Real animals do not eat one thing. Join all the chains in a habitat together and you get a food web — messier to look at, but far closer to the truth. Webs are where exam questions about knock-on effects come from, so the skill to practise is reading the arrows, not memorising the picture.
📘 What you need to know
A food web is a network of food chains linked together.
Webs are more realistic because most consumers eat several different foods.
Arrows still point towards the eater and still show energy and biomass moving.
Webs show interdependence — a change in one population affects others.
An organism can occupy more than one trophic level in the same web.
A species with several food sources is affected less by losing one of them.
What a web adds that a chain cannot
A food chain tells you one route the energy can take. A web tells you all of them at once. That matters because it lets you answer the question examiners love: what happens if this species disappears?
Trace grass to earthworm to sparrow to hawk and you have pulled a single food chain out of the web. There are several others hiding in the same diagram.
When you are handed an unfamiliar web in an exam, do not try to take it in all at once. Put your finger on the species named in the question and only look at the arrows touching it.
Predicting knock-on effects
This is the classic question. A species drops in number — what happens to everything else? The reasoning is always the same two moves: look at what it ate, and look at what ate it.
🧩 How to answer a “what if this species disappears” question
Look below it. Whatever it used to eat now has fewer predators, so that population increases.
Look above it. Whatever ate it now has less food, so that population decreases.
Check for alternatives. If a predator has other foods in the web, the fall is slight. If this was its only food, the fall is large.
Say the size of the effect, not just the direction. “Decrease slightly, because it can switch to caterpillars” is a full answer.
Using the web above, suppose the earthworm population crashes:
Grass increases — one of the things feeding on it has gone.
Mice fall sharply — earthworms are their only food in this web.
Sparrows fall slightly — they lose one food source but still have caterpillars.
Foxes fall sharply — they depend on mice, which have crashed.
Hawks fall a little — fewer mice, but sparrows are still around.
The pattern worth remembering: the more arrows going into a species, the safer it is. Species that feed on only one thing are the fragile ones, and they are usually the ones the question is really about.
One organism, two trophic levels
In a web, an omnivore often appears at more than one level. A fox that eats mice is a secondary consumer along one route; if the same fox also eats berries it is a primary consumer along another. Neither answer alone is complete — give both.
WORKED EXAMPLE
Using the food web above, write out two different food chains that each contain four organisms.
Step 1: Start at the producer every timeGrass is the only producer in this webStep 2: Follow the arrows without going backwardsgrass → earthworm → mouse → foxgrass → caterpillar → sparrow → hawkTwo valid four-step chainsgrass to earthworm to sparrow to hawk also works
WORKED EXAMPLE
In the same web, a disease wipes out the caterpillars. Explain the effect on the sparrow and hawk populations.
Step 1: Sparrows firstSparrows eat caterpillars and earthworms, so one of two foods is gonesparrows decrease, but only slightlyStep 2: Then the level aboveHawks eat sparrows and mice, and mice are unaffectedhawks decrease very slightly, or stay about the sameSmall drop in sparrows, smaller drop in hawksEffects fade as you move further from the species that changed
💡 Exam tip
Give the direction and the size of every change: increase, decrease sharply, decrease slightly.
Always justify with the web: “because earthworms are its only food source” is what earns the mark.
Count the arrows into a species before you answer. That single check decides sharply versus slightly.
When writing out a chain, never skip a step and never reverse an arrow.
If asked why webs are better models than chains, say consumers rarely feed on only one food source.
⚠ Common mix-up
Answering only one direction. Losing a species affects both what it ate and what ate it.
Forgetting alternative foods. A predator with three foods barely notices losing one of them.
Reversing arrows in a web. Easier to do here than in a chain because the lines are diagonal — check each one.
Giving an organism a single trophic level when the web clearly shows it feeding at two.
Saying a population “dies out”. Unless it had one food source only, a decrease is the safer and usually correct answer.
Up next: Why Energy Is Lost at Each Level — the four reasons a trophic level never passes on everything it receives.
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