BIOLOGY · Stage 7 · Chapter 3: Microorganisms and Decomposers
Biology 3.7 Food chains, food webs and decomposers
5 min read 6 parts Lesson 7 of 8
Lesson summary
3.7 Food chains, food webs and decomposers — Alhussan International Academy — Biology (Stages 7–9) notes: Build and interpret food chains and food webs and explain the importance of decomposers in them.Food chain — a simple model that shows the feeding relationships between organ…
- Further examples
- Primary consumer
- Secondary consumer
- Tertiary consumer
- Length limit
- Keystone species
Section 1 of 6Food chains
- Food chain
- a simple model that shows the feeding relationships between organisms in a habitat.
- Arrows
- they show the transfer of biomass and energy from one organism to the next, from the food to the feeder.
- (biological material)
- A rabbit eating grass is drawn as grass → rabbit.
- they show the transfer of biomass and energy from one organism to the next, from the food to the feeder.
- Example
- acacia tree → giraffe → lion.
- Further examples
- maize → chicken → human, and leaf → beetle larva → oriole.
- Length
- food chains may have two, three, four or even five links.
Section 2 of 6Roles in a food chain
- Producer
- the first organism, often a plant, which traps energy from the Sun and makes food by photosynthesis.
- Primary consumer
- a herbivore that eats producers, digests plants and uses the energy and materials for all the functions of life.
- Secondary consumer
- a carnivore that eats primary consumers.
- Tertiary consumer
- a carnivore that eats secondary consumers; the one at the end is the top predator.
- Prey
- an animal eaten by other animals.
- Predator
- an animal that eats other organisms.
| Role | Example |
|---|---|
| Producer | Algae, grass |
| Primary consumer | Rabbit, snail |
| Secondary consumer | Fox, thrush |
| Decomposer | Fungi, bacteria |
Section 3 of 6Energy and biomass in a chain
1Losses3 points
- at each stage some material cannot be digested and is lost as waste.
- Energy is transferred from the living organisms to their surroundings or used to move about.
- Roughly 10% of the energy at one level reaches the next.
2Length limit2 points
- the longer the chain, the less biomass reaches the organism at the top.
- Food chains rarely have more than four or five links.
3Numbers2 points
- there are fewer animals at the top than producers at the bottom.
- A pyramid of numbers shows the number of organisms at each level, although one large tree supporting many insects gives an odd shape.
Section 4 of 6Food webs
- Food web = a model that links several food chains together.
- Reason
- most animals eat more than one kind of organism, and most foods are eaten by more than one kind of animal.
- Chickens eat maize but also grass, almost any grain, worms and beetles.
- Lions eat giraffes but will eat many other animals if they get the chance.
- Realism
- food webs are still simple models, but they are more realistic than food chains.
- Changes
- removing or adding one species affects others.
- If frogs vanished, snakes would have less food and fall in number, while grasshoppers would increase and eat more grass.
- Keystone species
- one that many others depend on and so has a large effect if removed.
- Sea otters eat sea urchins and so protect kelp forests, which shelter fish.
Section 5 of 6Where decomposers fit
- Death
- every organism in a chain eventually dies, and its waste and remains are used by decomposers.
- Position
- decomposers feed on every level, so arrows can be drawn from every organism to them rather than putting them at one level.
- Action
- bacteria and fungi break down dead producers, consumers and waste.
- They release minerals into the soil and carbon dioxide into the air.
- Carbon
- the carbon compounds in dead material return to the air as carbon dioxide from respiration.
Section 6 of 6The circle of life
- Problem
- undigested material is lost along a chain, energy is transferred to the environment and plants keep taking mineral salts from the soil.
- If this continued, no minerals would be left, plants could not grow and all food chains and webs would fail.
- Solution
- decomposers break down the bodies of plants and animals and return nutrients to the soil.
- Plants take the nutrients up and use them to build new plants, the start of a new food chain.
- Energy and nutrients
- nutrients are recycled, while energy flows through the web and is eventually lost as heat.
- Models
- all models have limitations, so identify the strengths and weaknesses of any model used.
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