How does energy flow through food chains, and why are food chains short?
Food chains and food webs and the flow of energy from producers through consumers, pyramids of numbers and of energy, and the loss of energy at each level that limits the length of food chains.
An SQA National 5 Biology answer on energy in ecosystems, covering food chains and food webs and the flow of energy from producers through consumers, pyramids of numbers and of energy, and the loss of energy at each level that limits the length of food chains.
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What this dot point is asking
The SQA wants you to describe food chains and food webs and the flow of energy from producers through consumers, explain pyramids of numbers and pyramids of energy, and explain why energy is lost at each level and how this limits the length of food chains.
Food chains and food webs
The organisms in a food chain have roles:
- A producer is a green plant that makes its own food by photosynthesis, capturing energy from sunlight.
- Consumers eat other organisms. They are herbivores (eat plants), carnivores (eat animals) or omnivores (eat both).
A food web is several food chains linked together, showing the many feeding relationships in an ecosystem.
Pyramids of numbers and energy
The pyramid of energy is the more reliable picture, because it does not depend on the size of individual organisms.
Energy loss and short food chains
This is why food chains usually have only four or five links: by the top, almost all the energy captured by the producers has been lost.
Examples in context
Example 1. A grassland food chain. Grass (producer) is eaten by a rabbit (herbivore), which is eaten by a fox (carnivore). At each step most energy is lost as heat and movement, so the fox receives only a tiny fraction of the energy the grass first captured, which is why few foxes can be supported.
Example 2. Why no chain has twenty links. Because roughly only a tenth of the energy passes on at each level, after a handful of links there is almost no energy left. This is why you never see a food chain with fifteen or twenty stages: the energy runs out, limiting the chain length.
Try this
Q1. State what the arrows in a food chain represent. [1 mark]
- Cue. The direction of energy flow (from the eaten to the eater).
Q2. Name two ways energy is lost between levels of a food chain. [1 mark]
- Cue. Any two of: heat (respiration), movement, undigested material.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksExplain why most of the energy is lost at each stage of a food chain and what this means for the length of food chains.Show worked answer →
A 3-mark answer should give the routes of energy loss and the consequence.
At each stage, only a small percentage of the energy is passed on to the next level. Most of the energy is lost as heat (from respiration), through movement, and in undigested materials that are not eaten or passed out as waste.
Because so much energy is lost at each level, there is not enough energy left to support many more levels. This limits the length of food chains, which usually have only four or five links.
Markers reward (1) most energy lost at each stage, (2) named routes of loss (heat, movement, undigested material), and (3) the link to short food chains.
SQA N5 style2 marksExplain why a pyramid of energy is always pyramid-shaped, while a pyramid of numbers is not always.Show worked answer →
Two ideas: energy always falls, but numbers can be misleading.
A pyramid of energy is always pyramid-shaped because energy is always lost between levels, so each level has less total energy than the one below it.
A pyramid of numbers can be an odd shape because it counts organisms regardless of size. For example, one large tree (a producer) supports many insects, giving a narrow base, so the pyramid is inverted at that point.
Markers reward (1) energy always decreasing up the levels, and (2) the explanation that numbers ignore the size of organisms.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)