To compare sites fairly, convert each count to a mean per quadrat or to a density. Raw totals mislead when the sites were sampled with different numbers of quadrats.
This lesson is in ecosystems. Scaling a sample up to a whole-habitat estimate is covered in sampled count versus population estimate.
How do you calculate density from quadrat data?
Add the counts, divide by the number of quadrats for the mean, then divide by the quadrat area. Always show the unit.
For example, 5 quadrats of 0.25 m² give counts 3, 5, 4, 6 and 2. The total is 20, the mean is 4 per quadrat, and the density is 4 ÷ 0.25 = 16 per m². Check: 20 ÷ (5 × 0.25) = 20 ÷ 1.25 = 16.
Worked example: two sites, unequal effort
This data is invented. Site A used 6 quadrats of 1 m² and counted 30 plants. Site B used 10 quadrats of 1 m² and counted 40 plants.
| Site | Quadrats | Total | Mean per m² |
|---|---|---|---|
| A | 6 | 30 | 5 |
| B | 10 | 40 | 4 |
Site B has the larger total, but Site A has the higher density, 5 per m² against 4 per m². The raw totals would give the wrong conclusion.
Explaining a zone difference
A student samples a shaded and an open zone with the same number of quadrats. The mean count is 4 per quadrat in the shade and 9 in the open zone.
A full answer: “The open zone has a higher mean count, 9 against 4 per quadrat. This may be because more light reaches the open zone, so the plants can photosynthesise more and grow better.”
The word “may” is deliberate. The survey shows a difference, and the cause is an explanation, not a tested fact.
The mistake that costs marks
The common error is to compare totals when the number of quadrats differs. A second error is to give the density without a unit, or with the wrong unit.
| Step | Wrong | Right |
|---|---|---|
| Compare | B is denser because 40 is bigger than 30 | A is denser, 5 per m² against 4 per m² |
| Unit | 16 | 16 per m² |
Check yourself
Site C used 8 quadrats of 1 m² and counted 52 plants. Site D used 5 quadrats of 1 m² and counted 35. Which is denser?
Answer
Site C: 52 ÷ 8 = 6.5 per m². Site D: 35 ÷ 5 = 7 per m².
Site D is denser, although C has the larger total.
What to study next
Test the skill in the ecosystems practice set. The graph evidence comparison lab gives you more data to compare, and the mistake log tracks your slips.
If you want a teacher to check your comparisons, see online one-to-one Biology tuition.