To assess evidence for a habitat change, state what the data show, measure how big the change is, and say how far the data support a conclusion about cause. This appears whenever a question gives you survey results from a forest, wetland, reef or farmland and asks what has happened.
It belongs to human influence on ecosystems and uses the quadrat ideas from calculating a sampling estimate.
What are the three parts of a good answer?
- The observation: what changed, with figures and units.
- The size of the change: a difference or a percentage change.
- The limit: what the data cannot tell you, for example the cause.
Examiners reward all three together. An answer with only the first part describes. An answer with all three evaluates.
How do you calculate the change?
Use the same method every time.
- Difference = new value − starting value.
- Percentage change = difference ÷ starting value × 100.
A negative answer means a decrease. Always divide by the starting value, not the new one.
Worked example
Invented data for practice. A survey team counts mangrove saplings in ten 1 m² quadrats along the same stretch of shore, in the same month, in two years.
| Year | Mean saplings per m² |
|---|---|
| 2016 | 14 |
| 2026 | 9 |
Question: Assess the evidence that the mangrove habitat has changed.
Step 1, observation: the mean fell from 14 to 9 saplings per m².
Step 2, size: difference = 9 − 14 = −5 per m². Percentage change = −5 ÷ 14 × 100 = −35.7%, so about 36% fewer saplings.
Step 3, limit: the data show a fall at this stretch only. They do not show why. Clearing, a change in salt level or one dry year could each produce this result, so more evidence is needed before naming a cause.
Model answer: “The mean number of saplings fell from 14 to 9 per m², a decrease of about 36%. This suggests the habitat has become less suitable for sapling growth, but one site and two years cannot show the cause.”
The mistake to watch for
A common slip is to treat a change as proof of a cause.
Mistaken answer: “The number of saplings fell because people cut down the mangroves.”
The data show only that saplings fell. Cutting is one possible cause among several, and the table says nothing about it.
The correction is to separate the two steps: report the change, then say the data are consistent with a cause only if other evidence, such as a cleared area on a map, supports it.
Check yourself
Try these first, then open each answer.
1. An invented survey finds a mean of 40 fish per net in 2018 and 30 per net in 2025. Calculate the percentage change.
Show answer
Difference = 30 − 40 = −10. Percentage change = −10 ÷ 40 × 100 = −25%, a 25% decrease.
2. Why is percentage change more useful than the difference when comparing two sites?
Show answer
Sites can start with different numbers. A fall of 5 from 10 is much larger in proportion than a fall of 5 from 100. Percentage change puts both on the same scale.
3. A table shows bird species at one wetland fell from 18 to 12. A student writes “the drainage caused this.” Rewrite the claim so it fits the evidence.
Show answer
“Bird species fell from 18 to 12, a decrease of 33%. This is consistent with habitat loss, but the data alone do not show drainage was the cause.” (6 ÷ 18 × 100 = 33.3%.)
Where this leads next
Once you can report a change and its limits, follow one cause through to its effects in explaining eutrophication through a causal chain, then try the human influence on ecosystems practice set. The inheritance model board and probability tree board show the same habit of separating a model from a small observed sample.
Students who can calculate but still lose marks on wording are often missing the third part of the answer. A teacher can spot that pattern quickly in online one-to-one Biology tuition.