A cause starts a change, and an effect follows from it. In a chain, each middle link is an effect of the one before and the cause of the next.
This lesson opens environmental sustainability. The next lesson is interpreting conservation data.
How do you build a chain?
- Find the first change in the scenario.
- Ask “and so what happens?” to find the next link.
- Repeat until the last effect in the question.
- Join the links with “so” or “which leads to”.
Write the chain on one line first. It is also a plan for a longer answer.
Worked example: a river chain
This scenario is invented. A farm beside a river uses extra fertiliser, and rain washes some into the river.
- Cause: fertiliser runs off into the river.
- Effect, and cause of the next: extra nutrients make algae grow fast.
- Effect, and cause of the next: the dense algae block light, so plants below die.
- Effect, and cause of the next: decomposers use up a lot of the oxygen as they break down the dead matter.
- Final effect: fish and other animals lack oxygen, so their numbers fall.
Now read the chain backwards with “because”. Fish fall because oxygen falls, oxygen falls because decomposers use it, and so on back to the fertiliser.
The reversal check
| Statement | Check | Verdict |
|---|---|---|
| Fish die, so oxygen falls | Did fish dying come first? | Reversed: low oxygen killed them |
| Algae grow, so nutrients rise | Nutrients came first | Reversed |
| Nutrients rise, so algae grow | Correct order | Correct |
A reversed statement can sound reasonable, so test each one by asking which came first.
The mistake that costs marks
The common error is to list the effects without links, such as “fertiliser, algae, fish die”. The marker cannot tell whether you understand how one step leads to the next.
A second error is to put a consequence in the cause position, for example “fish die, which causes low oxygen”.
Check yourself
Invented scenario: a hillside is cleared of trees, and rain washes soil into a stream. Write a four-link chain.
Answer
Trees are cleared, so the soil is no longer held by roots. Rain then washes soil into the stream, which makes the water muddy and blocks light. Plants in the stream get less light, so they grow less and the animals that feed on them decline.
Any chain that keeps the correct order and uses linking words earns the marks.
What to study next
Go on to interpreting conservation data. The biological data interpretation trainer gives more chains to test, and the mistake log is a place to record reversed links.
If you want a teacher to mark your chains and point out reversed links, see online one-to-one Biology tuition.