Food chains (AQA GCSE Biology Combined Science): Revision Notes
Food chains
What are food chains?
Food chains show us how different living things feed on each other in nature. They help us understand feeding relationships within a community of organisms.
Each arrow in a food chain points from the organism being eaten to the organism that eats it. For example, if grass → rabbit → fox, the arrows show that rabbits eat grass, and foxes eat rabbits.
The direction of arrows in food chains is crucial - they always point from the food source to the consumer, showing the flow of energy and nutrients through the ecosystem.
Producers and consumers
Every food chain starts with a producer. Producers make their own food using sunlight through photosynthesis. Most producers are green plants or algae. They create glucose and biomass that supports all life on Earth.
The different levels of consumers are:
- Primary consumers - these are herbivores that eat producers (plants)
- Secondary consumers - these are carnivores that eat primary consumers
- Tertiary consumers - these eat secondary consumers
Worked Example: Identifying Trophic Levels
In the food chain grass → rabbit → fox:
- Grass = producer (makes its own food through photosynthesis)
- Rabbit = primary consumer (herbivore that eats plants)
- Fox = secondary consumer (carnivore that eats other animals)
All food chains must start with producers because they are the only organisms that can convert sunlight into usable energy for the entire ecosystem.
Predator-prey cycles
Predators are animals that hunt and kill other animals for food. Prey are the animals that get eaten.
In a stable community, the numbers of predators and prey go up and down in regular cycles. Here's how it works:
- Prey population rises - when there are few predators, more prey survive and reproduce
- More food for predators - as prey numbers increase, predators have more food and their population grows
- Predator population rises - more predators means they eat more prey, so prey population falls
- Less food for predators - with fewer prey available, predator population also falls
Then the cycle starts again! This creates a wave pattern when you plot the populations on a graph.
These population cycles are essential for maintaining balance in ecosystems. Without this natural regulation, one species could become too dominant and upset the entire food web.
Reading predator-prey graphs
When looking at population graphs, there are key patterns to recognise:
- The prey population peaks before the predator population
- When prey numbers are high, predator numbers start to rise
- When prey numbers fall, predator numbers fall too (but slightly later)
Worked Example: Zebra and Lion Populations
In a zebra and lion community:
- Lions are predators, zebras are prey
- A lion must eat several zebras each year to survive
- This means there are always fewer lions than zebras
- The populations change in cycles, with zebra numbers affecting lion numbers
The graph would show zebra populations peaking first, followed by lion populations rising as more food becomes available.
Always remember that predator populations are typically much smaller than prey populations because energy is lost at each trophic level - predators need to consume many prey organisms to survive.
Key Points to Remember:
- Food chains show feeding relationships - arrows point from food to feeder
- All food chains start with producers - usually green plants that make food by photosynthesis
- Consumers are organised in levels - primary (herbivores), secondary (carnivores), tertiary
- Predator-prey populations cycle - they rise and fall in regular patterns
- Prey populations peak first - then predator populations follow the same pattern