A food web shows several connected food chains, so changing one population can affect others in more than one way. The diagram supports likely effects, not a single certain result. You will see this in ecology and energy flow, usually as “suggest what may happen to the population of X if Y is removed”.
How do you reason through a change?
Every effect travels along an arrow, so follow arrows both ways.
- Downwards effect: the organism that fed on the removed one loses a food source. It may fall in number, or it may switch to another food.
- Upwards effect: the organisms that the removed one used to eat are no longer eaten by it. They may increase, unless something else eats them.
- Knock-on effect: a change in one population can change a third, such as a plant or alga that the increased organism feeds on.
Always ask whether the affected organism has other links in the web. More links usually mean more options.
Worked example
A pond food web (invented for practice) has these feeding links:
- algae → snail, algae → tadpole
- snail → fish, tadpole → fish, tadpole → dragonfly nymph
- fish → heron
A disease removes most of the fish. Suggest what may happen to the snails, the algae and the heron.
Step 1, snails: fish eat snails, so predation on snails falls. Snail numbers may increase, because fewer are eaten.
Step 2, algae: more snails may eat more algae, so the algae may decrease. Tadpoles also eat algae and fish also eat tadpoles, so tadpoles may increase too, which could reduce algae further.
Step 3, heron: the heron shown in the web eats only fish, so its food supply falls and its population may decrease. If the heron also eats other animals not shown, the effect could be smaller.
Step 4, state the limit: the web does not show every organism or how large each population is, so these are likely effects rather than certain ones.
The mistake to watch for
A common error is to give one certain chain of consequences as if the web were a machine.
Mistaken answer: “The fish are gone so the snails will increase, so the algae will all disappear, so the pond will die.”
The student used “will” and “all”, ignored that tadpoles, dragonfly nymphs and other links exist, and jumped to an extreme.
The correction is to use cautious words, give a reason from the web for each effect, and avoid extremes unless the data support them. A better answer: “Snail numbers may increase because fewer are eaten by fish. More snails may eat more algae, so algae may decrease.”
Check yourself
1. In the pond web above, a new pollutant kills all the algae. Suggest one effect on snails and explain it.
Show answer
Snail numbers may decrease because algae is their food source, so there is less to eat. Tadpoles, which also eat algae, may decrease too.
2. Dragonfly nymphs in the web eat only tadpoles. Suggest what may happen to them if tadpoles are removed, and why the answer is uncertain.
Show answer
Dragonfly nymph numbers may decrease because their only food shown has gone. It is uncertain because they might eat other prey that the web does not show.
3. Rewrite this so it is better: “If the heron is removed, the fish will take over the whole pond.”
Show answer
“If the heron is removed, fish numbers may increase because fewer are eaten, but other factors such as food supply could limit the increase.” It avoids “will” and “whole pond” and gives a reason.
Where this leads next
To check your reasoning against a labelled web, use the food-web and energy-flow explorer, which shows more than one plausible effect. Then look at calculating a sampling estimate to see how biologists get the population numbers behind such graphs.
If you keep writing answers that are right in idea but wrong in strength of claim, online one-to-one Biology tuition can help you practise the calibrated wording directly.