What is an Ecosystem?
An ecosystem is any functional unit where living organisms (biotic components) interact with each other and with their non-living environment (abiotic components) — exchanging energy and matter. A pond, a forest, a coral reef, even your school garden — all are ecosystems.
The key word here is functional. An ecosystem isn’t just a list of organisms; it’s a system where energy flows and nutrients cycle. NCERT Chapter 14 covers this as a central theme, and NEET dedicates roughly 4-5 questions per paper to this chapter alone.
Let’s understand why ecosystems matter beyond the exam: every crop failure, every flood, every locust swarm — these are ecosystem disruptions. Once you see that, the chapter stops being memorisation and starts making sense.
Key Terms & Definitions
Biotic components — all living organisms: producers, consumers, decomposers.
Abiotic components — non-living factors: temperature, rainfall, soil minerals, sunlight, wind. These set the carrying capacity of an ecosystem.
Producers (Autotrophs) — organisms that fix inorganic carbon into organic matter via photosynthesis or chemosynthesis. Green plants, algae, cyanobacteria. They form the base of every food chain.
Consumers (Heterotrophs) — organisms that obtain energy by eating other organisms.
- Primary consumers (herbivores): Grasshoppers, deer, rabbit — eat producers directly.
- Secondary consumers: Frogs, small fish — eat primary consumers.
- Tertiary consumers: Snakes, large fish — eat secondary consumers.
Decomposers (Saprotrophs) — fungi and bacteria that break down dead organic matter, releasing nutrients back to the soil. Often underrated in exams but crucial in nutrient cycling questions.
Detritivores — organisms like earthworms, millipedes, and dung beetles that fragment dead matter, increasing surface area for decomposers. Different from decomposers — they ingest the material rather than secreting enzymes onto it.
Trophic Level — the feeding position of an organism in a food chain. Producers = T₁, primary consumers = T₂, and so on.
Ecosystem Structure: The Two Functional Aspects
1. Energy Flow
Energy enters an ecosystem through photosynthesis. This energy flows unidirectionally — from producers → consumers → decomposers. It is never recycled. This is the single most important principle in this chapter.
NEET 2023 had a direct question: “Energy flow in an ecosystem is unidirectional because…” — the answer is that energy is lost as heat at each trophic level and cannot be reused. This appears almost every alternate year.
The 10% Law (Lindeman’s Law) states that only 10% of energy is transferred from one trophic level to the next. The remaining 90% is lost as heat through respiration, movement, and metabolic processes.
Energy available at trophic level (n+1) = 10% × Energy at trophic level (n)
If producers fix 10,000 kcal:
- Primary consumers receive: 1,000 kcal
- Secondary consumers receive: 100 kcal
- Tertiary consumers receive: 10 kcal
This explains why food chains rarely exceed 4-5 trophic levels — there simply isn’t enough energy left to sustain a viable population at higher levels.
2. Nutrient Cycling (Biogeochemical Cycles)
Unlike energy, matter is recycled. Carbon, nitrogen, phosphorus — these cycle between biotic and abiotic components. The word biogeochemical itself tells you: biological + geological + chemical processes are all involved.
Food Chain vs Food Web
A food chain is a linear sequence: Grass → Grasshopper → Frog → Snake → Eagle.
A food web is a network of interconnected food chains within an ecosystem. Food webs are more realistic — most animals eat multiple things and are eaten by multiple predators.
Food webs provide stability to ecosystems. If one species disappears, energy flow can be rerouted through alternate pathways. A simple food chain offers no such buffer — one extinction can collapse the whole system.
Grazing Food Chain (GFC) — starts from living green plants. Example: Plants → Deer → Tiger. This is the dominant energy pathway in most terrestrial ecosystems.
Detritus Food Chain (DFC) — starts from dead organic matter (detritus). Example: Dead leaves → Earthworm → Robin → Hawk. In aquatic and forest floor ecosystems, the DFC often transfers more energy than the GFC.
NEET frequently asks: “In which ecosystem is detritus food chain the major pathway?” Answer: aquatic ecosystems and forest floor. Many students write “terrestrial” — that’s the common error.
Ecological Pyramids
Ecological pyramids represent the trophic structure graphically, with producers at the base.
Pyramid of Numbers
Shows the number of organisms at each trophic level.
- Usually upright (many grass plants → fewer grasshoppers → even fewer frogs)
- Inverted in tree-based ecosystems: one large tree (T₁) supports hundreds of caterpillars (T₂)
- Spindle-shaped in parasitic food chains
Pyramid of Biomass
Shows the total dry weight (biomass) of organisms per unit area at each trophic level.
