How does energy flow through a food chain, and why are food chains short?
Food chains, food webs and trophic levels, pyramids of biomass, and the transfer and loss of energy and biomass between trophic levels.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on ecosystems, covering food chains, food webs and trophic levels, pyramids of biomass, and why energy and biomass are lost between trophic levels.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to interpret food chains, food webs and trophic levels, understand pyramids of biomass, and explain how energy and biomass are lost between trophic levels.
Food chains and trophic levels
A typical food chain begins with a producer (a green plant) that makes food by photosynthesis. It is eaten by a primary consumer (a herbivore), which is eaten by a secondary consumer (a carnivore), and so on. Decomposers (bacteria and fungi) break down dead organisms and waste, returning nutrients to the soil.
A food web is several food chains joined together, showing that most organisms eat, and are eaten by, more than one species. If one species changes in number, others in the web are affected.
Energy transfer and loss
Because so much energy is lost at each level, there is less and less available higher up the chain. This is why food chains rarely have more than four or five trophic levels: after that, there is not enough energy left to support another organism.
Pyramids of biomass
A pyramid of biomass shows the mass of living material at each trophic level. It is almost always a true pyramid shape, widest at the bottom (producers) and narrowing towards the top, because energy and biomass decrease at each level. Each bar is drawn to scale, so a quick glance shows how biomass falls as you go up.
Predator and prey numbers
In a food web, the numbers of predators and prey are linked and rise and fall together. If the prey increases, there is more food, so the predator numbers rise. More predators then eat more prey, so the prey numbers fall, after which the predators have less food and their numbers fall too, letting the prey recover. This produces cycles in the populations, slightly out of step with each other. Exam questions often give a graph and ask you to explain a rise or fall using the idea of food supply.
Why we farm efficiently
Understanding energy loss explains some farming methods. Because energy is lost at each trophic level, eating plants (being a primary consumer) wastes less energy than eating meat. Farmers can also raise the efficiency of meat production by keeping animals warm and still (for example in barns), so less energy is lost as heat from respiration and through movement, leaving more for growth. These methods raise yield but raise ethical questions about animal welfare, which is a common discussion point.
Try this
Q1. Name the first organism in any food chain. [1 mark]
- Cue. A producer (a green plant).
Q2. State two ways energy is lost between trophic levels. [2 marks]
- Cue. Any two of: respiration (heat), movement, and waste such as faeces.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain why a food chain rarely has more than four or five trophic levels.Show worked answer →
A Unit 1 explain question worth 4 marks. Reward: energy is transferred along the food chain when organisms are eaten (1), but most of the energy is lost at each level through respiration (as heat), movement, and in waste such as faeces and undigested material (1); only about 10 percent of the energy passes to the next level (1); after a few levels there is too little energy left to support another organism (1). Markers credit energy loss at each level, the small percentage passed on, and the conclusion. A common error is to say energy is destroyed.
WJEC style3 marksIn the food chain grass to rabbit to fox, name the producer and explain what would happen to the fox population if the rabbits caught a disease and died.Show worked answer →
A Unit 1 food-chain question. The producer is the grass (1). If the rabbits died, the foxes would have less food, so the fox population would decrease (1), because the foxes depend on the rabbits as their food source (1). Markers credit identifying the producer, the decrease in foxes and the reason (less food). A common error is to say the fox population would increase.
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