Food chains (AQA GCSE Biology): Revision Notes
Food chains
Food chains show us how different organisms feed on each other in nature. They help us understand the feeding relationships between living things in a community.
What is a food chain
A food chain shows the flow of energy from one organism to another. It starts with producers and moves up through different levels of consumers.
The arrows in a food chain show the direction of energy flow - they point from the organism being eaten to the organism that eats it.
Example: grass → rabbit → fox
- The arrow shows that grass is eaten by rabbits
- Rabbits are then eaten by foxes
Producers and consumers
Producers
Producers are the starting point of every food chain. They make their own food through photosynthesis.
Key facts about producers:
- Usually green plants or algae
- Make glucose using sunlight, carbon dioxide and water
- Create biomass that supports all life on Earth
- Don't need to eat other organisms to survive
Examples of Producers: Grass, trees, seaweed
Primary consumers
Primary consumers eat producers. They are also called herbivores.
Key facts about primary consumers:
- Feed directly on plants
- First level of consumers in the food chain
- Provide food for secondary consumers
Examples of Primary Consumers: Rabbits, sheep, caterpillars
Secondary consumers
Secondary consumers eat primary consumers. They are carnivores.
Key facts about secondary consumers:
- Hunt and eat herbivores
- Second level of consumers
- Can become prey for tertiary consumers
Examples of Secondary Consumers: Foxes, birds of prey, snakes
Tertiary consumers
Tertiary consumers eat secondary consumers. They are also carnivores and often top predators.
Examples of Tertiary Consumers: Hawks, sharks, big cats
Predator-prey cycles
Predators are animals that hunt and kill other animals. Prey are the animals that get eaten.
In nature, predator and prey populations rise and fall in regular cycles. This creates a pattern that repeats over time.
How the cycle works
Worked Example: The Predator-Prey Cycle
Step 1: Prey population increases
- When there's plenty of food and few predators
- More prey survive and reproduce
Step 2: Predator population increases
- More prey means more food for predators
- More predators survive and reproduce
Step 3: Prey population decreases
- More predators eat more prey
- Prey numbers fall
Step 4: Predator population decreases
- Less prey means less food for predators
- Predator numbers fall
- The cycle starts again
Why cycles happen
The populations are linked together. When one changes, it affects the other:
- More prey = More food for predators = More predators
- More predators = Less prey = Less food = Fewer predators
This creates a natural balance that keeps both populations from getting too high or too low.
Key Points to Remember:
- Food chains show energy flow from producers to consumers
- Arrows point from prey to predator (from what's eaten to what eats it)
- All food chains start with producers that make their own food
- Primary consumers eat plants, secondary consumers eat primary consumers
- Predator and prey populations rise and fall in linked cycles