- Usually upright in terrestrial ecosystems
- Inverted in aquatic (phytoplankton) ecosystems — phytoplankton reproduce so rapidly that the standing crop (biomass at a moment) is less than the zooplankton it supports
Pyramid of Energy
Shows the rate of energy flow at each trophic level (kcal/m²/year).
- Always upright — this is a universal rule. Energy always decreases as you move up. No exceptions.
Students often write “pyramid of energy can be inverted.” It cannot. Energy pyramids are always upright because energy is always lost at every trophic level. This is a guaranteed NEET trap question.
Biogeochemical Cycles
Carbon Cycle
Carbon moves between the atmosphere (CO₂), living organisms, and the earth’s crust (fossil fuels, carbonates).
Key processes:
- Photosynthesis: Atmospheric CO₂ → organic carbon in plants
- Respiration: Organic carbon → CO₂ released back
- Decomposition: Dead organic matter → CO₂ released by microbes
- Combustion: Burning fossil fuels releases stored carbon rapidly
- Ocean absorption: Oceans act as carbon sinks; CO₂ dissolves and forms carbonates
The problem with human activity: combustion and deforestation add CO₂ faster than photosynthesis and oceans can absorb it — driving climate change.
Nitrogen Cycle
Nitrogen makes up 78% of the atmosphere but plants can’t use N₂ directly — they need it as nitrates (NO₃⁻) or ammonium (NH₄⁺).
N₂ (atmosphere) → NH₃ (Nitrogen Fixation) → NH₄⁺ (Ammonification) → NO₂⁻ → NO₃⁻ (Nitrification) → N₂ (Denitrification)
Nitrogen Fixation (most important step):
- Biological: Rhizobium (symbiotic in legume roots), Azotobacter, Anabaena (free-living) — convert N₂ → NH₃ using enzyme nitrogenase
- Industrial: Haber process — N₂ + 3H₂ → 2NH₃
- Atmospheric: Lightning — N₂ + O₂ → NO₃⁻ (reaches soil via rain)
Nitrification: Conversion of NH₄⁺ → NO₂⁻ → NO₃⁻ by chemoautotrophic bacteria (Nitrosomonas, then Nitrobacter).
Denitrification: Pseudomonas, Thiobacillus convert NO₃⁻ back to N₂ — returns nitrogen to atmosphere.
NEET 2022 asked specifically about the organism responsible for nitrification. Nitrosomonas converts NH₄⁺ → NO₂⁻, and Nitrobacter converts NO₂⁻ → NO₃⁻. Don’t mix these up — write both.
Phosphorus Cycle
Phosphorus has no atmospheric reservoir (unlike C and N). It cycles only between rocks, soil, water, and living organisms. This makes it a sedimentary cycle.
Weathering of phosphate-rich rocks releases phosphate into soil → absorbed by plants → moves through food chain → returned by decomposition → some lost to deep sediments (this fraction is essentially removed from the cycle for millions of years).
Since there’s no atmospheric replenishment, phosphorus is often the limiting nutrient in freshwater ecosystems. This is why phosphate fertilisers washed into lakes cause algal blooms (eutrophication).
Solved Examples
Example 1 — CBSE Level
Q. A grassland ecosystem has producers fixing 40,000 kcal. How much energy is available to secondary consumers?
Solution: Using Lindeman’s 10% law:
- Energy at primary consumers (T₂) = 10% of 40,000 = 4,000 kcal
- Energy at secondary consumers (T₃) = 10% of 4,000 = 400 kcal
Example 2 — NEET Level
Q. Which of the following sequences represents a correct food chain? (a) Grass → Frog → Insect → Bird (b) Grass → Insect → Frog → Bird (c) Insect → Grass → Frog → Bird (d) Grass → Bird → Insect → Frog
Solution: Option (b).
A food chain must begin with a producer (Grass) and flow from lower to higher trophic levels. Insects (herbivores) eat grass, frogs eat insects, birds eat frogs. Option (a) is wrong because frogs don’t eat insects before insects eat grass.
Example 3 — NEET (Application Level)
Q. In a lake, the biomass of phytoplankton is less than the biomass of zooplankton. The type of pyramid this represents is:
Solution: Inverted pyramid of biomass.
Why this happens: Phytoplankton have extremely high turnover rates — they reproduce quickly and are consumed quickly. At any given moment (standing crop), their biomass is low even though their production rate is high. The pyramid of energy for the same lake would still be upright.
