Ecosystem: structure and components
A school pond has water, plants, fish, insects and microbes, all linked together. It works as one small unit of nature.
An ecosystem is a structural and functional unit of nature. Its abiotic components are inorganic materials (air, water, soil, light, temperature) and its biotic components are producers (autotrophs), consumers (heterotrophs) and decomposers. Two main structural features are species composition and stratification (vertical layering).
In the figure : Producers fix energy, consumers use it and decomposers recycle matter.
In the figure : Different species occupy different layers; this is stratification.
Rule 1: Components and function — The four functional components are productivity, decomposition, energy flow and nutrient cycling. Ecosystems vary in size from a pond to the biosphere, and can be terrestrial (forest, grassland, desert) or aquatic (pond, lake, wetland, river, estuary) or man-made (crop field, aquarium).
- Easy : Pond is an aquatic ecosystem.
- Medium : A crop field is a man-made ecosystem.
- Exam-type : Q: What are the four functional aspects of an ecosystem? A: Productivity, decomposition, energy flow and nutrient cycling.
Rule 2: Stratification and species composition — Species composition is the list of species present. In a forest, trees occupy the top layer, shrubs the second, and herbs and grasses the bottom. In a pond, vertical zones include plankton at the top and bottom organisms below. Stratification makes full use of space and light.
- Easy : Trees form the top vertical layer.
- Medium : Stratification reduces competition for space.
- Exam-type : Q: What is stratification? A: The vertical distribution of species at different levels in an ecosystem.
Ecosystem = abiotic + biotic components. Producers = autotrophs; consumers = herbivores, carnivores; decomposers = bacteria and fungi.
Leaving out decomposers when listing biotic components. They are an essential part of every ecosystem.
Memory clue: PCD — Producers, Consumers, Decomposers.
Spot the difference
| Point | Abiotic | Biotic |
|---|---|---|
| Meaning | Non-living components | Living components |
| Examples | Air, water, soil, light | Plants, animals, microbes |
| Role | Supply energy and nutrients | Use and recycle them |
Practice questions
- 1Define an ecosystem.
- 2Give two examples of abiotic components.
- 3What are producers?
- 4Name one man-made ecosystem.
- 5What is meant by stratification?
- 6Which organisms break down dead matter?
- 7Why are decomposers called the cleaners of nature?
- 8Why is stratification useful for an ecosystem?
Show answers (try them yourself first)
- 1A structural and functional unit of nature with biotic and abiotic components.
- 2Any two of air, water, soil, light, temperature.
- 3Autotrophs that fix solar energy, mainly green plants.
- 4A crop field (or aquarium).
- 5Vertical layering of species in an ecosystem.
- 6Decomposers (bacteria and fungi).
- 7They break dead matter into simple nutrients that producers can use again.
- 8Different layers use light and space efficiently with less competition.
Productivity
Two farms of the same size can produce very different amounts of crop; productivity measures how much plant material is made in a given time.
Primary productivity is the amount of biomass or organic matter produced per unit area per unit time by plants during photosynthesis, expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. Gross primary productivity (GPP) is the total production; net primary productivity (NPP) = GPP – R (respiration losses). NPP is the biomass available to consumers. Secondary productivity is the rate of formation of new organic matter by consumers.
In the figure : NPP = GPP – R.
In the figure : Oceans cover about 70 per cent of the surface but give only 55 billion tonnes (billion tonnes of dry matter per year).
Rule 1: Factors and values — Productivity depends on the plant species, environmental factors (light, temperature, water), availability of nutrients and photosynthetic capacity. The annual NPP of the whole biosphere is about 170 billion tonnes (dry weight); oceans contribute only about 55 billion tonnes despite covering 70 per cent of the Earth.
- Easy : NPP of biosphere: about 170 billion tonnes.
- Medium : Land makes the remaining 115 billion tonnes (170 – 55).
- Exam-type : Q: Which is higher, GPP or NPP? A: GPP.
Rule 2: Primary and secondary productivity — Primary productivity is done by producers; secondary productivity is by consumers. NPP is what is left after the plants use part of GPP for respiration.
- Easy : Plants use part of GPP in respiration.
- Medium : NPP can never exceed GPP.
