935 |
| Components | Abiotic + producers + consumers + decomposers |
| GPP | Rate of organic matter fixed in photosynthesis |
| NPP | GPP minus respiration; available to consumers |
| Light capture | Plants capture about 2–10% of PAR; PAR is under 50% of incident radiation (NCERT) |
| Ten per cent law | About 10% of energy passes to the next trophic level (Lindeman, 1942) |
| Energy | Flows one way; lost as heat |
| Nutrients | Recycled through biogeochemical cycles |
Food Chains and Pyramids
| Item | Key fact |
|---|---|
| Grazing food chain | Starts with living plants; main pathway in aquatic ecosystems |
| Detritus food chain | Starts with dead matter; larger share of energy in terrestrial ecosystems |
| Pyramid of numbers | Can be inverted (tree with insects; parasites) |
| Pyramid of biomass | Can be inverted (open sea: phytoplankton vs fish) |
| Pyramid of energy | Always upright |
Decomposition (NCERT order)
Fragmentation → Leaching → Catabolism → Humification → Mineralisation. Faster in warm, moist, aerated conditions; slower with lignin- and chitin-rich detritus and in waterlogged soils.
Biogeochemical Cycles
| Cycle | Type | Remember |
|---|---|---|
| Carbon | Gaseous | About 71% of carbon is dissolved in oceans (NCERT) |
| Nitrogen | Gaseous | Fixation, ammonification, nitrification, denitrification |
| Phosphorus | Sedimentary | No significant gaseous phase; rock weathering is the source |
| Sulphur | Sedimentary (with a gaseous component) | Released by volcanoes and fossil-fuel burning |
| Nitrogen step | Organisms |
|---|---|
| Fixation | Rhizobium, Azotobacter, Anabaena, Nostoc, Frankia |
| Nitrite formation | Nitrosomonas |
| Nitrate formation | Nitrobacter |
| Denitrification | Pseudomonas, Thiobacillus |
Succession
| Item | Key fact |
|---|---|
| Primary | Bare area with no soil; slow |
| Secondary | Soil present (abandoned farm, burnt forest); faster |
| Xerarch pioneer | Lichens |
| Hydrarch pioneer | Phytoplankton |
| End point | Climax community; mesic conditions |
Interactions
| Interaction | Sign | Example |
|---|---|---|
| Mutualism | +/+ | Lichen, mycorrhiza, fig and fig wasp |
| Commensalism | +/0 | Epiphytic orchid on mango |
| Amensalism | −/0 | Penicillium and bacteria |
| Parasitism | +/− | Cuscuta; cuckoo (brood parasite) |
| Predation | +/− | Tiger and deer |
| Competition | −/− | Gause’s competitive exclusion principle |
High-yield facts
- Habitat = address; niche = profession; ecotone = boundary zone with an edge effect.
- Keystone species: impact far greater than its abundance (Robert Paine’s starfish experiments, 1960s).
- Lichens indicate sulphur dioxide pollution.
- Logistic growth levels off at carrying capacity (K); exponential growth assumes unlimited resources.
- r-selected species: many offspring, little care; K-selected species: few offspring, high care.
Traps
- “Energy is recycled” — false.
- “All pyramids are upright” — false; only energy.
- “Phosphorus has a major atmospheric reservoir” — false.
- “Rhizobium converts nitrite to nitrate” — false; it fixes atmospheric nitrogen.
Related Topics
- Ecosystems: Structure, Energy Flow and Nutrient Cycles
- Ecology and Ecosystems — practice questions
- Biodiversity and Conservation
- All quick revision sheets
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