Exam-Specific Tips
For NEET
- Ecology (Ch 13 & 14) contributes 8-10 questions in NEET — one of the highest-scoring single chapters.
- Memorise: which pyramids can be inverted (numbers ✓, biomass ✓, energy ✗).
- Know specific organisms for each nitrogen cycle step — questions are increasingly organism-specific.
- The difference between detritivores and decomposers is a guaranteed 1-mark question every few years.
For CBSE Board (Class 12)
- 3-mark and 5-mark questions on nutrient cycles are common. Draw the cycle diagram — marks are awarded for the diagram even if your text explanation is weak.
- Questions on ecological pyramids often carry 3 marks: name, draw, example. Structure your answer that way.
- For the 5-mark “explain energy flow” question: write Lindeman’s law + calculation + why energy pyramids are always upright.
CBSE board 2024 had a 5-mark question: “Describe the carbon cycle and explain how human activities have disrupted it.” This appears in some form every 2-3 years. A diagram + combustion/deforestation analysis will fetch full marks.
Common Mistakes to Avoid
Mistake 1 — Confusing energy flow with matter cycling. Energy flows unidirectionally and is lost as heat. Matter cycles and is reused. Students often write “energy is recycled” — this is wrong and will cost marks.
Mistake 2 — Inverting the energy pyramid. No ecological condition makes the energy pyramid inverted. If an exam question shows an inverted energy pyramid and asks “which type is this?” — the answer is “this is not possible.”
Mistake 3 — Mixing up Nitrosomonas and Nitrobacter. Nitrosomonas acts first (NH₄⁺ → NO₂⁻), Nitrobacter acts second (NO₂⁻ → NO₃⁻). The mnemonic: Somonas comes before Bacter, just as S comes before B alphabetically.
Mistake 4 — Writing that DFC only occurs in aquatic ecosystems. DFC occurs in all ecosystems but is the dominant pathway in aquatic and forest floor ecosystems. In most terrestrial grasslands, GFC dominates.
Mistake 5 — Forgetting that phosphorus has no atmospheric component. Students include an “atmospheric reservoir” in their phosphorus cycle diagrams, copying from the carbon cycle by mistake. Phosphorus moves rock → soil → organism → soil only.
Practice Questions
Q1. Define ecosystem. Give two examples of natural and two examples of artificial ecosystems.
A functional unit where living (biotic) organisms interact with each other and their non-living (abiotic) environment, exchanging energy and matter.
Natural: Forest, Pond, Ocean, Desert. Artificial (man-made): Crop field, Aquarium, Garden, Reservoir.
Q2. Why are food chains generally limited to 3-4 trophic levels?
Due to Lindeman’s 10% law, only 10% of energy is transferred at each trophic level. By the fourth trophic level, only 0.1% of the original energy remains — too little to sustain a viable population of organisms. Energy limitation, not food availability, restricts chain length.
Q3. Distinguish between detritivores and decomposers with one example each.
Detritivores: Ingest dead organic matter and fragment it physically. Example: earthworm, millipede. They don’t break down organic molecules chemically — they increase surface area for decomposers.
Decomposers: Secrete enzymes onto dead matter and absorb the breakdown products (extracellular digestion). Example: Aspergillus (fungus), Bacillus (bacterium). They chemically break organic matter into inorganic nutrients.
Q4. The pyramid of biomass is inverted in a pond ecosystem. Explain why, and state whether the pyramid of energy for the same pond would also be inverted.
In a pond, phytoplankton (producers) have a very small standing biomass at any moment because they are consumed rapidly. Zooplankton (primary consumers) accumulate larger biomass. Hence the biomass pyramid is inverted.
However, the energy pyramid is always upright — phytoplankton have a very high rate of production (turnover), so the total energy fixed per unit time is greater at the producer level than at the consumer level. Energy cannot be inverted regardless of standing biomass.
Q5. What is the role of Rhizobium in the nitrogen cycle? Where does it live and how does it function?
Rhizobium is a nitrogen-fixing bacterium that lives symbiotically in root nodules of leguminous plants (peas, beans, soybean). It contains the enzyme nitrogenase which converts atmospheric N₂ into NH₃ (ammonia) — a process called biological nitrogen fixation. The plant provides carbohydrates to Rhizobium; the bacterium provides fixed nitrogen to the plant. This is why legumes improve soil fertility.
Q6. A forest has the following standing biomass: trees = 200 t/ha, herbivores = 20 t/ha, carnivores = 1.5 t/ha. Draw and label the pyramid of biomass. Is this upright or inverted?
