Trophic Levels and Food Chains — Ecology & Food Chains | IGCSE Biology

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This topic accounts for approximately 12% of your exam marks.

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Food chains, energy transfer, and ecological definitions are regularly tested, often as short-answer questions.

A food chain is a diagram showing the transfer of energy from one organism to the next as one organism eats another. Each step in a food chain is called a trophic level.

The trophic levels

Trophic level	What organisms eat	Examples
1st: Producers	Make their own food by photosynthesis, using light energy	Plants, algae, photosynthetic bacteria
2nd: Primary consumers	Eat producers (herbivores)	Cows, caterpillars, rabbits
3rd: Secondary consumers	Eat primary consumers (carnivores or omnivores)	Foxes, frogs, songbirds
4th: Tertiary consumers	Eat secondary consumers (top predators)	Eagles, sharks, lions
Decomposers	Feed on dead organisms and waste	Bacteria, fungi, earthworms

The first trophic level is always a producer because producers are the only organisms that can convert light energy from the Sun into the chemical energy needed by everything else.

Reading a food chain

A typical food chain is written as a sequence of organisms with arrows pointing in the direction of energy transfer:

grass → rabbit → fox

Grass (1st trophic level, producer) is eaten by the rabbit
Rabbit (2nd, primary consumer) is eaten by the fox
Fox (3rd, secondary consumer) is the top of this chain

Arrows point from the eaten organism to the eater (i.e. in the direction energy flows). The arrow means "is eaten by".

Most natural food chains have four trophic levels at most, because too much energy is lost between levels for a fifth one to be supportable.

Food webs

In any real ecosystem, organisms eat many different things, and most are eaten by many different predators. A food web is a diagram showing the network of all the food chains in an ecosystem. The same organism can occupy different trophic levels in different chains.

Species removal: knock-on effects in a food web

Removing a species from a food web can affect every other species in it, sometimes dramatically:

If a predator is removed, its prey species can multiply unchecked, eventually overgrazing the producers and crashing the whole system.
If a prey species is removed, its predators have less food and decline; species that depended on it for some other role (e.g. seed dispersal) are also affected.
If a producer is removed (e.g. an invasive species kills all the grass), every herbivore that ate it suffers, and every carnivore that ate the herbivore is hit too.

A well-known example: sea otters in the Pacific eat sea urchins. Sea urchins eat kelp. When sea otters were hunted to near-extinction in the 19th century, sea urchin numbers exploded, the urchins ate all the kelp, and the kelp forests collapsed. Restoring sea otters has restored the kelp.