- Exam-type : Q: Define secondary productivity. A: Rate of formation of new organic matter by consumers.
GPP – R = NPP. The biosphere NPP is about 170 billion tonnes, of which oceans 55 billion tonnes.
Writing NPP = GPP + R. NPP is GPP minus respiration.
Memory clue: G-R → N — Gross – Respiration = Net.
Spot the difference
| Point | GPP | NPP |
|---|---|---|
| Meaning | Total organic matter made | Matter left after respiration |
| Formula | Total production | GPP – R |
| Value | Higher | Lower |
Practice questions
- 1What is productivity?
- 2Write the relation between GPP and NPP.
- 3What does R stand for here?
- 4What is the annual NPP of the biosphere?
- 5Which ecosystem gives only 55 billion tonnes?
- 6What is secondary productivity?
- 7GPP of a field is 1500 and R is 400 (kcal m⁻² yr⁻¹). What is NPP?
- 8Why is the NPP of oceans low although they cover most of the Earth?
Show answers (try them yourself first)
- 1The rate of biomass production.
- 2NPP = GPP – R.
- 3Respiratory loss.
- 4About 170 billion tonnes of dry weight.
- 5The oceans.
- 6Formation of new organic matter by consumers.
- 71100 kcal m⁻² yr⁻¹.
- 8Low light penetration and nutrient limits reduce plant growth in water.
Decomposition
Fallen leaves vanish in a few weeks, turning into soil. Earthworms and microbes do this slow and silent work.
Decomposition is the breakdown of complex organic matter (detritus) into inorganic substances such as CO₂, water and nutrients. The steps are fragmentation (by detritivores like earthworms), leaching (water-soluble substances move into the soil), catabolism (enzymatic breakdown by bacteria and fungi), humification (formation of dark humus) and mineralisation.
In the figure : Humus is a dark, colloidal reservoir of nutrients that decomposes very slowly.
Rule 1: Steps — Fragmentation increases the surface area for microbes. Leaching carries soluble inorganic nutrients into the soil layer. Bacterial and fungal enzymes degrade detritus (catabolism). Humus is further degraded to release inorganic nutrients (mineralisation).
- Easy : Earthworm is a detritivore.
- Medium : Humus is slowly mineralised.
- Exam-type : Q: What is humus? A: A dark amorphous substance, resistant to microbial action, that serves as a reservoir of nutrients.
Rule 2: Factors affecting decomposition — The chemical composition of detritus: the rate is slow if it is rich in lignin and chitin and quick if rich in nitrogen and water-soluble substances like sugars. Climatic factors: warm and moist conditions favour decomposition; low temperature and anaerobiosis slow it down.
- Easy : Warm, moist: faster decomposition.
- Medium : Lignin-rich detritus is slow to decompose.
- Exam-type : Q: Name two factors that slow decomposition. A: Low temperature and anaerobic conditions (or lignin-rich detritus).
Decomposition needs oxygen (aerobic). Temperature and moisture are the main climatic controls.
Thinking leaching is done by microbes. It is a physical process by water.
Memory clue: F-L-C-H-M — Fragmentation, Leaching, Catabolism, Humification, Mineralisation.
Spot the difference
| Point | Fragmentation | Leaching |
|---|---|---|
| Agent | Detritivores | Water |
| Result | Smaller particles | Soluble nutrients move into the soil |
| Nature | Biological | Physical |
Practice questions
- 1What is detritus?
- 2Give an example of a detritivore.
- 3What is leaching?
- 4What is humus?
- 5Name two decomposers.
- 6Which climatic condition favours decomposition?
- 7Why is decomposition slow in cold places?
- 8Why is humus important for soil?
Show answers (try them yourself first)
- 1Dead plant remains and animal remains.
- 2Earthworm.
- 3Water-soluble inorganic nutrients move down into the soil.
- 4Dark colloidal organic matter resistant to decomposition.
- 5Bacteria and fungi.
- 6Warm and moist conditions.
- 7Microbial activity is low at low temperatures.
- 8It is a nutrient reservoir and improves soil structure.
Energy flow and food chains
The Sun's energy captured by a leaf moves on to a goat and then to a human. At each step, a lot of it escapes as heat.