Pyramid of Biomass (Terrestrial Forest):
Carnivores: 1.5 t/ha [narrow top]
Herbivores: 20 t/ha [medium]
Trees: 200 t/ha [wide base]This is an upright pyramid — biomass decreases from producers to consumers, which is the typical pattern in terrestrial ecosystems.
Q7. Why is the phosphorus cycle called a sedimentary cycle? What makes phosphorus a limiting nutrient in freshwater ecosystems?
Phosphorus has no atmospheric reservoir — it exists only in rocks, soil, water, and organisms. It cycles through geological (rock weathering, sedimentation) processes rather than atmospheric ones, hence called sedimentary.
In freshwater, phosphate is present in very small quantities. Plants and algae deplete it quickly. Since there is no atmospheric replenishment, the ecosystem’s productivity is limited by available phosphate. When excess phosphate enters (via fertiliser runoff), it causes explosive algal growth (eutrophication), depleting oxygen and harming other organisms.
Q8. Name the processes by which nitrogen is returned to the atmosphere from soil.
Denitrification — anaerobic bacteria (Pseudomonas, Thiobacillus denitrificans) convert soil nitrates (NO₃⁻) back to N₂ and some N₂O, which return to the atmosphere.
This process occurs in waterlogged, anaerobic soils and is actually a problem for agriculture — it reduces soil nitrogen availability. That’s why waterlogged fields are less fertile.
Q9. What is ecological efficiency? If 8,000 kcal is available at T₂, how much energy reaches T₄?
Ecological efficiency is the percentage of energy transferred from one trophic level to the next. By Lindeman’s 10% law, this is approximately 10%.
Energy at T₃ = 10% of 8,000 = 800 kcal Energy at T₄ = 10% of 800 = 80 kcal
Only 80 kcal out of 8,000 kcal at T₂ reaches the tertiary consumer level.
FAQs
What is the difference between a food chain and a food web?
A food chain is a single linear sequence of who eats whom (Grass → Deer → Tiger). A food web is a network of multiple interconnected food chains within an ecosystem — more realistic, since most organisms feed on and are fed upon by multiple species. Food webs provide greater stability; if one species is removed, energy can flow through alternative pathways.
Can an ecological pyramid ever be inverted?
Pyramids of numbers and biomass can be inverted under specific conditions. The pyramid of numbers is inverted in tree-based ecosystems (one tree supports many insects). The pyramid of biomass is inverted in aquatic phytoplankton ecosystems. The pyramid of energy is never inverted — energy always decreases up the food chain.
What is Lindeman’s efficiency?
Lindeman’s efficiency (or the 10% law) states that only about 10% of the energy at one trophic level is available at the next trophic level. The remaining ~90% is lost as heat through respiration, movement, and metabolic processes. It was proposed by Raymond Lindeman in 1942 based on studies of a Minnesota lake.
What happens when a keystone species is removed from a food web?
A keystone species has a disproportionately large impact on its ecosystem relative to its abundance. Removing it can cause a trophic cascade — populations of prey explode, overgrazing or overconsumption depletes producers, and the ecosystem structure collapses. Classic example: removal of sea otters causes sea urchin populations to explode, devastating kelp forests.
Why is biological nitrogen fixation important for agriculture?
Most plants cannot use atmospheric N₂ directly. Biological nitrogen fixation by bacteria like Rhizobium converts N₂ to usable NH₃, enriching soil naturally. This reduces dependence on synthetic nitrogenous fertilisers, which are expensive, energy-intensive to produce, and cause water pollution through runoff. Crop rotation with legumes (which host Rhizobium) is a traditional agricultural technique that exploits this.
What is eutrophication and how does it relate to nutrient cycling?
Eutrophication is the excessive enrichment of a water body with nutrients (especially phosphates and nitrates), leading to explosive algal growth (algal bloom). When these algae die and decompose, bacteria consume oxygen rapidly, creating hypoxic (low oxygen) conditions that kill fish and other aquatic life. It is a direct consequence of disrupted nutrient cycling — nutrients that should stay in soil are washed into water bodies through agricultural runoff.
Is the detritus food chain more important than the grazing food chain?
It depends on the ecosystem. In most terrestrial grasslands, the grazing food chain transfers more energy. But in forest floor and aquatic ecosystems, the detritus food chain is often the dominant energy pathway. Globally, more energy flows through decomposers than through herbivores — dead organic matter (detritus) is the largest energy pool available to consumers.