- An ecosystem is a community of living organisms interacting with each other and with their physical environment; the term was coined by A.G. Tansley in 1935.
- Every ecosystem has biotic parts (producers, consumers, decomposers) and abiotic parts (sunlight, water, soil, temperature, nutrients).
- Energy flows one way through an ecosystem and is lost as heat at each step, while nutrients cycle repeatedly between living and non-living parts.
Who Does What in an Ecosystem
| Component | Role | Examples |
|---|---|---|
| Producers (autotrophs) | Make food by photosynthesis — the first trophic level | Green plants, algae, phytoplankton |
| Primary consumers | Herbivores that eat producers | Deer, grasshopper, zooplankton |
| Secondary / tertiary consumers | Carnivores that eat other consumers | Frog, snake, eagle, tiger |
| Decomposers | Break down dead matter and return nutrients to the soil | Bacteria, fungi |
| Detritivores | Feed on dead organic matter (detritus) | Earthworm, termite, millipede |
Food Chains and Food Webs
- A grazing food chain starts with green plants (grass → grasshopper → frog → snake → eagle).
- A detritus food chain starts with dead organic matter; in forests, more energy flows through it than through grazing chains.
- Food chains interlink to form a food web, which makes an ecosystem more stable — if one species declines, consumers can switch to another.
Energy Flow and the 10% Law
Raymond Lindeman (1942) showed that only about 10% of the energy at one trophic level passes to the next; the rest is used in respiration or lost as heat. If plants fix 10,000 J, herbivores get about 1,000 J, primary carnivores 100 J and secondary carnivores 10 J. This is why food chains rarely have more than 4–5 levels and why top predators like tigers are always few in number.
Term "ecosystem"A.G. Tansley, 1935
Energy transfer~10% per level
Energy flowUnidirectional
Always uprightPyramid of energy
Pioneer on bare rockLichens
Symbiotic N-fixerRhizobium
Ecological Pyramids
| Pyramid | Shape | Exception to remember |
|---|---|---|
| Energy | Always upright | None — energy is always lost at each level |
| Numbers | Usually upright | Inverted in a tree ecosystem (one tree supports many insects) |
| Biomass | Usually upright | Inverted in aquatic ecosystems (phytoplankton biomass is smaller than that of zooplankton and fish at any moment) |
Biogeochemical Cycles
- Carbon cycle: plants absorb CO₂ in photosynthesis; respiration, decomposition and burning of fossil fuels release it. Oceans are the largest active carbon sink.
- Nitrogen cycle: air is about 78% nitrogen, but plants cannot use N₂ directly. It is fixed by Rhizobium (in root nodules of legumes), free-living Azotobacter, cyanobacteria such as Anabaena and Nostoc, and by lightning. Nitrosomonas converts ammonia to nitrite and Nitrobacter nitrite to nitrate (nitrification); Pseudomonas returns nitrogen to the air (denitrification).
- Phosphorus cycle: a sedimentary cycle with no significant gaseous phase; rocks are the main reservoir.
- Water cycle: driven by solar energy through evaporation, transpiration, condensation and precipitation.
Ecological Succession
Succession is the gradual, predictable change in the species of an area until a stable climax community forms. Primary succession starts on bare surfaces with no soil — bare rock, new volcanic islands — and begins with pioneer species such as lichens, which slowly create soil. Secondary succession occurs where a community was removed but soil remains (after a forest fire or an abandoned farm) and is much faster. Succession starting in water is a hydrosere; on dry rock it is a xerosere.
Terms Examiners Love
| Term | Meaning | Example |
|---|---|---|
| Ecotone | Transition zone between two ecosystems | Mangroves (between land and sea), grassland–forest edge |
| Edge effect | Greater species variety in an ecotone | More bird species at a forest edge |
| Keystone species | Has an effect far larger than its numbers; its loss changes the whole system | Tiger, fig trees, sea otter |
| Flagship species | A charismatic species used to win public support for conservation | Tiger, giant panda, Asian elephant |
| Indicator species | Its presence or absence signals environmental conditions | Lichens (sensitive to SO₂ air pollution) |
| Niche | The functional role of a species — what it eats, where and when it lives | — |
- Pyramid exceptions: "Which pyramid is always upright?" (energy) and "Where is the pyramid of biomass inverted?" (aquatic ecosystems).
- Organism ↔ process: Rhizobium/fixation, Nitrosomonas/nitrification, Pseudomonas/denitrification.
- Statement questions: "Energy flow in an ecosystem is cyclic" — false; it is unidirectional.
Q1. Which ecological pyramid is always upright?
✅ Answer: (c) Pyramid of energy
💡 Explanation: Energy is lost at every trophic level, so the energy pyramid can never be inverted.
Q2. According to Lindeman's law, roughly what share of energy passes from one trophic level to the next?
✅ Answer: (a) 10%
💡 Explanation: About 10% is transferred; the rest is used in respiration or lost as heat.
Q3. Which bacterium converts nitrite into nitrate?
✅ Answer: (b) Nitrobacter
💡 Explanation: Nitrobacter completes nitrification by oxidising nitrite to nitrate.
Q4. Lichens are known as pioneer species because they:
✅ Answer: (d) Colonise bare rock and start soil formation
💡 Explanation: Lichens break down rock surfaces and begin primary succession.
Q5. The pyramid of biomass is inverted in:
✅ Answer: (d) An aquatic ecosystem
💡 Explanation: In water, the standing biomass of phytoplankton at any moment is smaller than that of the consumers it supports.
🔁 Revise in 60 Seconds
- Ecosystem coined by Tansley (1935); energy one-way, nutrients cyclic.
- 10% law (Lindeman, 1942) → food chains of 4–5 levels.
- Energy pyramid always upright; numbers inverted in trees; biomass inverted in water.
- Rhizobium fixes, Nitrosomonas/Nitrobacter nitrify, Pseudomonas denitrifies.
📌 Prelims Traps to Avoid
- ✘ Energy does not cycle — nutrients do.
- ✘ The phosphorus cycle has no gaseous phase of importance.
- ✘ Secondary succession is faster than primary because soil already exists.
What is the 10 percent law in ecology?
Proposed by Raymond Lindeman in 1942, it states that only about 10% of the energy at one trophic level is passed on to the next; the remaining energy is used for life processes or lost as heat.
Why is the pyramid of energy always upright?
Because energy is lost as heat at each trophic level, every level has less energy than the one below it. So, unlike the pyramids of numbers and biomass, it can never be inverted.