A quadrat count is a sample. To estimate the population, find the mean count per quadrat and multiply it by the number of quadrat areas that fit in the habitat.
This lesson belongs to ecological evidence and population relationships. The next lesson, on energy transfer, asks for the same careful reading of data.
What is the route from count to estimate?
- Count the organisms in each quadrat and add the counts.
- Divide by the number of quadrats to get the mean per quadrat.
- Divide the mean by the quadrat area to get a density per m².
- Multiply the density by the area of the habitat.
Write the unit on every line. Density is per m², and the final answer is a number of organisms.
Worked example: a quadrat estimate
A student places 10 random quadrats, each 1 m², in a field of 400 m². The counts of one weed are 3, 4, 2, 5, 3, 4, 3, 2, 4 and 5.
The total is 35, so the mean is 35 ÷ 10 = 3.5 per m². The estimate is 3.5 × 400 = 1 400 weeds.
The count of 35 is the sample. The estimate of 1 400 is for the whole field, and it assumes the quadrats were random and the weeds are spread fairly evenly.
Worked example: capture-recapture
A biologist marks 40 beetles and releases them. A later sample has 30 beetles, and 6 of them are marked.
Estimated population = (40 × 30) ÷ 6 = 1 200 ÷ 6 = 200 beetles.
The method assumes the marked beetles mixed fully, no beetles left or arrived, and the mark did not harm them or make them easier to catch.
The mistake that costs marks
The usual error is to report 35 as the number of weeds in the field. That is only the count inside the ten quadrats.
A second error is to leave out the assumptions. A full answer says what the estimate depends on, such as random placement or no migration.
| Step | Wrong | Right |
|---|---|---|
| Report | 35 weeds in the field | About 1 400 weeds, estimated |
| Method | Sum only | Mean, then scale to the area |
| Assumptions | (missing) | Random quadrats, even spread |
Check yourself
A student uses 12 random quadrats of 1 m² in a field of 600 m² and counts 54 plants in total. Estimate the population and state one assumption.
Answer
Mean = 54 ÷ 12 = 4.5 per m². Estimate = 4.5 × 600 = 2 700 plants.
One assumption: the quadrats were placed randomly, so they represent the whole field. Another acceptable answer is that the plants are spread evenly enough for the sample to be typical.
What to study next
Go on to energy transfer without claiming that energy cycles. For reading more data tables, see interpreting population sampling data.
The biological data interpretation trainer gives you practice, and online one-to-one Biology tuition is there if you want a teacher to check your working.