Ecosystem: structure and components
An ecosystem is a part of nature where living things and their surroundings work together as one unit. It can be big (a forest, the sea) or small (a pond, a fish tank). A crop field or an aquarium is a man-made ecosystem.
Components
- Abiotic (non-living): sunlight, temperature, water, soil, air, minerals.
- Biotic (living): producers (green plants, phytoplankton), consumers (herbivores and carnivores) and decomposers (bacteria, fungi).
Structure
Structure means which species are present and how they are arranged. Species composition is the list of species. Stratification is vertical layering: in a forest, trees on top, then shrubs, then herbs and grass. Each layer gets a different amount of light.
Four functions
Every ecosystem does four jobs: productivity, decomposition, energy flow and nutrient cycling.
Pond: a simple example
Abiotic: water, dissolved minerals, sunlight. Producers: phytoplankton, algae, floating and rooted plants. Consumers: zooplankton, small fish, big fish. Decomposers: bacteria and fungi at the bottom.
Productivity: GPP and NPP
Primary production is the amount of biomass (organic matter) made by plants per area in a given time. The rate of making it is productivity, written as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹.
- Gross primary productivity (GPP): total rate of making organic matter in photosynthesis.
- Respiration (R): part of GPP used by plants themselves.
- Net primary productivity (NPP): what is left for animals and decomposers: NPP = GPP − R.
- Secondary productivity: rate at which consumers build new organic matter.
Productivity depends on the plant species, sunlight, temperature, water and nutrients. The whole earth makes about 170 billion tonnes (dry weight) of organic matter a year. The oceans cover about 70% of the surface but give only about 55 billion tonnes, because deep sea water gets little light and few nutrients.
Decomposition: five steps
Decomposers break down complex dead matter (detritus: dead leaves, bark, flowers, dead animals, dung) into simple inorganic things like CO₂, water and nutrients.
- Fragmentation: detritivores like earthworms break detritus into tiny pieces.
- Leaching: water-soluble nutrients go down into the soil and settle as salts.
- Catabolism: bacterial and fungal enzymes break detritus into simpler inorganic substances.
- Humification: a dark, sponge-like substance called humus forms. It resists microbes, decomposes very slowly and is a store of nutrients.
- Mineralisation: humus is slowly broken down by microbes, releasing inorganic nutrients.
What controls the speed
Fast when detritus is rich in nitrogen and sugars and when it is warm and moist. Slow when it is rich in lignin and chitin, and when it is cold, dry or has no oxygen (anaerobic). Decomposition needs oxygen.
Energy flow
The sun is the only source of energy for almost every ecosystem (deep-sea hydrothermal vents are an exception). Less than 50% of sunlight is photosynthetically active radiation (PAR), and plants trap only about 2–10% of PAR. That small share runs the whole living world.
Energy flows one way: sun → producers → consumers. It is never recycled. At each transfer much energy is lost as heat, as the second law of thermodynamics says.
Food chains and trophic levels
- Grazing food chain (GFC): starts with green plants: grass → goat → man.
- Detritus food chain (DFC): starts with dead matter: detritus → decomposers (fungi, bacteria) → detritivores. In a terrestrial ecosystem much more energy flows through the DFC than the GFC.
- Chains join to form a food web.
- Trophic levels: producers (1st), herbivores / primary consumers (2nd), carnivores / secondary consumers (3rd), tertiary consumers (4th).
- Standing crop: the mass of living matter at a trophic level at one time.
10 percent law
Only about 10% of energy at one level passes to the next (Lindeman). So 10,000 J in plants → 1,000 J in herbivores → 100 J → 10 J. This is why food chains rarely have more than 4–5 levels.
Ecological pyramids: number, biomass, energy
Draw the producers as the base and each higher trophic level on top. The pyramid can show number, biomass (dry weight) or energy.
- Pyramid of number: grassland is upright (many grass plants, fewer grasshoppers, fewer frogs). A single big tree with many insects and birds gives an inverted or spindle shape.
- Pyramid of biomass: usually upright on land. In the sea it is inverted: the mass of phytoplankton at one time is small because they grow and are eaten very fast, while fish mass is larger.
- Pyramid of energy: always upright, because energy is lost at every step and a level can never have more energy than the one feeding it.
Limits of pyramids
They ignore that one species can sit at two levels at once, they assume simple food chains (nature has webs), and they leave out saprophytes (decomposers) even though these are vital.
What the board exam asks
Common questions: define GPP and NPP and write NPP = GPP − R; list and explain the steps of decomposition with factors; explain why the energy pyramid is always upright; give an example of an inverted pyramid of number or biomass; calculate energy at a higher level using the 10% law; compare grazing and detritus food chains. Numericals are short but easy marks: always write the formula first.
Key formulas and definitions
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Worked examples
1. A field has GPP = 20,000 kcal m⁻² yr⁻¹ and plants use 4,000 kcal m⁻² yr⁻¹ in respiration. Find NPP.
NPP = GPP − R = 20,000 − 4,000 = 16,000 kcal m⁻² yr⁻¹.
2. A lake has NPP = 6,000 and R = 2,000 kcal m⁻² yr⁻¹. Find GPP.
GPP = NPP + R = 6,000 + 2,000 = 8,000 kcal m⁻² yr⁻¹.
3. Plants trap 50,000 J. Using the 10% law, how much reaches a secondary consumer?
Primary consumer: 10% of 50,000 = 5,000 J. Secondary consumer: 10% of 5,000 = 500 J.
4. A snake (tertiary consumer) gets 25 J. How much energy was in the producers?
Work backwards: × 10 at each step. Secondary: 250 J, primary: 2,500 J, producers: 25,000 J.
5. A forest receives 1,000,000 J of PAR and GPP stores 20,000 J. Find the photosynthetic efficiency.
Efficiency = 20,000 ÷ 1,000,000 × 100 = 2%. This is within the usual 2–10% range.
6. Herbivores in a meadow hold 1,500 kJ and carnivores eating them hold 120 kJ. Find the ecological efficiency and compare with the 10% law.
Efficiency = 120 ÷ 1,500 × 100 = 8%. It is a little less than 10%, which is normal: the 10% figure is only an average.
7. GPP of a crop is 30,000 kcal m⁻² yr⁻¹ and respiration is 30% of GPP. Cows eat 10% of the NPP. How much energy do cows take in?
R = 0.3 × 30,000 = 9,000. NPP = 30,000 − 9,000 = 21,000. Cows: 10% of 21,000 = 2,100 kcal m⁻² yr⁻¹.
8. Why can a food chain rarely have more than five levels? Show with numbers starting at 100,000 J.
100,000 → 10,000 → 1,000 → 100 → 10 → 1 J. By the sixth level only 1 J is left, far too little to support a population of animals.
9. Which pyramid in each case is inverted? (a) number pyramid of a banyan tree with insects, (b) biomass pyramid in the sea, (c) energy pyramid in a pond.
(a) Inverted (one tree, many insects). (b) Inverted (small phytoplankton mass, larger fish mass). (c) Never inverted: energy pyramid is always upright.
Common mistakes
- Writing NPP = GPP + R. Respiration is a loss, so NPP = GPP − R.
- Saying energy is recycled in an ecosystem. Nutrients cycle; energy flows only one way and leaves as heat.
- Thinking the pyramid of biomass is always upright. In the sea it is inverted.
- Mixing up humification (humus forms) with mineralisation (humus breaks down and releases minerals).