How does energy flow through living things, and how do food chains link the organisms in a habitat?
Describe food chains and food webs, explain the roles of producers, consumers and decomposers, and account for energy loss along a chain
A focused N(A)-Level answer on ecology. Producers, consumers and decomposers, food chains and food webs, why energy is lost at each step, and what happens when a chain is disturbed.
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What this dot point is asking
SEAB wants you to describe food chains and food webs, to explain the roles of producers, consumers and decomposers, and to account for why energy is lost along a chain. The central idea is that energy from the Sun flows through living things, but a lot is lost at each step.
The answer
Food chains
A food chain shows how energy passes from one organism to the next as food. The arrows point in the direction the energy flows (from the eaten to the eater). For example:
Producers, consumers and decomposers
Each organism has a role:
- producers: green plants that make their own food by photosynthesis. They start every food chain and capture energy from the Sun.
- consumers: animals that eat other organisms. A primary consumer (herbivore) eats plants; a secondary consumer eats the primary consumer; and so on.
- decomposers: organisms such as bacteria and fungi that break down dead bodies and waste, returning nutrients to the soil so plants can use them again.
Food webs
In a real habitat, most animals eat more than one type of food and are eaten by more than one predator. A food web joins many food chains together to show these links. If one organism in a web changes in number, it affects the others connected to it.
Energy loss along a chain
Energy enters the chain as sunlight, captured by producers. At each step, much of the energy is lost rather than passed on, because organisms:
- use energy in respiration, much of which is lost as heat,
- use energy for movement and other life processes,
- do not eat or cannot digest every part of their food (energy lost in waste).
Because so much energy is lost at each step, food chains rarely have more than four or five links: there is not enough energy left to support many more.
Examples in context
Example 1. Why removing one species harms a whole web. If a disease wipes out the rabbits in a habitat, foxes that fed on them lose food and decline, while the plants the rabbits ate may grow unchecked. Because organisms are linked in a food web, a change in one affects many others.
Example 2. Why composting works. A compost heap relies on decomposers (bacteria and fungi) breaking down food scraps and garden waste. They return the nutrients to a form plants can absorb, which is why compost makes a good fertiliser.
Try this
- Cue. State what a producer is. A green plant that makes its own food by photosynthesis and starts a food chain.
- Cue. Explain why energy is lost at each step in a food chain. It is used in respiration (lost as heat), in movement, and in undigested waste.
- Cue. Name the organisms that recycle nutrients from dead material. Decomposers (bacteria and fungi).
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksA food chain is: grass to grasshopper to frog to snake. (a) Name the producer. (b) Name a secondary consumer. (c) State where the energy in the grass originally came from.Show worked answer →
(a) The producer is the grass.
(b) The secondary consumer is the frog (it eats the grasshopper, which is the primary consumer).
(c) The energy originally came from the Sun (light energy trapped by the grass in photosynthesis).
What markers reward: grass as the producer, the frog as the secondary consumer, and the Sun as the original energy source.
Original3 marks(a) Explain why a food chain rarely has more than four or five links. (b) State the role of decomposers in an ecosystem.Show worked answer →
(a) Energy is lost at each link (as heat from respiration, in movement, and in undigested waste), so there is too little energy left to support many more levels.
(b) Decomposers break down dead organisms and waste, returning nutrients to the soil so they can be used again.
What markers reward: energy lost at each step limiting the number of links, and decomposers recycling nutrients by breaking down dead material.
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