CDS Biology · Ecology and Environment
Ecosystems and Energy Flow
Population, community, ecosystem and biome, how a community develops by succession, how food chains pass on food and poisons, and why energy dwindles at each step.
Why this matters
Eight CDS questions from 2017 (II) to 2026 (II). Two ask simply what a food chain is, two are about energy flow, and one HARD question asks the order of the stages of succession.
Concept 1 of 3: Population, community, ecosystem and succession
Start with one deer. All the deer in a forest are a population. All the living things in the forest together, deer, trees, birds and fungi, are a community. Add the soil, water, air and sunlight they depend on, and you have an ecosystem. A community also changes over time: bare ground is slowly taken over by plants, each set changing the place for the next, until a stable community settles.
Definition
Levels of ecology (NCERT Class 10 and 12):
- Population: organisms of one species living in the same area at the same time.
- Community: populations of many species living together in one area.
- Ecosystem: a community together with its non-living (abiotic) environment: a pond, a forest, an ocean, even an aquarium (a man-made ecosystem).
- Biome: a very large region with its own climate and vegetation, such as a desert, grassland or tropical rain forest.
- Ecological succession: the gradual change of the community in a place, in the order nudation (a bare area forms), migration (seeds and spores arrive), ecesis (they settle), reaction (the living things change the place), stabilisation (a stable climax community).
- On bare rock the first settlers, the pioneer species, are usually lichens.
| Level or stage | Meaning |
|---|---|
| Population | One species, same area, same timeQ CDS 2021 (II): a population is a group of organisms of one species living in the same area at the same time. |
| Community | Many species living together |
| Ecosystem | Community plus its abiotic environment |
| Biome | Large region of one climate and vegetation type |
| Succession | Nudation, migration, reaction, stabilisationQ CDS 2019 (II), HARD: the order is Nudation, Migration, Reaction and Stabilization. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2017 · CDS (II) 2017 — General Knowledge · Q10Easy
Example 1 · Ecology and Environment · Ecosystems and Energy Flow
A biological community in its environment such as a pond, an ocean, a forest, even an aquarium is known as
Community has no soil or water in it
A community is only the living part. Add the abiotic part (soil, water, air, light) and it becomes an ecosystem. A population is one species only.
Succession starts with nudation
Succession cannot begin until there is a bare area: nudation comes first and stabilisation last, with migration and reaction in between in that order.
Concept 2 of 3: Food chains and biomagnification
A food chain shows who eats whom. It always starts with a green plant, which makes food from sunlight, and each next link eats the one before it. Some poisons, such as DDT, are not broken down or passed out of the body. So each animal keeps all the poison from everything it ate, and the amount climbs higher at every step up the chain.
Definition
Food chains (NCERT Class 10, Our Environment):
- A food chain is the passage of food, and so of energy, from one organism to the next.
- Each step is a trophic level: producers (green plants) first, then herbivores (primary consumers), then carnivores (secondary and tertiary consumers). For example grass, insect, frog, snake, hawk.
- Decomposers (bacteria and fungi) break down dead bodies and return nutrients to the soil.
- Bioaccumulation: a heavy metal or pesticide builds up inside one organism over its life.
- Biomagnification: its concentration rises at each higher trophic level, so a large fish carries more pesticide than the plankton. With DDT, fish-eating birds get thin eggshells that break.
| Trophic level | Role | Example |
|---|---|---|
| First | Producer | Grass, phytoplankton |
| Second | Primary consumer (herbivore) | Insect, deer, zooplankton |
| Third | Secondary consumer | Frog, small fish |
| Fourth | Tertiary consumer | Snake, big fish |
| Top | Top carnivore; gathers the most pesticide | Hawk, fish-eating birdQ CDS 2018 (I): bioaccumulation is build-up in one organism; large fish holding more pesticide than plankton is biomagnification, so Statement II does not explain Statement I. |
| All levels | Decomposers return nutrients | Bacteria, fungiQ CDS 2019 (II): a food chain is the passage of food, and so energy, from one organism to another. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (II) 2026 — General Knowledge · Q38Easy
Example 2 · Ecology and Environment · Ecosystems and Energy Flow
Which one among the following constitute a food chain ?
