An ecosystem is a community of living organisms plus their non-living environment, interacting as a functional unit. CBSE Class 10 and 12 both cover it; NEET expects you to know the standard trophic structure and the Indian examples. We will break ecosystems into their two component types and the roles each plays.
The word ecosystem was coined by Arthur Tansley in 1935. The key insight is that organisms and their environment are inseparable — sunlight drives the plants, plants feed the herbivores, herbivores feed the carnivores, and when everything dies, decomposers return the nutrients to the soil for the plants. It is a closed loop of matter powered by a one-way flow of energy.
Core Concepts
Biotic components
All living organisms in an ecosystem, organised by how they obtain energy:
Producers (autotrophs) — organisms that fix energy from sunlight (or rarely, from chemical reactions). On land: green plants, mosses, algae. In water: phytoplankton (diatoms, dinoflagellates, cyanobacteria). They form the base of every food chain.
Consumers (heterotrophs) — organisms that eat other organisms:
- Primary consumers (herbivores) — eat producers. Examples: grasshoppers, deer, zooplankton.
- Secondary consumers (primary carnivores) — eat herbivores. Examples: frogs, small fish.
- Tertiary consumers (top carnivores) — eat other carnivores. Examples: eagles, sharks, lions.
- Omnivores — eat both plants and animals. Examples: humans, crows, bears.
Decomposers (saprotrophs) — break down dead organic matter (detritus) into inorganic nutrients. Primarily bacteria and fungi. They complete the nutrient cycle by returning carbon, nitrogen, phosphorus and other elements to the soil and atmosphere.
Detritivores — animals that feed on dead matter but do not fully mineralise it. Examples: earthworms, dung beetles, millipedes. They fragment detritus and increase its surface area for microbial decomposition.
Abiotic components
The non-living factors that set the stage and determine which organisms can survive:
| Factor | Examples | Influence |
|---|---|---|
| Light | Sunlight intensity, photoperiod | Drives photosynthesis, controls flowering, migration |
| Temperature | Ambient, seasonal range | Sets metabolic rates, limits species distribution |
| Water | Rainfall, humidity, water table | Most critical on land, limits terrestrial biomes |
| Soil | pH, mineral content, texture | Determines which plants grow where |
| Atmosphere | CO, O, N | Raw materials for photosynthesis and respiration |
| Topography | Altitude, slope, aspect | Creates microclimates within a region |
Abiotic factors create the niche — the total set of conditions in which a species can survive. Xerophytes evolve in low-water deserts (thick cuticle, sunken stomata), hydrophytes in water-rich environments (aerenchyma, floating leaves).
Trophic levels and food chains
A trophic level is a feeding position in a food chain:
- T1: Producers
- T2: Primary consumers
- T3: Secondary consumers
- T4: Tertiary consumers
A food chain is a linear sequence of who eats whom. Example:
Two types of food chains:
- Grazing food chain (GFC) — starts with living producers. Most energy flows through this in terrestrial ecosystems.
- Detritus food chain (DFC) — starts with dead organic matter. In a forest, more energy actually flows through the DFC than the GFC because so much leaf litter accumulates.
A food web is a network of interconnected food chains. In reality, most animals eat multiple prey types and are eaten by multiple predators.
Energy flow
Energy enters the ecosystem as sunlight. Only about 1-5% of incident solar radiation is captured by producers (Gross Primary Productivity). Energy flows one way — from producers through consumers to decomposers, with roughly 90% lost as heat at each trophic transfer. This is the 10% rule (Lindeman’s efficiency).
If producers fix 10,000 kJ, primary consumers get ~1,000 kJ, secondary consumers ~100 kJ, tertiary consumers ~10 kJ. This is why food chains rarely have more than 4-5 links — there is simply not enough energy left.
This one-way flow has a critical consequence: energy does not cycle. Unlike nutrients (which are recycled), energy enters as light and exits as heat. The ecosystem needs continuous solar input to function.
Nutrient cycling
Unlike energy, nutrients cycle between biotic and abiotic components:
Carbon cycle: CO is fixed by photosynthesis, passes through food chains, and returns to the atmosphere via respiration and decomposition. Fossil fuel burning and deforestation add extra CO, driving climate change.
Nitrogen cycle: Atmospheric N is fixed by bacteria (Rhizobium, Azotobacter, cyanobacteria) into ammonia. Nitrification converts NH to nitrites and nitrates. Plants absorb nitrates. Animals get nitrogen by eating plants. Denitrification returns N to the atmosphere.
Phosphorus cycle: No atmospheric phase — the reservoir is in rocks and sediments. Weathering releases phosphate, which is absorbed by plants. It cycles through organisms and returns to soil/water via decomposition. It is often the limiting nutrient in aquatic ecosystems.
Water cycle: Evaporation, transpiration, condensation, precipitation, runoff, infiltration. Plants contribute significantly through transpiration — a single large tree can transpire 400 L of water per day.
Types of ecosystems
| Type | Characteristics | Examples |
|---|---|---|
| Forest | High biomass, complex food webs | Tropical rainforest, deciduous, coniferous |
| Grassland | Dominated by grasses, seasonal rainfall | Savanna, steppe, prairies |
| Desert | Low water, extreme temperatures | Thar, Sahara |
| Aquatic freshwater | Standing or flowing water | Ponds, lakes, rivers, streams |
| Marine | Saltwater, covers 71% of Earth | Estuary, coral reef, open ocean |
| Wetland | Saturated soil, high productivity | Mangroves, marshes, Ramsar sites |
Ecological succession
The gradual change in species composition of an ecosystem over time.
