Food chains and food webs revision notes

Edexcel International GCSE Biology (4BI1), chapter 14: Ecosystems, specification point 4.6, 4.7. Free lesson.

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Food chains and food webs show how energy moves between organisms. Knowing the trophic levels and arrow direction helps you explain feeding relationships and predict what may happen when a species declines.

How to name the levels in a food chain

A trophic level is an organism’s feeding position in a chain. Start with the producer, then name each consumer by what it eats. Grass is the producer because it makes its own food. Every food chain starts with a producer. A rabbit eats the grass, so it is a primary consumer and a herbivore. A fox eats the rabbit, so it is a secondary consumer and a carnivore. An eagle eats the fox, so it is a tertiary consumer and the top carnivore in this chain. Decomposers feed on dead material from organisms at every trophic level.

Which way food-chain arrows point

Arrows show the direction that energy travels. They mean “is eaten by”, so the arrow points from the food towards the organism that feeds on it. In the chain grass → rabbit → fox → eagle, each arrow points away from the producer. The arrow does not show the direction the animals move or the order in which they eat.

How a food web links chains

A food web shows several food chains that share species. Organisms can feed at the same trophic level when they eat similar kinds of food. Grass and shrub are producers. Rabbits eat grass and insects eat the shrub, so both are primary consumers. Foxes eat rabbits and birds eat insects, so both are secondary consumers. An eagle eats both foxes and birds, making it a tertiary consumer. The two feeding routes form branches that meet at the eagle.

Why a food web is more stable than a chain

A food web has alternative feeding routes. This can make it more stable than a single chain because a predator may switch to another prey species if one becomes less available. If one prey species declines, a predator with other food sources can switch to an alternative. With fewer alternatives in a simple chain, a change to one species can affect the organisms linked to it.

How to avoid reversing an arrow

A common error is to draw the arrow from predator to prey because the predator eats the prey. Instead, read each arrow as “is eaten by”. For a rabbit and fox, the correct arrow is rabbit → fox. Drawing fox → rabbit would show energy travelling from the predator to its prey, which is the opposite direction.