How does energy pass along a food chain, and why do predator and prey numbers rise and fall together?
Food chains and food webs, the roles of producers, consumers and decomposers, the transfer and loss of energy along a food chain, and predator-prey cycles.
A focused answer to the OCR Gateway GCSE Biology A topic B4 on feeding relationships, covering food chains and webs, producers, consumers and decomposers, the loss of energy along a food chain, and predator-prey cycles.
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What this dot point is asking
OCR wants you to interpret food chains and food webs, state the roles of producers, consumers and decomposers, explain why energy is lost along a food chain, and explain predator-prey cycles.
Food chains
Every food chain begins with a producer:
- Producers are usually green plants or algae. They make their own food by photosynthesis, capturing light energy, so they are the source of energy for the whole chain.
- Consumers are organisms that eat other organisms. A primary consumer (herbivore) eats producers; a secondary consumer eats primary consumers; and so on.
- Decomposers are bacteria and fungi that break down dead organisms and waste, releasing nutrients back into the environment.
Food webs
A single organism is usually part of several food chains, because most animals eat more than one type of food and are eaten by more than one predator. A food web is a set of interconnected food chains, showing the feeding relationships in a community more realistically. If one species in a web changes in number, it can affect many others, which shows the interdependence of the community.
Energy transfer and loss
At each step of a food chain, only a small part of the energy is passed on. Energy is lost because:
- Organisms use energy in respiration (for movement, and to keep warm in mammals and birds), and this energy is eventually transferred to the surroundings.
- Energy is lost in waste (faeces and urine) and in parts that are not eaten (such as bones or roots).
Because so much energy is lost at each step, less is available the further up the chain you go. This is why food chains are usually only four or five organisms long, and why there are fewer organisms (and less total biomass) at each higher level.
Predator-prey cycles
In a stable community, the numbers of a predator and its prey rise and fall in linked cycles:
- When prey are plentiful, there is lots of food, so the predator population rises.
- As predators increase, they eat more prey, so the prey population falls.
- With less prey to eat, the predator population then falls.
- With fewer predators, more prey survive, so the prey population rises again, and the cycle repeats.
The predator peak usually comes slightly after the prey peak, because it takes time for the predators to respond to the change in food.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksIn the food chain grass to rabbit to fox, name the producer and explain why there is usually less energy available to the fox than to the rabbit.Show worked answer →
A 4-mark question on energy transfer.
Producer: the grass is the producer (it makes its own food by photosynthesis, capturing light energy).
Less energy to the fox: energy is lost at each step of the food chain, so less is passed on each time. Energy is lost through respiration (used for movement and to keep warm), and in waste (faeces and urine) and uneaten parts. So only a small fraction of the energy in the rabbits is passed on to the fox. This is why food chains are usually short and there are fewer foxes than rabbits. Markers reward the producer and at least one reason energy is lost (respiration or waste) plus the conclusion that less is passed on.
OCR 20214 marksA graph shows the populations of a predator (lynx) and its prey (hares) over many years, rising and falling in repeating cycles. Explain why the two populations rise and fall in this linked way.Show worked answer →
A 4-mark question on predator-prey cycles.
When the prey (hares) are plentiful, there is lots of food for the predators (lynx), so the predator population rises. As the number of predators rises, more prey are eaten, so the prey population falls. With less prey (food), the predators then have less to eat, so the predator population falls. With fewer predators, more prey survive, so the prey population rises again, and the cycle repeats.
Markers reward the linked sequence: plenty of prey leads to more predators; more predators leads to fewer prey; fewer prey leads to fewer predators; fewer predators leads to more prey. A strong answer notes the predator peaks slightly after the prey peak.
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