The Sun is the only source of energy for almost all ecosystems. Of the incident solar radiation less than 50 per cent is photosynthetically active radiation (PAR), and plants capture only 2–10 per cent of PAR. Energy flow is unidirectional. The grazing food chain (GFC) starts with producers; the detritus food chain (DFC) starts with dead organic matter. According to the 10 per cent law, only 10 per cent of the energy passes to the next trophic level.
In the figure : Each step passes on about 10 per cent of the energy; the rest is lost as heat or used in respiration.
In the figure : Illustration of the 10 per cent law (relative energy units).
Rule 1: Food chains and food webs — In the GFC, energy comes from producers to herbivores to carnivores. In the DFC, detritus is eaten by detritivores and then by predators; in many ecosystems, more energy flows through the detritus food chain than through the GFC. Food chains join to form a food web. The number of trophic levels in a chain is limited (usually 4 or 5) because of energy loss.
- Easy : Grass → deer → tiger is a GFC.
- Medium : Dead leaf → earthworm → bird is a DFC.
- Exam-type : Q: Why can a food chain not have too many levels? A: A large part of energy is lost at each step, so little remains for higher levels.
Rule 2: Trophic levels and standing crop — The organism occupies a trophic level based on its source of food. The mass of living material at a trophic level at a given time is the standing crop. One organism can occupy more than one trophic level (a sparrow is a primary consumer when it eats seeds and a secondary consumer when it eats insects).
- Easy : Standing crop is measured as biomass or number.
- Medium : A sparrow can occupy two trophic levels.
- Exam-type : Q: If a producer has 20,000 J, how much is available to the secondary consumers? A: 200 J (10 per cent of 10 per cent = 1 per cent).
Energy flow is one-way (producer → consumer → decomposer). Matter cycles, energy does not.
Thinking energy can go back to producers. It is lost as heat and cannot be reused by producers.
Memory clue: 10 per cent rule — Each step passes only a tenth on.
Spot the difference
| Point | Grazing food chain | Detritus food chain |
|---|---|---|
| Starts with | Green plants | Dead organic matter |
| First consumer | Herbivore | Detritivore |
| Energy flow | Less in most ecosystems | More in most ecosystems |
Practice questions
- 1What is PAR?
- 2How much PAR do plants capture?
- 3What is a food web?
- 4What is the 10 per cent law?
- 5What does the detritus food chain begin with?
- 6What is standing crop?
- 7A producer level has 5000 kJ. Find the energy that reaches the tertiary consumers.
- 8Why is energy flow called unidirectional?
Show answers (try them yourself first)
- 1Photosynthetically active radiation.
- 2About 2–10 per cent.
- 3A network of interconnected food chains.
- 4Only about 10 per cent of the energy passes from one trophic level to the next.
- 5Dead organic matter.
- 6Mass of living organisms at a trophic level at a particular time.
- 75 kJ (5000 × 10% × 10% × 10%).
- 8Energy moves from producers to consumers and is lost as heat; it cannot flow back.
Ecological pyramids
If you stack the producers, herbivores and carnivores of a grassland as blocks, the stack looks like a pyramid: wide at the bottom and narrow at the top.
An ecological pyramid shows the relationship of trophic levels in numbers, biomass or energy. The base is the producer level. The three kinds are the pyramid of numbers, pyramid of biomass and pyramid of energy. The pyramid of energy is always upright. Pyramids of numbers and biomass may be inverted in some ecosystems.
In the figure : Pyramid of energy cannot be inverted since energy is lost at each step.
Rule 1: Types of pyramids — Pyramid of numbers: count of individuals. In a grassland it is upright; in a tree ecosystem with many insects it is inverted. Pyramid of biomass: usually upright, but in the sea it is inverted because the standing crop of phytoplankton is small while zooplankton and fishes have more biomass (phytoplankton reproduce very fast). Biomass is better measured as dry weight, since fresh weight includes variable water.
- Easy : Sea biomass pyramid is generally inverted.
- Medium : Pyramid of energy is always upright.
- Exam-type : Q: Why can the pyramid of energy never be inverted? A: Energy decreases at every trophic level because of losses.
Rule 2: Limitations — Pyramids do not account for the same species occupying more than one trophic level, do not include decomposers, and assume simple food chains. Food webs are not shown.
