Food Chains, Food Webs, and Trophic Levels

A food chain shows a single path of energy from a producer through a series of consumers, a food web shows many linked food chains, and trophic levels are the feeding steps along these paths. In brief: in the chain grass → grasshopper → frog → snake → hawk, grass is at trophic level 1 and the hawk is at level 5. Only a small fraction of the energy at one level reaches the next, usually about 10 percent, so food chains are short.

Food chains and webs are among the most useful models in ecology, because they show who eats whom and how energy moves. This guide explains how to read and draw chains and webs, defines trophic levels, explains energy flow and the ten percent rule, works through a numerical example and corrects common mistakes.

What is a food chain?

A food chain is a sequence of organisms in which each one is food for the next. It starts with a producer, usually a plant or alga, and continues through consumers. An example in a meadow is: clover → rabbit → fox.

The arrows show the direction of energy transfer, so they point from the organism that is eaten to the organism that eats it. They do not mean “eats”. Decomposers can be added at the end, because they feed on the remains of every organism in the chain.

Most chains have three to five links. Longer chains are rare, because too little energy remains at the top to support more organisms.

What is a food web?

In real ecosystems, most animals eat more than one type of food, and most are eaten by more than one predator. A food web is a diagram that combines many food chains to show all of these links.

Food webs are more realistic and more stable than single chains. If one prey species disappears, a predator that has other food sources can survive. However, some species have an especially large effect. A keystone species, such as the sea otter in kelp forests, controls the numbers of other species so strongly that its removal can change the whole web.

Webs also help to predict the effect of changes. If a pesticide kills insects, birds that eat them lose food, and the plants that insects used to eat may increase.

What are trophic levels?

A trophic level is the position of an organism in a food chain, counted by the number of steps from the producers.

• Level 1, producers: Plants, algae and other photosynthetic organisms.
• Level 2, primary consumers: Herbivores that eat producers.
• Level 3, secondary consumers: Carnivores that eat herbivores.
• Level 4, tertiary consumers: Carnivores that eat other carnivores.
• Level 5, quaternary consumers: Top predators in long chains.

Omnivores belong to different levels depending on what they eat. A person eating wheat is at level 2, and the same person eating beef from a cow is at level 3. In a web, an organism can therefore have a fractional trophic level.

How does energy flow through ecosystems?

Energy enters most ecosystems as sunlight. Producers convert a small fraction, often only a few percent, into chemical energy stored in organic compounds. When a consumer eats a producer, it receives only part of that energy.

At each step, a large amount of energy is lost. Organisms use energy for respiration, movement and keeping warm, and this energy is released as heat. Some parts of the food are not eaten, such as roots or bones, and some are not digested and leave the body as waste.

As a rough rule, about 10 percent of the energy at one trophic level is stored in the bodies of the next level. The exact percentage varies widely, from a few percent to around 20 percent, depending on the organisms and the ecosystem. The rule explains why there are many more plants than herbivores, and many more herbivores than carnivores.

Energy flows in one direction and does not recycle. Matter such as carbon and nitrogen, in contrast, is recycled by decomposers.

Trophic levels compared

Worked example: energy along a food chain

Suppose the grass in a field captures 20 000 kilojoules (kJ) of energy per square meter in a year. Use the ten percent rule to estimate the energy available at each level of the chain grass → grasshopper → frog → snake.

1. Level 1, grass: 20 000 kJ.
2. Level 2, grasshoppers: 10 percent of 20 000 is 2 000 kJ.
3. Level 3, frogs: 10 percent of 2 000 is 200 kJ.
4. Level 4, snakes: 10 percent of 200 is 20 kJ.

The snakes receive only 20 kJ out of 20 000, which is 0.1 percent of the original energy. This explains why a field can support thousands of grasshoppers but only a few snakes. The calculation is an estimate based on a simplified rule, and real transfer efficiencies differ from one link to another.

Now check a related idea: if the same grass energy fed a herd of cattle (level 2) that people eat, 2 000 kJ would remain for people at level 3. If people ate the grain directly at level 2, they would have access to ten times more energy from the same area. This is one reason why plant-based diets use land more efficiently.

How did these models develop?

The term food chain was used by Charles Elton in his 1927 book Animal Ecology, where he also described the pyramid of numbers. Raymond Lindeman developed the trophic-dynamic idea in 1942, tracing energy through the levels of a lake.

The ten percent figure became popular through teaching, but it is only an approximation. Measured efficiencies vary and are often reported as roughly 5 to 20 percent.

Why are food webs useful today?

Fisheries managers use food webs to decide catch limits, because removing a large predator can allow smaller fish to increase and then reduce plankton. Conservationists map webs to predict what a species loss will do to the rest of the ecosystem.

Webs also show how toxins travel. Pollutants that do not break down can accumulate in the tissues of animals at higher trophic levels, a process called biomagnification.

Which mistakes are common?

Mistake 1: Drawing arrows in the wrong direction

Arrows point from the food to the feeder, showing the movement of energy. A rabbit eats grass, so the arrow goes from the grass to the rabbit.

Mistake 2: Starting a chain with an animal

Every chain begins with a producer, because energy enters the living world through photosynthesis or chemosynthesis.

Mistake 3: Saying that energy is recycled

Energy is transferred and eventually lost as heat. Nutrients are recycled, not energy.

Mistake 4: Thinking that the 10 percent rule is a fixed law

It is a guideline. The exact fraction depends on the organisms and conditions.