Primary succession — on bare rock or new land (lava flows, glacial retreat). Begins with pioneer species (lichens, mosses) that weather the rock into soil. Progresses through herbaceous plants, shrubs, and finally trees (climax community). Takes hundreds to thousands of years.
Secondary succession — on land where a community was removed but soil remains (after fire, logging, abandoned farmland). Faster because soil nutrients and seed banks already exist. Reaches climax in 100-200 years.
Worked Examples
Grass captures 10,000 kJ → grasshoppers get ~1,000 kJ → frogs get ~100 kJ → snakes get ~10 kJ → eagle gets ~1 kJ. The 10% rule is an approximation — actual transfer efficiencies vary between 5% and 20% depending on the ecosystem and organism.
In reality, most animals eat multiple prey types and are eaten by multiple predators. A grassland eagle eats snakes, frogs and small birds. A food web captures these cross-connections; a single chain does not. Food webs also explain why removing one species can have unpredictable cascading effects.
A forest ecosystem has GPP = 20,000 kJ/m/year. Plants use 12,000 kJ for their own respiration. NPP = GPP - R = 20,000 - 12,000 = 8,000 kJ/m/year. This NPP is the energy available to herbivores and decomposers.
When wolves were removed from Yellowstone, elk populations exploded. Elk overgrazed riverbank willows. Without willows, beavers had no dam material. Streams became shallow and fast, fish habitat declined. Reintroducing wolves (1995) reversed all of this. This is a trophic cascade — a change at one trophic level ripples through the whole food web.
In a freshwater lake, adding nitrogen fertiliser has little effect on algal growth, but adding phosphorus causes an algal bloom. Phosphorus is the limiting nutrient because the natural supply (from rock weathering) is slow. Agricultural runoff adds excess phosphorus, causing eutrophication. This is why phosphate-free detergents were introduced.
Common Mistakes
Calling fungi producers. They are decomposers (or saprotrophs) — they break down dead matter for energy. They do not fix energy from sunlight. Some students confuse fungi with plants because both are sessile.
Saying energy cycles in an ecosystem. Energy flows one way (sunlight → chemical → heat). Only nutrients cycle. This distinction is a guaranteed one-mark question.
Confusing a food chain with a food web. A chain is linear (one path). A web is a network of interconnected chains. Real ecosystems have webs, not chains.
Forgetting that abiotic components include not just climate but also soil pH, minerals, and topography. NEET questions sometimes describe a limiting factor and ask you to classify it as biotic or abiotic.
Assuming the 10% rule is exact. It is an approximation — actual efficiency ranges from 5% to 20%. Aquatic ecosystems tend to be more efficient (10-15%) than terrestrial ones (5-10%).
Exam Weightage and Strategy
Ecosystem components are tested in CBSE Class 10 (3-5 marks) and Class 12 (5-7 marks). NEET asks 2-3 questions per year from the Ecosystem chapter, making it one of the highest-yield ecology topics. Questions cover trophic levels, energy flow, productivity, decomposition, and nutrient cycling.
Draw one grassland food chain with the 10% energy loss marked at each step. Below it, write the formulas for GPP, NPP, and the 10% rule. That single diagram plus three formulas covers most PYQs on this chapter.
Practice Questions
Q1. Distinguish between GFC and DFC with examples.
GFC (Grazing Food Chain): starts with living green plants as the energy base. Example: Grass → Rabbit → Fox → Eagle. DFC (Detritus Food Chain): starts with dead organic matter (detritus). Example: Dead leaf → Earthworm → Frog → Snake. In forests, more energy flows through DFC because of the large amount of leaf litter. In grasslands and aquatic ecosystems, GFC dominates.
Q2. Why is the pyramid of energy always upright?
Because energy is lost as heat at each trophic transfer (roughly 90%). Each successive level always has less total energy than the one below it. This is a thermodynamic necessity — the second law guarantees energy dissipation. Pyramids of number and biomass can be inverted in special cases, but the energy pyramid never can.
Q3. What is the role of decomposers in an ecosystem?
Decomposers (bacteria and fungi) break down dead organic matter into inorganic nutrients (mineralisation). These nutrients are released into the soil and water, making them available for uptake by producers. Without decomposers, nutrients would remain locked in dead bodies and the ecosystem would run out of raw materials for growth. Decomposers are the essential recyclers.
Q4. Explain why food chains rarely have more than 4-5 trophic levels.
Due to the 10% rule, each trophic level receives only about 10% of the energy from the level below. By the 4th or 5th level, the available energy is too low to support a viable population of predators. For example, if producers fix 10,000 kJ, the 5th level would receive only 1 kJ — not enough to sustain any organism.
FAQs
What is the difference between a habitat and a niche?
A habitat is the physical place where an organism lives (a pond, a forest canopy, a sandy shore). A niche is the organism’s functional role in the ecosystem — what it eats, when it is active, how it interacts with other species. Two species can share a habitat but not the same niche (competitive exclusion principle).
What is biomagnification?
The increase in concentration of a persistent toxic substance (like DDT or mercury) at each trophic level. Producers absorb a small amount, herbivores accumulate more, and top predators end up with dangerous concentrations. This is why eagles and tuna have higher mercury levels than the organisms they eat.
Who proposed the 10% rule?
Raymond Lindeman (1942), based on his study of Cedar Bog Lake in Minnesota. He showed that energy transfer between trophic levels is roughly 10% efficient, with the rest lost as heat through respiration. This is also called Lindeman’s trophic efficiency.
An ecosystem is a balance sheet — inputs of sunlight and nutrients, outputs of heat and decomposition. Understand the flows and the components take care of themselves.