- Easy : Decomposers have no place in pyramids.
- Medium : One species may feed at two levels.
- Exam-type : Q: Name one limitation of ecological pyramids. A: They do not include decomposers (or food webs).
Energy pyramid: always upright. Biomass in the sea: inverted. Numbers on a tree: inverted.
Thinking that every pyramid is upright. Number and biomass pyramids may be inverted.
Memory clue: E-Always-Up — Energy pyramid is always upright.
Spot the difference
| Point | Pyramid of numbers | Pyramid of energy |
|---|---|---|
| Measures | Number of individuals | Energy at each level |
| Shape | May be inverted | Always upright |
| Example inverted | Many insects on one tree | Never inverted |
Practice questions
- 1Name the three types of ecological pyramids.
- 2Which pyramid is always upright?
- 3Which is more accurate for biomass: fresh weight or dry weight?
- 4Give an example of an inverted pyramid of numbers.
- 5Which pyramid is inverted in the sea?
- 6Name one limitation of ecological pyramids.
- 7Why is the pyramid of biomass in the sea inverted?
- 8Why must dry weight be used for biomass measurement?
Show answers (try them yourself first)
- 1Numbers, biomass and energy.
- 2The pyramid of energy.
- 3Dry weight.
- 4A big tree with many insects feeding on it.
- 5The pyramid of biomass.
- 6They do not include decomposers.
- 7Phytoplankton are small in standing crop but reproduce rapidly, so zooplankton and fishes have more biomass.
- 8Water content varies, so fresh weight is unreliable.
Nutrient cycling and ecosystem services
A forest cleans the air and holds the soil. These free services quietly support every city.
Nutrient cycling is the storage and movement of nutrient elements through the components of an ecosystem; nutrients are repeatedly used. It is of two types: gaseous (reservoir is the atmosphere or hydrosphere, for example carbon) and sedimentary (reservoir is the Earth's crust, for example phosphorus). Products of ecosystem processes are ecosystem services, for example purification of air and water by forests.
In the figure : Nutrients move between living and non-living parts again and again.
Rule 1: Nutrient cycles — Plants take up nutrients from soil and air; animals eat plants; decomposers return nutrients to soil and air. The cycle of an element depends on its reservoir. Unlike energy, nutrients are recycled.
- Easy : Carbon cycle is gaseous.
- Medium : Phosphorus cycle is sedimentary.
- Exam-type : Q: What is the reservoir of phosphorus? A: The Earth's crust (rocks).
Rule 2: Ecosystem services — Healthy ecosystems purify air and water, mitigate floods and droughts, cycle nutrients, pollinate crops, generate soil and preserve biodiversity. These are called ecosystem services; loss of ecosystems reduces them.
- Easy : Forests purify air and water.
- Medium : Wetlands reduce floods.
- Exam-type : Q: Give two ecosystem services. A: Purification of air and water, pollination.
Gaseous cycle: atmosphere/hydrosphere reservoir. Sedimentary cycle: Earth's crust reservoir.
Confusing energy flow with nutrient cycling. Energy flows in one direction; nutrients cycle.
Memory clue: C-gas, P-rock — Carbon = gaseous; Phosphorus = sedimentary (rock).
Spot the difference
| Point | Gaseous cycle | Sedimentary cycle |
|---|---|---|
| Reservoir | Atmosphere/hydrosphere | Earth's crust |
| Example | Carbon | Phosphorus |
| Speed | Faster | Slower |
Practice questions
- 1What is nutrient cycling?
- 2Name the two types of nutrient cycles.
- 3Give an example of a gaseous cycle.
- 4Give an example of a sedimentary cycle.
- 5What are ecosystem services?
- 6Name one service given by forests.
- 7Why is nutrient cycling important?
- 8How is the nutrient cycle different from energy flow?
Show answers (try them yourself first)
- 1Storage and movement of nutrient elements in the ecosystem.
- 2Gaseous and sedimentary.
- 3Carbon cycle.
- 4Phosphorus cycle.
- 5The products of ecosystem processes useful to people.
- 6Purification of air and water.
- 7It makes nutrients available again and again for living organisms.
- 8Nutrients are reused repeatedly; energy flows one way and is lost as heat.