Food chains and food webs

A series of organisms feeding on one another forms a food chain. Each step is a trophic level.

Producers come first in every food chain, because they alone fix solar energy and make it available to everything above them. That phrasing — fix up the solar energy and make it available to the heterotrophs — is what the marking scheme looks for.

In reality an organism eats more than one thing and is eaten by more than one thing, so chains interconnect into a food web.

The ten per cent law

Green plants capture only about 1% of the sunlight falling on their leaves — so about 99% is not converted into food energy, which is a favourite MCQ.

Of the energy that is captured, only about 10% passes to the next trophic level. The other 90% is lost as heat, and used in the organism's own life processes.

Working the standard numerical: 20,000 J at the producers gives 2,000 J to herbivores, 200 J to small carnivores, 20 J to large carnivores. Divide by ten at each step.

This is why food chains are generally only three or four steps long. After four levels so little energy remains that it cannot support another one. It is also why the number of organisms shrinks at each level, and why there are far fewer top carnivores than plants.

Biomagnification

Pesticides sprayed on crops are washed into the soil and water, taken up by plants, and eaten. They are not broken down and not excreted — so they accumulate in the body at each level, and the concentration rises as you go up.

Human beings, at the top of most food chains, get the highest concentration. In an ordinary chain the tertiary consumer carries the most pesticide, the producer the least.

Watch the assertion–reason trap here. "A vulture has the least pesticide because it occupies a high trophic level" is false reasoning — a high trophic level means more, not less. Read which direction the claim runs before answering.

Removing a trophic level

Pull out one level and the effect runs both ways, which is where the 3-mark answers are won:

In plants → deer → lion, remove the deer and the lions starve for want of food, while the plants overgrow with nothing eating them.

Remove the lions instead and the deer multiply unchecked, overgraze, and the vegetation is destroyed — so the food chain is affected, even though the level above is gone.

Remove the plants and the whole chain collapses, because nothing else can fix solar energy.

What the board actually asks

About 3 marks a paper across 2023–2026, and the most reliably examined topic in Our Environment.

• Construct a food chain from a jumbled list (hawk, snake, plants, rat → plants → rat → snake → hawk) and name the trophic level of one organism.
• Which group does not form a food chain? — look for a set with no producer, or with the second trophic level missing.
• Why are food chains limited to three or four steps? — the ten per cent loss at each transfer.
• Energy numerical — divide by ten per level.
• Biomagnification — how harmful chemicals accumulate progressively, and who ends up with the most.
• Assertion–reason on pesticide levels, where the reason is often right but pointed the wrong way.

The commonest lost mark is naming producers without saying why they are first. "They are plants" earns nothing; "they fix solar energy and make it available to consumers" earns the mark.