Food chains and food websIB MYP Sciences: Revision notes

A food chain shows what eats what in a habitat.

• Producers make their own food by photosynthesis, such as grass and algae.
• Consumers get energy by eating other organisms. A primary consumer eats producers, a secondary consumer eats primary consumers, and a tertiary consumer eats secondary consumers.
• Decomposers, such as bacteria and fungi, break down dead organisms and waste.

Each step in the chain is a trophic level. Producers are the first trophic level.

• Herbivores eat only plants (rabbit, grasshopper).
• Carnivores eat only animals (lion, eagle).
• Omnivores eat both plants and animals (humans, bears).

A predator hunts and eats other animals, which are its prey.

In a food chain, the arrows show the flow of energy. They point from the organism being eaten to the organism that eats it.

grass → grasshopper → lizard → eagle

Here grass is the producer, the grasshopper is the primary consumer, the lizard is the secondary consumer and the eagle is the tertiary consumer.

Most organisms eat more than one thing, so real feeding links form a food web, many linked food chains. One organism can occupy different trophic levels in different chains. A fish that eats a snail (primary consumer) is a secondary consumer, but if it eats a dragonfly larva (a secondary consumer) it is a tertiary consumer.

The numbers of predators and prey in a habitat are linked. Population data often show a repeating cycle:

1. Prey numbers rise when there is plenty of food.
2. More prey means more food for predators, so predator numbers rise after a delay.
3. More predators eat more prey, so prey numbers fall.
4. Less food means predator numbers fall, and the cycle repeats.

On a population graph, the predator peak comes after the prey peak.

Because organisms are linked, a change to one species affects others.

• Removing a predator: its prey increase, and then eat more of what they feed on.
• Removing a producer: the herbivores lose their food and decline, then the carnivores that eat them.
• Adding a new organism: it may eat or compete with existing species and change their numbers.

When you answer, follow the chain of effects step by step.