Ecosystems
An ecosystem is a place plus its energy flow. Learn the flow direction and the trophic ladder, and questions about food chains, pyramids and biomes stop being separate topics.
Mental model
An ecosystem is one arrow of energy and one circle of matter. ENERGY flows one way only — sun → producer → herbivore → carnivore — and about 90% is lost at each step, which is why food chains rarely exceed four or five links and why the biomass pyramid is upright on land. MATTER, by contrast, cycles endlessly (carbon, nitrogen, water). Arrow for energy, circle for nutrients: that single picture answers most ecosystem questions.
Associations
A short trigger on the left, everything it pulls with it on the right.
Energy flows one way; matter cycles.
Sunlight enters through producers and is lost as heat at every step — it never returns. Carbon, nitrogen and water instead cycle through the system repeatedly. That single contrast explains why energy pyramids are always upright while nutrient cycles are drawn as loops.
10% rule: most energy is lost at every step up.
Roughly ten per cent of energy passes from one trophic level to the next; the rest is lost as heat through respiration. This is why food chains rarely run past four or five levels and why the biomass of predators is always far smaller than that of prey.
Things to remember
The night-before list. Short enough to recall cold.
- 1Producers (green plants), consumers (herbivores, carnivores, omnivores) and decomposers (fungi, bacteria) are the three functional groups of an ecosystem.
- 2A pyramid of numbers can sometimes be inverted (one tree, many insects), but a pyramid of energy is always upright.
- 3A biome is a large regional ecosystem type defined by climate and dominant vegetation, such as tropical rainforest, grassland or desert.
- 4An ecotone is the transition zone between two ecosystems, often richer in species than either side.
- 5A keystone species has an ecological effect far larger than its numbers would suggest — removing it can collapse the community structure.
In detail
- Lindeman's ten percent law: only about 10% of energy at one trophic level passes to the next.
- Green plants are producers (autotrophs); animals are consumers; bacteria and fungi are decomposers that return nutrients to the soil.
- The pyramid of energy is always upright; the pyramid of numbers can be inverted, for example a single large tree supporting many insects.
- Biomagnification means a persistent pollutant such as DDT becomes more concentrated at each higher trophic level, hitting top predators hardest.
- Wetlands, forests, grasslands, deserts and marine areas are the broad ecosystem types; a wetland's key services are flood control, groundwater recharge and habitat for migratory birds.
Where people slip
- Decomposers are not consumers in the food-chain sense; they break down dead matter and are usually shown outside the main chain.
Check yourself
Nothing here is scored — it will not change your mastery figures.
In a food chain, approximately what percentage of energy is transferred from one trophic level to the next?
From the question bank
What is the term for a community of interacting organisms and their physical environment functioning together as a unit?
From the question bank
Which organisms are known as 'producers' in an ecosystem?
Books to read
- NCERT Class XII — Biology
- Environment for Civil Services — Shankar IAS Academy
Official sources
Hand-checked official domains only — the notes above carry no links of their own.