A food chain starts with a producer
Every food chain begins with a green plant and then runs through animals that eat each other. A set of only herbivores, or one that puts the grass last, is not a food chain.
Accumulation is one body, magnification is up the chain
Bioaccumulation happens within one organism. Biomagnification is the rise from one trophic level to the next. The plankton-to-fish observation is biomagnification.
Concept 3 of 3: Energy flow and the ten per cent law
Of all the energy a plant stores, an animal eating it keeps only about a tenth in its own body; the rest is used for living and lost as heat. The same happens at every step. So energy shrinks tenfold at each level, and after four or five steps there is too little left to feed another level. Energy only flows forward and is lost as heat, but the nutrients in dead bodies are returned to the soil and used again.
Definition
Energy flow (NCERT Class 10 and 12):
- Ten per cent law (Lindeman): only about 10 per cent of the energy at one trophic level is passed to the next; the rest is lost, mostly as heat in respiration.
- So food chains are short, usually 3 or 4 (at most 5) levels: the energy available falls at each higher level.
- Energy flow is unidirectional: from the Sun to producers to consumers, and it is never reused.
- Nutrient flow is cyclic: decomposers return matter to the soil, and it goes round in the biogeochemical cycles (carbon, nitrogen, water).
- The pyramid of energy is therefore always upright.
Energy at a trophic level
- E1energy fixed by the producers (first level)
- Enenergy available at level n
- nnumber of the trophic level
Worked example
The plants of a field fix 20,000 J of energy. About how much reaches the secondary consumers (the third trophic level)?
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2021 · CDS (II) 2021 — General Knowledge · Q13Moderate
Example 3 · Ecology and Environment · Ecosystems and Energy Flow
Which of the following limits the number of trophic levels in a food chain ?
Energy one way, nutrients in a cycle
Energy flows one way and is lost as heat. Nutrients cycle, returned by decomposers. Options that make energy cyclic, or both flows the same, are wrong.
Energy, not food or parasites, limits the chain
A food chain stops at 4 or 5 levels because the energy available falls tenfold at each step. Shortage of food in general, polluted air or parasites are not the reason.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Energy flow and the ten per cent law
Energy at a trophic level
Reference tables (2)
Population, community, ecosystem and succession5 rows
| Level or stage | Meaning |
|---|---|
| Population | One species, same area, same timeQ CDS 2021 (II): a population is a group of organisms of one species living in the same area at the same time. |
| Community | Many species living together |
| Ecosystem | Community plus its abiotic environment |
| Biome | Large region of one climate and vegetation type |
| Succession | Nudation, migration, reaction, stabilisationQ CDS 2019 (II), HARD: the order is Nudation, Migration, Reaction and Stabilization. |
Food chains and biomagnification6 rows
| Trophic level | Role | Example |
|---|---|---|
| First | Producer | Grass, phytoplankton |
| Second | Primary consumer (herbivore) | Insect, deer, zooplankton |
| Third | Secondary consumer | Frog, small fish |
| Fourth | Tertiary consumer | Snake, big fish |
| Top | Top carnivore; gathers the most pesticide | Hawk, fish-eating birdQ CDS 2018 (I): bioaccumulation is build-up in one organism; large fish holding more pesticide than plankton is biomagnification, so Statement II does not explain Statement I. |
| All levels | Decomposers return nutrients | Bacteria, fungiQ CDS 2019 (II): a food chain is the passage of food, and so energy, from one organism to another. |
Watch out for (6)
- Community has no soil or water in it→ Population, community, ecosystem and succession
- Succession starts with nudation→ Population, community, ecosystem and succession
- A food chain starts with a producer→ Food chains and biomagnification
- Accumulation is one body, magnification is up the chain→ Food chains and biomagnification
- Energy one way, nutrients in a cycle→ Energy flow and the ten per cent law
- Energy, not food or parasites, limits the chain→ Energy flow and the ten per cent law
Test yourself on Ecology and Environment
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.