Real animals do not eat one thing. Join the food chains of an ecosystem together and you get a food web — messier to draw, far more honest, and the tool examiners use to ask the best question in this topic: what happens if one species disappears?
📚 What you need to know
A food web is a network of interconnected food chains.
Webs are more realistic than chains because consumers rarely rely on a single food source.
Arrows still show the direction of energy flow, from the eaten to the eater.
Food webs show interdependence: a change in one population affects others.
A species with only one food source is hit hard if that source declines; a species with alternatives is affected less.
An organism can sit at more than one trophic level in the same web.
More links generally means more resilience, because energy can reach a consumer by another route.
A woodland food web
Trace a single path through this web and you have a food chain. The web is just all the chains drawn at once.
🧩 How to read a food web in an exam
Find the producers at the bottom and work upwards.
For the species in the question, list what it eats and what eats it.
Count how many food sources it has. One source means high risk; several means it can switch.
Follow the effect one step at a time, and say whether each change is large or small.
Remember producers respond too: fewer herbivores usually means more plants.
Interdependence: pulling one thread
Suppose disease wipes out most of the voles in the web above. Do not guess — trace the arrows.
Notice the answers point in both directions: species above the vole decline, species below it increase.
Do not stop at the first ring of effects if the question offers more marks. Fewer foxes means more rabbits, which means more grazing pressure on the grass again. Two or three steps of reasoning usually matches the marks available.
Why webs are more useful than chains
They show all the feeding relationships, not one selected path.
They reveal which species are vulnerable because they depend on a single food source.
They show which species are important connectors, with many arrows in and out.
They make resilience visible: a web with many links can absorb the loss of one species better than a simple chain.
Worked examples
WORKED EXAMPLE
Using the web above, state what the owl eats and give its trophic levels.
Step 1: read the arrows into the owl
Arrows come from the vole and from the robin.
Step 2: work out each path
Grass → vole → owl puts the owl at level 3. Bramble → caterpillar → robin → owl puts it at level 4.
The owl feeds at trophic levels 3 and 4Two feeding routes means two trophic levels. Give both, and say which path gives which.
WORKED EXAMPLE
Predict and explain the effect on rabbits if all the bramble were cleared from the wood.
Step 1: who loses food first
Caterpillars lose their food source, so caterpillar numbers fall. Voles lose one of two food sources, so they fall a little.
Step 2: follow it up
Fewer caterpillars means fewer robins, so the owl relies more heavily on voles.
Step 3: reach the rabbits
With fewer voles available, foxes hunt rabbits more, so rabbit numbers are likely to fall.
Rabbit numbers fall, through increased predation by foxesThe rabbit never ate bramble. The effect reaches it entirely through the predators.
WORKED EXAMPLE
Explain why a food web with many links is more resilient than a simple food chain.
Step 1: what happens in a chain
In a chain each consumer has only one food source, so losing one species removes the food supply of everything above it.
Step 2: what happens in a web
Consumers have alternative prey, so if one species declines they can switch to another.
Step 3: the conclusion
Energy can still reach the higher levels by another route, so the ecosystem absorbs the disturbance instead of collapsing.
More links means more alternative energy routesThis is the same resilience idea used in the sustainability pages — use the word.
💡 Exam tip
Answer with direction and size: “falls sharply” or “falls slightly”, with the reason attached.
Count a species’ alternative food sources before predicting how badly it is hit. That is where the explanation mark sits.
Remember effects travel downwards too. Fewer herbivores means more plants.
Only use the information in the web given. Do not add feeding relationships from your own knowledge.
If asked to add a species, draw arrows in both directions as needed — what it eats and what eats it.
⚠ Common mix-up
Reversing arrows. They show energy flow, so they point towards the consumer.
Saying a species will die out when it has other food sources in the web.
Ignoring the producers when a herbivore declines. Plant numbers usually rise.
Assuming one trophic level per species. In a web an animal can feed at two or more.
Stopping after one step when the question is worth three marks.
Treating a food web as a food chain and reading only the path you noticed first.
Up next: Why Energy Is Lost at Each Level — the four places energy leaks out between one trophic level and the next, and how to calculate the efficiency.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
